DSVELOPiMENT OF PORTABLE ELECTROCHEMICAL TRANSDUCERS
                         FOR
THE DETECTION OF SULFUR DIOXIDE AND OXIDES OF NITROGEN
                   CONTRACT NUMBER
                    CPA 22-69-118
                    FINAL REPORT
      PERIOD COVERED:  JUNE 1969 TO AUGUST 1970
                      AUTHORS
         RAMESH CHAND     ROLAND V. MARCOTE
                   SEPTEMBER, 1970
                    PREPARED FOR
    DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
               DURHAM, NORTH CAROLINA
              DYNASCIEKCES CORPORATION
            INSTRUMENT SYSTEMS DIVISION

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                                                                    NSTRii^EHT SYSTEM
CORPORATION
                                 TABLE OF CONTENTS

     Section                                                                Page

         1.0  INTRODUCTION	.  .  .  .	    1

         2.0  GENERAL DISCUSSION .  .	    2

              2.1  Sweep Voltaimietry	    2
                   2.1.1  Instrumentation. .  .  .  '.	    2
                   2.1.2  Experimental Procedure  	    5
              2.2  The Electrochemical Transducer.	    5

         3.0  EXPERIMENTAL	11

              3.1  Sv/eep Voltammctry  Studies	   11  .
              3.2  Experimental S02 and NOX Transducers	17
                   3.2.1  Evaluation  of Semipermeable Membranes	17
                   3.2.2  Transducers Employing Aqueous
                          Sulfuric  Acid Electrolyte.	19
                   3.2.3  Transducers Employing KPF6  in
                          Propylene Carbonate as  Electrolyte 	   20
                   3.2.4  Transducers Employing Dryed KPFg and
                          Cl^COOH in  Propylene  Carbonate as Electrolyte.  .   22
                   3.2.5  Transducers Employing Other Electrolytes	   23
              3.3  Selected NOX and S02 Transducers	25
              3.4  Sulfur Dioxide Scrubbing Systems	26
                   3.4.1  Electro-oxidation Systems	26
                   3.4.2  Wet Scrubbing Systems	   27

         4.0  PROTOTYPE MONITORS PERFORMANCE  DATA	30

              4.1  Sensitivity	   30
                   4.1.1  Sensitivity and Operational Temperature	30
                   4.1.2  Sensitivity Ranges  	   30
              4.2  Selectivity	30
              4.3  Response and Recovery Times  	   34
              4.4  Stability of NOX and S02 Prototype Monitors	34
              4.5  Transducers Life Data	38
              4.6  Temperature Control and Compensation	38


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RPORATIOIM
                                                                                            i
                                LIST OF FIGURES                           -                  \
                                                                                            *
     -  '                          '                      -                                     s
   Figures                                                          .      Page              j
                                                                .••'''                     ^
      1        Oxidation of N02, NO, and S02> in l.ON HUSO,          .                        |
              at a Gold Sensing Electrode	   2            •   j

      2        Cyclic Voltammetric Instrumentation  	   4               i

      3        Sweep Voltammetric Cell  	   6               !

      4        Electrochemical Transducer  (Schematic)	   8               ,
                                                                                            i
      5        Potenti.ostatic Control of an Electrochemical Transducer  .  .   9

      6        Linear Response of an NOX Prototype Monitor	31               :

      7        Characteristic Response  of Monitor SS-330	  35


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ORPQRATIQN
                                  LIST OF TABLES

     Table"                •                                                Page

       1        Sweep Voltaimnetry Evaluations	    12

       2        Sweep Voltammetry Evaluations 	  ....    13

       3        Sweep Voltammetric Evaluations of
                Potential Interferences 	    15

       4        Sweep Voltammetric Evaluations of
                Potential Interferences 	    16

       5        Selective Membranes	    18

       6        Evaluation of NOV Transducers Employing KPF.--PC  ....    21
                                x                          o

       7        Evaluation of Various Organic Electrolytes..	    24

       8        The Effect of Temperature Upon Transducers
                Sensitivity	    32

       9        Evaluation of Various Interferences .  .	33

      10        Stability of NO  and S00 Prototype Monitors	    37
                               X       £*

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ORPO RATION
 t>                                  .
   "l.O  INTRODUCTION
 o                          '    '
    This is  the Final Report  on the Dept.  of Health,  Education and Welfare
    Contract CPA-22-69-118, covering the period from  June  1969 to  August  1970.
    The objective of this  effort was to develop and provide  portable  electro-
    chemical transducers for  the detection of sulfur  dioxide and oxides of
    nitrogen in the industrial stack.
    The v?ork carried out involved the  screening of different aqueous  as well
    as  nonaqueous electrolytes for the purpose of developing selective  sulfur
    dioxide  and oxides of  nitrogen transducers.  Though many organic  electrolytes
    were found to be feasible based on the sweep voltammetry studies,  they  were
    all abandoned from consideration in the final design because the  transducers
    employing these' electrolytes gave  very limited useful  life, slow  response
    time and erratic behavior.
    A dilute solution of sulfuric acid (IN H2SO^) was selected as  the  electrolyte
    for both sulfur dioxide and oxides of  nitrogen transducers.
    The selectivity of the sulfur dioxide  transducer  was obtained  by  subjecting
    its sensing electrode  to  a fixed potential and by the  use of polyethylene
    as  the gas permeation  membrane. The total interference  from 1000  ppm NO
    and 100  ppm N0£ was found to be about  5% full scale on a 0-1000  ppm S02
    transducer.
    The selectivity of the oxides of nitrogen transducer could not be  obtained in
    the presence of sulfur dioxide and a scrubber was designed to  remove  S0?
    without  appreciable affecting NOV  concentration.   The  scrubber consisting of
                                    X
    aqueous  solution of I^CO-j, KHCO^j  KNOo and KNO~ was found to absorb less
    than 1%  of 2000 ppm of NO and less than 20% of 100 ppm N02- However,
    considering the low concentration  of NOo in the stack, the overall  effect
    on  the NO  concentration  is insignificant.

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                                                                         SYSTEMS Bi
a2.0  .-GENERAL DISCUSSION    ..___.                      •'.."'
                         '                      •                •
 2.1  JWEEP VQLTAMMETRY                 "               .    "     •
 The oxidation and reduction potentials  of different  electroactive  gases   _
 in different aqueous  and nonaqueous  electrolytes  were  determined using-
 sweep voltammetry .  This technique,  which permits the  scanning of  an entire
 voltage range in a  matter of seconds, not only shows the  potentials  at
 which reactions  occur, but also their rates  and efficiencies  in terms of
 current generated.  Moreover,  information regarding  electrolyte stability
 and electrode kinetics also result from sweep voltammetric studies.   The
 existence of an  electrode reaction is indicated by a peak in  the voltage-
 current curve traced  out by an x-y recorder.   The potential at which the        .
 peak occurs is related to the  specific  electrode  reaction, whereas the psak
 height (current) is proportional to  the concentration  and diffusion  coefficient
 of the reacting  species.  The  efficiency  of  the electrode reaction,  in terms
 of electrochemical  reversibility,  is also reflected  in the peak height and
 the shift of peak potential.   The  purpose of this study was to find  an
 electrolyte which showed selective oxidation peaks for either SO,  or NO  , or
                                                                 £*      ft.
 which showed their  respective  oxidation peaks at  substantially different
 values.  It was  understood at  this stage  that both NOX and S02 being highly
                                                                                          F;'
 electroactive are likely to offer  the most interference with  each  other. As              K
 an example, Figure  1  shows the voltammograms  for  S02,  NO  and  N02 obtained in              \
 IN H2SO/,. electrolyte.                                •       "
 2.1.1   INSTRUMENTATION
 The  instrumentation  employed  for  the  cyclic  voltammetric measurements  is
 schematically shown  in Figure  2.   Basic  to the method  is a  potentiostat
 which maintains  the  voltage between  the  reference-working electrode  couple
 at a value  equal  to  the sum of the input reference voltages  represented
 by the  dc reference  and function  generator values.  The potentiostat
 accomplishes  this  by controlling  the  current between the working  and counter-
 electrodes.   The  potentiostat  senses  the difference between  the reference-
 working electrode  voltages and the sum of the reference input voltages by.
 means of a  high  gain amplifier system, and feeds  the output  through  the
 working electrode  -  counterelectrode  couple.  This amplifier has  high  input
 and  low output impadance, and  a rise-time less than 10  l sec.  The system
 permits the measurement of current-voltage-time relationships under
                                      2


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  POTENTIOSTAT
    [REFERENCE
       VOLTAGE
 REFERENCE
 ELECTRODE
      WORKING
      ELECTROD

       FUNCTION
      GENERATOR   f
POWER  ' '
AMPLIFIER
•PC-Y RECORDER
  COUNTERELECTRODE
   .CELL'

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well-defined conditions.  No voltage change is possible at the reference
.electrode, and  the measurements reflect only the voltage changes at the
working electrode-electrolyte  interface independent.of ohmic losses through
the  electrolyte and  polarization  at  the counterelectrode.
The  potentiostat was.designed  and built at this  laboratory.  Employing solid
state  operational amplifiers and  a Harrison type 6824A current amplifier,
the  instrument  is capable of an output of ±50 volts at 1.2 amp.  The  function
generator is a  Hewlett Packard Model 3300, with  a  Hewlett Packard Model
3302A  Trigger,  phase lock plug-in unit.
The  two-chamber sweep  voltammetry cell  (Figure  3)  employs three  electrodes.               j
These  electrodes are the working, counter  and reference  electrodes.   The                  »
two  chambers are connected  to  each other  by an  electrolyte bridge.  The                   J
working or sensing  electrode was  a 1.27cm x 0.127cm piece of gold mesh  99.9%             .>
 pure.   The counterelectrode was  a platinum (99.9%) mesh, and the reference               jj
 electrode was  cadmium/cadmium fluoride  (Cd/CdF2) obtained by direct fluorination         i>
 of 99.9% pure  cadmium wire.                                                              ji

 2.1.2  EXPERIMENTAL PROCEDURE        .                                                    |(
 The sweep voltammetry cell  was cleaned  and dried.  The gold  electrode was              .  jj
 cleaned by immersion  in hot 50% aqueous HN03 followed by water rinse, immersion          jj
 in hot 20% aqueous KOH, water rinse, another brief immersion in the 50% HNO_3             j
 and rinsed with distilled water.  This electrode was further rinsed with the
 electrolyte.
 The Cd/CdF7 reference electrode was cleaned by brief immersion  in diluted
 HN03  followed  by rinses with  distilled water and  the electrolyte.  The
 platinum counterelectrode was anealed at a flame  before using it.
 Individual gases were bubbled through the electrolyte for three minutes,
 and sweep curves were recorded on the third sweep cycle.  The scanning
 range was -0.5 volts  to -KL.25 volts at a sweeping rate  of 40 mv/second.

 2.2  	
 The electrochemical transducer  is a sealed faradaic  device  in which  the
 electro-oxidation  or  electro-reduction of absorbed gas  molecules at  a
 sensing electrode  results  in  a  current directly proportional  to the  partial
  pressure of  the gas.


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 WORKING ELECTRODE LEAD
REFERENCE ELECTRODE

        \
         GAS INLET  TUBE
            (TEFLON)
                                  <™»GAS INLET
                                     PORT .
                                     -COUNTER -
                                      ELECTRODE
                                      LEAD
                                -WORKING
                                  ELECTRODE
    PLATINUM
GAUZE  COUNTER-
ELECTRODE

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BJlillll
CORPOfiATION
INSTRUMENT SYSTEMS DiViSiO;]
   ' Figure 4  shows a  schematic drawing of an electrochemical transducer.  The
     pollutant molecules diffuse  through  the semi-permeable membrane, dissolve
     in  the film of electrolyte,  and diffuse to the surface of the sensing electrode
     where they undergo electro-oxidation or electro-reduction, depending upon the
     nature of the gas molecules  and the  transducer operational mode.
     The  counterelectrode material has an oxidation potential higher than that of
     the  pollutant.  When the  two electrodes are joined together externally
     through a small resistor, the sensing electrode which is a polarizable
     electrode assumes the  potential of the counterelectrode less the iR drop
     across the resistor.   The pollutant  molecules, upon reaching the sensing electrode
     encounter a potential  higher than required for their electro-oxidation.  The
     net  result is lowering of the potential of the sensing electrode proportional
     to  the concentration of the  pollutant molecules and equivalent release of
     electrons from the sensing to the counter electrode.  Electrochemically,
     while the pollutant is being oxidized at the sensing electrode, a corresponding
     reduction reaction occurs at the counterelectrode.  Because the current is
     diffusion limited, the resulting electrical current is directly proportional
     to  the pollutant  partial  pressure.   The magnitude of the concentration gradient
      at  the  sensing electrode  is directly proportional to the gas concentration in
      the electrolyte  layer  (Pick's  law of diffusion), and therefore to the partial
      pressure of  the  pollutant in the sample.  The diffusion current  is expressed
      by:
                                            nFADC
                                         x e_

      Where i is the current  in amps, n is the number of exchanged electrons par
      mole of pollutant, F is the Faraday constant  (96500 coulombs), D is  the
                                            f\
      diffusion coefficient  of  the gas in cm /sec., C is the concentration of
      the gas dissolved  in the  electrolyte layer  (moles per cm-*) and £  is the
      thickness of the diffusion layer in cms.
      The potential exerted  upon the gold electrode by the counterelectrode was
      regulated by use of potentiostatic  control.  This is schematically shown
      in  Figure 5. A  stable D.C. power source, such as a mercury battery  across
      a potentiometer  was used  to lower or increase the effective potential on
      the sensing  electrode.  We have used a lead dioxide counterelectrode for
      the sensors  employing  aqueous  sulfuric acid electrolytes and a cadmium

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   THIN FILM ELECTROLYTE
     SENSING ELECTRODE
 /


''/.
     BULK ELECTROLYTE
 77T7// '777/T7
/ /  / /  / //  / f / <  / //
     COUNTERELECTRODE  / //
                                                TO



                                               AMP

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           FIGURE 5      SCHEMATIC SH.Ovr.tHG POTEHTIOSTATIC CONTROL OF ELECTRO-
                /' ".   . .  CHEMICAL TRANSDUCER TO OBTAIN S02 SELECTIVITY
   SENSING ELECTRODE
OUNTER- ELECTRODE
                                              M.35 VOLTS
                                    TRANSDUCER
                                                                             -o
                                                                             -o

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CORPORATION
     The lead dioxide electrodes were made by the anodic electroplating on
     platinum mesh  (52 mesh size, Englehard Industries).  The plating solution
     consisted of 350 gms/liter of Pb(NO~)0 and 2 gms/liter of Cu(NOo)9% H.O.
                                        3 f.                         ~> /  2.
     The plating was carred out at 15 amp/ft  current density and at a solution
     temperature of 120°F.
     The cadmium fluoride electrodes were prepared by mixing 85% CdF9 (Research
     Inorganic, 98%), 5% carbon (Research Inorganic, 99.9997=) and 10% polypropylene
     poxtfder  (Hercules Co.).  About 6 gms of this mixture was evenly spread on
     both sides of a platinum mesh and pressed to 3,000 psig for three minutes
     at 110°C.

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CORPORATION
     ,3.0  EXPERIMENTAL

     3.1
     In the evaluation of sweep voltanmograms the most useful parameters are the
     potential and current flow values at which the oxidation and reduction of
     the gas being studies occurs. • These values are generally referred to as the
     peak potentials.  The peak potential values represent the minimum voltage
     required to obtain the maximum current f'low.
     The primary objective of these sweep voltammetry studies was the discovery
     of electrolytes with selective response for NOX and S0?.  With this purpose
     in mind, a number of electrolytic systems were characterized.  The oxidation
     and reduction potentials of NO, N0?, S0_ and other electroactive species
     which could interfere with NOX and S02 were obtained.  Tables 1 and 2 show
     the oxidation and reduction,potential of various gaseous pollutants in each
     of the electrolytes studied.
     The selective oxidation or reduction of a pollutant in the presence of other
     electroactive speci.es is obtained if either its oxidation potential is lower,
     or its reduction potential is higher than those of the interfering gases                 •
     (Stockholm Convention).  The electro-oxidation of sulfur dioxide in the                 i:
                                                                                             I '
     presence of oxygen and oxides of nitrogen clearly exemplifies the selective             '• *
                                                                                             i !
     oxidation of pollutants.  The electrochemical half-cell reactions, in                   ij
     IN H^SO,, for these gases are shown below.  The potential values (E°)                   S
                                                                                             i
     throughout this report are in volts relative to the normal hydrogen                     v
     electrode  (NHE)
                                                            p
                                                             298° ' Volts
•p
& -~00 »
          S02+2H20  —*  S04 44H+42e~                         40.17
          N024H 0   —»?   NO "+2H+4e~                          40.80
          N0421I20   -^»  NO "44li+4-3e~                         40.96
                              , 4                                                             {
                                                                                             i
                                                                                             }
     The sweep voltammetry studies  indicated  the existence of several electrolytes           j.
     potentially suitable for the selective sensing of NO  and SO  .  The most                £
                                                         x       i                           £
     promising systems  were for SO-  IN 1I_SO,, and  for NO  undistilled propylene              ?
     carbonate saturated with undried KPF, .                                                  *'
                                                                                         •    l>
                                                                                             ?

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                                        •  TABLE 1
                               SWEEP VOLTAMMETRY EVALUATIONS

Electrolyte
IN H SO, aqueous
IN H_SO, aqueous
Saturated with Tri-
nitrobenzene
7.8 N H2S04

36 N H2S04 '

Distilled propylene
Carbonate -0.75M
Dry KPF,
Undistilled propylene
carbonate -0 .7511 Undried
KPF,
o
Undistilled propylene
carbonate -0.75M Undried
KPF, +10% H00
D /
Peak Potentials, Volts (NKE)
OXIDATION
NO
0.94
1.20
0.94
1.14

0.96
N0£
1.10
1.30
0.97'

1.25
1 . 20 j
0.67
1.55
0.66
1.07

0.85
1.35

1.10

1.70

0.70
1.26

1.00
1.50

1.03

so2
0.85
1.16

1.00
NH3
NPO
NPO

NPO
!
0.75
1.55
0.78
•1.20

1.30

1.09

NPO

-0.17 '

NPO

NPO

REDUCTION
NO
NPO
NPO

NPO

0.60
-
0.56

. ' 0.75

NPO

N02
NPO
NPO

NPO

0.90

0.56

0.70

0.83

so2
NPO
NPO

NPO

NPO

-0.50

NPO

NPO

NH3
0.45
0.50

l.OS

NPO

-0.50

-0.85

0.15
-0.83


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                                SWEEP VOLTAMMETRY EVALUATIONS
	 	

Undistilled propylene
carbonate saturated
with undried KPF,
Distilled gaircna-
butyrolactone -0.75M
dry KPFg
Distilled gamma-
butyrolactone -0.75M
undried KPF-
5
Distilled gamma -
butyro lac tone -0.75M
undried KPFg +10% H.,0
N,N-Dimethyl Formamide
-0.75M dry XPFg
Distilled Formamide
-0.75M dry KPFg
Distilled Formamide
-0.75M undried KPF,
+10% H20
Glycerol - H»0 (1:1)
saturated wiuh KPF-
6
Peak Potentials, Volts (NKE)
OXIDATION
NO
0.86
0.77
1.07
0.98
1.00
0.77
0.97
0.95
1.18
0.87
NPO
K02
0.72
0.77
0.94
0.99
1.00
1,25
1.03
0.70
1.04
so2
1.25
0.88
0.30
1.00
0.87
0.55
1.02
0.90
0.90
0.65
NH3
-0.11
NPO
NPO
NPO
NPO
NPO
NPO
NPO
REDUCTION
NO
0.73
0.41
0.98
NPO
NPO
NPO
NPO
0.62
N°2
0.55
0.75
0.52
0.82
0.85
1.08
NPO
NPO
NPO
so2
-0.44
-0.43
-0.50
NPO
-0.30
NPO
NPO
-0.45

NPO
NPO
NPO
NPO
NPO .
. NPO
NPO
NPO.

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CORPORATION
   •  Sulfur dioxide in IN aqueous  H_SO,  has  an oxidation peak at  0.85 volts.                  »
     This  potential is lower than  those  of NO and  N0_.   Both, NO  and  N02>  have
     two oxidation peaks at 0.94 and 1.20, and at  1.10  and  1.30 respectively.
     Therefore,  IN aqueous HJ50, can be  employed for the selective  sensing of
     S02 (Figure 1).   .       '   '
     In 7.8 N aqueous H_SO,  and 36 N H?SO, nitric  oxide undergoes electro-
     oxidation at a lover potential than sulfur dioxide and nitrogen  dioxide.
     These electrolytes  could be utilized in an NO monitor, but they  are  not
     suitable for use in either NO  or S00 monitors.
                                  Jt      ^
     Nitric oxide, in N -H«SO, aqueous electrolyte  saturated with  trinitrobenzene,
     was found to have an oxidation potential of 0.94 V. Another oxidation peak
     was observed at:  1.14V.   Nitrogen dioxide showed an oxidation potential of
     0.97V and sulfur dioxide of 1.16.  This electrolyte is thus  of possible use
     in an NO  sensor.  A moderate oxidation maximum current flow for 00  was
             X                                                   .      £.
     observed at 1.4V.  Three large reduction peaks were observed at  0,0  volts
     for II , 0  and CO (Tables  3 and 4).
     The 0.75M KP1;'  in undistillecl f ormamide ,with  10% H-O by weight had a
     potential use in an S0? monitor. Sulfur dioxide in this electrolyte is
     oxidized at a lower potential (0.85V) than NO and  N0?.  Formic acid  and
     H«S are oxidized at 0.85V  and 0.35V respectively and thus they would
     interfere.   Small reduction maximums were measured at  -1.0V  for  formalde-   •
     hyde, H90,  0., and CO.   A  moderate  current flow was observed for !!„  at
            £•    £•                                                     £.,
     -1.0V.  Large reduction maximums were measured for C09 and E S at -1.0
     volts and for S00 at -0.6V.   The 0.75M  dryed  KPF,  in distilled formamide
                     3                               6
     also was found to be suitable for the selective sensing of S09 (Table 2).
     Gamma-butyrolactone (distilled) - 0.75M dryed KPF,. electrolyte appeared
     to be useful for an N02 monitor. The addition of  10%  of HO by  weight to
     this electrolyte causes a  reversion of  its selectivity.  In  other words,
     it becomes  a selective electrolyte  for  S09.
     The N, N-dimethyl formamide - 0.75M KPF- electrolyte was found to be selective
     for sulfur dioxide,  S09 (0.55V) being  oxidized below  NO and NO.. A small
                            ^                                      ^
     oxidation peak for formic  acid was  observed at 1.24V,  and two  large   peak
     potentials  for H?S at -0.21 volts and 1.40 volts were  observed.   Formic acid
     reduces at -0.80 volts giving a large current flow. Sulfur  trioxide and
     H«S gave large reduction peaks at -0.937 and  -0.3V respectively  (Table 3).


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                                           TABLE 3
                  SWEEP VOLTAMMETRIC EVALUATION OF POTENTIAL INTERFERENCES

Electrolyte
Forrnamide (undistilled)
-0.75M KPF, +107. (by
weight) HO
& -Butyrolactone (Distilled)
-0.75HXPF, +107, (by -weight)
H20
Propylsne Carbonate
(undistilled) saturated
with KPF,
IN K2S04 saturated with
Trinitrobenzene
Spectro-grade N,N-Dimethyl
Forrnamide -0.75M KPF,
6
Peak Potentials, Volts (NEE)
OXIDATION
YJL~~'
0.35

NPO

-0.21
1.40
NPO
NPO
o? | so3 co
NPO
NPO
1.19 NPO
NPO NPO
NPO 1 NPO j NPO NPO
! !

NPO
-1.40
NPO
. NPO
NPO NPO
' ' REDUCTION
E2S
-1.0

NPO

-0.93
H2°
-1.0
NPO
NPO

NPO
°2
-1.0
0.0
NPO
0.0
v*PO
so3
-0.6
NPO
-0.8

-0.3
,co
-1.0
NPO
NPO
0.0
NPO

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                                           TABLE 4
                 SWEEP VOLTAMMETRIC EVALUATION OF POTENTIAL INTERFERENCES

Electrolyte
IN H.SO^ saturated with
Trinitrobenzene
Spectro-Grade N,N- Dimethyl
Formsraide - 0.75M KPF,
0
Formamide (Undistilled)
-0.75M KPF6 +10% (by
weight) H20
<£ -Butyrolactone (Distilled)
-0.75M KPF, +10% (by weight)
H20
Propylene Carbonate (Undis-
tilled) saturated with KPF,
0
Peak Potentials, Volts (NKE)
OXIDATION
N2
NPO
NPO
+1.8
NPO
NPO
co2
—
NPO:.
NPO
NPO
NPO
HCOOK
— —
1.24
0.85
NPO
NPO
H£CO
—
NFO
NFO
NPO
NPO
H2
NPO
NPO
NPO
NPO
NPO
REDUCTION
N2
NPO
NFO
-0.5
-1.5
NPO
co2
—
NPO
-1.0
NPO -
NPO
HCOOH
•* •
-0.80
NPO
0.0
-1.0
H2CO
—
NPO
-1.0
NPO
NPO
H2 .
0.0
NPO
-1.0
-0.75
NPO

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fffiWiffliK
CORPORATION
    ,'Voltammo grams of undis tilled propylene carbonate saturated with KPF, showed
                 *
     the possibility of utilizing this electrolyte in NO  transducers,  both NO
                                                        X      • •
     (0.86V) and N02 (0.72V) being oxidized at a lower potential than S02 (1.25V).
     Sweep voltammetric curves of C02, H-, H2S, H_0, 0 ,.. SO-, CO, formic acid and
     formaldehyde indicated that these gases would not interfere with the oxidation
     of NO and N0.       •
     3.2  EXPERIMENTAL SO  AND NO  TRANSDUCERS

     3.2.1  EVALUATION OF SEMIPERMEABLE MEMBRANES
     The concept of selective permeation, which dates back to the nineteenth
     century, is based on the phenomenon that gases diffuse at a different rate
     through certain polymeric films.  A membrane or polymeric film, through
     which a gas diffuses at a rate several times larger in order of magnitude
     than another gas, is said to be selective for the former.
     From the above discussion, the importance of semipermeable membranes in
     the development of S09 and NO  selective sensors seems evident.  Electro-
                          £•       X
     chemical transducers were assembled employing different semipermeable
     membranes, PbO_ and Au electrodes, and IN aqueous H_SO, electrolyte.  The
     net selectivity (potentiostatic control was not employed) of these permeation
     membranes toward N02, NO and SO- are shown in Table 5.
     These studies revealed the existence of several semipermeable membranes
     highly suitable for SO- transducers,  they are:  MEM 213, polyethylene,
     cellophane, 'poly-vinylchloride and j>ellicon.  Unfortunately not a single
     NO  selective membrane was found.
       x
     The polyvinylchloride and cellophane membranes are absolutely impermeable
     to NO and N0_, but they present severe drawbacks.  The polyvinylchloride
     membrane has a sluggish response.  The cellophane membrane is prepared by
     imbibing the natural polymer, cellulose, in glycerol, glucose or polyhydric
     alcohol.  These compounds, which act as plasticizers, were found to slowly
     hydrolyze in acidic solutions.  Once the plasticizer is totally removed, this
     membrane becomes brittle and loses its selectivity towards S0_.
     The pellicon membrane, which responds almost equally to NO- and S0?, could
     be employed in either S0« or NO- transducers.  The utilization of this
     membrane would require potentiostatic control in an SO- transducer and would
     require an SO- scrubber in an NO- sensor.  Pellicon hydrolyses in diluted

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• . TABLE 5 . •
• SELECTIVE MEMBRAKES

MEMBRANES'
Teflon, 1/4 mil
MEM. 213, 1.2 mils
Polyethylene, 1.2 rails .
Polyethylene, 0.3 mils
Polyvinylchloride, 1.5 mils
Tedlar, 1.2 mile
Saran Wrap, 1 mil
Pellicon, 0.5 mils
Zitex 12-137B, 2 mils
Cellophane, 1 mil. .
Polyethylene and Pelicon
Double Membrane, 1.5 mils
Selectivity Ratios
so2
0.83
1.00
1.00
1.00
1.00
0.0
0.0
0.94
1.00
1.00
1.00
NO
0.73
0.08
0.08
0.14
0.0
0.0
0.0
0.0
0.30
0.0
0.0
K02
1.0
0.39
0.03
0.0
0.0
0.0
0.0
1.0
0.69
0.0
0.0







V- "\

|


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'JORPORATION
    /solutions of strong acids (H-SO , ) ,  but it is very stable in diluted and
     even reasonably concentrated solutions of weak acids (CHiGOCl!) .   Transducers
                                                             J
     employing IN acetic acid electrolyte were assembled and tested.   It was
     found that IN CH^COOH can successfully replace the IN H-SO, electrolyte.
                    ' -J                                       £.  H-
     The polyethylene and MEM 213 (a silicone-polycarbonate copolymer) membranes
     are the most suitable for S0_ transducers.  MEM 213 responds to  S0_ at a
     slower rate than the polyethylene membrane, and its sensitivity  is temperature
     dependent.  Sensors employing polyethylene membranes require potentiostatic
     control.  By adjusting the applied potential upon the gold sensing electrode
     to 0.550V, the N0_ interference is totally eliminated.  The interference
     from NO cannot, at this potential,  be eliminated, 'but it is very small
     (2 to 5%).  At an applied potential of 0.533V the NC>2 is reduced and its
     interference is of the order of -2 to -5%, while that of the NO  is 0.4 to
     1%.
     3.2.2  TRANSDUCERS EMPLOYING AQUEOUS SULFURIC ACID ELECTROLYTE
                             - 1     - 2
     Transducers employing 10  N, 10  N, IN, 25% and 50% aqueous II SO, were
                                                                  f-~  M*
     assembled and tested.  In all cases Pb09 counter and Au sensing electrodes
     were used.  No substantial differences -- selectivity, response and
     response time wise -- were observed for S0?, N0_ and NO in the first four
     aqueous H9SO. electrolytes.  But in the fifth one, 50% aqueous H SO. ,
              £>  *\                     •                              £  T-
     sluggish responses were observed.
     Electrolytes composed of IN H9SO, with various additives were tested with
                                  £.  *"T
     the objective of improving the N09:NO ratio, the selectivity for NO , and
                                      £•*       •                          2C
     the response time.  Attempts were made to reduce the S0? solubility by
     employing the principle of "common ion effect".  The ideal ion for this
     purpose would have been SO,-,  because it affects the first order reaction:
     Unfortunately, this cannot be accomplished because, if S0_  was added to
     the electrolyte, it would also undergo electro-oxidation.  Therefore,
     efforts were made to reduce the S0? solubility by employing the same
     principle, common ion effect, upon the second order reaction.
                                            = + 4ll+ + 2e~
     This was done by employing electrolytes composed of K0SO. or Na SO  in
            .                                              2424

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CORPORATION
     IN II' SO. at concentration ranging from 5%  to saturation.  The addition of
     sulfate ion did not have any effect upon the sensors selectivity.
     Electrolytes composed of IN H^SO, saturated with  three nitroderivates of
     benzene, C,HCN00, D-C..H, (N0_)0 and C,H0(N00) 0 were evaluated.  The C,HCNO  '
               O _>  2     OH/i^      O J    £. O       •                O J  /.
     and the CfiH.>(N(V)- exhibited neither positive, nor'negative effects upon
     selectivities, sensitivities and response  times.  The IN H9SO. saturated
                                                               £.  ^T
     with p-C,H. (NO,.") „ electrolyte showed a positive effect upon the response
             O *T    £* f~
     times of oxides of nitrogen, and made the  NO^rNO  selectivity ratio almost
     equal to one.
     Polyalcohols are known to inhibit the oxidation of sulfur dioxide.  For
     this reason, sensors utilizing 5% and 10%  of glycerol and mannitol in
     IN H-SO^ respectively were evaluated.  Glycerol showed a medium detrimental
     effect upon the S0~ and NO- responses, and a very large negative effect
     on the NO response.
     3.2.3  TRANSDUCERS EMPLOYING KPF6 IN PROPYLENE CARBONATE AS ELECTROLYTE
     Transducers utilizing various concentrations of  potassium hexafluorophosphate
     in propylene carbonate, CdF? counterelectrode and Au  sensing  electrode
     were assembled and tested.  'Dr3' gaseous mixtures of 2000 pprn  NO,  2000 ppm
     NO- and 2000 ppm SO., were used in the evaluation of these sensors.  The
     range of applied voltage was +1.45V to -1-1.60V anodic  to the CdF~  counter-
     electrode .
     The experimental work performed employing these  transducers corroborated
     the results obtained from the sweep voltam.aie.tric studies, i.e., electrolytes
     composed of PC-KPF, are selective for NO  .  No interferences  from CO  ,
                       O                     X                           tL
     formic acid, formaldehyde, II , HpS, 0?, CO and NH~ were observed.  A small
     response to SO^ was observed.  Table 6 shows the selectivity  ratios of
     these sensors under various conditions, for S0_, NO and N0_.
     The effect of electrolyte additives to the electrolyte comprising 0.25M KPF,
     in propylene carbonate were evaluated.  The addition  of 1.3%  (by  volume)
     nitrobenzene and 10% HO to the 0.25X KPF  in P.C. resulted in a  faster
     response  (2.5 times faster) to N0_.  But  the backgi-ound current was increased
     from 7.8uA to 84.5uA for nitrogen, and the response to NO and NO  decreased
     from 23uA to 2.5uA approximately.  The addition of 1.3% HK03  to the 0.25M KPF,
     in P.C., 1.3% nitrobenzene, and 10% II 0 electrolyte,  raised the background
     current to approximately 125 uA and the NO and NO  response time  increased  to

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                     TABLE 6
. EVALUATION OF NO-  TRANSDUCERS EMPLOYING K?F,-PC
                 x                          6
ELECTROLYTE
Saturated KPFg in Propylene
Carbonate
Saturated KPF, in Propylene
Carbonate
0.5M KPF, in Propylene
Carbonate (160°F)
0.25H KPF in Propylene
Carbonate
MEMBRANE
1/4 mil
Teflon
1/4 mil
Teflon
1/4 mil
Teflon
1/4 mil
Teflon
POTENTIAL
+1.560
+1.45
+1.53
+1.59
CURRENT FLOWS (ua)/2000 ppai
N2
11.0
1.36
27.8
7.8
so2
12.65
1.4
27.8.
7.83
NO
23.85
10.3
83.2
21.8
NO 2
27.30
17.2
60.4
23.9
SELECTIVITY • '
RATIOS
S°2
9.S
0.46
0
.21
NO
76.7
100
100
87.0
N02
100
92.5
58.9

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CORPORATION
     .Electrolytic systems employing KPF, in P.C., despite their selectivity for
     NO , present severe drawbacks, the most important being response and recovery
       2t
     times.  The average time to reach 90% response was 5 minutes for NO and 30/40
     minutes for NO .  The average time required to reach 90% recovery was 1 hour
     and 1.. 3 hours respectively.  These sensors also require long periods of time
     to achieve stable responses.  This time may vary from a few hours to several
     days. .The other drawback of this electrolyte is that it loses its selectivity
     for NO  when H90 is present.
           X       £+

     3.2.4      f^.        ^COOH IN PROPYLENE CARBONATE
     A single experiment performed utilizing P.C. saturated with KPF, and doped
                                                                    o
     with CELCOOH revealed a system with apparently large potentialities.  The
            J
     addition of CH'COOH caused a reversion of the NO-NO selectivity, i.e., the
     systems became moire sensitive to NO than to NO,.,.   It also results in improved
     response and recovery times.
     A considerable effort was placed upon the development of an NO  sensor
                                                                   JC
     utilizing the KPFg-PC-CH COOH electrolytic system.  In the course of these
     studies, Pt and Au sensing electrodes and Ag?S, CdF~ and PbO,, counterelectrodes
     were evaluated.  Approximately thirty sensors using four different concentrations
     of CiUCOOH in 0.75M KPF  in propylene carbonate were exhaustively tested.  The
     acetic acid concentrations were:  three drops per 30 mis. of electrolyte,
     2% by weight, 5% by weight and 10% by weight.  Previous findings, i.e.,
     reversed NO,:NO ratio and faster response times,  were corroborated; but
     the number of difficulties encountered were sizable.  Among them ware the
     following:
          a.  Response times two to three times slower than in NTUSO, electrolyte
              sensors .
          b.  Large zero and span drift.
          c.  Background current twice as large as the span current for 1000 ppm
              of NO.
          d.  Sensor exhibited erratic behavior.
          e.  None of the sensors assembled were operational for periods longer
              than two weeks.  No reasons for this behavior were observed.
          f.  Sensors were very noisy.

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CORPORATION

    •Due to time shortage,     the study of the KPF^-PC-CH^CQOH electrolytic
     system was abandoned. '                                            •    .

     3.2.5  TRANSDUCERS EMPLOYING OTHER ELECTROLYTES
     Table 7 summarizes the electrolytic systems studied in this phase.  The
     transducers, in most cases, had gold sensing and cadmium fluoride counter-
     electrodes.  Gaseous dry mixtures, in the 2000 ppm range, of NO, N0_ and S0_
     were employed.
     Sweep voltammotrie studies indicated the possibility of employing N, N dimethyl
     formamide, 0.75M KPF, electrolyte in SO, sensors.  Sensors were evaluated
     at applied anodic to counterelectrode voltages of 4-0.75V, +1.00V, +1.10V
     and -1-1.20 volts; but no S0? selectivity was observed.  Apparently, the
     oxidation peak values for NO, NO, and S0? differ but their oxidation curves
     overlap.  Nitrogen dioxide was reduced at voltages of +1.10V or lower.  At
     1.10V the current flows were 30ua for 1987 ppm NO, 26 ua for 2054 ppm S02
     and 24 ua for N?.
     Transducers were assembled using the electrolyte gamma-butyrolactone 0.75M
     KPFg +10% H2<3.  Teflon. (1/4 mil), pellicon (0.5 mil), polyethylene (1 mil),
     and MEM 213 (1 mil) were used as membrane materials.  Nitrogen dioxide
     was reduced and nitrogen oxide was oxidized at potentials lower than 0.93V.
     This makes the electrolyte unsuitable for use in NO  sensors.
     Different permeation membranes were used in the evaluation of gamma-
     butyrolactone -0.75M KPF, +10% H_0 electrolyte.  The pellicon membrane
     dissolved in the electrolyte.  The silicone membrane became brittle and
     disintegrated.  A transducer assembled using a 1/4 mil teflon membrane  .
     performed well but there was too much interference from nitric oxide.
     Another transducer using 1 mil polyethylene as a membrane showed good
     selectivity towards S0? in the presence of NO and N0?.  However, rapid
     loss of electrolyte during life data evaluations at 160°F resulted in
     salt crystallization under the 0.2-0.5 mil polyethylene membrane causing
     the transducer to become inoperative.
     Transducers were also assembled utilizing 0.375M KPF, gamma-butyrolactone
     •1-5% 11JD as electrolyte.  Decreasing the salt concentration reduced
     crystallisation as electrolyte was lost, but the response current and
     response rate to NO  were significantly decreased.  The use of 0.2-0.5 mil
                        X              •
     polyethylene, 2 mil polyvinyl chloride membranes, and 1/2 mil teflon

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                                            TABLE  7
                          EVALUATION OF VARIOUS ORGANIC  ELECTROLYTES
Electrolyte
Formaruide -0.75M KPF, +
10% H20
N, N- Dimethyl Formamide
-0.75M KPF,
D
Propylene Carbonate
Saturated with- KPF,
o
% Butyrolactone -0.75M
KPF6 4-10% H20
.375M KPF, in "o Bu ty-
ro lactone-^5% H£0 *
.375H KPF, in ^ Buty-
rolactone +5% EJD •>''>'
Transducer
so2
so2
NO
X
so2
so2
S02
Potential
+1.05
•fl.lOv
+1 . 60v
-rO.92
+0.92
+0.92
so2- .
+100%
+100%
0.48%
+100%
63.5%
100%
NO
0
+300%
+100%
+12.3%
100%
0
N02
+0.935%
Large Reduction
92.5%
-3%
-5.15%
0
 -•> With I/2m Teflon membrane

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U irillWl. ^OiitiMiit'l!:^
COKPGRATIOiM
     membranes did not  substantially  affect  the  entent  of  the  response  to
     S02*
     In  the evaluations of  transducers using formamide  -0.75M  KPF,. 4-10% H_0,
     the voltage applied  to the  gold  working electrode  was varied  from  -KL.OOV
     to 41.10V to achieve a large  response to S09 without  interference  from NO
     and N02<  At -1-1. 05V  the current  flow was 61.5 ua  for  1941 ppm SO  ,  33 ua
     for 2011 ppm NO, 33.6  ua for  2410 ppm N02,  and  33  ua  for  dry nitrogen.   The
     interference was 0%  for KO  and 0.90% for NO .   At  applied voltages of +1.00V
     and balow, N0?  is  reduced,  but at potentials of -i-1.05 and above  it is
     oxidized.  This electrolyte was  considered  unsuitable because of  the  large
     background current .

     3.3  SELECTED NO  AND  S00 TRANSDUCERS
          "           X        2.  ,-.— ———
     The transducer utilizing l.ON HJSO, electrolyte,  gold sensing and  lead
     dioxide counterelcctrodes, and a  1 mil polyethylene membrane was  selected
     for employment  in  the  S0~ prototype monitor.  The  transducer utilizing
     aqueous l.ON H^SO, was selected  over the transducer utilizing formamide
     -0.75M KPF,. -M0% HJD primarily because  of the latter 's small  response
     to SOy in comparison to background current. The  transducer utilizing gamma -
     butyrolactone -0.75M KPF, -!-10% H?0 was  unsatisfactory primarily due to
     the rapid loss of  electrolyte and the crystallization of  salt.  The 1 mil
     polyethylene membrane  material was selected over  other materials  because
     of  its stability,  selectivity for S0« and permeation  characteristics.
     Sweep voltaianetry  studied revealed the  existence  of only  one  electrolyte,
     KPF,. in propylene  carbonate,  suitable for use in  NO  selective transducers.
        6    l  L J               '                        x                     .
     Further evaluation of  this  electrolyte,  with and  without  additives
     employing the actual electrochemical sensors, showed  the  existence of severe
     drawbacks.  The overcoming  of the problems  encountered with this  transducer
     would most likely  require a considerable length of time.   Studies  performed
     employing a wide variety of simepermeable membranes did not uncover the
     existence of NO  selective  membranes.   Therefore,  because of  lack  of alterna
                    x                                                    '
     tives, the NO   transducer to  be  used in the prototype monitor will employ
                  X
     IN II^SO, , gold  sensing  electrode, lead dioxide counterelcctrodes  and 0.25
     mils teflon semipcrmeable membrane.  This sensor  is not selective  for NO
           •                                                                   *••
     ancl therefore a scrubber for  the removal of S0_ is a  necessary part of the
     NO  sensing system.                           :
       X                                         •

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COHPORAYICKO

    , '3.4  SULFUR DIOXIDE .SCRUBBING SYSTEMS
     Once the necessity of an SO  scrubber was established, various SO  scrubbing
     systems were designed and evaluated.  Scrubbing is a basic chemical engineering
     unit process which consists in the absorption of gases-" in liquid or solid
     phases.  The process is favorably affected by turbulence, high inter-                   1
     facial surface areas, and high diffusion coef f iciencies .  In order to have               jj
     efficient absorption the gas must either react irreversibly with, or be                 f
     very soluble in the absorption liquid or solid.  The studied scrubbing systems          I
     may be grouped in tv?o categories, electro-oxidation and wet scrubbing.  The             I
                                                                                             I
     latter clascif ication, wet scrubbing, can also be divided into two sub-
     groups, oxidants and alkaline absorbants .
                   v
     3. A.I  ELECTRO- OXIDATION SYSTEMS                                                        I
     The possibility of scrubbing SO  by electro-oxidation was considered because            I
     sulfur dioxide and nitrogen oxides can be readily oxidized to sulfuric acid             \
                                                                                             V
     and nitric acid respectively.                                                           i
     The reactions take place on a solid electrode and the principles involved               jj
     are similar to the gas diffusion fuel cells.   Porous electrodes are used to             C
                                                                                             fi
     provide large contact areas between the gaseous fuel (in this case, pollutants),        f:
     the liquid electrolyte and the solid electrode.  However, there are some
     important differences.  Hydrogen as a fuel in a typical H?-0  fuel cell requires
     a platinum surface which needs to be rejuvenated often. On the other hand,  we
     have abserved that NO  and S09 may be readily electro-oxidized on a variety
                          X       £»
     of surfaces including porous nickel, carbon,  gold and platinum.  Also, unlike
     in a fuel cell, the cathode may be contained  as an integral part of the system.
     Thus,  in place of the  0_ electrode, we could  have a strong oxidizing lead
                                             •
     dioxide electrode.  The reaction taking place at the lead dioxide electrode
     is given below:           .       .
                Pb02 H- SO^," + 4H+ + 2e   -  PbSO^ + 2^0        1.685 Volts

     The reaction product,  PbSO,,  can be converted back to Pb09 in a fashion
     similar to that of a lead-acid battery.

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CORPORATION

    . '3.4  -SULFUR DIOXIDE SCRUBBING SYSTEMS
     Once the necessity of an S0» scrubber was established, various SO. scrubbing
     systems were designed and evaluated.  Scrubbing is a .basic chemical engineering
     unit process which consists in the absorption of gases"' in liquid or solid
     phases.  The process is favorably affected by turbulence, high inter-
     facial surface areas, and high diffusion coefficiencies .  In order to have
     efficient absorption the gas must either react irreversibly with, or be
     very soluble in the absorption liquid or solid.  The studied scrubbing systems
     may be grouped in two categories, electro- oxidation and wet scrubbing.  The
     latter classification, wet scrubbing, can also be divided into two sub-
     groups, oxidants and alkaline absorbants .

     3.4.1  ELECTRO- OXIDATION SYSTEMS
     The possibility of scrubbing S0_ by electro-oxidation was considered because
     sulfur dioxide and nitrogen oxides can be readily oxidized to sulfuric acid
     and nitric acid respectively .
     The reactions take place on a solid electrode and the principles involved                5
     are similar to the gas diffusion fuel cells.  Porous electrodes are used to              [..
                                                                                              I;
     provide large contact areas between the gaseous fuel (in this case, pollutants),         i
                                                                                              $
     the liquid electrolyte and the solid electrode.  However, there are some                 ||
     important differences.  Hydrogen as a fuel in a typical H^-O  fuel cell requires         I
                                                                                              t
     a platinum surface which needs to be rejuvenated often. On the other hand, we            \
     have abserved that NO  and SO- may be readily electro-oxidized on a variety              |
                          X       £•*                                                           t
     of surfaces including porous nickel, carbon, gold and platinum.  Also, unlike            i
     in a fuel cell, the cathode may be contained as an integral part of the system.          jj
     Thus, in place of the 0_ electrode, we could have a strong oxidizing lead                i.
     dioxide electrode.  The reaction taking place at the lead dioxide electrode  '  '          [<
                                                                                              £
     is given below:           -                                                               ?
                Pb02 H- SO,  -f 4H  -h 2e  =  PbSO, + 21^0        1.685 Volts

     The reaction product, PbSO, , can be converted back to PbO  in a fashion
     similar to that of a lead-acid battery.

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FTiP'Irl'lfi	:	;	______	.JEE^^Hiii^EiSi
CORPOKATIOPJ

     The electro-oxidation cell was  assembled using a porous nickel screen as
     the electrode.  The electrode was  made hydrophobic on one side by a thin
     teflon coating.  The cathode used,  was a lead dioxide electrode obtained
     from a lead-acid battery.   Dilute  sulfuric  acid was used as the electrolyte.
     The cell was assembled such that the gas sample could be passed over the
     hydrophobic side of the electrode  and not come into direct contact with the
     electrolyte.  Applied potentials were varied between 0.0V and 0.4V.  Sulfur
     dioxide was most efficiently scrubbed at an applied voltage of 0.0V.  Oxides
     of nitrogen (NO and N09) were also electro-oxidized at this potential.  The
     S09 removal efficiency was of the  order of  60-70 percent.  This electro-
     chemical oxidation system  could most likely be made more efficient and
     selective.  The system could be made selective by employing a more selective
     membrane, by the addition  of nitrate and nitrite ions, or by finding a mean
     to keep the potential difference bet\?een the electrodes selective and efficient
     for S0~.  The complete development of this  scrubbing system would require an
     effort beyond the scope of the  original contract.

     3.4.2  WET SCRUBBING SYSTEMS

     3.4.2.1  The __Fe (OH).- iJKaNO3_OxMSH1L.S_cr
     The ferric ion selectively oxidizes  SO,,  in  the  presence of oxides of
     nitrogen according to the following  half-cell reactions:
            +3    -           +2
          Fe   -f e   -:.--—>-  Fe                                0.77
          NO + 2H20  ^r±:  NO " 4- 4H+ + 3e"                   0.96
          N02 -I- HO  ^=ZZ  N03" + 2H"'" + e-                    0.80
          S0? + 2H20 ?L=.-£.  S04"2 + 411* + 2e"                  0.17

     The absorption and ulterior oxidation of  S0? occurs according to the
     following reactions;
                -I- 2Fe (OH)3 ^rz±:   2Fe  (OH)2
                H- Fe (Oil)   =ivrr±:   . 2\i,   + Fe

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           Y-w;                                                         IKSTRIIM SYSTEMS BSVISiCfi
          tiil!:©	__.	__	.——-——	—
COKPORATSON

    'The overall reaction  can  be.  summarized  by:
          2Fe(QU), -I-  SO  	5>  2LI 0 + FeSO. -I- Fe(OH),
                 J     £»           •  £•        r -         ^£
     The addition of  NaNO~ is  necessary because  of the large solubility of N0~.
     Sodium nitrate and nitrite decrease the solubility  of N0? by means of the
     "common ion effect" principle..  A large nitrate ion concentration causes
     the NOp solvation reaction to shift to  the  left;  hence causing the N0?
     solubility to decrease accordingly with:
            .:      :   J>         + j>      „    H+shift
          H,0: -I-  ;N-N   ;s=5r 1L.O  -N    + NO   -£——!?  HONO  + UNO
        .         0     D        ^0             .
     The scrubbing solution  was  prepared by slowly adding Fe (SO,)„ (100 g) to
     a solution containing KaOlI and 250 g of  NaNO~ in 400 mis of water.  The
     scrubber container was  a standard,  glass  bubbler with 1/2-inch  diameter
     disperser.  This scrubbing solution removed  2% of 2000 ppm KO, 30% of
     257 ppm NO   and was 100%  efficient in the removal of 2000 ppm SO  at a
     flow rate of 2 SCFH.  The major  drawback of this  scrubbing solution is
     its bulkiness, i.e, for one week operation at  2 SCFH of 2000 ppm SO ,
     400 g of Fe (SO,)0  in two liters of water would be necessary.
     3.4.2.2  The_H 0 7^NaNO jOx idant JSc^uJbb ing_Svj;J: £m
                   /C. £.     J)
     This sc7:ubbing solution was  composed  of  250g of NaNO»,  320 mis of II 0 and
     80 mis of 30% HO.   Sodium nitrate  is  also used  in this  solution to reduce
     the solubility of N0_.  The  gaseous mixtures employed in the evaluation of
     this scruber were 2000 ppm NO, 2000 ppm  SO   and 257 ppm NO .  The SO
     removal was 100%, the NO's 4% and  the N0»  removal  12%.   The main drawback
     of the H_0?-NaNO_ scrubber is that its decomposition would be catalyzed
     by organics present  in the stack gas.
     3.4.2.3  lhe_Na CO«1NaNO»_.Alkaline  Scrubber
                    2   ~>   ~  ~~>
     A scrubbing solution  containing  112g  of  Na  CO   and  320g of NaNO,, in 500 mis
     of water was evaluated  employing 2000 ppm NO,  1049  ppm NO ,  100 ppm NO  and
     2000 ppm S0».  The SO  was  totally  removed.  This  scrubber also absorbed 2%
     of the NO, 13.7% of the 100  ppm  NO  and  18% of the  1049 ppm NO .   It must b
                                       f.                           2
     noted that this scrubber does not oxidize the  S0_,  but merely neutralizes
     the sulfurous acid 'formed by the dissolution of S0? in water.

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            -h
                       Na  + OH" -h NaHCO,
NalIC0
                                -I- Oil
     H2co3  ^
               H20 + CO
           H- 2XaOH
                                2H0
This scrubber was selected, with some modifications, for use with the NO
prototyps monitors.
3.4.2.3  The_KrCOoHKHGO[,-KNOoiKNO  A Ucalina_Scrubber
This scrubbing solution is basically like the aforementioned one.  The
NaJX),. was substituted by K CO,, because of the latters larger solubility;
this will extend the useful life of the scrubber.  In addition, the solution
also contains KKCO^ for the. purpose, of reducing the absorption of the C0_
contained in stack gases.  Because experimental data showed that the addition
of KNO,, reduces the absorption of N0_, this chemical is also contained  in
the scrubbing solution.  The composition of the scrubber, par 100 gr of           I
water, is;  10.6 grs. KHC03> 75.7 grs. K^O , 3.2 grs. KNO , and 8.17 grs. KNO  . '
Laboratory tests have shows that this scrubber will remove 2000 ppm S0_n_
completely while absorbing less than 1% of 2000 ppm NO and less than 20% of
100 ppm N0?.  Because of the low NO  concentration (with respect to NO) in
stack gases, the overall effect on the NO  concentration is insignificant.

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                             '    '                        .             iriSTjUJ?,;Ef!T SYSTEMS DP«'ISiD:i
              i ._.--. ....._ -i— r-i-_ ._ .. . -._... -••_-..  .. .. .-— ___ -.-u.r--.f-TT - - — axu- _LI.--.- • i~  ' .  ..... "  ' -------------- --------- - — ~»...a-mJ»..i».MD«mi^.. ••.-ma .• -*...s^.i ..• ......... aiia.im^
-ORPORATIOIM

     4.0  PROTOTYPE MONITORS PERFOR^IAHCE DATA
     The S02 and NO. transducers response is linear throughout the  0  to  5000 ppm
     concentration range.  Figure 6 shows a plotting of NO  concentrations versus
                                                          X
     the sensor's output and versus the amplified output, thereby  illustrating  the
     linear response of an NX- 130 transducer.  Similar results are  obtained  with
     S0_ transducers.
     4.1.1  SENSITIVITY AND OPERATIONAL TEMPERATURES
     Table 8 shows the effect of temperature upon the  sensitivity  of  NO   and SO
                   •                                                    X
     transducers, for various pollutant concentrations, and  at  three  operational
     temperatures --  70°F, 120°F and 160°F.  The values shown in Table 8 are
     mean valuar, obtained! from long- exposure data of a large number of transducers
     It is interesting to note that although the background  (zero) currents
     increase at a rate larger than those of the spaa  currents, the transducers
     sensitivity increases, with increasing temperatures.

     4.1.2  SENSITIVITY RANGES
     Originally the NO  and S00 prototype analysers were  to  have three ranges of
                      x       /
     sensitivity:  0-50, 0-1000 and 0-3000 parts per million.   This is equivalent
     to span ratios of 1:30:60.  Two analyzers, SS-330 and NX- 130, capable of
     sensing currents of the order of 150ua, without saturating the operational
     amplifier, and having the aforementioned span ratios were  evaluated.  It
     was found that at the lower range (0-50 ppm) the  instruments  zero and span
     pots, had poor resolution.  It was therefore, necessary to increase the lower
     range from 0-50 ppm -to 0-100 ppm, the new  span ratios being 1:10:30.

     4.2 .iELj^rVTrY
     The effect of various potential interferences was determined  employing the
     final version of the S0~ and N0_  prototype analyzers (Table 9).  The
     selectivity ratios of various permeation membranes for  NO, NO and  S0_ ware
     shown in Table 5.
     Four different pollutants were found to interfere with  the selective sensing

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0
1000
2000
3000
4000
5000
                           PPM NO
            Figure  6. Response of KO   Prototype-Monitor
                                    X

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                       TABLE  8




TIB! EFFECT OF TEMPERATURE UPON TRANSDUCERS SENSITIVITY
STACK
MONITOR
SS-330
NX- 130
POLLUTANT
CONCENTRATION
20 ppm SO „
.'.
300 ppm SO
1000 ppm SO
2000 ppm SO
N2
50 ppm NO „
2000 ppm NO
	 	 CURRT
70°F
0.15
1.75
6.95
15.8
0.03
0.7
2.84
1000 ppm NO | 12.10
2000 ppm NO
N2
23.0
0.03
:NT, .MIC ROAM
120°F
__
7.3
25
48
1.2
--

21.4
44 . 1
1.0
PERES 	 _
160°F
. __
13
45
95
8- '-
1.8
6,5
32.7
64.0
5.5

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                                       'TABLE 9

                           EVALUATION OF VARIOUS INTERFERENCES
MONITOR

NX130
SS330
.
CO
121ppm

0.0
0.0

266ppm

45.0 '
30.2

lOOppm

0.0
0.0
]
Q
2
217.

0.0
0.0

INTERFERENCES PERCENT*
HCOOH ; H CO
600ppni ; 500?pn:

0.0
0.0

0.0
0.0
C°2
m-
,

0.0
0.0
* /
548ppm

2.00
3.34
E2

0.0
0.0
•
.
03 ! 10%
SOOppsd : H,0
£

0.0
0.0

0.0
0.0
* •
Monitor
J NX130 respect  1000 ppn NO at 70°F
^ SS330 respect  1000 ppm S0? st 70°F

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                                              ^                         J5SI5HIE!!
CqitPORATIOM

     control (Figure 5) the interference from 300 ppm N09 is almost wholly
     eliminated (0-0.5%), and the interference from 2000 ppm NO reduced to 7%.
     The interference from 548 ppm SO- and 266 ppm E9S were found to be 3.37o
     and 30.2% respectively.  All of the aforementioned degrees of interference
     are expressed with respect to 1000 ppm of S09.  Among these interferences
     only that of II93 may be said to be significant.  Should it become desirable,
     the H9S 'can be readily and selectively absorbed in a column containing silica
     gel impregnated with Ag SO..  Ammonia interferes with the selective absorption
                                          (1)
    -of 1I_S from SO -H S gaseous mixtures.  '  It is important to bear in mind
     that 11 S-SQ  mixtures are not stable because of the following probable
     reactions:
          US 4- 5SO  4- HO  rrr——>•  2S 0 "2 4- 411*
          8H2S -I- 4S02   -rs=±^   SO -I- 4lI20 4- Polythionic Acids
     The NO  (KX-130) analyzeris, despite of the results shown in Table 9, free
     of interference from S0», 11 ~S and S0_ because this analyzer employs a scrubber
     which absorbs SO,., SO,, and H^S with an efficiency of 100%.
                                                            	

     4. 3  RESPONSE AHD RECOVERY TIMES

     Figures 7 and 8 illustrate the response and recovery times characteristic
     of S00 (Model SS-330) and NO  (Model NX-130) respectively.  The S09 transducer
          £••                      X                  '                   f~
     has a response time of 90% of full scale (F.S.) in less than forty-eight
     seconds and a recovery time of 90% of F.S. in less than eighteen seconds.
     The NO  electrochemical transducers have a response time of 90% F.S. in
           x •
     five to twelve seconds for NO, and thirty to forty seconds for NO,.,.  The
     recovery times (90% F.S.) for these two pollutants are twenty-four and
     forty seconds respectively.

     4-4  ST^BILITY_pj;_KO  AMD .-SO  PROTOTYPE JHNITORS

     The zero and span drifts per day of NO  and SO  selective analyzers were
     measured at temperatures of 40°F, 100°F and 160°F (Table 10).  The values
     shown in this table were obtained under the most adverse conditions, i.e.,
     no temperature control was employed.  As it may be expected, the instrument's
     drift increases with increasing temperature.  Still, even at 160°F the daily
      (1)

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                           FIGURE 7
           CHARACTERISTIC RESPONSE OF MONITOR SS-330
                 TO VARIOUS SO  CONCENTRATIONS
 CHART SPEED:   1  INCH/HOUR
29 PPM
 SO,,
             II'
10 PPM
2000 PPM
    2
.LOW.:			LOW			„_	.....HIGH.
                                             A.P.M.
                                             RANGE

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                                FIGURE 8

                 CHARACTERISTIC RESPONSE  OF MONITOR NX-130

                              TO NO AND NO,,
2000
  KO
HIGH
2000 PPM
  •NO,,
                                         CHART SPEED:  1 INCH/HOUR
20 PPM
  NO
                           LOW
                             LOW
              A.P.M.HANG IS

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                            TABLE 10




            STABILITY OF NO^ AND S02  PROTOTYPE MONITORS
:-: "." " ~~ ' '
MONITOR

NX- 130 .
•

SS-330

TEMPERATURE
"F
40
100
160
40
100
160
±%, CURRENI
ZERO
0.15
0.50
0.9
0.23
0.61
0.95
DRIFT /DAY *
SPAN
0 . 24
0.51
1.3
0.5
0.61
1.2
'Relative  to  340 PPM SO  -Full Scale,  in  the  instruments' most




 sensitive range.

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CORPORATION
     drifts do not exceed ±1.3%.  At temperatures below 110°F. (the most widely
     vised temperature) the instruments' drift is well below ±17o of full scale per
     day.  With the use of temperature control, the span and zero drifts are well
     below ±0.5% F.S. per day.

     4.5  TRWSDUCERS LIFE DATA
     The transducer's life is a direct function of the diffusion rate of the
     electrolyte through the permeation membrane, and therefore also a function
     of the pollutants' flow rate through the sensor.  The same liquid electrolyte
     diffuses at different rates through different'membrane materials.  The
     evaporation rate of IN H-SO^ electrolyte through 0.25 mil Teflon and one
     mil polyethylene membranes were determined at two different temperatures,
     70°F and 160°F.  The electrolyte losses were found to be 0.04 grs/day at
     70°F, and 0.25 grs/day at 160°F for the polyethylene membrane.  Through the .
     0.25 mil Teflon membrane, the losses were 0.26 grs/day at 70°F and 1.86
     grs/day at 160°F.  The gas flow through the cells was one S.C.F.H.  It is
     necessary to emphasize that thase. values were obtained utilizing dry
     gases.  Under standard conditions, the losses of electrolyte would be
     substantially smaller.

     4•6  TEMPERATURE COMPENSATION AND CONTROL
     Because of the large effect of temperature upon the sensors output  (Section
     4.1.1), the necessity of temperature compensation or control is evident.
     Attempts were made to temperature compensate the sensors output by means of
     a thermistor and associated electronics.  Various different thermistors,
     positioned in the top half of the sensors, were evaluated.  The best results
     obtained were with GA45P1 (50K, Fenwal Electronics Inc.) thermistor.  This
     thermistor and associated electronics were found to effectively compensate
     (±87, of full scale) for temperature changes within the 40°F to 120°F range.
     Since larger accuracy and stability were desirable, a temperature controlled           f
                                                                                            K
     environment for the sensor was designed.  The heater and sensor assembly are            f
     discussed in detail in the Appendix.  The heater and associated electronics             :
     effectively control (±0.5CF) the temperature of the sensor within the 40° to           |
     140°F range.  This small temperature variation (±0.5°F) to which the sensor is         is
                                                                                            3
     subject to, has no significant influence upon the sensors output.                      /

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COPPOrtATION
                                       APPENDIX
     Figure 1A shows a front view of the newly developed  -  second  generation  -
     Dyriasciences Air Pollution Monitor.  Front,  top  and  side  schematic  views
     of the. Monitor are shown in Drawing No.  620432.   Four  of  these  instruments
     (two NX-130 and'two SS-330) were supplied to H.E.W.  as part of  the  Contract
     CPA-22-69-118.  The monitors are compact and portable, and  they were
     designed to continuously measure the concentration of  different pollutants
     in a gas stream.  The pollutants that can be measured  are sulfur  dioxide
     and oxides of nitrogen.  The outstanding .feature of  the monitor is  that  any
     one of the six sensors developed by Dynasciences can be incorporated  in  the
     same monitor. _ The model numbers,  ranges, and the recommended applications
     of the different sensors are as follows:
          MODKL
          1.  NX--130 SP
          2.  SS-330 SP
GAS MEASURED
""Total oxides of
 nitrogen
Sulfur dioxide
.RANGES
0-100, 0-1000
and 0-3000 ppm
0-100, 0-1000
and 0-3000 ppm
APPLICATION
Industrial stacks,
automobile exhaust,
process control,
development studies
Industrial stacks,
process control,
development studies
          "Requires scrubber if S0? present.

     Table 1A shows the specifications for Monitors  NX- 130 and  SS-330.
     Each sensor is a sealed  unit  and  no  addition  of  any  electrolyte  is ever
     required.   The sensor operating life may be as long  as  one  year  or more
     depending upon the condition  of the  sample.   Generally  spsaking, hot, dry
     and dirty samples will shorten the sensor life.   The sensor may  be easily
     replaced with a new sensor as described  below.   Drawing 672020 and 672021
     show the design and specifications of the transducers polypropylene  body.

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                      WIJASOTS
                               AIR POLLUTION MONITOR
                BAMPLB FLOW
                                         40 .  EQ/  BO
                                         •	•'•  -.., . mo
                                         OF RANGE
            SYSTEM    HEATER     ZEHO       RANOB       SPAN
             POWER    POWER                „__
                              Figure 1 -A.
Figure 2-A.

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COHPORATIOW
                                      TABLE 1A

                      MONITORS SS-330 AND MX- 3.30 SPECIFICATIONS
      SENSITIVITY RANGES;
      DETECTION LIMIT:
      RESPONSE:
      RESPONSE AND RECOVERY TIMES;



      STABILITY:



      SPECIFICITY:



      TEMPERATURE CONTROL:



      OUTPUT:



      SENSOR LIFE:


      POWER SUPPLY:
      MONITOR WEIGHT:
      FLOWRATE:
0-100,0-1000 and 0-3000
parts per million
•2% of full scale
Linear response over the entire
range of the pollutant.  Overall
accuracy:  ±2% full scale .with the
use of integral meter, ±1% full
scale with the use of external
potentiometrie recorder
907o of full scale in less than
60 seconds
Better  than ±1% of full scale
per day
No response to N2, 0~> CO, COo,
water vapor or hydrocarbons
About 120°F ±1%F, adjustable
(see Text)
Meter readout and 0-10 mv output
for recorder
One year at 70°F, moist gases
105 to 125V 60 Hz 5 VA required for
powering solid state amplifier or
12 V.D.C.
Less than 15 Ibs.
                                                   Keep  the  flow  rate within  the
                                                   green indicator  on the  flow metc-.r

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                                                                                                 A| -5.EE DON f^JtfM IOAU&70   \-'ff
                                                                                                                             ,"ArL-t*i
                VIEW B' B
                                                           OPTION CHART
                                                            -I SENSOR AMD HEATER (MODEL £OO)
                                                            -Z. SENSOR AND HEATER (MODEL4OO)
                                                            -3 SENSOR MODEL 2OO
                                                            -4 LESS SENSOR AND HEATER(5TOCK OMLY)
                                                            -5 LESS SENSORCSTOCK ONLY>
                                       COVER MOUKITING
                                       SCREWS-2 PX.ACE.SIREF)
                                                                                                        S2 REOO
                                                                                                          lEO'APART
                                                                                                          TMRO \TEM IA\
                                        -CHASSIS MOUMT
                                         SCREWS-2 PLACESCREF)
                                                                                              FOP PARTS LIST SEE PL620432
VIEW A-A
                                                                         mcr tan  I  uso> o
                                                                                           u
                                                                                              HR»J ft-18-fo
                                                                                                         f nppnDATinM '
MON ITOR,POLLUTION,
AIR

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                                                                                                    •®-\   K-®«-   ;
   A PARTS LISTS FOR DASM NUMBERS NOT DRAWN; INrORMATION
  ^"^  FOR REF ONLY.
   /§> ARROWS INDICATE DIRECTION OF FLOW

  ^ HEM 16 IS REPLACEMENT LAMP FOR ITEMS 135118.
   A, ITEM 45 IS 7/16 LONG AND CENTERED ON'S' POSITION.
    P^ITEM 143 USED w ITH HEATER,
     ITEM 88USEDVW/O HEATER

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  ,,
cor:rjor.ATioi\!

    • SE-Ng Ojj, REPLA CEMENT
      "
     The instrument is designed to be maintenance free.  Tha sensor must never
     be taken apart.  A fault in the sensor usually shows up as slower response  .
     time and/or inability to calibrate the 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
     stabilize before it is ready for use.  Before replacing the sensor, make
     sure that the slide switch  located near the battery on the chassis is in
     the proper position.  The use of models Nil- 210 and NR-230 for NO- monitoring
     requires the slide switch moved towards the front of the monitor.  All other
     models require slide, switch in the opposite direction as marked.
     The procedure . for sensor replacement is as follows:
          Step 1 - Disconnect 115V input power cord.
          Step 2 - Remove (2) screws, Item 65, and remove cover/rear
                   panel assembly.
          Step 3 - Disconnect (2) tube fittings, Item 152.
          Step l\ - Unplug electrical leads running from sensor to chassis.
          Step 5 - Remove insulator, Item 130.
          Step 6 - Remove (2) screws, Item 150, which hold cover, Item 141,
                   to enclosure, Item 140.
          Step 7 - Remove cover/sensor assembly 602580 and replace with
                   new assembly.
          Step 8 - Reverse Steps 1 through 7.
          NOTE:  Fittings, Item 152, shall be firmly Jhand tightened only.

     HEATER
     A heater assembly with the appropriate electronics is provided on all the
     monitors.  The purpose of the heater, as it was previously stated, is to
     maintain a constant temperature environment for the sensor, thereby improving
     the stability and accuracy of the system.  Figures 2A and 3A show "the sensor
     and heater enclosure in different stages of assembly.  Drawing 620432
     schematically shows the sensor and heater assembly.

                                         A-5                                                 1

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             ;	.	______	inSTngOJT$Vg£gD;yi?g
COJU'ORATIOW

    5 "The heater can be switched on by pushing the HEATER POWER switch  on the front
     panel. . About 3 1/2 to 4 hours are required for the sensor to come to equilibrium
     at a factory-set temperature  of about 120°F.  Laboratory tests have shown that
     heating the sensor at temperatures higher than 120°F result in shorting its
     operating life.  Continuous heating to 160°F for 4 hours resulted in permanent
     damage to the sensor.  The sensor may be operated at 140°F for a  short
     duration of up to 8 hours at  a time, without damaging the sensor.   To readjust
     the temperature set-point, proceed as follows:
          Remove the printed  circuit board from its receptacle.
          Using an Ohmmeter,  check the resistance between Points A and B
          (see Drawing  No. 602517, Printed Wiring Assembly)  and adjust
          pot R41 to obtain 25K ohm resistance.
          Install the printed circuit board back in its receptacle.
     For the applications where the ambient sample and room temperatures are
     fairly constant or where the  monitor is required to be. used for a spot test,
     the heater is not required, thus avoiding the delay for the sensor to come
     to equilibrium.
     The heater is proportionally  controlled thus eliminating the possible inter-
     ference from an On/Off action.  The HEATER red light on the front panel
     indicates the level of power  to the heater.  In tha beginning when the
     HEATER POWER is switched on,  the HEATER light is very bright.  However,
     within the first two hours it starts dimming and stays fairly dim for the
     continuous operation.  CAUTI.ON - If the HEATER light remains bright for more
     than two hours, turn the power off and check for possible malfunction in the
     heater circuit.  The two white leads from the heater should be firmly plugged
     in the white jacks on the chassis which supply the power to the heater.  Check
     the resistance across the two green leads after removing them from the chassis.
     These loads are joined to a thermistor which should read about 100 Kohrn
     resistance at room temperature and about 45 Kohm resistance at the controlled
     temperature of 120°F.  A broken thermistor  would show an infinite resistance
     avid would keep the heater power on all the time.

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s  , i •                                •                 .        •
  The meter  reading  is  directly  proportional  to  the  concentration of the
  pollutant.   In  addition,  a  10  mv  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  fitting.
  115/230  volts  and  100  volts,  50  or  CO  Hz  powered  from the  three-prong
  receptacle  on  the  back of  the monitor.  Also  a  12 V.D.C. capability is
  provided.   The 12  V.D.C. input jack is  provided on the back of the  instru-
  ment next to the fuse.
 A  size  "D" mercury  battery  is  used  to  bias  the  sensor.   Its  voltage at all '             I
 times should  remain 1.35V ±10%.  The battery voltage  may be  checked by                  *
 holding the RANGE switch on the  front  panel to  the  BATT TEST position.                  [
 Replace the battery if  the  output on the  meter  shows  less than 90%. full    •,             L
 scale deflection.                                                               .         f
                                                                                          ?'
                                                                                          I:
 .SCRUBBER                                                                                I'
 For  the applications where  the analysis of  NO  using  NX-130  is desired in               JT
 the  presence  of  SO,., a  liquid  scrubber is provided  for  the removal of SO                >
 before  the sample is passed to the  monitor.  The  scrubbing solution is
 especially prepared to  remove  SO- without affecting the NO  concentration.
 A  cartridge containing  glass wool  is attached on  the  exit of the scrubber
 to trap axiy liquid  carry-over.

 SAMPLE _ C01TDI TIONIJgG AND gRESSURE AD.JITS THEM TS ,
 The  gas sample should be free  of suspended  material and the  moisture
 content of the sample should be  such that it does not condense in the sensor.
 Except  for this  restriction, it  is  actually beneficial  in terms of life-
 time of the sensor  to use moist  gases  or  gases  with a dew point slightly
 below the operating temperature  of  the sensor (relative humidity of 40 to
 80%  is  recommended).  The temperature  of  the sample should be kept less
 than 100°1;1 and preferably at room  temperature.  Higher  sample temperatures
 to 140°F may  be  used for a  short duration of up to  8  hours.   Longer exposure

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CORPORATION
     be ;passed over the sensor by connecting it to the input 1/4" polypropylene
    i »   -
     tube fitting provided at the right side when facing the back of the instru-
     ment.  The sample passes through the sensor, flowmeter and exits from the
     fitting on the left side which should be connected to the vent line.  The
     instrument reading is independent of the flow rate.  However, it is suggested
     that to get the optimum response time and a long operating life, a flow rate
     of 0.5 to 2.0 SCi7H be kept steady throughout the run.  If the flow indicator
     is kept in the blue area, the flow rate will be satisfactory.
     The sample should be preferably "pushed" rather than "pulled" through the
     sensor.  If it is desired to pull the sample by creating a suction at the
     exit, care must be taken to assure that the vacuum created in the
     sensor is not more than 6" of H90.
             i '                      £
     The sensor is designed for operation at atmospheric pressure.  It can be               .
     operated "in line4." at pressures slightly above or below atmospheric, but
     it is of u^ino£t_im]>Drtance that any pressure changes occur slowly in order
     to prevent damage to the sensor.                                                       j
                                                                                            !
     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            5
     calibrated at one pressure and is used at another.  For example, calibration
     of the sensor at 740 mm Hg of ambient atmospheric pressure and 1009 ppm of
     NO  gas results in a full scale meter reading equivalent to 740 mm Hg.
       J\                          "•
     If the total "in line" pressure is changed to say 700 mm Hg, and the ratio
     of molecules of NO  gas to total gas is kept constant at 1000 ppm, then
     the new reading will by ~ — of full scale.
     The instrument may be calibrated in any RANGE position.  However, for
     maximum accuracy, calibrate in the RANGE most commonly used.  Turn on
     power to the instrument by pushing in the SYSTEM POWER switch.  Set the
     RANGE switch to the desired position.  Turn the SPAN knob completely
     clockwise.  Wail about 15 minutes before proceeding with the calibration.
     If the heater is used, wait 4 hours before, proceeding.  Flow the zero gas
     (N9 or NO , and SO, free air) through the instrument and wait for 5 minutes
       *~      5t        /-
     Then bring the reading to zero by adjusting the ZERO knob.  Lock the ZERO
     knob.  Pass the calibration gas over the sensor.  Wait: 5 minutes.  Adjust
     the SPAN knob so that the meter reading corresponds to the concentration


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         rill                                            ____ l?ISTnijLM^5fE^
COFIPOKATIOW
     in  the calibrated sample.  Lock the SPAN knob.
     The instrument is designed to be essentially drift free and will retain
     its calibration for a long time.  However, for optimum results,  calibrate
     the instrument at regular intervals.

     TjlE ELECTRONIC PACKAGE
     The following material was prepared by Mr. J. W. Schmidt,  Supervisor of
     Laboratory Instruments.
     During system operation the cell output current, is converted to  a proportional
     voltage by the Current Amplifier.  This voltage signal is  then scaled by
     the span control and amplified by the scaling amplifier.  The scaling amplifier
     gain is determined by the range control (High - Keel - Low) which provides  a
     proper output for full scale meter deflection and recorder output.
     .The current amplifier input is provided by the cell (MTJ.)  and bias network
     composed of resistors RGO through RG4 and battery (B) .   The function of the
     bias network is to provide the proper cell bias for system application
     required.
     The front panel zero control circuit consists of pot R66,  resistors R3, R4
     and R5 and 6.2V zeners VR, and VR?.   This provides a ±  current to offset
     residual cell currents.
     Q, , R- and R  are creating a short circuit to cell currents when the power
     is in the off state.  This maintains the cell in a ready state for rapid
     stabilization.
     The current amplifier is basically a low noise FET input amplifier which
     utilizes the ua 741 for high open loop gain and complimentary output
     drivers Q. and Gv for linearity.  -Potentiometer R,„ and resistors R   and
              *~r      *J                                i. -J                i.\J
     R.„ are used to zero the amplifier during initial alignment.  The amplifier
     gain is determined by RT, , RI and R  .  R  and R   are  used to modify the
     temperature characteristic of thermistor RT .  RT  and  R  are physically
     located in the cell to obtain temperature tracking gain which will provide
     the best system accuracy during heater warm-up.  The basic gain equation
     is:
                                 Vo =  Iin RF

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UNLESS OTHEHWISE SPCCIFIED
INTPR DWG P£R MIL-STU-100
OiM.AflE IN INCHES
DO NOT SC*t£ DRAWING
DIM. TO BE MLT AFTER FINISH
STANDARD HOLES PFR AHD1038
                                                                                              HSTWUIKT SlTTtWS BYIPOW  CHATSWOB'
SCHEMATIC   D1AGE.A/Y\
      MOX    MOMITOE.
                                                   kPPROVED HR.W  4 - 19 - TO
                                                   4i.
                  G6024&

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                                                             II	  	 ^^.ZVPO 	
 7.FOR ASSY  SEE DWG N0.62043E.

^^COMPONENTS IN THIS AREA DELETED ON UNITS W/0 HEATER.
   C23 IS REPLACED WITH IOO MF,^^5^25 v CAPACITOR.

^^P3 WILL MATE WITH 0 3, J4, J5,J £,J~7 OR J8 DEPENDING ON
    APPLICATION.
               GROUND POINT AT PINS C.D.&E OFCONMECTORSOKI
       CHASSIS ADJACENT TO PIN C .
      --...  -.. GROUND POINT AT PINS  5 S. 10 OF TRANSFORMER
      AND ON CHASSIS ADJACENT TO TRANSFORMER.
	"           RT'S>RT£ ARE LOCATED irl CELL MT I. RT3& RT<5
   MAY BE REQUIRED FOR TEMPERATURE COMPENSATION ON
   UNITS Y7/O HEATER. OPTION POR MOUNTING THEM ON
   CIRCUIT BOARD SHALL BE PROVIDED.
 2. CaPACITANCt VAl_UE6 AKE IMjjF, IO'/., IOO V

 I. ftiliToK  VALUES  AE6 IN OHM1,±l?i,  Vtl<*jf/

 NOTES: UIILESS OTMESWISE SPECIFIED

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                                    Rl  RT]
lin, :•.;• the cell, current  Rf = R,  4- «~T~i7^"~
'  '                              J. L   KI T Jxi,




The scaling ampli.fj.er AR_ is an inverting operational amplifier with  four


selectable gain settings determined by the range  switch.  The basic gain


equiation is:

                              .    Rf                      .
                           • - Av ~ — -
                                   Rin



where Rf is R 0 for all ranges and Rin composed of one of the series


resistors R.(7> R-.O or R,, and the equivalent  source  resistances.  This


source resistance is the effective resistance of  the span not R,0 and
                                                      1        68                       ,

any parallel resistance such as R, , or R, ,_•                                             I
  J                              16     45

                                                                                        I
To maintain an accurate raster reading, the meter  is  electrically contained              '.


in the feedback loop and the system is therefore  immuned to  changes in  the              \

                                                                                        t
mater resistance as a function of time and temperature.                                 t



Overload protection for the circuit is provided by the diode network  of


CR.. through CR  and R9r|.  This  limits the amplifier  to ±1.4V.          •                 F



The recorder output is developed by the resistive divider R  ' and R9o-                  f,
                                                           rf— <—       t*J                  I

This provides a lOmv full scale output with a low output impedance .                     £



Initial ^ero adjust for AR0 is set by R n .                      •                       i;
                          /L            /.L                                               v:

                                                                                        (.'
Temperature control of the cell heater is determined by resistive bridge                ij


amplifier AR_ and associated driver transistors R through R .                          |

                                                                                        i

The resistive bridge is composed of resistors R/0> R/?> ^YiV ^9  (thermistor)           [


and potentiometer R , , .  Bridge unbalance is sensed by. amplifier AR  which


proportionally changes the output voltage to  the  heater.  At 25CC the


bridge unbalance causes pin 3 to be positive  with respsct to pin  2.   The


output at pin 6 is then at a maximum positive voltage.  This voltage  is


buffered by emitter followers Q., and Q  which control the high current
                                /,      O

pass transistors Q .  As the heat increases,  the  thermistor  RT   (located


on the cell) v?j'.ll continue to decrease in valxie until the bridge becomes


balanced.  The high gain of the amplifier maintains  the heater at a high


input current for fast warm-up  and then maintains a  proportional control


within a few tenths of a degree.



Circuit protection is obtained  by the heater  power switch S_ and diodes
                                   A-11
CR  and CR. . .

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C O ft ij ORATION
     The heater  light DS3  is directly  in  parallel with  the  heater  blankets
     ana 'has a brilliance  which  is  proportionate to  the applied  voltage.
     Monitor pov?er can be  provided  by  cither  115 VAC line voltage  or  +12  TOC.
     The -I-12VDC  operation  is dependent-on the  PSI DC to DC  converter.  This  is_
     a special module which provides ±22V of  partially  filter  voltage in
     parallel with the rectified AC line  voltage.
     The filtered DC voltages  are regulated by integrated circuit  regulators
     Z. and Z  .  The regulated ±15  VDC is used to virtually eliminate the
     effects of  line voltage and. frequency.   They also  provide short  circuit
     protection  to preve.nt propagation failures.  Initial adjustment  is set
     by potentiometer R _  for  the H-15VDC  and  R 3 for the -15VDC.

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