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
                                            0.5C
     4.  Response Time:                     95% of full scale in 15
                                           seconds
     5.  Recorder Output:                   0-50 mv
     6.  Operating Temperature:            0-50C (compensated)
     7.  Ambient Temperature Effect:        Temperature compensated within
                                           1% from -1C to 37C

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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+   (@25C)
                     ^        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
25C.  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 5C,
20C, and 35C.  A D.O. calibration was performed in the 20C bath
after stable temperature and D.O. saturation had been attained.
     The D.O. sensor was transferred from the 20C sample to the 5C
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 20C 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 20C sample to the 35C
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 20C sample from the
35C sample.  The response is illustrated in Figure 8.  A similar res-
ponse, of shorter duration, to that of the 20C to the 5C 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 20C.  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
                     5.0C                    5.1C
                    35.0C                   35.25C

     The response time in reaching steady state after transferring the
sensor from a 35C bath to a 5C 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 5C to 35C.
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 5C, 20C, and 35C 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
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     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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 8
                              Actual D.O. Level
                        1
                          1
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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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     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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    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
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                   D.O. Recorder
PANEL METER FULL SCALE
                   D.O. Level at 5C
     D.O. Level at 20C
        0
                                             14
                                     16
18
       24     6     8     10     12

                OPERATING TIME,hours

Figure 5. Sensor Transfer from 20C Bath to 5C Bath

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   16
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           D.O. Level at 5C
                        D. O. Recorder
              D.O. Level at 20C
                                  I
                                      1
            0                     1

                       OPERATING TIME, hours

       Figure 6. Sensor Transfer from 5C Bath to

                 20C  Bath

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  20
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                        D.O. Recorder
                2O at 35C Added
                                   D.O. Level at 35C
        A. D.O. Level at 20C
                                                  16
                4           8           12

                        OPERATING TIME, hours

       Figure 7. Sensor Transfer from  20C Bath to 35C Bath
20

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   28
   24
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       Wiped Surface
       of Thallium Ring
                                        D.O. Recorder
Panel Meter Full Scale
     D.O. Level at 20C
         D.O. Level at 35C
           0
             1
                                        8
                    234567

                  OPERATING TIME, hours

Figure 8. Sensor Transfer from 35C Bath to 20C Bath

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E
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   12
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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 20C Baths
                                           25

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LU
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        0
                 25
          5        10       15       20
                 OPERATING TIME, minutes
Figure 10. Transfer  of Temperature Sensor from 35C
           Bath to 5C Bath

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             OPERATING TIME, minutes

Figure 11. Transfer of Temperature Sensor from 5C
          Bath to 35C 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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