DEVELOPMENT OF A FORMALDEHYDE ANALYZER
FOR MOTOR VEHICLE EXHAUST EMISSIONS PERIOD COVERED:
JULY 1, 1970 TO JUNE 30, 1971
Tifnothy H. Johnston, et al
30 August 1971
DISTRIBUTED BY:
Kfiri
National Technical Information Service
U. S. DEPARTMENT OF COMMERCE
5285 Port Royal Road, Springfield Va. 22151
This document has been approved for public release and sale.
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DYKASCIENCES CORPORATION
INSTRUMENT SYSTEMS DIVISION
.SUBSIDIARY OF WHITTAKER CORPORATION
JINAL REPORT
DEVELOPMENT OF A FORMALDEHYDE ANALYZER
FOR
MOTOR VEHICLE EXHAUST EMISSIONS
PERIOD COVERED:
JULY 1. 1970 TO JUNE 30, 1971
CONTRACT NUMBER: CPA 70-170
PREPARED FOR
DEPARTMENT OF HEALTH, EDUCATION AND WELFARE
DURHAM, NORTH CAROLINA
AUTHORS: TIMOTHY R. JOHNSTON
ROLAND V. MARCOTE
BAMESH CHAND
APPROVED:
5. EISENBERC, KAHAGER
ENVIRONMENTAL PRODUCTS D
REPORT NO.: 171
DATE: AUGUST 30, 1971
BIBLIOGRAPHIC DATA I'- Rtpm No.
SHEET V tAPTD|0955
d. Yiik .cd Sokicle
Development of a Formaldehyde Analyzer for Motor Vehicle
Exhaust Emissions
Timothv H. Johnston. Roland V, Marcote. Ramesh Chand
Pefforaiag Organizalton Nane and Adojcia
Dynascienees Corporation
Instrument Systems Division
Subsidiary of Whlttaker Corporation
IX. SpoOBOrifig OlcaaiKatioa Name and AddrcBi
ENVIRONMENTAL PROTECTION AGENCY
Durham, North Carolina
X Hocipient'a Acceaiiop No.
'import Bat.
August 30. 1971
I- Perfofoiiag OgafLization Rept.
"" 171
10. PrOfeel/Taซk/Worb Una No
CPA 70-170
. Type of Report & Period
Covered
Final
15. Supplenenterjr Nate*
The purpose was to develop an electrochemical transducer which would selectively oxidize
formaldehyde in the midst of all of the other gas species present In the exhaust stream
of internal combustion engines. The current generated by this device would be amplified
and displayed In a manner which would permit quantitative analysis of the formaldehyde
concentration. The sensor which was developed was not as selective as proposed. However,
its lack of selectivity applies only to members of the aldehyde family. It is equally
sensitive to all aldehydes. Hundreds of potential electrode-electrolyte-membrane combina-
tions were evaluated and assessed for potential utility. Electro-oxidation was possible
only in a basic electrolyte. Since this solution vas..not compatible with-a severely aci-
dic combustion environment certain compromises were made to effect a workable sensor.
Basically, the sample was diluted and the sensitivity of the sensor waa Increased. The
problem of evaluating the sensor's performance necessitated the completion of two other
significant achievements: production of a reliable formaldehyde standard in the Ippm
concentration range; and development of a wet method of analysis which would give accur-
ate yet fairly rapid measurements at this low level of pollutants The first requirement
iiajL aSficftaHKp Ehป oraaUnn nf an alnha-polvoxvmethvlene permeation tube, the second
he chromotropi
17. Key Vordl aBd Docoacm Aoalyaia17o- DeKiiptois
Air pollution
Toraaldehyde
Chemical analysis
Motor vehicles
Exhaust emissions
Electrochemistry
Oxidation
Internal combustion en{
Callbratini
17k Identifiera/Opei
by a unique application of t
method of analysis.
omotroplc acid.
I7c. COSATI Field/Group
IB. Amiability Statement
Unlimited
FOHM NTI*->I (Iff-TOJ
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TflLE Of CONTENTS
1.0 INTRODUCTION
2.0 DISCUSSION
2.1 E lectrochenicet Transducers
2.2 Sweep Vottasmuetry
2.2.1 Principles
2.2.2 Instnmantation
2.2.3 Experiecntat Procedure
3.0 EXPERIMENTAl. RESULTS
3.1 Sweep Votts try Studies
3.1.1 Acidic Electrolytes
3.1.2 Basic and geutrel Solutions
3.1.3 Selected Electrolytes
3.2 Stsudsrd Porrsidehpda Source
3.2.1 Evaluation at Various Sources
3.2.2 thsracteristics of Pormsldshyde Source
3.2.3 Wet Otemical Anetysis of Formaldehyde
3.3 Evaluation of X e
3.3.1 Screening Studies
3.3.2 Development of final Membrane
3.4 Evaluation of Electrolytes
3.4. 1 Screening Test and Rasulto
3.4.2 Discussion of the Results
3.4.3 Electrolyte-Exhaust Screen Compatibility tests
3.5 Evstustioo of Coucter Electrodes
3.5.1 SeC f* W E1 Results
3.5.2 Compettbititp end Pabrication Studies
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TA3LR OF CONTENTS - Continued
3.6 Evaluation of Sensor Electrodes
3.6.1 Pletinum Electrodes
3.6.2 Cold Electrodes
3.6.3 Psllsdiun sad Rhodium Electrodes
3.6.4 Sensing Electrode-Exhaust Stres a Compatibility Studies
3.1 Expericssntel Transducers
3.8 Selection of the Vinal Cell Configuration
4.0 PROTOTYPE CRARACTERIZATION
4.1 Interference Studies
4.1.1 Signal Interference
4.1.2 Physical Interference
4.2 Stability Studies
4.2.1 Span Stsbility
4.2.2 Zero Stsbitity
4.2.3 Noise Level
4.3 Linsarity
4.4 Temperature Effects
4.4.1 Sigoal Interference
4.4.2 Physical Interference
4.5 Life tine
4.6 Evaluatton of Sampling Techniques
4.6.1 Redesign of Cell Sample Area
4.6.2 Ambient Sampling Conditions
4.6.3 Auto Exhaust Sampling Conditions
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INSmUMWrSYSJUlsrnva!: J
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LIST OF FIGURES
Figure No . L Es t
1 Electrochemical Transducer (Schematic) 3
APPENDIX
2 Potentioatetjc Control of An Electrochemica). Transducer 4
A Instrument Operation and Design 66
3 Cyclic Voltemmetric Instrunantation 7
B Parts List 79
4 Sweep Voltanunetric Call 8
5 Oxidation of Formaldehyde in 1.0 N Acetic Acid 11
6 Black Platinum Electrode Kinetics in 1.0 N 13
7 Oxidation of Formaldehyde in 1.0 N R 2 S0 4 , Platinum 14
Electrode
8 Oxidation of Formaldehyde in 1.0 N u 2 so 4 , Cold Electrode 14
- 9 Oxidation of NO 2 , NO, and $02 in 1.0 N 11 2 S0 4 , Gold 16
Electrode
Voltaimnograra of Formaldehyde in 10% NaOR 18
11 Voltenmogran of Formaldehyde in 2% NaOR 19
12 Formaldehyde Oxidation in 10% 1(2003 21
13 Oxidation of Formaldehyde in 10% 1(2003 and lOt 101003 23
14 Formaldehyde Permeation Rate versus Temperature 28
15 cm Permeation Tube
15 Formaldehyde Permeation Tubas, Flow Rate versus 29
Temperature
16 Formaldehyde Concentration versus Detection Current 61
17 Current Output versus Temperature 62
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Q4SWUMBU smmsorais osi
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t.IST OP TABLES
Electrolyte Screening Study
Counter Electrode Screening Study
Effect of Pt Plating on Response Time
Chemical Treatment of Pletinine Electrodes
Signal Interference Detertsinations, Exhaust Cell
Signal Interferences, Ambient Cell
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1.0 wnoDuCTl i
This is the final report sumnarizing the activities end results in the
development of a formaldehyde analyzer for motor vehicle exhauat emissions. This
work proceeded under the authority of the Environmental Protection Agency. The
contractoal designation wee A 10-170.
The purpose oS ak4eeUort was to develop en elactrochenical transducer
which would eelective iy oxidize formaldehyde in the midst of all of the other gee
species present in the exhaust stream of Internet combustion engines. The current
gensrated by this device would be amplified and dieplayod in a manner which would
permit quantitative analysts of the formaldehyde concentration. -
The sensor which was developed was not as selective as proposed. However,
ita lack of selectivity appliae only to members of the aldehyde family. It is
equally seneitive to all aldebydee.
Development of the final sensor was accomplished by means of a thorough
screening program in whictr hundreds of potential electrodeelectrolyte-membrane
combinations were evaluated and assessed for potential utiIity. The nta่er of
alternatives in the selection of the final configuration was ec-verely limited by
the fact that electro-oxidation was possible only in a basic electrolyte. Since
thia solution was not compatible with a severely acidic combustion environment
certain compromises were made to effect a worksble sensor. Basically, the aanrple
was diluted and the sensitivity of the sensor was increased.
The problem of evaluating the senaors performance necessitated the com-
pletion of two other significant achievements: I. Production of a reliable
formaldehyde standard in the Ippa concentration range; and 1. Development of a
wet method of analysis which would give accurate yet fairly rapid maaauremanta
at this low level of pollutant. The first requirement was satisfied by the crea-
tion of an alphapolyoxynetbytena permeation tuba, the second by a unique applica-
tion of the chronotropic acid method of analysis.
Z a k
1
2
3
4
3
6
37
43
4 5
49
36
37
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2.0 DISCUSSION
2.1 Electrochemical Transducers
The heart of the formaldehyde detection system is the transducer; a self-
contained. long-life, totally enclosed electrocheeical cell. It ia a current
generating device in which the absorbed gas molecules are oxidized at a sensing
electrode. The current which is produced is directly proportional to the partial
pressure of the gas in the mixture under analysis.
ฃ amplified schematic of en elect rochemical tranaducer is depicted in
Figure 1 the forealdehyde molecules diffuse through the protective semipermeable
membrane into the thin film of electrolyte. The dissolved species then diffuse to
the eenaing electrode where they undergo electrooxidation. Since the rate of
response is dependent on the rate of diffusion through these two mediums, it is
neceeasry to keep the total thickness to a reinimom.
The eensict electrode is connected externally through the bias network
depicted in Figure 2 to the counter electrode. The counter electrode materiel
usually has a higher oxidation potentiel than the pollutant. The aensing electrode
is a polarizeble electrode which easumes a selective potential maintained by the
counter electrode and adjusted through the use of e stable D.C. power source. ฃ
mertuxy battery ie need so that the potential remains constant when the instrument
power is off. The pollutant molecules upon reaching the sensing electrode encounter
a potential higher than that required for their oxidation. Electrochenicelly. while
the pollutant io being oxidized at the sensing electrode, a corresponding reduction
reaction occurs at the counter electrode. The resulting current is diffusion
limited end is a function of the concentration gradient between the membrane and
the sensing electrode. Since this gradient is directly proportional to the partial
preasure of the furmoldehyde in the sample (a relationship derived through Ficka
Law 2 Diffusion), chga onidation current is directly proportional to the formalde
hyd cc- attn 1La. ถibuo me arc ct lc to exproeo the formaldehyde concentration by
FIGURE 1 ELECTROCHEMICAL TRANSDUCER
3
/ 1//I SENSING ELECTRODE 7/
/ 1 1// i 1 / A
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measuring the generated current. The diffusion current is expressed by the equation:
j nFADC
6
FIGURE 2.
Where S ie the current in ezsps, n is the number of exchanged electrons
SCHEMATIC SIICAJ1NC POTENTIOSTATIC CONTROL OF
per mole of pollutent, F is the Paredey conetent (96,300 coulonbe), D is the dif-
ELECTROCHEMICAL TRANSDUCER TO OSTA1N 0(20 SELECTIVITY 2
fusion coefficient of the gas in cm ieee. , C is the concentration of the formalde-
hyde in the sample and 6 is the thickness of the diffusion layer.
1.35 VOLTS
2.2 Sweep Voltazsnetry
2.2.1 Principles
The use of sweep voltazmnetry as a tool in selecting the most promising
candidate electrolytes has been adopted es standard procedure in this laboratory
for sometime. The technique permits the scanning of en entire voltage range in
a matter of minutes. The same process with an experimental transducer would take
days. The resultant scan is a record of the current changes, due to slectrochemical
reactions at the sensing electrode, as a function of the potential of the electrode.
In the evaluation of sweep voltssssogrems the most useful parameters are
the potentials at which oxidations or reductions occur and the rate of current
chsnge.
The approximate potential of en reduction or oxidation reaction is deter-
mined at the nsximust current value for thet reaction. This is usually the value
at the apex of a peak, which is referred to in the text of this report as the peak
potential. This current peak results from a localized depletion of the reacting
species at the electrode site and is not to be confused with a discontinuance of
the reaction at ensuing potentiale. For an oxidation reaction the peak ia a good
indication of the minimise potential which must be used to achieve efficient oxida-
tion. In the case of a reduction it represents the maximum potential which can
be applied before the reduction process becomes inefficient. The value of the
peak potential is a function of the scan rate. In comparison analysis this
5
SENSDG
(Ag/Ag 2 O )
TRANSDUCfl
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- I l
parameter should be kept constant.
The efficiency end rate of a reection can be evaluated b 7 the ehspe of
the reaction curve. An efficient reaction is characterized by e peak ae oppoaed
to a plateau or continuous slope. A fast reaction rate is indicated by a sharp
peak. S alow reaction is characterized by a broad hump.
Sweep voltauseecry is elso useful in evaluating electrode kinetics and C
C- ,
the competibility of the electrodeelectrolyte couple. Sweep voltainsetry wee
utilized during this coutrect to find en electrolyte which showed selective oxide
4
tion for formaldehyde end which displayed the properties of am efficient oxidatio 1>
medium.
4
2.2 .2 Instrumencstioo
C,
A schematic of the instrumentation package assembled for cyclic voitseine
measurements ia ehown in Figure 3. S potentioatat maintains the voltage between
reference electrode end the working electrode at a value equal to the sum of the I
reference and function generator volteges. The potentiostat accomplishes thie by
controlling the curreet between the working and counter electrodes. It senses thi
difference between the reference and working electrode voltages and compares it tu
the reference input voltegee by means of a high gain smplifier system. The outpml
is fed through the working electrode-counter electrode psir. The system permits Z
the measurement of current-voltagetime relationships under well-defined conditioi
No voltage change is possible st the reference electrode. Thus, the measurements
reflect only che voltsge changes et the working electrodeelectrolyte interface.
independent of ohmic tones through the electrolyte end polarization at the count
electrode.
Voltege emeep measurements ore made in a two conpartemnt cell with three
etectrodas (Figure ). The refecenco chamber contains the reference electrode anc
is tnz -tad to Ow m ln compnrtmet via a small cepillery which acts es the
-3-
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FIGURE 4. SWEEP VOLTAMMETRIC CELL
DTI1 SCIOICES PSTRUMENT SYSTEMS DIV .
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electrolyte bridge. For most electrolytes a saturated calouiel reference electrode
is satisfactory. The working electrode and counter electrode are situated in the
main compartment. The working electrode is usually a noble metal such as a gold
or platinum sines the same terial would be used as the sensing electrode in the
final transducer. The dimensions are held constant at 1.0 x 0.3 cm for ease of
comparison between different materials. The tip of the Luggin capillary is notched
in a vertical direction to allow reproducible positioning of the working electrode.
The working electrode is totally enclosed by a platinum counter electrode to ensure
an omnidirectional charge distribution. Both compartments of the cell are filled
with the same electrolyte.
2.2.3 Experimental. Proced e
The sweep voltamnetry cell is initially washed with an industrial deter-
gent, rinsed with distilled water, imsersed in 507. 111403 acid, and cleaned again
with distilled water. It is then rinsed with the electrolyte to be used in the
experiment and filled with electrolyte. Novually, for subsequent determinations,
the cell need only be rinsed with distilled water and treated with the electrolyte
to be investigated.
The working electrodes are cleaned in hot 507. aqueous BNO , rinsed with
water, imersed in hot 457. sqmeous 7DB, rinsed with water, and washed finally with
the electrolyte.
The reference electrode is washed with distilled water and electrolyte
before use. The platinum c ter electrode is flame annealed and then rinsed.
In the process of gemarating sweep curves the first step is usually to
record the background current for the plain electrolyte. Next, known amounts of
the molecule to be studied axe added either as a solid or a gas. The solution is
then aerated with nitrogen to get rid of all traces of oxygen . The nitrogen flow
La stopped and the sweep curmo is recorded.
-9-
WORKING ELECTRODE LEAQ_,.
REFERENCE ELECTRODE
GAS INLET
PORT
..COUNTER
ELECTRODE
LEAD
GAS INLET TUBE
(TEFLON)
ELECTRODE
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CURRENT, mAJcw. 2
___________________ MSTRIJMENI SYSTEMS DIVISION
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3.0 EXPERIMENTAL RESULTS
3.1 Sweep Voltaninetry Studies
Many sweep curves were made to evaluate the feasibility of rapid, selec-
tive oxidation of formaldehyde. The shape of the various curves allowed us to
predict which electrolytes would be most promising for the rapid detection of
formaldehyde. The potentials of the reactions allowed us to select those electro-
lytes which would permit selective oxidation of formaldehyde. Background traces
were used to determine possible interference from the electrolyte itself.
Determinations were made with gold and platinum electrodes to determine
the relative value of each. All curves were made at a sweep rate of 50 seconds per
cycle. Voltausnograms were also made of 02 and NO 2 containing solutions since these
two exhaust stream components, by themselves, would most likely determine the poten-
tial at which selective oxidation of formaldehyde would have to take place. A
brief explanation of this statement should suffice. Oxygen has a definite reduction
potential which cannot be eliminated. Thus all determinations must be made above
this potential. Nitrogen dioxide is usually both reduced and oxidized over a very
narrow potential range. The transition potential or plateau between the oxidation
and reduction is usually the only voltage where selective oxidation can be recorded
unless the NO 2 can be scrubbed from the sample. With formaldehyde this option was
nut available because the chemical reactivity of the molecule prohibits the presence
of any type of scrubber in the sampling sy8tem.
3.1.1 Acidic Electrolytes
The initial supporting solutions studied were acids.
a.) Aqueous organic acids - Typical curves for the 1.0 N acetic acid
system are shown in Figure 5. The solid curve shows the oxidation-
reduction reactions of 1.0 N acetic acid at a gold electrode after
with nitrogen. At 0.4 V acetic acid is reduced to ethanol.
0.O is oxidized a< eta1dehyde. The upper limiting potential
-10-
S
R JCTION OXIDATIOI
S I S
-. 5 ta M .a 9
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REDUCTION OXIDATI
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in this medium is 0.88 V, a value well below the potential of the most
comonly used counter electrode. As can be seen by the broken line curve
the formaldehyde oxidiation overlaps the acetic acid at +0.6 V. Since
the formaldehyde concentration in the curve is 1000 times larger than
the concentrations found in the sensor it can be seen that if acetic acid
were employed in the transducer an abnormally high and unstable background
current would make the detection of formaldehyde difficult. Electrolytes
composed of 1.0 N formic acid and 1.0 N butyric acid gave very similar
results. It is apparent that most partially oxidized organic molecules
are electrochemically active in the 0.4 to 0.8 potential range in an
aqueous acidic medium. This property makes the use of aqueous organic
electrolytes as a sensing medium impractical.
b. Sulfuric Acid
1.) Platinum sensing electrode - It was reported by Ruck and Griffith* )
that the reproducibility of the black platinum electrode in el .ectro
chemical applications was seriously impeded by the formation of
platinum oxides. For sweep voltananetry evaluations this property
was not an obvious impairment. A sweep of 1.0 N 11 2 S0 4 at a platinum
electrode after treatment with nitrogen is presented in Figure 6.
The oxidiation of platinum coomences at 0.75 V. Reduction of the plat-
mum oxides occurs at 0.72 V. Absorbed hydrogen peaks can be plainly
seen at the lower potentials, 0.1 0.3 V. The sensitivity for this
curv. was 100 tia greater than normal.
Tb. curve for formaldehyde at a platinum electrode is shown in
Figure 7. The primary oxidation peak occurs at 0.89 V during the posi-
tive sweep. Th. positive peak which appears during the negativ, sweep
(1) Journal of the !1.ctroch.mical Society, Nov. 1962 - p. 1005.
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is not part of the oxidation sequence of formaldehyde. It is an
anomaly which was observed throughout the sweep studies. Its poten-
tial was found to be independent of p11 and solute species. Its
disappearance was observed only in hydroxyl bases.
The oxidiation peak at 0.89 V was sharp enough to be indicative
of a fast reaction. However, the oxidation potential was about the
same as carbon aonoxide. Unless a highly selective membrane could
be found this intsrference would make the use of a pl tinuin electrode
inipractical,
2.) Cold Sensing Electrode - A potential sweep of a formaldehyde
solution in 1.0 N H 2 S0 4 with a gold electrode yielded a curve
(Figure 8) very similar to that obtained with platinum. The gold
electrode was not as efficient however. The area under the oxidation
peak was four times smaller than platinum. It should be noted that
the electrode ystem for the detection of formaldehyde must be very
efficient. The absorption factor for formaldehyde in most aqueous
solvent systems is l00L. If all of the absorbed formaldehyde is not
imaediately oxidized at the sensing electrode,the system will be non-
linear and plagued by very slow recovery rates. The gold electrode
was superior to platinum in that interference from carbon monoxide
was not as great a problem.
Interference from NO and NO 2 would not be a problem either as
evidenced in Figure 9, but it was certain that in instances wlmre
SO 2 was contained in the gas stream a gross interference would be
confronted.
OXIDATION OF YOR IALDEHYDE IN 1.0 N H 2 S0 4 AT A PLATiNUM SE: S INC ELEClWD
I.
/
F0RM .DZHTDE
/
N
/
:1
___
0.29 0.4 0.51 0.62 0.73 0.84 0.89 0.95 1.06 1.17
POTENTIAL, VOLTS (N.R.s..)
FIGURE 8
OXIDATION OF FORMALDEHYDE IN 1.0 N H 2 S0 4 AT A COLD SENSING ELECTRODE
:ii __
1
0.5
C 0.62 0.7 0.8 0.87 0.9 1.0
)TENTIAL, VOLTS (N..LE.)
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0X ATI0N CURR T, at/c 2 3.1.2 Msic and Neutral Solutions
a.) 107. Aqueous Sodium I4ydroxide - Formaldehyde exhibits one ox1d.t nn
peak with a gold electrode at -0.172 V in 107. NaOli (Figure 10). The peak
at 40.1 V is not due to formate oxidation. Later studies with sodium
hydroxide and sodium fox-mate did not reveal any substantial fox-mate oxi-
dation peaks in this region. The forinate reduction which starts at -O.55V
was verified with a formate solution. It was also observed that the re-
du tion did not occur until after a formaldehyde oxidation sweep was made.
A voltage sweep of oxygen in this electrolyte revealed that a lower limit-
ing potential of +0.1 V would prevent oxygen inteference in the detection
of formaldehyde.
b.) 27. NaOR plus 0.5 N Sodium Fox-mate - Sweep voltametric studies of this
electrolyte using a gold electrode, Figure ii , revealed that fox-mate oxida-
tion or reduction peaks are absent in the potential range -0.45V to -I0.5V.
The oxidation of formaldehyde in this electrolyte occurs at -0.188V. The
0 oxygen reduction wave begins at +0.lV.
c.) 107. Aqueous Potassium Carbonate - Investigation of formaldehyde oxide-
tton in carbonate and bicarbonate mediums was undertaken because of the
anticipated incompatibility of a hydroxide medium with the CO 2 in the gas
stream. Carbonate and bicarbonate solutions derive their basicity from
disassociation equilibriums due to the weak acidity of carbonic acid.
(1) K 2 C0 3 + B 2 0 p KMcO 3 + KOli
(2) 1C0 3 + 1120 b KOIl + 11 2 C0 3
(3) 11 2 C0 3 ) 1120 + CO 2
Th. possibility that the carbonate and bicarbonat. mediums would be
neutralized by CO 2 was considered to be unlikely, or at th. worst, minimal.
17
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FIGJRE It
VOLTA?4IOGRM4 OF 20, 02 L N 2 IN
0.5N FORMATE + 27 NaOH
AT A COLD ELECTRODE
LUUK LU
3.0
2.0
1.0
0.0
-1.0
-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1
POTENTIAL VC LTS (N.R.E.)
0.2 0.3 0.4 0.5
VOLTA)*fOGRAM OF 20, N 2 AND
02 IN 1O7 NaON
AT A GOLD ELECTRODE
hO
0.8
0.6
0.4
0.2
0.0
0.2
-0.4
0.6
-0.8
1.0
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Voltanisograins of formaldehyde oxidiat ion in these tv solutions were
made with gold and platinum electrodes (Figure 12). The formaldehyde
oxidation curve for gold is broad with a gentle slope indicating a slow
reaction. Th. curve for platinum is quite the opposite. The peak at
0.001 V is fairly sharp. In addition, an oxidation peak occurs at 0.4V.
This is most likely an overvoltage catalysed oxidation which would be
fairly fast. The negative sweep oxidation peak was missing with platinum
but was conspicuously present with gold. This was not expected since
the pH of the carbonate solution was greater than 11. The only difference
between the carbonate and the sodium hydroxide electrolytes was the in-
direct origin of the hydroxide ion. The sensitivity of l07 K 2 C0 3 to
formaldehyde was sufficient enough to warrant further investigation.
Reduction of 02 did not occur above 4O.2V and a plateau at +O.45V was
observed between the oxidation and reduction of N02. Selective oxidation
of formaldehyde in a cmrbonate media would most likely take place at this
potential.
d.) 507. Aqueous Potassium Carbonate. - The pH of this solution was slightly
greater than 12. A potential shift of -0.2 V was observed from the voltages
obtained with 1.07. 1 (2003. The slope and magnitude of the peaks were roughly
equivalent.
e.) 107. Potassium Bicarbonate - Oxidation of formaldehyde in 107. bicar-
bonate at a gold electrode was a very slow reaction. The hump was so broad
that a peak as such vas not clearly defined. At a platinum electrode the
reaction appeared to be more reversible. The primary oxidation peak was at
.225V, th. secondary peak was at 0.68V. The shape of the curve was almost
identical to that obtained in 107. 2 C0 3 .
-20-
> . .
I
C d
.0
14
0
1.
14
5.
4.
3.
2
1
0-.i
0.0
Figure 12. Formaldehyde Oxidation in 10% K 2 C0 3
Formaldehyde Oxidation
o .11 .2 .3 .4 .5 .6
Formate Oxidation
Formaldehyde
PLATINUM ELECTRODE
Formaldehyde
Oxidation
) .
i;
)
Forma te
Oxidation
.2
.1
.2
Potential (NHE)
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f.) 107. Potassium Bicarbonate plus 107. Potassium Carbonate - Sweep curves
obtained in this mixture at a gold electrode were for some reason much
better than those obtained in the individual solutions (Figure 13). The
primary oxidation peak was at O.6V. The peak, although sharper than those
obtained in K CO 3 or KHCO 3 still did not have the properties associated
with a rapid reaction.
g.) IN Potassium ailoride - Sweeps conducted in this neutral electrolyte
did not reveal any distinguishabit oxidation or reduction peaks.
3.1.3 Selected Electrolytes
The most promising electrolyte-sensing electrode combinations as determined
by sweep voltairanetry were:
a.) Platinum Electrode - iN H2S04
b.) Cold Electrode - 107.
c.) Platinum Electrode - 107. K2C03
d.) Platinum Electrode - 101 )0 CO 3
Platinum was selected with 112504 because of its efficiency. Potassium
hydroxide was selected in lieu of sodium hydroxide because of its greater solubil.ity.
3.2 Standard Formaldehyde Source
3.2.1 Evaluation of Various Sources
Pure, dry formaldehyde is stable only at temperatures of 80 to l00C. At
ordinary temperatures the dry gas polymerizes slowly depositing a white film of
polyoxysiethylene. This polymerization reaction is best described as a polymolecular
surface reaction at pressures lover than 200 om. It must also be noted that the
polymerization process is strongly catalyzed by trace amounts of water.
The analytical properties of a formaldehyde detector could not be evaluated
without the use 3f a dependable formaldehyde source. Thus, in the course of this
attry : wa. e -e sar to preparf xormaldehyde gas streams of known concentrations.
CURR.ENT (mA/cm 2 )
b - I
88
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INSHWMIIIT SYSTEMS DMS?39
CORPORAT ION
The possibility of preparing atandards by passing nitrogen over or through
a formaldehyde solution (132 methanol) was evaluated. This source of formaldehyde
was found 1 as previously reported, unsuitable. The main drawback was the methanol
interference on the selective aensing of formaldehyde.
ฃ second method was based on the generation of a C5 2 0 preasure by heating
paraformaldehyde (P(CR20)n) to 120ฐC in a stainless steel tank. The liberated
formaldehyde was then transferred to another heated etaieleea ateel tank which had
been evacuated. Once the deeired CR20 pressure was obtained, the tank was pressur-
ized to 1500 pounds with nitrogen.
Various difficulties ware encountered in the preparation of CR 2 O gas
streams by this method. The results obtained ware not consistent indicating the
possibility of polymerization.
Another of the methods investigated consisted of passing dry nitrogen over
permeation tuhes maintained at constant temperature. Initial tests on permeation
tubes involved the use of paraformaldehyde. A reproducible formaldehyde standard
cound not be obtained iron paraforemldehyde for two important reasons:
1. Paraformaldehyde baa a water content of 2 to 42 even after desiccation.
This hig$i amount of water caused the polymerization of formaldehyde in the
gas stream.
2. The structure of parafonealdehyde is characterized by tow molecular
weight oligomers, which lend amorphous random properties to the crystal-
line etate.
The partial preeaure of paraformaldehyde and other polymeric forms of
formaldehyde is due to unaippering depolymerization reactions at the hydroxyl end
groups. The rate of thermal decomposition is accordingly dependent on the number
and availability of end groups. The attainment of equilibrium partial pressure is
extremely difficult with a polymer such as psraformsldehyde since it is characterized
by widely varying chain lengths and ill-defined solid state eurface structuree.
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gJ B STEUMEflTSYS1tMSDIVi5
CORPORATION
Accordingly a search was made for a polymer characterized by uniform
cbein lengths, a high degree of polymerization, a well defined crystalline struc-
ture, and low water content. These properties were found in slpha-polyoxymathytene
Properly prepared, it typically bee a formaldehyde content of 99.7 to 99.92. The
degree of polymerization is believed to be an average chain length of 200.
The alphs-polyoxymethylene was prepared by adding potassium hydroxide to
402 methanol-free aqueous formaldehyde solution. The KOR pellets were slowly added
until a 1 lOR/ Sฎ 20 molar rstio was reached. The alpha-polyoxymethylene so
obtained was vacwss filtered, washed with ethanol and ether, and stored in a
desiccator. The operation wasrspested twice at oa/ca 2 o molar ratios of 1:100
and l20.
The equilibrium pressures have already been well-defined in the range gO
to 100ฐC by Iwses and Imoto . These pressures follow the equation:
lO
log Pa 12.02 - 3.57 ...
where Pa io the CR 2 0 partial pressure and Tฐ is temperature.
Permeation tubes ware conetructed with iS cm lengths of 1/4 commercial
teflon tubing with a wall thiclcnsss of 0.03 inches. The tubes were filled with
atpha-polyoxynethylane. The ends were closed with teflon plugs and clamped shut.
Tests were conducted at temperatures of 100 and 130ฐC. The gas atream was nitrogen
used at a flow rate of 1.0 SCPR. Calculations of the permeation rates were made
via weight toss maaeuremante every 24 hours at 100 end 130ฐC. A gsa stream con-
taining 97 ppm R 2 00 was obtained at 130ฐC with a variance of ฑ22. At 100ฐC a
concentration of 11.5 ppm was obtained. Tests with a 10 cm length of 1/8 tubing
at 100ฐC yielded a gas stream containing 1.0 ppm R 2 CO.
3.2.2 Characteristics of Formaldehyde Source
A test program usa set up to establish the reliability of the alpha-
polyoxymethylene peameatton tubes.
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DRUMUITSYS1EMS9Ms ; 1
NSIRUMENT SYSTEMS DiV S J F 11Zr2IhI1I S
CORPORATION
Permeation tubes containing a lpha-polyoxymethylene were prepared by a
standard method for the evaluation. All of the tubes were identical in configura-
tion. They were prepared fron 114 0.1). commercial teflon tubing with a wall
thickness of 0.03. They were designed to have a sample chamber exactly 15 cm
long. A typical tube contained 1.2 grams of alphapotyoxyeethylene.
Tests were conducted to characterize the following four properties of the
tubes:
I. Variation in permeation rates from tube to tube.
2. stability of the permeation rates of selected tubes for one week at
temperatures of 100C and 130ฐC.
3. variation in permeation rate with respect to flow rate.
4. Variation of permeation rate with respect to temperature. Except
when noted tests were conducted with dry nitrogen at a flow rate of 1.0
SCFII at 130ฐC.
Variation in the permeation rate between different tubes was found to be
ฑ7% with respect to the arit ticel mean. This deviation is attributed to the
physical properties of the tmbing since the vapor pressure of sipha-polyoxymethylene
is well characterized fit thie temperature region.
The permeation rate stability was determined individually for three tubes.
The rates were found to be constant within ฑ2% for each of the tested tubes at
both 100 end 130ฐC. This variation was decreased to ฑ0.5% by placing a stainlese
steel coil in front of each of the tubes to minimize the effect of the carrier gas
temperature. During this test it was determined that each permeation tube has a
life expectancy of 12-14 days when used continuouely at t30ฐC and 6-8 months at
100 C.
By varying the dry nitrogen flow rate from 0.5 to 5 SCFR it was demonstrated
that tha parncation rate in this flow range is independent of the rate of removal of
fr .s nc.:ydQ frir- thu c3rface of tli permeation tube.
-26
Permeation rates do very with tempereture end the proporttee of the teflon
tubing. Thus it is necessary to calibrate each tube in order to obtain accurate
formaldehyde concentrations. A typical weightloss temperature curve ie presented
in Figure 14. A complete calibration curve is outlined in Figure IS. Since the
permeation rate is not a function of the carriers flow rate, a single point cali-
bration say be sufficient for a tube which is to be used to supply formaldehyde
over a marrow concentrstion range.
3.2.3 Wet Chemical Analysis of Formaldehyde
Evaluation of the permeation characteristics of the pernaation tubes wae
achieved primarily through the uee of weight lose techniques. At the lower concen-
tration levele this method wee not always reliable. Weight losses were often lees
than 1 og/day and the weights obtained were accurate only within 0.2 mg. Por this
reason en effective wet chemical method for the direct determination of formalde-
hyde from tow concentration gee streams wee developed.
The epectrophotoeetric anslysie developed for this purpose was baeed on -
the reaction of chromotropic acid with formaldehyde.
The normal procedure for employing this method ie to remove the fornatde-
hyde from the gas stream with a eintered glass gas scrubber filled with water.
This method has many drawbacks, chief among them being the following:
1.) water-filled scrubbers are inefficient, ebeorbing only 70-75%.
addition of a basic medium increases the efficiency somewhat but it
interferes with the eubeequent analyeie which takes place in a strongly
acidic nolution.
2.) Final analysis of the scrubbed sample requires at least e 50 to I
dilution. This increases the ei sa requirements of the eempte tremendously.
27
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FIGURE 14
Tsp.ratur. C
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NOT REPRODUCIBLE
FIGURE 15
IORMAI.DEMYDE PERMEATION TUBES
-29-
FORM .LDEHYDE PERMEATION RATE
VERSUS TEMPERATURE
15 CM PERIIEATLOt4 TUBE
FLOW RATE VERSUS CONC 4TRATION
-
- t - t rr
1=
w
z
14
I
4 1
5.0
4.0
3.0
2.0
1.0
1r n
EtI
jJ jii
/
/
/
_
- - - ---I
-f-fฑ +-H-t- - - -
t 434 4
F ill! tT
1O.Q_
9 .
I
7
I
4
2
1 _0
100 105 110 115
120 125 130
tft
0
: ft if
+-f-+-t- -t-
-4 -.
3
N 2 FLOW RATE, S.C.F.H.
6
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VISTRUMEMT SYSTEMS DIVISION
CORPORATION
An innovative method of analysis was evolved and substituted to eliminate
these problems. The chrostotsopic acid is dissolved directly in concentrated
sulfuric scid. The scrubber is filled with this solution. A 125 ml scrubber
with 35 ml of solution wee found to be adequate.
Concentrated 112504 is very viscous and has a high surface tension. As
the gas stresm bubbles through, a fosm of bubbles 2-3 thick forms on top of the
mixture. This increases the scrubbing efficiency to almost 100%. (A second
scrubber placed in series with the first was found to be void of formaldehyde.)
The scrubber solution ia washed into a 50 ml volumetric and the color development
is made directly on this sample without dilution. Results were found to have
very little variance. Linearity of the standards was perfect over the range
tested. The complete procedure is given in detail below.
Reagents :
Chromotromic Acid : Dissolve lg chromotropic acid in 500 ml concentrated
R2S0 4 with heat and magnetic stirring. Dilute to 1000 ml with concen-
trated 1 12S04.
Standard : Dissolve 100 mg. a lphe-polyoxymethylene in 100 ml concentrated
112504. Dilute 5100 with concemtrated 112504. This is the standard stock
solution.
Sample Preparation : Place 35 ml of sample solution in e 125 ml scrubber.
Bubble the sample through to get 23 g of formaldehyde. For a 2 ppm gas
stream thin takes about 18 minutes. Wash the solution into a 50 ml volu-
metric and bring to volume.
Analysis Procedure :
I.) Dilutc the standard stock solution 1:100 and 2:100 with chromotropic
acid,
2.) toat oqual portionc of n chromotropic acid reagent blank, the sample,
d th3 tcSards for 20 ainutes in boiling water.
INSTRUMD(1 SYSTEMS DIVIs.u;i
3.) Cool end read the absorption at 570 iqi with a colorimeter or
Beckman D i i.
4.) Construct a standard curve and extract the concentration of the
sample simply as total grams formaldehyde in the sample.
5.) Calculations:
Cas Stream Cone. (PPM) 2O in sample (g) X 106
Flow rate k 1 X Time (hr.)
hr 22.41 m
Agreement between this method of analysis end the weight loss method was
exceptionally good. The direct method usually ran 47, lower then the weight loss
method at small concentrations.
3.3 Evaluation of Membranes
3.3.1 Screening Studies
The membrane is an integral part of the electrochemicel transducer.
Although its most important property is the permeability of the gas to be analyzed,
it has many other important functiona. It must: contain the electrolyte resovoir
of the cell; have a low evaporation rate to insure the life of the cell; have a
certain degree of selectivity for the pollutant; be chemically and physically inert
to the electrolyte end the sample stream; and have a feet diffusion rate.
A fairly complete selection of membranes was assembled for evaluation.
a,) Permeability The criterion of permeability eliminated many of these
at the beginning of the tests. Formaldehyde would not permeate through:
1.) 1 nil polyvinyl chloride
2.) 1 mil polyvinyl acetate
3.) 2. nil cellophane
4.) 1 nil polyethylene
5.) 0.5 nil ifapton or B-Film
6.) 1.5 nil 1ar
7.) 114 nil T af Ion
ORPORATION
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31-
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CORPORATION
U&PUMM SYSTEMS OMSION
Membranes which were found to be permeable end their sensitivities are
listed:
1.) 1 cii single backed dimethyl silicone 1 1.5 pS/ppm
2.) 1 cii siticone-polycarbonate QdEH 213) 2 pA/ppm
3.) Permion 2291 -catl.on exchange membrane, drifted.
4.) 4 nil R.T.V. Silicone Rubber Cement, 1 pS/ppm
5.) Zitex (teflon hydrophobic fiber) 12-137*, pore size 1-2 p,
pore volume 122, 2.7 MA/ppm
6.) Zitex l2137B, pore size 3-6 i, pore volume 202. 25 isA/ppm
7.) Zitex - E606-223, pore size 12 i , pore volume 552, 2 pA/ppm
these values were obtained using a 30 ml cell body which contained a 1.9
gold sensing electrode, lOt ROE electrolyte, a pressed Ag/Ag 2 0 counter-
electrode, end an applied potential of 40.1 volt.
Ziter is a trade name uaed by Chemplest ฑor their membranes made of
fibrous hydrophobic telfom. It is a porous medium which seals the cell
through a capillary effect. The mechanism by which a gas reschea the
electrolyte is simple physical transport. There ia no esiectivity, e l i
gaaes permeate at the same rate.
b.) Leakase - Of the permeable membranes, HEM 213 sod lVlV did not suffer
any kind of leakage. The Zitex membranes often developed pinhole leaks
especially at the 0-ring seal of the cell. Permiom did not hold beck
electrolytes at all. Single backed dimethyl silicone leaked consistently.
c,) Evaporation Rates
1.) 1 mit ? I 213 0.7g/dry
2.) 4 mit LLV. Not determined.
3.) Zitex 12137* m 7g/day (1.9 diameter)
4.) Zitex 12 13Th - 12g/day (1.9 diameter)
5.) titer E6O6223 Not determined.
CORPORATION
IISTIWMEHT SYSTEMS Dfl
It vss observed that membranes which tranasittnd C8 2 0 efficiently also
allowed water to pass from the electrolyte to the sample stream. This
property accounts for the relatively high evaporation rates. The tests
were conducted with dry nitrogen at g oCy, at a flow rate of 1,0 SCyH.
the cells comtained 180 ml of electrolyte end hsd 1.9 diameter membranes
Use of partially humidified eir would decreese these velues by as much
as 802.
d.) Selectivity Of the membranes listed in part c.) above, only HEM 21
and ELY. demonstrated any degree of selectivity. This selectivity was
most apparent in the reduction of carbon monoxide and nitric oxide levels
but it did not eliminate either of them completely. All of the pollutsnt
gases permeated at rates capable of causing interference.
e,) Electrolyte Compatibility Zitex membrsnes are composed of teflon
fibers amd thus are compatible with all aqueous electrolytes,
REM 213 has long term stability in the pH range 4-9. However, its
life time in IN H 2 SD is at least 6 months based on previous studies.
In 102 K 2 C0 3 it waa one month (pH 11.4); in 102 RON it was only three dsyi
The fabricated Et.V. silicone membrane was stable in all of the
electrolytes.
f.) Relative Diffusion Pates Based on tests in a variety of e lectrolyu
the acceptable membranes demonstrated maximum response times of:
1.) iei 213 902 im 1 to 3 minutes.
2.) LT.V. 902 in 30 minutes,
3.) titer - 12-137 * - 901 in 2 minutes.
4.) titer - 12-13Th 907. in 3 minutes.
S.) titer E6OS 223 - 907. in 30 minutes.
Thsee response times were obtained in cells containing 102 ICON, a
Ag/Mao cotmiterai rode, a Au sensing electrode, end an applied potential
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of +0.1W. The membranes were all 1.9 in diameter. The concentra-
tion was 2.2 ppm 20 at a flow rate of 1.0 CPU. All tests were run at
room temperature.
3.3.2 Development of Final Membrane
None of the membranes teatS wae suitable for formaldehyde detection.
R.T.V. was too slow. MEN 213 was probsbly the best available but it was not
compatible with any of the proposed electrolytes. This left only Zitex as a
reasonable membrane surface.
The use of Zitex as a membrane has many drawbscks. The four moat impor-
tant are listed below.
1. Zitex is a porous medium. It is by this means that the sample gases
are transported to the electrode. The partially vaporized electrolyte
is transported with equal facility back to the sample gas stream. This
heavy electrolyte loss drastically reduces the life-time of the cell.
2. It is very difficult for the manufacturer to maintain the uniformity
of the teflon fiber structure of Zitsx. Very often pin hole leaks
develop which soon render the cell useless.
3. Ziten, being a teflon compound, is characterized by weeping at
sealed surfaces. Thus the 0-ring seal of the cell eventually becomes
ineffective,
4. A high percentage of the sample is absorbed in the electrolyte when
a Zitex membrane is used. This very often results in ton-linearity at
low concentrations where absorption decreases due to reduction in
colliaion cross sections. The high surface area also is responsible
for slow response times.
It usa apparent that none of the three types of Zitex were acceptable as tested.
An alceTnative n thod of approach ucn aelected. Since the current was very high
INSTRUMENT SYSTEMS DIVISISI
for 12l37B it seemed obvioua that a 90%. reduction in surface area would still
leave plenty of sensitivity. A membrane of 1 mil polyethylene, which is imper-
meable to formaldehyde 1 was cut to fit the cell. A 0.5 hole was cut in the
center. A 0.7 diameter piece of 12-13Th was bonded across the hole with epoxy
cement. It was determined that this configuration had the following practical
advantages over those previoualy tested.
1. The evaporation rate was reduced by an order of magnitude.
2. Weeping waa not apparent eince the 0-ring seat vae on the polyethyl-
ene membrane.
3. Linearity at smeller concentrations waa achieved.
4. Response time wee increased to 90%. in 45 seconds.
5. Seepage was reduced significantly. It was determined that alot of
the previous trouble was asaociated with the 0-ring seal crushing the
membrane.
Obviously this type of membrane was far superior to those previously tested.
However, there were still some problems in the selection of the final support
material. Polyethylene was found to make a weak bond with epoxy cement.
Other materials impermeable to formaldehyde were tested. 1.5 mil Mylar
was strong but was subject to chemical corrosion. Polyvinyl acetate end poly-
vinylchloride stretched too nach. The best support material was determined to be
2 nil teflon film. The surface of this membrane was treated with a sodium suspen-
sion. This treatment left a carbonized surface layer which provided a good ad-
hesion ares for epoxy cement. Other deaireable propertiea were the strength and
inflexibility of the 2 mil thickness which contributed greatly to the stability
of the electrolyte layer between the membrane and the electrede.
fflfSS1 ll I ! iiC S _________
CORPORATION
INSTRUMENT SYSTEMS DIVISION !I1AS KIt IUS
CORPORATION
-34-
35
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4 4 4 (4
r It I t
n a it
5 C S
It it ft 0.
5 5 5
p_ I C
C 4 4 *
so N N It
ID ID S %
a a a
. . ft
I-
5
5
C C 4 4 Iso
a m
4 4 (0 a
III S
It p M
a S i N
I it I
22 n
ID 1
S 4
?
S
a
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it
it
It
a
S
i f
CO RPO RAT iO N
3.4 Evaluation of Electrolytes
3.4.1 Screening Teat and Results
The electrolyte is one of the moat Important components of the cell.
Its selection is determined largely on the baste of the chemical and electro
chemical properties of the electrolyte relative to formaldehyde. Chief among
these are:
1.) Potatial of thidation and Reduction Reactions - The electrolyte
coupled with the sousing electrode determines the oxidation or reduction
potential of formaldehyde.
2.) Reaction Rate - The electrolyte contributes generally to the im-
proved kinetics of electrochemical reactione either through chemical
reaction with the species, participation in the electrochenical reaction
or as a catalytic agent.
3.) Reaction Scheme - The electrolyte determines, usually through
chemical interaction, the extent to which an oxidation or reduction
viii proceed, that te to say, the number of electrons involved.
4.) Diffusion Rate - The rate of diffusion of formaldehyde through the
electrolyte effects the rate of response. By holding the thickness of
the electrolytic layer and the membrane saterial constant, comparative
rates can be detarmined as a function of ion species and concentration.
these determinations were not necesaery since only the responae rates
vera of interest.
5.) Absorption Rate - The magnitude of the response is a function of
the percentage of pollutant which is absorbed at the membrane electrolyte
interface.
Several electrolytes were evaluated as candidate mediaa for the selective oxidation
of formaldehyde. Representative data is tabulated in Table I. Two types of sensing
electrodes were used, 1.9 die, gold end platinum. The platinum electrodes were
. , Pa 04 P4 P4 , , Pa P4 -. - Is p- I s o
I
D
r
CC 0 p O
Is 1 4 I& I31 t F
lf
C C C C C
,9 0 C C C
- . 4 14.4 14 > - 44 I :
C 4 4 0 n it a C .4 a c c 4 4 a C In .
n if if if I .I
so C C C C C C C C
so so so so so so so so so so
I I 4 4 e
C It It C C C C - a 0 g , S i
so so so so g , P4
0 P 4 19 Pa Pa P4 Pa P4 P9
0 0 0 0 0 0 0 0 , 1 oe
104 a
5
I -
so
I ฑ * Z I 6 6 Z so so so so I CC g
?030?. . .
g _ , , 5 a e e a .
U
, a
so
C ,
a u, I- 14 * U U a U p (4 so so a so l .I so
0 0 U 0 0 0 so
U, 5 5 3 5
, a a it I 5 5 5 r
S I - t 0 5 10 0
a e 5 w. n S ID 0 5 s a s
fD ID 1 S (0 (I
S S . . . Os
1 , _s I
flU 9
00
-36-
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INSTRUMENT SYSTEMS DIVISION MA%OIEbiItIES
CORPORATION
platinum plated 333 line gold mesh. The reaponae timea reported were the fasteet
onee obtained. Each of the evaluetions were conducted at room temperature. The
membranes and counter electrodes were varied while recording these response times.
The membranes were not reported because }eI 213 and reduced Eitex were the only
ones used. The counter electrodes,although reported, were not necessarily the
ones best suited to the electrolyte. The Pb0 2 electrode was plated on a platinum
mesh. All of the other counter electrodes were pressed electrodes, contained with-
in a polypropylene skin, and supported by a suitable metallic mesh.
The concentration of formaldehyde used for the teats was 2 ppm in a
stream of dry 142. Exceptions to this are noted. The formaldehyde source in all
cases was a permeation tube. Electrolyte additives such as gelatin, mannitol,
and potassium formate were ineffective in all of the tested solutions.
3.4.2 Discussion of Results
Sulfuric acid was found to be an entirely unacceptable oxidation medium.
Preliminary studies with a cosssercial 372 formaldehyde solution used as tha source
of formaldehyde had not ehown this. Subsequent work with this electrolyte revealed
that the response attributed to formaldehyde oxidiation was due entirely to the
presence of the 132 methanol used as a stsbilizing agent. Tests conducted with a
solution containing only 132 methanol gave exactly the same response as the cosisar-
cial formaldehyde.
Evaluation of 1.0 N 112504 sensors was then conducted with formaldehyde
permeation tubes. This electrolyte wss found to be insensitive to formaldehyde
at concentrations below 10 ppm. At 50 ppm the oxidation at platinum and gold was
very inefficient.
During the evaluation of these experimental transducers, it was observed
that, after responding to formaldehyde, the cells never wholly recovered, i.e.,
an acr. cc of tho boceline current tas always registered.
INSTRUMENT SYSTEMS DIVISION
A cells recovery, in some cases, would be smeller than 202 of the total
response to formaldehyde. It wee believed that the cause of the continuously in-
creasing baseline current wee unreactad formaldehyde. In other words, not all of
formaldehyde permeating through the membranes was being electro-oxidized. In order
to corroborate this assumption, CH 2 O analyses using the chromotropic acid proce-
dure were performed upon a m er of electrolytes extracted from the experimental
transducers. Although color development difficulties were encountered, it was
possible to determine the existe kce of formaldehyde (5 to 80 ppm) in 311 of the
tested electrolytes.
Elimination of sulfuric acid as an electrolytic medium created a need
for additional development effort.
From the screening test results a very predictable problem was shaping
up. The final sensor would be subjected to a 12-152 CO 2 environment. The only
electrolytes which showed some promise for the rapid detection of formaldehyde
were potassium carbonate eolutions with platinum electrodes and potassium hydrox-
ide solutions with gold on platinum electrodes. Both of these electrolytes are
neutralized by CO 2 to bicarbonate. Potassium chloride and bicarbonate eolutions
which are not affected by co 2 were not affective in the detection of formelde-
hyde. These findings were met surprising because they coincided very well with
the sweep voltasmistry studies.
The effect of electrolyte concenLrstion on response time was determined
from both K 2 C0 3 and KGB solutions. The results were quite opposite. As the K 2 C0 3
concentration increased the response time became slower. For lOB the response
became dramatically faster. The answer to these results is found in the hydroxyl
ion concentration which effects the rate of reaction at the sensing electrode.
An the carbonate ion increaeea, the hydroxyl ion concentration does not become
significantly greater and the high concentration slows the diffusion rate. An
increase in the ICR concentrntion renults in a direct increase in the hydroxyl
A CV1E iCIES
ORPO RATION
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INSTRUMEI IE SYSIEMS DIVISION
ion concentration. A corresponding increase in the rate of reaction was observed.
The role of the hydroxyl ion as an oxidation expiditer also explains
the results obtained in the screening taste end the sweep voltaimnatry experiments.
As the p1 1 value increases the electrolytes become increasingly effective in the
detection of formaldehyde.
Formaldehyde goes into solution by combining with water to form the
gem-diol.
The gem-diol subsequently polymerizes and becomes unavailable for oxidation or
it reacts to the potential at the sensing electrode. In a basic media reaction
1.) is imnedietely followed by or substituted by the reaction:
In this ionic form the diffusion rate is more rapid.
The basic media also holds the oxidation reaction scheme of formalde-
hyde to a one step process, the production of the formuste.
This 2 electron oxidation was verif led in a separate faradic experiment. In en
acidic media the reaction proceeds alt the way to the formation of CO 2 . a 4 elec-
tron oxidation. This would seemingly enhance the eensitivity but the second
oxidation step is so slow that it Spades the rate of response.
3.4.3 ElectrolyteExhaust Stream Compatability Tests
Tests were conducted with a sample stream containing 12% CO 2 in dry N 2
to determine the neutrelization rate of 10% tOll end 10% t 2 C0 3 in magnetically
stirred sensor cells.
During the first two weeks, it was observed that the neutralization
rate of 10% 110!] by Ut CO 2 decreased as a function of time from 0.23%/hr. to
0.05%/hr., calculations based on the original t on configuration. It then stabil-
ized at this rate and 58.4% was neutralized after six weeks. It was estimated
thst complete convara ion would tske place in about 4 to 5 months. It was postulated
on the results of the concentrntion-response time tests that until complete con-
version was achieved the oxidation potential or response time would not be dramat-
ically affected. However, once complete conversion bad bean achieved e dramatic
oxidation potential shift would occur. This shift might or might not be accompanied
by an equivalent shift in the potential of the counter electrode. Most likely,
neutralization would affect the selectivity of the cell and probably would effect
the electrode kinetics.
In addition neutralization would not stop at this point. The resulting
K 2 00 3 eolution would be converted to the bicarbonate at a slightly slower rate.
This rate is slow enough to guarantee a lifetime of 6 months before 50% of the
electrolyte is converted to the bicarbonate. The formaldehyde response rate
remains unaffected at this composition. With a Pt electrode the formaldehyde
oxidation is retarded to 90% in five minutes in a solution completely converted
to the bicarbonate. The exact point at which the bicarbonate conversion begins
to affect the response time is not known.
This data applies only to neutralization of the entire cell. Oxidatiom
and neutralization actually take place only in the thin electrolyte layer between
the sensing electrode and the permeation membrane. Therefore, in the absence of
agitation the complete neutralization of the cell would require a much longer
period of time while complete neutralization of the thin electrolyte layer would
depend on the Co 2 absorption rate, the hydroxyl ion concentration, and the dif-
fusion rate of fresh hydroxyl ions to the electrode surface.
CORPORATION
mSmUMauSYslwsoIVIS z: ;
RPORATION
H UI
1.) C 0tH2O ) C
H iV on
H. 4 ,OH B% ,fO +
+ tOll) C. + K + 1 120
HOH gG!
0
C + 201(____)H-C + 28 2 0+2e
HOH 0
41
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__________ INSTRUMENT SYSTEMS DIVISION
CORPORATION
3.5 ______________________________
3.5.1 Screening Tests and Results
A good counter electrode has at least three important characteristics:
1.) It has a stable potential, preferably close to or larger than that of the
species to be oxidized. 2.) It is an efficient electrode made from an easily
reduced species (in oxidation applications), having a large electrolyte-electrode
interface surface. 3.) It is chemically and electrochemically compatible with
the electrolyte.
Prom the results obtained in the electrolyte evaluations, it was clear
that the final counter electrode would be used in only three solutions, KOR, K 2 CO 3
I and 101C0 3 . All three of these solutions are basic and unfortunately very few
electrodes have stable potentials in the basic p H range. Among the best couples
are Hgfugo and AgIAg 2 O. The potentials of these two may vary as much as 1.0 my. - -
Since our need was not for precise reference electrodes this variance was
tolerable. The combinations which were evaluated are presented in Table 2. All
of the cells were assembled using reduced zytex membrane and a pletinun sensing
electrode.
The csndidate counter electrodes were all pressed electrodes with the
exception of Pb0 2 which was plated on a platinimi mash. Each was fabricated from
Anslytically Pure Reagents, pressed at 8000 psi onto an appropriate natal support
mesh, and enclosed within s polypropylene filter paper skin. The Hg/HgO electrode
was tested both as a pressed electrode and in the form of a mercury pool.
Prom the raaults obtained it was concluded that either the bicarbonate
media was a poor one for the oxidation of formaldehyde or that lack of a comnon
ion effect was severly hampering the counter electrodes.
Pot the most part the lead electrodes exhibited steady potentials and
response times but ae a group they were classified as inefficient electrodes.
Thc tost efficient alectrodes were mercury oxide and the silver compounds.
Evaluation of Counter Electrodes
C ,
In
m > 5. 5. 2 5 5 2
N It N N N N It It It 55 N N It It It
C - 0 01 0 It . - 01 a 01 It
0 5 5 5. 2
QI 5. 5 n N
N ra N N 05 N 8 & S o N N ta o o o
o fl N 0 5 4 0 0 5
0 0 0
I
p a I-
p. p. p. p. p. p. I - p.
o o C ) 0
o o . o o88888 ..
ia ia i , - iai i (4 I., ia, ia N
N
p . 4
p.
8
w
2 2 2 I N I I 2 N I 2 N 2 N 2 2 N 0
o 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 rIt N
o a a . . a a a a a n I
a is t., is n to is ,., is is a is is m a cn E
N
g
Ir N
N
0
0 14 p. p . I N
* 5 5 U . . fJ . lIt N
N N N (a ผfl N N 14 14 N N , N 10 5
p. IN
N N N 0 2 2 N N
p. pa l.a 2 2 2 2 i w ..c
IN q
2 2 2 is is a c a pa pa a
aa
C ) N N N N C) N N N r It C It
i-. 0 0 0 0 rt 0 pa 0 0 0 p . 5 0
N pa 0
N 0 I & a 5
a 0. 0. 5 4 5 Ii 0 a
I 0 0 a 0 4 0 p. 0. 5 a it
i 0 0 0 0 I 0
It Ft pa
pp pa a I it 0. C 0 N is a is
a a m I I 5 is i t 0 5 p. I
0 5 a a a a 00 0 i i . 0.
it it I it is N 0 0. 0 C 0.
pa pa is cs pa 5 I I Ft I
is is a 5 is a C l 5 pa
p. p. 0. it 0. is a l I t
0 0. P 1
a -
N 05
S IN
it 0 pa
is C
a N
a
pa 0. a
a p.
is
5 0.
pa a
0 0
a a
C
-42-
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mw nsvswasoreisi u
CORPORATION
However, the mercury electrodes were prone to potential drift and the silver
electredee severely contaminated the cells.
3.5.2 Cotspatebility end Pebricetion Studies
The mercury-mercury oxide couple was ccmipatible with ROB and 1(2003
electrolytes; however, it was eubject to response time decey end potantiel drift.
The mercury couple usuelly bee a fairly etsble potentiel so it was poetulated
that the method of construction was reeponsible for the drift. xperistentation
was conducted to determine the best method of febricetion. A pressed electrode
was found to be far superior to the mercury pout type. An electrode which main-
tained a relatively constant potential along with a rapid response time was
essemb led by the following method:
I. The support mesh was 33 line gold which had been treated with chromic
acid cleaning solution followed by immersion in nitric acid. This clean-
ing procedure ensured a proper eurfece for subsequent amalgamation with
mercury.
2. ฃ platinum wire teed was then spotweided to the gold.
3. the mesh wee insnereed in triple distilled mercury. The excess
mercury wee jarred loose from the mesh by impact.
4. The mesh was pressed at 5000 lbs/fe. in a 1.4 die, mold with a
mixture of 4 gm red mercuric oxide end 200 eg of polypropylene p3wder
(Bsrcoflat 233), at 100ฐC for 15 minutes.
5. The whole electrode was pressed inside a polypropylene fitter paper
skim to keep it intact.
This electrode had a potential of 0.12V in 10% ROB and 0.27V in 10% K 2 C0 3 versus
the hydrogen electrode. Response times of 45 seconds were obte,.ned in cells made
with both solutions for 1 ppm cH 2 O. the cells had platinum sensing electrodes
end reduced Zytex membranes. No applied potential was used in 1(0K. A potential
of -0.LBV usa used with 1(2003.
NSTRWWff SY STE MS
CORPORATION
Improvement of the aitwer compound electrodes was e more difficult task.
It was discovered that Ag O and AgCI were not compatible with 10% R 2 C0 3 . these
ccispounde were slowly converted to silver carbonate. in addition, the silver
electrodes ware quite soluble in both R oll and R Co 3 . The silver ion would then
migrate to the sensing electrode area and react with formaldehyde to deposit free
silver on the membrane end sensing electrode. Contamination of the sensing elec-
trode sod membrane was stopped by inserting s layer of permion 2291, a cation
exch.nge siasbrsne, between the two electsodea. This arrangement temporarily
solved the problem until the peretion was saturated. It was quite apparent that
the silver was much more aolub le under these conditions than the literature bed
reported.
During the course of these experiments it was noticed that the Ag 2 CO 3
electrodes were discoloring as if they hsd been exposed to a light source. It
was evident that the silver ions were being expoaad to light through the opaque
polypropylene cell body. This caused the formation of free silver, which in turn
caused a concentration gradient of silver ion, which ultimately resulted in in-
creased solvation of the counter electrode, to circumvent this prohlexe the cell
body-a were paimted black, to further lower the solubility a counter electrode was
made from MCI pressed onto a silver mash and NOt was added to the ROB and 1(2003
electrolytes to make the silver ion concentration smelter through the comeon ion
effect. This last step did help clean the cell issizeasurably. Bowaver, the
presence of chloride in the electrolyte made the cell very noisy end increased
the background current to 3 iA. Thie was due to oxidation of the sensing elec-
trode which occurs at much lower potsntisla in a chloride media. This eensitivity
eliminated the ฃgCl counter electrode since it waa not stable in the absence of
chloride ion.
Silver carbonate usa eventually eliminated because its eolubitity product
was too high even in a 107. R 2 C0 3 solution.
-45-
-44-
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The Ag/Ag 2 0 electrode was clearly the only silver electrode which was
useful and it could only be used in a 201! medium.
The best Ag/Ag 2 0 couple was obtained by pressing 4 gm of Ag 2 o onto a
silver mesh which had been i rsed in 50% nitric to psrtially oxidize the surface.
It was pressed at 18,000 lbFSa 2 for 15 minutes.
The resulting electrode was noise free. The black cell reduced the
aolubili y of the silver as anticipated. Even with the decreased aolubility Per-
mion 2291 still had to be inaerted between the two alectrodea. It waa eventually
diacovered that this membrane inhibited adequate diffusion of the electrolyte to
the membrane. This waa characterized by 0.02 pA spikes at 20 to 30 minute intervals
due to localized depletion of electrolyte at the aenaing electrode surface. These
apikea ware equal to a 1% noise level at the moat aenaitiva range.
A separate experiment was conducted to determine the effect of applied
potential on the fig/ligO and AgIAg 2 O electrodea. It was found that the potentials
remained unaffected by exterior applied potentials. The only affects on the rata
of response ware due to the added resistance in the exterior circuit from the pot
used to adjust the voltage.
3.6 Evaluation of Sensor Electrodes
3.6.1 Platinum Electrodes
Platinum electrodea ware proven to be faster and more efficient than
gold electrodes in every electrolyte excepc 10% ROB. Because of this superiority
en axperimantal program was completed to bring the rate of response to a maximum
with respect to the platinum electrode. The madium chosen was R 2 C0 3 stnce it was
the fastest electrolyte next to ROB and it offered more resistance to CO 2 posioning
than ROB. All of the platinum sensing electrodes tested were platinum plated gold
with te a zception - on 80 tine plntinum mesh. The test was conducted in two phases:
INSTRUMENT SYSTEMS DIYISISH
the effect of plating on response time end the effect of post-plating electrode
conditioning on response time.
Plating conditions were varied quite widely during the test. Two differ-
ant types of baths were used. The first was chioroplatinic acid at various concen-
trations in a solution buffered with diamonium phosphate and dieodium phosphate.
The second was a aolution of the dinitroemmine in eulfamic acid and water. This
is manufactured by Sal-Rex Corp. under the trademark Platinex III. The teat con-
ditions and findings are presented in Table 3.
The best results were obtained with a 15 gIl chloroplatinic acid bath
at a current density of 3OA/ft 3 . The finish obtained under these conditions was
found to be dark spongy platinum black. Electrodes plated under other conditions
were, for the most part, irreproducibla and exhibited erratic behavior during
testing.
A series of experiments was initiated to chemically tieat the platinum
electrodes to improve their behavior. The specific areas where improvements were
needed were: 1. The background current, which was vary high. 2. The noise
level, which wee also too high, and 3. The response rate. The tests are outlined
in Table 4. All of the experimente were conducted on platinum electrodes plated
under the conditions in Table 3, item I.
The beet results were obtained with method 5 where the electrode was
oxidized and cleaned in hot chronic acid and reduced in 202 FeSO 4 . Celia made
with this electrode, a Ag/Ag 2 0 counter electrode, reduced zyter membrane and 102
R 2 CO 3 had response times of 902 in 40 seconds and ware virtually noise free.
A cell was constructed using a sensing electrode composed of 80 line
platinum mesh. The purpose of this experiment was to deternine the reaction time
of a shiny, unblemished platinum surface. The response time for this electrode
was extreisely slow, 90% in one hour. This result lends credence to the possibility
AS LE ICIES
:ORPORATI0N
INSTRUMENT SYSTEMS oivisw:i
CORPORATION
-46
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TABLE 4 CHEMICAl. TREATMENT Dl PLATINUM ELECTRODES
Oxtdiaing, Passivation
Ctaanins Solutiun Method Reduction Method Remarks
1. Boiling 30% 111103 Hot 40% ROll None Slower response, Lower noise leve l.
and backs round.
2. Botling 50% RHO 3 Hot 40% ICON Cathodic in 40% XCII High background, 90% in 2 minutes.
3. Boiling 50% IINO None Cathodic in iN 112504 High background, 90% in 40 seconds.
4. Rot Chromic Acid Hone Boiling MCI Low noise end background, slower
response.
5. Rot Qwomie Acid Hone 207. PeSO 4 Very low background, low noise level,
907. in 40 seconds.
L
a
TABLE 3. EFFECT OP PLATINUM PLATING ON RESPONSE TIME
90%
SUBSTRATE PlATING SOLUTION CURRENT DENS ITT RESPONSE TIME RENA lIES
1. 333 Line Au 15g/l M 2 PtC1 6 30 A/ft 2 3750 sec. Initial high background current.
2. 333 tine Au lOg/I R 2 PtCl 30 A/ft 2 I m m. Very conaistent
3. 333 tine Au 4.Og/t H 2 PtCt 6 to A/ft 2 15 lain. Hot reproducible
4, 333 line Au Platinex In 10 A/ft 2 2 mm.
5. 100 line Au tog/I R 2 PtCI 6 10 A/ft 2 12 mm. Not reproducible
6. tOO line Au Platinex III 10 A/ft 2 2-3 m m. Not reproducible
7. 333 line Au 6 g/L II 2 PtC1 6 1 A/ft 2 35-nm. Slow and erratic
8. 333 tine Au 6g/l N 2 PtCl 6 3 A/ft 2 3 5 am, Slow and erratic
9. 33 line Au LOgII H 2 PtCl 6 3 A/ft 2 30 n m. Much too slow
-------
INSTRUMENT SYSTEMS LlVitt
ORPORAT IOF I I
VIS4EIII SYSTEMS II I VIS!3N
that the detection surface be of a fine grained nature either through depoaition
or chemical etching.
3.6.2 Cold Electrodes
Previous experiments with gold electrodes demonstrated their relative
lack of efficiency when compared to platinum electrodes in 10% 1C 2 C0 3 . The best
response time obtainable was 907. in five minutes. In a 10% 1(011 medium their per-
formance is comparable.
The chief value of a gold electrode lisa in its low background current
and noise level. Since these properties ware of chief concern, electrodea of 333
line and 100 line gold were physically, chemically, and electrochsmically treated
with the hope of increasing the rate of response to formsldehyde. This goal was
not achieved in the 107. K 2 C0 3 electrolyte.
The explanation of this was demonstrated previously in figures 7 and 8.
The formaldehyde oxidation curve for gold is brosd with a gentle slope indicating
a slow reaction. The curve for platinum is quite the opposite. The peak at
0.001 V is fairly sharp. In addttion, a catalytic oxidation peak occurs around
0.4 V. Above this voltage the formaldehyde response is very rapid for platinum.
Below 0.4 V the response time decreases to 90% in two to three mioutes.
The methods for conditioning the gold for this test were essentially
the same aa for platinum with two exceptions: the gold was mechanically cleaned
by scraping and thermally cleaned by pyrolization st 900C. The calcinated gold
electrode was found to be unresponsive to formaldehyde. The fastest rats of response
was the limiting value of 90% in Live minutes. It was clear that gold was of little
use in 10% 1c 2 C0 3 solutions. Its chief value was in applications in 10% 1(011.
3.6.3 Palladium and Rhodium Electrodes
Palladium and Rhodium electrodes were prepared from solutions of Palls-
dei ant. Sodex ranufccturad by Sel let Corp. These electrodes were similar to
gold io response times but were characterized by high background currents. Sweep
volteemtetry curves using these electrodes indicated that oxidation of the base
metals was occuring.
3.6.4 Sensing Electrode-Exhaust Stream Compatibility Studies
With the zitex membrane the sensing electrode is exposed to all of the
contaminants in the exhaust stream. Thus the only selectivity exhibited by the
cell would he due to:
1. The absorption rate of the electrolyte.
2. The potential of the sensing electrode.
3. The sensitivity of the sensing electrode.
A preliminary interference test was conducted with the major components
of the exhaust atrean: 2000 ppm 140, 200 ppm 1102, 100 ppm SOz, 2% Ca, 3% 02, and
127. CO 2 . The teats were conducted with a gold electrode in 10% and 45% 1 (011 and
with platinum and gold electrodes in 102 K 2 C0 3 .
a.) CO 2 Interference - Solutions of 457. and 10% ROll were not compatible
at all with 12% CO 2 . With 45% F U the carbonate was forming inside the
membrsme. This broke the surface tension of the solution which destroyed
the hydrophobic nature of the sytex membrane, As a result crystals of
carbonate grew right out of the top of the membranes. With 10% lOll the
solution was converted to the bicarbonate medium as feet as the hydroxyl
ions could diffuae back into the electrolyte layer. The same results
were obtained with 10% K 2 CC 3 . The sensing electrode in each case lost
aensitivity. This vas not due to the reaction of C02 with the electrode
but to the reaction of CO 2 with the electrolyte layer.
Since there was no alternative in the choice of electrolytes or
membranes, experiments were conducted to see if this problem could be
oo lved through sampling techniques. It uas determined that s ten-fold
dilution of the exhaust stream would lower ths CO 2 level to the point
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INSTRUMEIII SYStEMS DM: n
CORPORATION
where neutralization of the electrolyte was not faster than diffusion
of hydroxyl ions to the electrode oxidation site. This method wee deemed
adequate in view of the fact that a ten-fold dilution of the sample
would also result in Lowering the dew point of the sample (in this
case, automobile exhaust) below room temperature. This would prevent
water condensation end subsequent formaldehyde loss in the condensate.
b.) CO Interference - The platinum electrode in the X 2 C0 3 cell was found
to be extremely sensitive to carbon monoxide whereas the gold was not.
The interference from 2% CO was equivalent to 10 ppm C11 2 0 and could not
be eliminated through potentioatatic control. This turn of events elim-
inated K 2 CO 3 as a sensing media due to the 5 minute time limit imposed
by oxidation kinetics at the gold electrode. The sensitivity of the
platinum electrode to carbon monoxide was found to be due to sn inter-
mediate formed with platinum oxide at the electrode site.
c.) 110, 1102, 02, and SOz Interferences - In 10% XCII, interference from
all of these components was eliminated at e controlled potential of
O.l7V (N i lE). In 45% XCII, the potential was O.lV with respect to the
hydrogen reference electrode. The interference from $02 could not be
eliminated in the 10% K 2 00 3 medium. Interference from NO, NO 2 and 02
were eliminated at +0.45 V (NilE),
3.7 Experimental Traneducers
The difficulties encountered during the early stages of development
prompted some experimentation with transducers of various configurstiona. While
none of these configurations were successful they are recorded here for reference.
a.) Indirect Electrochenical Detection of Yorsialdebyde - The purpose
of this cell was to enhance the response time to formaldehyde by means
of the chemical reaction of formaldehyde with silver ion to form free
silver. The silver formed would be reoxidixed to silver ion at the
iNSTRUMENT SYSTEMS C I V
CORPORATION
sensing electrode in proportion to the formaldehyde concentration.
This test was conducted with two separate approaches: the first with
Tollens Reagent as the electrolyte, and the second with A O as the
sensing electrode. The indirect concept was abandoned because the
potential at the sensing electrode necessary to reoxidize the silver
was alec , sufficient to oxidize all of the other pollutant gases. This
property did not permit the cell to be selective.
b.) The Gsl,anic Cell - The Galvanic concept of tvo separate compart-
ments for the senaor and counter electrodes was investigated for two
main reasone: I) it would permit the construction of wet counter
electrodes which would not be hampered by the effects of high tempera-
ture compression end organic binders, and 2) the electrolytic bridge
between the two compartments would prevent contamination in either
direction. The experiment was concluded when it was found that the
impedance caused by the electrolytic bridge made the cell very sluggish.
3.8 SeLection of the Final Cell Configurstion
by combining the results gsthered in sections 3.3 3.6 reasonable con-
figurations cam be assigned to the final formaldehyde transducers At least two
different configurations are necessary, one for an anslysis in a CO 2 environment,
namely automobile exhaust, and another in the absence of s CO 2 environment, namely
ambient air.
i s a ion:
The following cell configurations were selected for prototype character-
a.) CO 2 Enviw,wcut -
Ne,sbrsee - Reduced Zitex 12-1373 OS dia.
Sensing Electrode - 333 line Au treated with 50% IWO 3 and 457. XC II
Electrolyte - 107. XCII
Counter Electrode - Rg/RgO or Ag/Ag 2 0
53..
52-
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Applied Potential -O.04V for Hg/EgO or 40.15 for Ag/AggO.
Precautions Black cell, end Permion 2291.
INSTR W UU SYSTEMS OUiSlI
CORPORATtON
b.) Ambient Environment
Membrane Reduced Zitex 12-137 5 0.5 die.
Sensing Electrode - 333 line Aug.
Electrolyte - 30-45% IWH
Counter Electrode - llg/RgO
Applied Potential - 0.1 V
For the CO 2 environment the option of ueing a Ag/Ag 2 0 electrode was
included. Ag/Ag 2 0 although elightly noisy is a much better working
electrode under a wide variety of electrolytic conditions. It nay also
be necessary to change the applied potential as the electrolyte layer
becomes neutralized.
4.0 PROTOTYPE CRAMCTERIZAT ION
Interference Studies
Signal Intorference
a.) Cell for Auto Exhaust - The initial transducer contained a 10% 1(014
electrolyte and a Hg/EgO counter electrode. The cell wee preconditioned
with 1% COg in between interference tests with varioue gases. It was
determined that the msgnitude of interference due ts each pollutant for
a 10% tO M cell did not change appreciably in the presence or absence of
CO 2 . However, to achieve selective oxidation it was necessary to in-
creaae the applied potential of the cell during CO 2 exposure from -0.04 V
to -Cd v for aUg/EgO counter electrode. During the course of the study
it was noticed that the Hg/EgO electrode often became sluggish when faced
with the teak of oxidizing fornaldehyde in a partially carbonated media.
wsTpuMagsysTa s DIVISION
Tests with a Ag/AggO electrode end an applied potential of 40.13 V
indicated that it was not affected by partially carbonated conditions.
The interference data is presented in Table 5. Results were the same
for both counter electrodes.
The quantitative results of the atudy proved to be very satis-
factory. Interference from the major contaminants proved to be almost
nil. The largest interference was obtained from NO 2 . Rowevcr, the
maximum value for this proved to be only 2% on the most sensitive scale.
The partially oxidized hydrocarbons bad high interference values.
This was to be expected. However, it wes determined that alcohols and
ketonea have a higher oxidation potential than aldehydea at a gold
electrode. Thus interference froe these compounds is minimal. The
detection cell was equally sensitive to all of the aldehydea included
in the test. Each was found to have a responae comparable to formalde-
hyde. Response rates were elso the same.
b.) Ambient Cell Interference studies with the ambient cell contain-
ing a Hg/EgO counter electrode yielded even better results. A 307. ROB
solution was used for the electrolyte, eince response tines of 15 sec.
for 90% were obtained using this electrolyte.. The signal-interference
test results for the ambient cell are listed in Table 6.
Interferences from SO 2 and HO 2 were greatly reduced, as were
signal levels from other pollutant speciea. The number of partially
oxidized hydrocarbons was reduced for thie study. Only ooe compound
representing each of the species was evaluated. The Eg/HgO electrode
appeared to be a much faster and more stable ccunter electrode in the
more concentrated basic electrolytes than it had been in the partially
carbonated dilute bases.
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TABLE S
SIGN&L INTERFERENCE DETERMINATIONS
EJO IAUST CELL
Concentration tquiv. ppm of CR 2 0
CO 2t 0.0
02 St 0.0
$02 100 ppm 2.97 X 10
NO 2 200 ppm 2.0 X to-i
NO 2000 ppm 0.0
CO 2 it 0.0
CR 4 tOO ppm 0.0
C 2 R 6 100 ppm 0.0
C 2 R 2 100 ppm t x 10
Bezama 500 ppm 6.8 X 10
Acetaldehyde 10 ppm 10.0
Propional 10 ppm 10.0
Acrolein 10 ppm 10.0
Acetone 10 ppm 0.0
2-Butanone 10 ppm 6.1 it io2
Benza ldehyde 10 ppm 10.0
Methanol t o ppm 6.4 it
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NIMA $ IIB I S UGTRUMEtII SYSTEMS DM5 151
:0RPORATION
TABLE 6
4.1.2 Physical Interference
S tONAL INTERFERENcES,
Physical interference from pollutants indigenous to an exhaust stream
AMBIENT CELL
was observed in only two cases. The first was in the presence of water, the
ฃquiv. ppm of second was due to c0 2 .
Gas Concentration Formaldehyde
a.) Water Water interferes with the detection of formaldehyde when
CO 2% 0.0
the relative humidity is higher than 30-50%. If the gas stream tempera-
02 5% 0.0
ture is below its dew point condensed moisture in the sample chamber of
$02 100 ppm I a 10
the cell completely prevents the detection of formaldehyde at any con-
N02 200 ppm 2 i c r 4
centration. Rumidity levels at 90 to 95% reduced the signal level at
NO 2000 ppm 0.0
1 ppm by 25% end slowed the responee time to 907. in 10 minutes. A
a ! 4 100 ppm 0.0
relative humidity of 70% caused a 7% signal loss at the 1 ppm level.
C 2 ) ! 6 100 ppm 0.0
Since the exhaust stream dew point Ia about l20F problems with moisture
C 2 )! 2 100 ppm 0.0
are anticipated unlesa the sample ie diluted. Removal of water from
Rexene 500 ppm 1 x 10
the eample etream would also remove the formaldehyde.
Benseldehyde 10 ppm 10.0
b.) CO 2 Interference from CO 2 wee determined at the 1% level. It
Methanol 10 ppm 8 x IO
had been previously determined that a level of 12-15% CO 2 could not be
Acetone 10 ppm 0.0
tolerated in direct contact with the cell. At 1% concentrations, the
behavior of the 10% cell with a Ug/ilgO counter electrode was steady for
a continuous period of four hours. After this period the formaldehyde
signal level was observed to drift. A 24 hour exposure caused the
signal to decrease to 20% of its original value. The reepoose rate
also decreased to 90% in 10 minutes. With a Ag/Ag 2 0 electrode the
signal decay was much less intense for long periods, about 10%, and
the 90% response tima remained feirly constant at 1 minute. Diecontinu-
ous uee of either cell in the presence of 17. CO 2 die not cause any of
these problems. It was also observed that cells be Ly damaged through
continuous expoeure would recuperate after a period of flushing with a
zero gas of air or mitrogen.
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Ambient cells were exposed to 300 ppm CO 2 and 1000 ppm CO 2 for
long periods of time without any apparent damage.
4.2 Stability Teats
Results of testa with 1% CO 2 carrier gas streams were reported in
section 4.1.2. Both cells were also tested with uncontaminated N 2 carrier gases.
The exhaust stream cell bad a Ag/AgaO electrode, the ambient, a EglIlg o electrode.
the calls were evaluated by testing them in a temperature chamber which was ant
at 80ฐF. Variation in temperature waa occasionally +5ฐ ?. The cells were continu-
ously exposed to a N 2 gsa stream containing 2.2 ppm formaldehyde at 1.0 SCFH. The
signal waa continuoualy monitored with a 10 mY recorder. Twice a day the celia
were flushed with pure nitrogen and the zero aignal was recorded.
4.2.1 Span Drift
Span drift for both cells was almost negligible. Initial varLationa
were ฑ2%/day. However, after 4 days the calls stabilized to ฑ1%/day.
4.2.2 Zero Drift
Initial zero drift was due to stabilization of the call after construc-
tion. Upon reaching equilibrium both the 10% KOli and 30% KOR cells ahoved very
little tendency towards isro drift. Values of ฑ1%/day were recorded.
4.2.3 Noise Level
The noise level for the ambient cell was negligible. The exhaust call
contained a Ag/Ag 2 0 counter electrode which was prone to random electro chemical
discharges. After equilibration these indiecriminate noise levels were reduced
to a 21 aignal leval.
4.3 Linearity
Tests were conducted to demonstrate the linearity of the detection
current generated by the cell at varying concentretiona of formaldehyde. The
transducer was the Hg/HgO-it0N standard ambient call. The membrane used was the
reduced area Zitex 12-137B en a teflon substrate.
The teat was conducted at 80ฐ ?. The results are presented in Figure 1.
Values were taken at 0.5, 1.0, 2.0, 5.0, end 10.0 ppm formaldehyde. When normal-
ized to a signal for I ppm fcrmatdehyde the variation was found n be ฑ0.5%, in-
creasing from a value of 2.16 pA/ppm at 0.5 ppm to 2.18 pA/ppm at 10.0 ppm.
4.4 temperature Zffects
4.4.1 Signal Interference
Electrocheaical transducers are very sensitive to temperature variations.
An experiment waa conducted in an environmental chamber on two samples of each
formaldehyde monitor. Current readings were taken in the range 65ฐF to 110?.
The sample strum waa 2.2 ppm formaldehyde in nitrogen. Typical current-tempera-
ture curves era shown in Figure 17.
4.4.2 Physical Interference
At temperatures Lower than 60ฐF forealdehyde condensation end poly.
iserizetion take place. This phenomenon is due both to the propertree of formalde-
hyde and to the lowering of the dew point of the gee stream. Because of this
characteristic the cell will be temperature controlled at 80ฐ ?. Above 80ฐ? tempera-
ture compensation will be utilized. Use of temperature compensation corrected the
curve detailed in Figure 17 to a straight line normalized to tha current value at
17.5ฐF. This was accomplished through the use of a thermistor in series with a
resistor of equal value in the feedback ioop of the first ataga amplifier in the
inatrumentation package.
NTRONHU 5TBaSDMS 3 t M INT SYSTEMS D i
ORPORATION
59
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NOT REPRODUCIBLE
FIGURE 16 FORMALDEHYDE CONCENTRATION
VERSUS DETECTION CURRENT
18
I
6
4
2
0
2
4
PPM FORMALDEHYDE
6 8 10
6 1
-------
FIGURE 17
CURRENT OUTPIrZ VERSUS TEMPERATURE FORMALDEHYDE CELL
S
CORPORATiON
4.3 Life Time
The life time of a cell is dependent on each of the individual components.
me electrolyte supply in the cell was determined to be adequate for eight months
of continuous use. This figure is based on a weight loss of 0.5g/day with dry N 2 .
With a semi-humid gas stream the cell should last longer.
Samples of the counter electrodes have been operational for aix months
without any signs of deterioration.
The life time of he sensor electrode depends on the :ate at which it
is poisoned. The Hg/HgO counter electrode and the gas stream pollutants have not
affected the electrode. The AgIAg 2 O might possibly present some problems but as
yet these have not materialized.
The membrane is composed almost entirely of teflon and is not expected
to be damaged.
4.6 Evaluation of Sampling Techniques
4.6.1 Redesign of Cell Sample Area
The sample chanber of the standard cell was not adequate for the deter-
minเtion of formaldehyde. It bad too many surfaces where formaldehyde could
condense Out, too long a residence time, and it allowed most of the formaldehyde
to pass through without coming into contact with the membrane surface. Response
times using this chamber were 90 in 10 minutes at 0.1 iSA/ppm. See drawing 672020
in the appendLx.
The chamber was redesigned to eliminate these deficiencies. The volume
of tb. chamber was decreased in size by 807. to lower the residence time. The
electrode support screen was removed to decrease the available condensation sur-
face area and to allow the entire formaldehyde sample to flow over the membrane
surface. Support for the sensing electrode was achieved through the use of a
porous 1.5 teflon pad for the exhaust cell and a 1.9 diameter nylon mesh for
the a ient cell.
I
80
TEMPERATURE (Y)
-62-
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uisimjijitn SYSIEMS DiVISION
C o i l PORA11ON
The diameter of the reduced zytex membrane was enlarged to 0.75 for
the auto exhaust cell. This increased the sensitivity of the cell to 5 i sA/pp s so
that the sample could be diluted. The diameter of the ambient cell membrane was
decreased to (J, 4 Pt This increased the response time, decreased the evaporation
rate, end reduced the seneittsity for s.ribtent studies to 1.4 sA!ppm.
4.6.2 Ambient Sampling Conditions
For the ambient cell there are no special problems. Sampling should
be conducted under the following conditions.
I.) The sample should be in the 80l20F temperature range, preferably
around 80ฐF,
2.) The sample stream should be free of particles larger than 104s.
3.) The flow rate should be 12 CPU.
4.) The relative humidity should be around 50% or lower. Above 507.
some formaldehyde condensation takes place.
5.) The sample should be pushed through rather than pulled, lulling
the ssmple through may increase the response time.
4.6.3 Auto Exhaust Sapling Conditions
The exhaust stream cell is quite adequsts when used in the proper manner.
1.) The exhaust sample should be diluted to lower the CO 2 level to 17..
This can be accomplished by physically mixing a 1 SCPU sample of the
exhaust stream with a 9 SCFU air flow and subsequently metering a
I scm sample of the mixture through the cell.
The preferred method would be to utilize a venturi type pump which
would suck in a 1 Cr11 sample of the exhaust at a flow rate of 9 Cr11.
A 1 Cr11 sample of this mixture could then be pushed through the cell.
DiSTRUM INI SYSTEMS OMSION
ฃ second alternative would involve sampling the gee stream with a
permeetion membrane such as NEl l 213. The diffused gas would then be
passed through the cell with an sir stream at 1-2 CPU.
The final condition of the sample should be the same as for the
ambient cell.
The testing should be conducted on a discontinuous basis. The
cell will usually function without probe lss if it is exposed to the
diluted exhsu it stream for 0-30 minute periods foalowed by a 10 minute
purge with sir.
CORPORATION
-a-
-65
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msmu ua s siuas mr i
INflMJMEJU SYSTEMS DIV t ESIUWUIIt55 W _______________________________________________________________
81ASฉIIIIC(S CORPORATiON
CORPORATION
APPENDIX A
INSTRUMENT OPERATION AND DESiGN
the Dynasciences Formaldehyde Monitor is a compact, portable instrument
designed for rapid discontinuous measurementa of the concentration of formalde-
hyde in a diluted automobile exhaust stream (AL-620). It may also be used for
continuous measurements under ambient conditions (AL610). Front, top, and side
schematic views of the monitor are shown In drawing No. 620531. Other pollutants
that can be measured by substituting the appropriate selective transducer into
the instrument package ere: oxtdes of nitrogen, nitrogen dioxide, sulfur dioxide,
and undiluted ambient formaldehyde. This interchangeability is an outstanding
feature which makes the monitor a highly versatile tool.
APPENDIX A
The model numbers, standard ranges and the recoesnended applications of
INSTRUMENT OPERATION AND DESIGN the different sensors are as follows:
) CDEI. GAS MEASURED RANGES: APPLICATION
1. 11 1-130 Total oxides of 0500, 0-1500 Industrial stacks, eutoisobile
nitrogsn* and 0-5000 ppm exhaust, process control,
development studies.
2. SS-330 Sulfur Dioxide 0-500, 01500 industrial stacks, process
and 0-5000 ppm control, development studies.
3. NR-230 Nitrogen Dioxide 0-200, 0-600 Process Control
and 0-2000 ppm
4. NE-l b Total oxides of 0-5, 0-15 and Plume and ambient measurements,
nitrogen* 0-30 ppm exposure studies.
5. SS-3b0 Sulfur Dioxide 0I, 0-3, end Plume and ambient measurements,
0-10 ppm exposure studies.
6. NR-210 Nitrogen Dioxide 0-5, 0-15, and Plums and ambient messurements,
050 ppm exposure studies.
7. AL-610 Formaldehyde 0-1, 0-3, and Ambient studies.
0-10 ppm
8. AL-620 Formaldehyde 0 0.l, 00.3, Diluted automobile exhaust,
end 01.0 ppm ambient studies,
*Reqnires scrubber if so2 present.
Table IA shows the specifications for Monitors AL6l0 and AL-620.
66- 67
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iNSifl uMilfi 515*1 13 DIVISION
CORPORATION
TABLE IA
MONITORS AL-610 AND AL-620 SPECIFICATIONS
Sensitivity Ranges: 0- i, 0-3, 0-10 ppm (AL-610)
0-0.1, 0-0.3, 0-1.0 (AL-620)
Detection Limit: 2% of full scale
Response: Linear response over the enc te range of the
pollutant. Overall accuracy: ฑ2% full
scale with the use of integral meter, ฑ1%
full scale with the use of external
potentiocetric recorder.
Response and Recovery Tines: 907. of full scale in less than 30 seconds
( A L- G b ) -
90% of full scale in lees than 60 seconds
(AL-620).
Stability: Ret tar than ฑ17. of full scale per day.
Specificity: No response to N 2 , 02, 00, C0 2 , SOp, NO,
water vapor or hydrocarbone.
Temperature Control: About 80 ? ฑ1 %.
Temperature Compensation: 80 ? - 120ฐF .
Output: Meter readout and 0-10 eN output for retorder.
Sensor Life: One year at 80?, noist gases
Power Supply: 105 to 125? 60 Re 5 VA required for powering
solid state amplifier (12 V.D.C. optional).
Monitor Weight: Less then 15 lbs.
Plowrete: Keep the flow rate within the green indicator
on the flow mater. Recommended flow rate: I SCFII.
-68
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PfSTP&MII11 SYSTELISOWISI
INSTRUMENt SYSTEMS DIVISION
CORPORATION
SENSOR
The sensor operates on the principle of a fuel cell. The formaldehyde
diffuses through a sszeiperisesbls membrane and undergoes electra-oxidation si a
special sensing electrode. The resulting current is directly proportional to
the partial pressure of the pollutant in the gas mixture. This current is ampli-
fied and the output of the amplifier is displayed on the meter. The selectivity
of the sensor to formaldehyde is obtuined by the unique mesibranslelectrolyte/
electrode combination.
Each sensor is s sealed unit end no addition of any electrolyte is ever
required. The sensor operating life may vary from 3 to 9 months depending upon
the condition of the sa mple. Generally speaking, hot, dry sod dirty samples
will shorten the sensor life. The sensor may be conveniently replaced with a
new sensor as described later. Drawings #672592 and 672021 show the design and
specifications of the polypropylene transducer body.
SAMPLE CONDITIONING AND PRESSUI ADJUSTMENTS
The electrolyte used for the rapid, selective oxidation of formaldehyde
imposes certain restrictions on the nature of the ssmpls during automobile exhsust
analysis. The gas stresa should not contain more then it CO 2 and the relative
humidity of the gas should not be higher than SOt.
The sensitivity of the AL-620 instrument has been increased to allow the
sample to be diluted to meet these requirements. A standsrd procedure based on
a tenfold dilution of ths sample has proven to yield satisfactory results. Con-
tinuous exposure to automobile exhaust for periods longsr than four hours may
cause some drift in the calibration spec.
The recomasndsd method of analysis is exposure to the diluted sample ges
for 30 minutes or lsss followed by flushing with a zero gas for 10 minutes. This
procsdurs will guarantee trouble-free operation.
CORPORATION
The formsldshyds smalent sensor AL-6l0 is designed for application in an
environment charsctarizi.S by a C og content of 1000 ppm or less. It hss a faster
response time, lower sensitivity, and does not require sample dilution.
The followimg sampling requirements apply squally to both models. The
te,spsraturs of the samepls should be less than 100 ?, preferably in the range 80-l00 P
The conditioned sample can be passed ovsr the sensor by connecting it to the input
114 polypropylene tubs fitting provided it is et the right side when facing the
back of the instrument. Ths sample passes thiough the sensor, flowmeter, sod exits
from the fitting on the left side which ahould be coenectsd to the vent line. The
instrument reading is independent of the flow rate. However, it is suggssted that
to get the optimum response time and e long operating life, a flow rats of 0.5
to 2.0 SCFH be kept study throughout the rim, if the f low indicetor is kept in
the blue ares, the flow rats will be satisfactory.
The sample should be pushed rather than pulled through the sensor.
However, if it is desirable to pull the sample by cresting a suction st the exit,
cars must be taken to essurs that the vacuum crested in the sensor is not more
than 3 of Ego.
The sensor is designed for operation at atmospheric pressure. It can be
operated in line at pressures alightly above or below atmospheric, but it is
of utmost importance that any pressure changes occur slowly in order to prevent
damage to the sensor.
Since the sensor measures the partial pressure of the gas being monitored,
it must be realized that a correction should be made if the sensor has been cali-
brated at one pressure and is used at another. For example, calibration of the
sensor at 740= Mg of ambient atmospheric pressure and 10 ppm of QIgO gas results
in a full scale meter reading unique to 740= 11g. If the total in line pressure
is changed for example to 700mm Hg, end the ratio of moleculss of CE 2 O gas to tots l
gas is kept constant at 10 pile, them the sew reading will be of full seals.
70
71
-------
oirr trr
The meter reading is directly proportional to the concentration of CH 2 O.
In addition, a 10 nV recorder output is provided in the back of the monitor for
continuous recording. The output Jack is available on the back of the instrument
next to the sample outlet fitting.
POWER REQUIREMENTS
11:1230 volts and 100 volts, 50 or 60 as powered from the three-prong
receptacle on the back of the monitor. An optional 12 Vdc capability is available.
The 12 Vt input jack is provided on the beck of the inatrument next to the fuse.
HEATER
A heater assembly with the appropriate electronics is provided ss a
standard feature for the formaldehyde model. The purpose of the heater is to
maintain a temperature environment for the sensor highei than 80ฐF, thereby improv-
log the stability and accurscy of the system. At tower temperatures formaldehyde
is absorbed by the surfaces in the sampling system.
When the outside temperature is below 80ฐF the heater must be utilized.
The heater can be switched on by pushing the BEATER POWER switch on the front
panel. About 3-1/2 to 4 hours are required for the sensor to cans to equilibrium
at a factory-sat temperature of about 80ฐF.
To rsadjust the tamparaturs setpoint, proceed as follows:
a.) Remove the printed circuit board from its receptacle.
b.) Using an ohmmeter, check the resistance bstwaen Points A and B
(sea Drawing No. 602517, Printed Wiring Assembly) and adjust pot Ml
to obtain 41K ohm resistance.
c.) install the printed circuit board beck in its receptacle.
mSTRuMEIa sisia s DIViSION
Ths heater is proportionally controlled thus eliminating the possible
interference from an On/Off action. Tha HEATER red light on the front panel
indicates the level of power to the heater. in the beginning when the HEATER
POWER is switched on, the BEATER light is very bright. However, within the first
two hours it starts dimming and stays fairly dim furing the continuous operation.
CAi Jri O N - if the BEATER light remains bright for more than two hours,
turn the power off and check for possible malfunction in the heater
circuit. The t m white leads from the heater should be firmly plugged
in the white jacks on the chssais which supply the power to the heater.
Check the resistance across the two gresn leads after removing them
from the chaaais. These leads are joined to a thermistor which should
read about 100 K ohm resistance at 77ฐF and about 45 K ohm resistance
at the controlled temperature. A broken thernietor would show as
infinite resistance and would kssp the heater power on all the time.
CALIBRATION
The instrument ray be calibrated in any BMCE position. However, for
maximum accuracy, calibrate in the High Range. Turn on power to the instrument
by pushing in the SYSTEM POWER switch. Set the RAN switch to the desired
position. Turn the SPAN knob completely clockwise. Wait about 15 minutes be-
fore proceeding with the calibration. If the heater is used, wait 4 hours before
proceeding. Plow the zero gas (N 2 or formaldehyde free air) through the instrument
and wait for 5 minutes, Them bring the reading to ssro by adjuating the ZERO knob.
Lock the ZERO knob. Pass the calibration gas over the sensor. Wait 5 minutes.
Adjust the SPAN knob so that the mater reading corresponds to the concentration
in the calibrated sample. Lock the SPAN knob.
CORPORATION
INSTIWMB4TSYS IEMSDIVIS IWI
CORPORATION
72
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CORPORATION
INSTIU JMENT SYSTEMS UIV1SWN
_________________ CORPORATION
WSTRtJMEt ITSY SffJ&5c:!;:
BATtERY CHECK
The inatrimient is designed to be essentially driftfree (ฑ 1% full scale
per day) and wilt retain its calibration for a long ties. However, for optinun
results, calibrate the instr nt at regular intervals.
SEWSOR REPlACEMENT
The instr,szmnt is designed to be maintenance free. The sensor must never
be taken apart. ฃ fault in the sensor usually shows up as slower response time
and/or inability to calibrate tie instrument.
The faulty sensor may be easily replaced with a new sensor in the manner
described below. The newly replaced sensor may take 6 to 8 hours to stabilise
before it is ready for use. Before replacing ths sensor, make sure that the
slide switch located maar the battery on the chassis is in the proper position.
The use of m dsls NE-210 and NR-230 requires the slide switch moved towards the
front of the monitor. All other modsls require slide switch in the opposite
direction as marked.
Installation of the ALEdO cell raquirsa manual switching of the battery
polarity using the switch located among the black cell Jacks. The black cell-
lead should be plugged into the terminal marked AL-6l0.
PROCEDURE FOR CHARGING SENSOR (UNITS WITH HEATER )
Disconnect llSV input power cord.
Remove (2) screws, item 65, and remove cover/rear panel assembly.
Disconnect (2) tube fittings, item 152.
Unplug electrical leads running from sensor to chassis.
Remove insulator, Item 130.
Remove (2) screws, Item 150, which hold cover, Item 141, to enclosure,
Item 140.
Remove cover/sensor assembly 602580 end replace with new assembly.
Reverse Steps I through 7.
aft: Fittings, ita 152, shall be firmly Iant tightened only.
A size B mercury battery is used to bias the sensor. Its voltage at
el I times should remain 1.35 V ฑ10%. The battery voltage may be checked by hold-
ing the RANGE switch on the front psnel to the BATtERY TEST position. Replace
the battery if the output on the mater show, less than 90% full scale deflection.
ThE ELECTRCRIIC PACKAGE
During system operation the cell output current is convertsd to a pro-
portional voltage by the Current Amplifier. This voltage signal is then scaled
by the span control and amplified by the scaling smplificr. The scsling amplifier
gain is determined by the rsnge control (High - Med - Low) which provides a proper
output for full scale meter deflection snd recorder output.
DETAILED OPERATION
The current amplifier input is provided by the csll (PITT) and bias net-
work composed of resistors EdO through 164, R68, R69 end battery (B). The function
of the hiss network is to provide the proper csll bias for system applicetion re-
quired.
The front pansl zero control circuit consists of pot 166, resistors R3,
BA and R5 and 6.2V seners VU 1 and PR 2 . This provides a ฑ current to offset
residual cell currents.
Q 1 . snd P 2 era cresting a short circuit to all currents when the
power is in the off state. This maintains the cell in a ready state for rapid
stabilization.
The current amplifier is basicelly a low noise PET input smplifisr which
utilizes the RA 741 for high open loop gain end complementary output drivers 04
and for linearity. Potentiometer 113 end resistors Rto and R 12 are used to
zero the amplifier during initial aligtzaent. The amplifier gain is determined
by R I 1 , U 1 and U 1 1 , U 1 and U 11 are used to modify the temperature characteristic
Step 1:
Step 2:
Step 3:
Step 4:
Step 5:
Step 6:
Step 7:
Step 8:
- 14-
75
-------
of thermistor R I 1 . RT 1 and R are physically located in the cell to obtain temper-
ature tracking gain which will provide the best system accuracy during heater
vern-up. The basic gain equation is:
To tin R I
where Vo is the voltage at the emitters of 04 end Q 3 .
tin is the cell current
RP 1111 + R 1 + RT 1
The scaling amplifier ARt is an inverting operational esiplifier with four select-
able gain settings determined by the range switch. The basic gain equation is:
Av Rf
Rin
where Rf is R 19 for aLl ranges and J Un composed of one of the series
resistors R 17 , R 18 or RM snd the equivalent source resistances. This source
resistance is the effective resistance of the span pot R and sny parallel
resistance such as R 16 or
To msintein en accurate meter reading, the meter is electrically contained
in the feedback loop and the system is therefore immune to changes in the meter
resistance as a function of time and temperature.
Overload protection for the circuit is provided by the diode network of
CR 1 through CR 1 and R 20 . This limits the amplifier to ฑ1.4V.
The recorder output is developed by the resistive divider 1122 and R 23 .
This provides a 10 mV full scale output with a low output impedance.
Initial 5 ero adjust for Alt ie set by 11 2L Temperature control of the
cell heater is determited by resistive bridge amplifier Al and associated driver
transistors R 7 through 19.
The resistive bridge is composed of resistors 1140, k b ผ . (therm-
istor) and potentiometer 14t Bridga unbalance ia senaed by amplifier AR 3 which
INSTRUMENT SYSTEMS DIVISION
proportionally changes the output voltage to the hester. At 25ฐC the bridge
unbalance causes pin 3 to be positive with respect to pin 2. The output at
pin 6 is then at a maximum positive voltage. This voltage is buffered by
emitter followers and which control the high current pass transistors Qp.
As the heat increases, the thermistor RT 2 (located on the cell) will continue
to decrease in value until the bridge becomes balanced. The high gain of the
amplifier maintains the haater at a high input current for fast wan-up and then
mainteina a proportional contro. within a few tenths of a degree.
Circuit protection is obtained by the heater power switch S 2 end diodes
CR 9 and CR14.
The heater light D53 ia directly in parallel with the heater blankets
end has a brilliance which is proportionate to the applied voltage.
Monitor power can be provided by either 115 VAC line voltage or +12 VDC.
The +12 VoC operation is dependent on the 951 DC to DC converter. This is e
special module which provides ฑ22 V of partially filtered voltage in parallel
with the rectified AC line voltage.
The filtered DC voltages are regulated by integrated circuit regulators
Z 1 and Z 2 . The regulated ฑ15 VDC is used to virtually eliminate the effects of
line voltage end frequency. they also provide short circuit protection to pre-
vent propagation failures. Initial adjustment is set by potentiometer 112 1 for
the +15 VDC end for the 15 VDC.
CORPORATION
! STRUMUIT SYSTEMS DIVISION SฉI E S
________________ CORPORATION
7 1
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DISTRUMENISYSTENS DIVISION DISTUMUffSTS1D SD
CORPORATION
CORPORATION
I APPENDIX B
I PARTS LIST
I _
Appendix 3 CO taiitS a list of replaceable parts for the Dynasciencea
I Air PoLlution Monitor.
I To obtain replacement parts. addtess order of inq@iry to:
Dynasciencea Corporation
Instr mient Systems DivL ion
9601 Canoga Avenue
Chatsworth, California 91311
APPENDIX B
PARTS LIST
79 80
-------
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