EPA 670-4-73-018
Environmental Monitoring Series
PERFORMANCE OF THE UNION CARBIDE
DISSOLVED OXYGEN ANALYZER
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
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EPA-670/4-73-018
July 1973
PERFORMANCE OF THE
UNION CARBIDE DISSOLVED OXYGEN ANALYZER
by
Robert J. O'Herron
Methods Development and Quality Assurance
Research Laboratory
Program Element 1HA327
NATIONAL ENVIRONMENTAL RESEARCH CENTER
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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REVIEW NOTICE
This report has been reviewed by the National Environmental
Research Center - Cincinnati and approved for publication. Mention
of trade names or commercial products does not constitute endorse-
ment or recommendation for use.
11
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FOREWORD
Man and his environment must be protected from the adverse effects
of pesticides, radiation, noise and other forms of pollution, and the
unwise management of solid waste. Efforts to protect the environment
require a focus that recognizes the interplay between the components
of our physical environment--air, water, and land. The National
Environmental Research Centers provide this multidisciplinary focus
through programs engaged in
• studies on the effects of environmental
contaminants on man and the biosphere, and
• a search for ways to prevent contamination
and to recycle valuable resources.
This report is part of a continued effort by the Instrumentation
Development Branch, Methods Development and Quality Assurance Research
Laboratory, NERC, Cincinnati, to evaluate instruments and provide
information to both users and suppliers. It is also intended that
instrumentation be upgraded and that a choice of the most suitable
instrument can be made for a particular application.
A. W. Breidenbach, Ph.D.
Director
National Environmental
Research Center, Cincinnati
111
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PERFORMANCE OF THE
UNION CARBIDE DISSOLVED OXYGEN ANALYZER
The Union Carbide Dissolved Oxygen Analyzer, Model 1101, was
evaluated to determine the effectiveness of the thallium electrode
in the measurement of dissolved oxygen. This report summarizes the
results of performance tests which included, stability, transient
response, linearity, and temperature compensation. Data obtained
indicated long-term instability due to dissolved oxygen (DO) sensor
drift, instrument calibration dependence upon dissolved solids con-
tent of the sample, instrument ground loop problems, and excessive
transient response to changes in sample temperature.
INSTRUMENT DESCRIPTION
The Union Carbide Dissolved Oxygen Analyzer consists of a sensor
assembly and a signal conditioning unit located in an electronics
cabinet. The dimensions are as follows:
1. Electronics Cabinet 21cm x 29cm x 10cm deep
2. Sensor Assembly 25cm long x 9.8cm diameter
The general specifications for this instrument as is listed in
the Union Carbide Operating Instructions are as follows:
1. Power: 115 V, 60 Hz, 1/10 ampere
2. Range: 0.1 - 15 ppm D.O.
3. Accuracy: ±0.2 ppm over full range
± 0.5°C
4. Response Time: 95% of full scale in 15
seconds
5. Recorder Output: 0-50 mv
6. Operating Temperature: 0-50°C (compensated)
7. Ambient Temperature Effect: Temperature compensated within
1% from -1°C to 37°C
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8. Maximum Recommended Distance
Between Probe and Electronics
Cabinet: 152 meters.
The sensor assembly contains a thallium electrode formed in a ring
about a reference electrode. A temperature sensitive resistor (Sen-
sistor) is housed within the cylinder. The thallium ring and reference
electrode is a galvanic cell with the half-cell potential of the thal-
lium being responsive to D.O. content in solution. The Sensistor is
said to perform the dual function of temperature compensation and
temperature measurement.
The signal conditioning unit consists of a solid state (FET)
operational amplifier complete with power supply, calibration poten-
tiometers, and an output linearizing module. The linearizing module
converts a negative logarithm output to a linear output proportional
to D.O. in solution. This linear output is connected to a meter
calibrated for D.O. in ppm and temperature in degrees centigrade. The
meter is backed by a mirror to reduce reading errors due to parallax.
A 0-50 mv recorder can be connected to a terminal strip mounted within
the electronic cabinet.
THEORY
The operating principle of this measurement is: D.O. in solution
will react with thallium in the reversible reaction forming thallium
hydroxide.
4 Tl + 02 + 2 H20?^4 Tl (OH) (1)
The thallium hydroxide, in turn, will dissociate
4 Tl (OH) ^4 Tl+ + 4 (OH)" (2)
The result is that a half-cell potential is produced by the thal-
lium in contact with its ions (Tl ). The half-cell potential will vary
as the D.O. in solution varies. The half-cell potential produced by
the thallium and that of the reference electrode is Nernstian and
can be described by the equation:
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E = Eo - 0.592 log Tl+ (@25°C)
———^— d,
n
where E0 is the standard potential of the cell, Tl is the activity
3-
of the thallium ion, and n is the valence. The potential produced
forms the input to the signal conditioning unit.
PERFORMANCE
The first objective was to determine the long term stability of
the D.O. sensor. To accomplish this objective the following apparatus
were utilized:
1. a temperature controlled water bath (Forma Scientific)
equipped with an internal pump to provide continuous
sample flow across the sensor
2. a recorder (Esterline-Angus) with variable span capability
of 0 to 100 mv
3. titration apparatus and the necessary chemicals to perform
Winkler determinations
4. a compressed air source diffused to produce saturated D.O.
in solution.
The temperature of the water bath (tap water) was controlled at
25°C. Repeated observations were made of the D.O. sensor's perfor-
mance after calibration. In each case, an upward drift resulted in
the recorded output of the essentially constant dissolved oxygen in
the sample. Figures 1, 2, and 3 illustrate three of these tests.
Figure 2 illustrates the characteristic drift after substitution of a
silver-silver chloride reference electrode in the circuit. (The meas-
ured potential of the Ag - Ag Cl reference electrode is within 10 mv of
that used by Union Carbide.) The thallium electrode was filed smooth
and immediately placed in the sample except for the results shown in
Figure 2, which is merely a continuation of the calibration of Figure 1.
Since similar results were obtained (drift) with the use of the
Ag - Ag Cl and the two reference electrodes provided by Union Carbide,
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the drift was obviously the result of the thallium electrode. Figure
3 indicates the recorded D.O. to be 7 ppm high after a 7-day interval.
This drift was accelerated if the thallium was exposed to air for a
period of time. Thus, the thallium is quite susceptible to oxidation
in air.
A literature review* of the properties of thallium indicated that
the chloride ion would possibly interfere with the reaction of equation
(2). Thallium chloride does not dissolve easily and its formation in-
terferes with the oxidation of thallium. Five-year averages with
maximum and minimum values for pH, dissolved solids, and the chloride
ion were obtained from the Cincinnati Water Works for finished water.
5-year
average Maximum Minimum
pH 8.6 8.1 8.9
Dissolved solids (Total), ppm 327 533 230
Chloride ion, ppm 47 87 23
A stability test was performed in distilled water to minimize any
effect of dissolved solids. A very unstable output (noisy) was over-
come by preconditioning the thallium surface in tap water through the
usual initial transient interval. When steady state was attained, the
sensor was returned to the distilled water sample and a stable output
was obtained. The distilled water has a pH of 6, total dissolved
solids of less than 2 ppm and chloride ion of less than 1 ppm. Figure
4 illustrates the results of this test.
The indicated, upward drift was very much less than that of the
tap water tests. After 8-1/2 days of operation, the recorded indica-
tion was too high by 2 ppm. Although this test in distilled water
neither proved nor disproved chloride ion interference, it did present
a more stable medium in which the D.O. sensor could be observed in
further performance tests.
*Encyclopedia of Science and Technology, Volume 13, McGraw Hill.
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Distilled water samples were used in further testing to avoid
excessive sensor drifts. Three sample baths were controlled at 5°C,
20°C, and 35°C. A D.O. calibration was performed in the 20°C bath
after stable temperature and D.O. saturation had been attained.
The D.O. sensor was transferred from the 20°C sample to the 5°C
sample to establish
1. the effectiveness of temperature .compensation
2. the transient response to a temperature change.
As illustrated in Figure 5, an exceedingly long transient interval
resulted before the steady state value was reached (15 hours). In ad-
dition, an erroneous D.O. indication in excess of 3 ppm resulted at the
steady state value.
The D.O. sensor was returned to the 20°C sample, and the response
is illustrated in Figure 6. The transient response was 40 minutes,
and the D.O. indications returned approximately to that of the calibra-
tion value of the previous day.
The D.O. sensor was transferred from the 20°C sample to the 35°C
sample, and the response is illustrated in Figure 7. A long term drift
was experienced to reach a steady state value. Some of this may be the
result of adding distilled water to replenish the evaporating sample.
The final value was erroneous (too high by 2.5 ppm).
Finally the D.O. sensor was returned to the 20°C sample from the
35°C sample. The response is illustrated in Figure 8. A similar res-
ponse, of shorter duration, to that of the 20°C to the 5°C exchange.
occurred. The total response was 7 hours. Vigorous wiping with tis-
sues midway through this response had no effect in reducing the tran-
sient interval.
From the foregoing results, it was determined that temperature has
a marked effect on the reaction rates of Equations (1) and (2).
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Generally, the reaction rate is affected most by an exchange of the
D.O. sensor from a higher to a lower temperature sample. Equilibrium,
apparently, can be disturbed for hours by sudden changes in temperature.
The cause of erroneous steady state indications from sample to sample was
thought to be the result of a ground loop problem between the test ap-
paratus and the D.O. analyzer. If this were true, inadequate temperature
compensation could not be implied from the foregoing results.
To determine if a ground loop existed, the three water baths were
used to control distilled water samples at 20°C. Two of these baths
were allowed to saturate with D.O., and the third was purged of oxygen
with nitrogen gas. Figure 9 illustrates the results of this test. It
is readily apparent that the 4 ppm difference between water baths num-
ber one and number two is the result of ground loops . This sample is
distilled water from the same still and controlled at the same tem-
perature with Winkler determinations differing by only 0.05 ppm. The
only difference, then, is that of the water baths and the indeterminacy
of their respective ground loops with that of the signal conditioning
unit. Note also that the 15 second response time listed in the Union
Carbide specifications is met only when changing from a higher to a
lower D.O. sample at the same temperature. When changing from a lower
to a higher value, the response time is greater than 1 minute (Figure 9).
A test for linearity in the distilled water sample indicated that
a recalibration was necessary. The complete procedure given in the
operating instructions were followed in an attempt to calibrate the
instrument. However, the span values" were 8.8 ppm (679.15 mv) and
1.5 ppm (707.15 mv), which were determined in the distilled water sample.
When the calibration was performed using these voltage inputs, the D.O.
zero control could not be adjusted for proper agreement at 1.5 ppm.
The minimum attainable panel meter reading was 2.0 ppm with the D.O.
zero control at its lower mechanical stop. Calibration dependence
upon sample characteristics was confirmed by successfully calibrating
the instrument with a tap water sample.
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The Union Carbide Dissolved Oxygen analyzer was installed and
operated under field conditions at the test station on the Little
Miami River. The tests were inconclusive because a portion of the
thallium ring totally dissolved within the test interval. D.O. off-
sets and drift however, were observed during the 1 week of operation.
The temperature system was calibrated prior to the collection of
the D.O. data. The panel meter versus an NBS calibrated thermometer
were:
NBS thermometer Panel meter
5.0°C 5.1°C
35.0°C 35.25°C
The response time in reaching steady state after transferring the
sensor from a 35°C bath to a 5°C bath is illustrated in Figure 10. The
time required to reach within 95% of the full scale value was in excess
of 4 minutes. Figure 11 illustrates the exchange from 5°C to 35°C.
Again, the time required to reach within 95% of the full seale value
was in excess of 4 minutes.
DISCUSSION
In summary, there are problems associated with the Union Carbide
D.O. sensor that would require correction. These problems are:
1. long-term instability because of sensor drift
2. calibration dependence upon dissolved solids content of the
sample
3. ground loop between the sample medium and the signal condi-
tioning unit
4. excessive response times of signals because of temperature
changes.
The drift encountered in repeated tests of the sensor's perfor-
mance in tap water samples would suggest an interference with the
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reactions of Equations (1) and (2). A selective membrane approach
would apparently be needed to overcome this problem.
The instrument calibration depended upon dissolved solids content
of the sample. An interference is indicated to the reaction of thal-
lium and oxygen in solution. Possibly, a stable, noninterfering
electrolyte could be used with the conventional galvanic cell approach
to the measurement of D.O. A membrane, an electrolyte, and a two-
element electrode could be used. This, in turn, would overcome the
ground loop problem of having the thallium in direct contact with
the sample.
The excessive response time of signals because of temperature
changes has been indicated by sequentially transferring the D.O. sen-
sor through 5°C, 20°C, and 35°C baths. It has been determined from
this that the chemical reaction rates are adversely affected. Cor-
rection for this may be insurmountable for the sensor in its present
form. Again, if the conventional galvanic cell approach is pursued,
the problem may be avoided.
The accuracy of the temperature system was adequate and within
Union Carbide's specifications. The response time, however, was
excessive. The response time can be improved if the Sensistor is
moved from within the probe body to closer contact with the sample.
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24
20
E
Q.
Q.
LU
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X
o
Q
LJU
O
CO
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16
12
Calibration
D.O. Recorder
8
Winkler Determinations
0
8 16 24 32 40 48 56 64
OPERATING TIME, hours
Figure 1. Sensor Drift after Calibration
72
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20i-
E
Q.
0.
LJJ
O
>
X
o
Q
LJJ
O
CO
CO
Q
16
12
D:O. Recorder
8
Actual D.O. Level
1
1
I
0
8
16 24 32 40 48
OPERATING TIME, hours
56
64
Figure 2. Sensor Drift after Insertion of Ag-AgCI
Reference Electrode
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20
E
0.
a
LJJ
X
o
o
LJJ
O
CO
CO
o
16
12
8
Winkler Determinations
D.O. Recorder
Actual D.O. Level
0
8
16
24 32 40 48 56 64
OPERATING TIME, hours
Figure 3. Sensor Drift after Filing Thallium Ring
72
80
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24
E
Q.
Q.
LU
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>-
X
o
Q
LU
O
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20
16
12
8
D.O. Recorder
Winkler Determinations
Actual D.O. Level
18 24 40 56
72 88 104 120 136 152 168 184 200
OPERATING TIME, hours
Figure 4. Sensor Drift in Distilled Water
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Recorder Full Scale
28 r
24
E
§ 20
LJJ
o
>-
X
o
Q
LJJ
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16
12
8
D.O. Recorder
PANEL METER FULL SCALE
D.O. Level at 5°C
D.O. Level at 20°C
0
14
16
18
24 6 8 10 12
OPERATING TIME,hours
Figure 5. Sensor Transfer from 20°C Bath to 5°C Bath
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16
E
Q.
Q.
LD
X
o
Q
LU
O
CO
CO
o
12
8
D.O. Level at 5°C
D. O. Recorder
D.O. Level at 20°C
I
1
0 1
OPERATING TIME, hours
Figure 6. Sensor Transfer from 5°C Bath to
20°C Bath
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20
E
Q.
Q. 16
LLJ
X
o
12
Q
LU 8
O
<° A
co 4
D.O. Recorder
2O at 35°C Added
D.O. Level at 35°C
A. D.O. Level at 20°C
16
4 8 12
OPERATING TIME, hours
Figure 7. Sensor Transfer from 20°C Bath to 35°C Bath
20
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28
24
a 20
z
LLJ
O
>-
X
o
Q
LLJ
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D
16
12
8
Wiped Surface
of Thallium Ring
D.O. Recorder
Panel Meter Full Scale
D.O. Level at 20°C
D.O. Level at 35°C
0
1
8
234567
OPERATING TIME, hours
Figure 8. Sensor Transfer from 35°C Bath to 20°C Bath
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E
a
Q.
LJJ
O
>-
X
O
Q
LJJ
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12
8
Bath
No. 1
Bath
No. 2
Bath
No. 1
Winkler Bath No. 1=8.8ppm
Winkler Bath No. 2 = 8.9ppm
Winkler Bath No. 3 = 1.8ppm
Bath Bath
No. 2 No. 1
Bath No. 3
0
20
10 15
OPERATING TIME, minutes
Figure 9. Ground Loop Problems in Transferring
Sensor Between 20°C Baths
25
-------
LU
1
I
0
25
5 10 15 20
OPERATING TIME, minutes
Figure 10. Transfer of Temperature Sensor from 35°C
Bath to 5°C Bath
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O 30
o
LU~
DC
ID
20
DC
LU
Q_
2
UJ
10-
J_
0
1
5 10 15 20 25
OPERATING TIME, minutes
Figure 11. Transfer of Temperature Sensor from 5°C
Bath to 35°C Bath
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BIBLIOGRAPHIC DATA
SHEET
1. Report No.
EPA-670/4-73-018
4. Title and Subtitle
PERFORMANCE OF THE
UNION CARBIDE DISSOLVED OXYGEN ANALYZER
3. Recipient's Accession No.
5- Report Date
1973-issuing date
6.
7. Author(s)
R. J. O'Herron
8- Performing Organization Kept.
No.
9. Performing Organization Name and Address
Methods Development and Quality Assurance Research Laboratory
U.S. Environmental Protection Agency
National Environmental Research Center
Office of Research § Development, Cincinnati, Ohio 45268
10. Project/Task/Work Unit No.
11. Contract/Grant No.
12. Sponsoring Organization Name and Address
SAME AS ABOVE
13. Type of Report & Period
Covered
14.
15. Supplementary Notes
16. Abstracts
The Union Carbide Dissolved Oxygen Analyzer, Model 1101, was evaluated to determine
the effectiveness' of the thallium electrode in the measurement of dissolved oxygen.
This report summarizes the results of performance tests which included, stability,
transient response, linearity, and temperature compensation. Data obtained indicated
long-term instability due to dissolved oxygen (DO) sensor drift, instrument calibra-
tion dependence upon dissolved solids content of the sample, instrument ground loop
problems, and excessive transient response to changes in sample temperature.
17. Key Words and Document Analysis. 17a. Descriptors
performance evaluation, signal stabilization, transient response, linearity,
oxygen electrodes (thallium), galvanic cells, drift (sensor), temperature
coefficient.
17b. Identifiers/Open-Ended Terms
dissolved oxygen, thallium electrodes, instrumentation.
17c. COSATI Field/Group
_ Dissolved GaSCS
18. Availability Statement
Release to public
19. Security Class (This
Report)
UNCLASSIFIED
1EI
(Th
20. Security Class (This
Page
UNCLASSIFIED
21. No. of Pages
22. Price
FORM NTIS-35 (REV. 3-72)
U.S. GOVERNMENT PRINTING OfflCE 1974- 758-490/1086
USCOMM-DC 14952-P72
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