National Oceanic
and Atmospheric
Administration
National Ocean Survey
Test and Evaluation Laboratory
Rockville MD 20852
United States
Environmental Protection
Agency
Office of Energy, Minerals, and
Industry
Washington DC 20460
EPA-600'7-79-058
March 1979
Research and Development
Evaluation of Calcium
Sensors in Fresh-
and Saltwater
Interagency
Energy/Environment
R&D Program
Report
-------
RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7 Interagency Energy-Environment Research and Development
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9. Miscellaneous Reports
This report has been assigned to the INTERAGENCY ENERGY-ENVIRONMENT
RESEARCH AND DEVELOPMENT series. Reports in this series result from the
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health and welfare from adverse effects of pollutants associated with energy sys-
tems. The goal of the Program is to assure the rapid development of domestic
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essary environmental data and control technology. Investigations include analy-
ses of the transport of energy-related pollutants and their health and ecological
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This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/7-79-058
March 1979
EVALUATION OF CALCIUM SENSORS IN FRESH- AND SALTWATER
by
Gary K. Ward
National Ocean Survey
National Oceanic and Atmospheric Administration
Rockville, Maryland 20852
Interagency Agreement No. D5-E693
Project No. EAP-78-BEA
Program Element No. 1 NE 625C
Project Officer
Gregory D'Allessio
Office of Energy, Minerals, and Industry
U.S. Environmental Protection Agency
Washington, D.C. 20460
This study was conducted
as part of the Federal
Interagency Energy/Environment
Research and Development Program
Prepared for
OFFICE OF ENERGY, MINERALS, AND INDUSTRY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
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DISCLAIMER
This report was prepared by the National Ocean Survey's Test and
Evaluation Laboratory, National Oceanic and Atmospheric Administration,
reviewed by the U.S. Environmental Protection Agency, and approved for
publication. Approval does not signify that the contents necessarily
reflect the views and policies of the National Oceanic and Atmospheric
Administration or the U.S. Environmental Protection Agency, nor does
mention of trade names or commercial products constitute endorsement
or recommendation for use.
-------
FOREWORD
The Test and Evaluation Laboratory of the National Ocean Survey's
Office of Marine Technology, National Oceanic and Atmospheric Administration,
conducts work to:
"test and evaluate new or state-of-the art sensors for use in the
marine environment
"determine the error bounds on chemical sensors' performance to
obtain data quality assurance information
"determine suitability of new sensors for in situ or field use
"evaluate new methods for the chemical analysis of seawater
"develop standards and calibration equipment and procedures to main-
tain a quality assurance program for measurements in the marine
environment.
New chemical sensors are under continual test and evaluation to assure
that the most accurate results possible are obtained. If specific ion
electrodes are to be used to measure chemical parameters in seawater, they
must be evaluated directly in the medium of interest, i.e., seawater. This
report investigates the basic characteristics of specific ion electrodes in
a variety of water types to determine their suitability for in situ or
monitoring applications.
Eugene M. Russin
Chief, Sensor Test Branch
Test and Evaluation Laboratory
m
-------
ABSTRACT
The Orion Calcium Ion Electrode #93-20 was evaluated for suitability
as a calcium ion sensor for either monitoring or in situ marine
applications. The electrode was tested with three separate sensor modules
for the following parameters: accuracy, precision, temperature dependence,
short- and long-term stability, durability, sensitivity to fluctuations in
light intensity and flow conditions, response time as a function of
temperature and concentration, and variability between modules. The three
sensors of the "liquid ion-exchange" type were evaluated at 10°C and 25°C
in freshwater, synthetic seawater (35-, 20-, and 5-ppt salinity), and natural
waters (IAPSO standard seawater, Atlantic Ocean water, and Chesapeake Bay
water). A description of the sensor, theory of operation, and a summary of
the test results are included. The electrode response for two of the
modules was a linear function of the logarithm of calcium concentration.
All three modules failed prematurely (i.e., before the 6-mo guaranteed
period). While the accuracy in high salinity samples (35 and 20 ppt) was
poor, the concentration measurement was better than 7% in 5-ppt salinity
seawater and indicates a susceptibility of the sensor to the "salt-extrac-
tion effect," which results in electrode drift and abnormal inaccuracy in
salt solutions. The long response times (i.e., average of 33 min in
freshwater) and sensitivity to flow conditions make the practical appli-
cation of this calcium sensor to in situ measurements a somewhat dubious
proposition.
IV
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CONTENTS
Foreword Hi
Abstract iv
Figures vi
Tables vii
Abbreviations and Symbols viii
Acknowledgement ix
1. Introduction 1
2. Conclusions 2
3. Experimental Procedure 3
Instrument description 3
Test procedure and rationale 4
4. Results 6
Calibrations 6
Accuracy 9
Response time 10
Drift 12
Environmental effects 13
References 14
Bi biography 15
Appendix -Instrument Theory . -. 16
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FIGURES
Number paqe
1 Sensor module for the Orion #93-20 calcium ion electrode .... 19
2 Calibration curves for Orion calcium electrode at 25°C in
freshwater - Evaluation #1 20
3 Calibration for Orion calcium electrode at 25°C in freshwater-
Evaluation #2 20
4 Orion calcium electrode response as a function of temperature
and concentration in freshwater - Evaluation #2 21
5 Calcium electrode response as a function of temperature and
concentration in freshwater - Evaluation #1 21
6 Orion calcium electrode response as a function of concentration
at 10°C and 25°C - Evaluation #2
22
7 Orion calcium electrode response as a function of concentration
in 35% 20", and 5~ppt salinity seawater at 25°C-
Evaluation #1 22
8 Calcium electrode response as a function of Ca concentration
in 35~, 20~, and 5~ppt salinity seawater - Evaluation #2 ... 23
9 Time response of the Orion calcium ion electrode #93-20 in
freshwater Run #1 as a function of calcium concentration at
25°C - Evaluation #1 23
10 Time response of the Orion electrode in freshwater Run #4 at
25°C - Evaluation #1 24
11 Time response of the Orion calcium electrode in freshwater
Run #4 at 25°C - Evaluation #2 24
12 Electrode response as a function of time and concentration in
freshwater at 10°C - Evaluation #1 25
13 Electrode time response as a function of concentration in
freshwater at 10°C - Evaluation #2 25
14 Time response of the Orion calcium electrode in 20-ppt
salinity seawater - Evaluation #1 26
15 Time response of the Orion calcium electrode in 35-ppt
salinity seawater - Evaluation #2 26
16 Drift in electrode potential at 25°C over a 50-day period at
constant calcium concentration ^'
vi
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TABLES
Number Page
1 Manufacturer's Specifications 3
2 Coefficients for Calibration Curves for Orion Calcium
Electrode 8
3 Accuracy of the Orion Calcium Electrode in Seawater 10
4 Response Times for the Orion Calcium Electrode 11
5 Drift of the Orion Calcium Electrode 13
6 Environmental Effects on the Orion Calcium Sensor 14
vii
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LIST OF ABBREVIATIONS AND SYMBOLS
ABBREVIATIONS
ACS
cm
h
IAPSO
ISA
log
min
ml
mV
ppt
RSE
sat'd
American Chemical Society
centimeter
hour
International Association for the Physical Sciences
ionic strength adjuster
logarithm
minute
milliliter
millivolt
parts per thousand
residual standard error
saturated
of the Ocean
SYMBOLS
A
Ag+
°C
E
E°
F
I
°K
KC1
M
R
S
T
T!
T95
2
Y
0
activity
silver ion
degrees Celsius
calcium ion
electrode potential
electrode constant
Faraday's constant
ionic strength
degrees Kelvin
potassium chloride
molarity
universal gas constant
electrode slope
temperature
response time within 1 mV
95% response time
ionic charge
activity coefficient
standard deviation
approximately
vm
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ACKNOWLEDGEMENT
The author wishes to acknowledge the professional and dedicated
efforts of Charles White, Jerald Peterson, and John Lawler, without whose
help this study could not have been completed. The author gratefully
acknowledges the support of the U.S. Environmental Protection Agency and the
National Ocean Survey.
IX
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SECTION 1
INTRODUCTION
Recently developed ion-selective electrodes have received considerable
attention in the past few years for a variety of applications. Their
selectivity, simplicity, and low cost make these sensors attractive devices
for water quality measurements in a wide range of natural waters. Although
ion electrodes are available for many chemical species, little work has been
done on the performance and characteristic 'behavior of the sensors in media
other than pure water (e.g., in seawater). The accuracy, precision, and
reliability must be known before they can be used for meaningful measure-
ments.
The Orion Calcium Ion Electrode can theoretically be used to measure
directly the calcium activity in seawater or freshwater without sample pre-
treatment and thereby becomes a candidate for in situ application in water
quality measurements. Since calcium is a major constituent of seawater, the
element has been the subject of many studies in the marine sciences due to
the involvement of calcium in many biological, geological, and chemical
processes in the oceans.
In this study, the Orion Calcium Ion Electrode #93-20 was evaluated for
its performance and accuracy in samples from freshwater, estuarine, and sea-
water environments.
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SECTION 2
CONCLUSIONS
(1) The sensor modules failed to operate for the entire "6-month" period
guaranteed by the manufacturer even though they were used well before the
"expiration date." They generally lasted only about 3 months. Since the
sensors have a relatively short shelf life, it took a significant amount of
time to obtain replacements from manufacturers.
(2) The electrodes are physically durable since the body is made of hard
plastic.
(3) The height of the filling solution in the reference electrode had no
effect on the electrode potential as long as it was above the sample solution
(required to maintain a positive pressure).
(4) The amount of available light had no effect on the sensors.
(5) While two sensors had a linear response in freshwater, one sensor
required a 2d-degree calibration curve. All modules tested had a linear
response in seawater.
(6) Although the calibration curves in some cases were linear, the
residual standard error (scatter about the curve) indicates that at least
a three-point calibration is required to obtain useful results.
(7) The reproducibility of +4% (+0.5 mV) claimed by the manufacturer only
holds if the sensor is calibrated hourly. Since the response times were
quite long (average of 33 min and 18 min in freshwater and seawater, respect-
ively), this specification was difficult to check directly, and in light of
the response times is rather meaningless. However, the sensor drift from
Table 5 does indicate the change in electrode potential over the 1-hour time
period. In Evaluation #1, the average drift was a total of 0.8 mV/h
(+0.4 mV/h ) and therefore, for these two sensors the specifications are
correct. The sensor in Evaluation #2 with an average drift of _+0.9 mV/h
('v 6%), did not meet the specifications.
(8) The results indicate that the calcium electrode should not be used in
freshwater without an ionic strength adjuster (ISA). Direct measurements
with some sample pre-treatment is possible only in low salinity (5 ppt)
seawater. The accuracy at higher salinities was too poor for reliable results
to be obtained with the electrode, even if the sensor is calibrated in
synthetic seawater solutions. For this reason, the calcium electrode is
generally unsuitable for direct in situ measurements in the marine environment.
The sensor could be used in conjunction with some type of sample pre-treatment
and frequent multi-point calibrations, but its sensitivity to flow variations
and long response time would not make these electrodes easily adaptable to
continuous monitoring applications.
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SECTION 3
EXPERIMENTAL PROCEDURE
INSTRUMENT DESCRIPTION
The Orion Calcium Ion Electrode #93-20 was manufactured by Orion
Research, Inc., with the specifications given in Table 1. The electrode is
the liquid ion-exchange type with a replaceable sensor module (see Figure 1),
similar to the Orion Divalent Ion Electrode, (Ward, 1978a), and is described
in further detail in the Appendix. The Model #93-20 sensor replaces the
older Orion #22-20 electrode and requires an external reference electrode to
measure calcium in aqueous solutions. The reference electrode used in this
study was the Orion #90-91 Single Junction Reference electrode with the
Orion #90-00-11 Filling Solution (4M KC1 sat'd with Ag+) as recommended by
the manufacturer. The reference electrode was the "sleeve" type and is
described in detail elsewhere (Ward, 1978b).
TABLE 1. MANUFACTURER'S SPECIFICATIONS
Concentration range 8 x 10~6 to 1M
pH range 5.5 to 11 pH
Temperature range 0° to 50°C
Electrode resistance 1 to 4 megohms
Reproduci bi1ity +4%
Sample aqueous solutions only
Minimum sample size 3 ml in a 50-ml beaker
0.3 ml in Orion Microsample Dish
(Cat. No. 92-00-14)
Storage store in air
Module life 6 months under normal laboratory
conditions
Size electrode length: 13.5 cm
diameter: 1.1 cm
cap diameter: 1.6 cm
cable length: 75.0 cm
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The electrode potential developed by the electrode system was measured
with an Orion Model 801A Digital pH/mV Meter with a range of +1999.9 mV in
0.1 mV increments and with a repeatability of +0.1 mV. The meter features a
high input impedance, extremely low drift, and an output display directly in
mV. The meter drift was less than 0.2 mV over a 6-month period.
The electrode was secured in the top of a polypropylene container,
which was submerged in a well-insulated constant-temperature bath controlled
to +_0.01°C. The sample container was sealed to prevent sample evaporation,
and the bath was constructed to eliminate possible electrical ground loops
or interferences which frequently plague electrode measurements. The sensor
was kept at a constant 4 cm below the surface of the solution, which was
agitated with a Teflon star-head magnetic stirring bar and a Troemner
variable-speed submersible stirrer. The temperature was monitored constantly
with a Hewlett-Packard quartz-crystal thermometer that was periodically
calibrated against a platinum resistance thermometer. The sample solutions
were suspended in the temperature bath immediately prior to each run to
eliminate possible electrode response to temperature equilibration.
The salinities of the seawater samples were measured with a Guildline
Model 8400 Laboratory Salinometer (AutoSal) which has been described in
detail elsewhere (Boyd, 1976). The natural seawater samples were also
analyzed for calcium with a Perkin-Elmer Model 503 Atomic Absorption Spectro-
photometer. The calibration curves and fits were determined by a least-
squares method with a Hewlett-Packard 9825A Programmable Calculator.
The freshwater standard solutions were prepared for Fisher Certified
ACS Reagent Grade calcium chloride and Fisher Certified Atomic Absorption
Standard Calcium Reference Solution. The synthetic seawater solutions were
also prepared from Fisher Certified ACS Reagent Grade chemicals using the
formula of Kester (1967). The 20- and 5-ppt salinity synthetic seawater were
prepared from the 35-ppt seawater by weight dilution with pure water. The
natural seawater samples were: (1) IAPSO standard seawater
(PfiS 27/7, 1974 - Cl = 19.3675) at 35-,20-,and 5-ppt (from weight dilutions);
(2) Atlantic Ocean water (32.2-ppt salinity from 38° 40.3'N, 74° 20.0'W) at
32-and 20-ppt salinity (also diluted with pure water); and (3) Chesapeake
Bay water at 4.2-ppt salinity. All dilutions were made with Millipore
ion-exchanged (18-megohm) water.
TEST PROCEDURE AND RATIONALE
The electrodes were evaluated with three separate sensor modules (see
Figure 1) at 10°C and 25°C in five different water types: pure water,
synthetic seawater, IAPSO standard seawater, Atlantic Ocean water, and
Chesapeake Bay water. The pure water tests were performed to establish
basic electrode characteristics without chemical interferences and only
slight ionic strength effects. The sensor was calibrated with synthetic
seawater to determine the electrode performance in a complex solution of
known composition that simulated natural seawater and its matrix effects.
The subsequent electrode measurements in 35-,20-,and 5-ppt salinity IAPSO
4
-------
seawater were made to determine the electrode accuracy after proper cali-
bration with synthetic seawater standards and to observe the effect of
salinity variations on the electrode response without relative compositional
changes. Measurements were made in unpolluted Atlantic Ocean water to
determine variations in electrode response between IAPSO seawater (treated
Atlantic Ocean water) and natural untreated ocean water. Finally, the
Chesapeake Bay water provided samples with an estuarine matrix and possible
pollutant interferences.
The calibration curves in freshwater were obtained by measuring the
calcium activity in standard solutions ranging from 0.1 M to 1 x 10-3 M -
Ca . Two consecutive runs were made at 25°C: the first in decreasing con-
centration step-changes, the second in increasing concentration steps. With
this procedure the effects of directional concentration steps could be
determined for response times, electrode calibrations, and reproducibility
(short-term drift). The electrodes were subsequently calibrated at 10°C and
then again at 25°C in the same standard solutions as the other runs. From
these tests, the effects of positive or negative temperature variations could
be measured for time response, calibration curves, electrode slopes,
long-term drift (between the two 25°C runs), and durability to temperature
changes. In each calibration run, the sensor was tested for: (1) short-term
drift (3 hours); (2) sensitivity to flow variations; (3) sensitivity to
changes in ambient light; (4) response due to motion of the electrode and
wire connections; (5) variations in electrode potential due to different
filling-solution heights; and (6) electrode response as a function of time.
The calibrations in synthetic seawater were performed by measuring the
calcium activity in four standard seawater solutions of known calcium content
at the same salinity as the natural water samples. Immediately after cali-
bration, the values for calcium activity were determined in two different
natural water samples. Since the relationship of activity to concentration
is not accurately known in seawater (see Appendix), the test results and
calibration curves were calculated in concentration units. This procedure
was completed at 35-9 20-, and 5-ppt salinity.
The concentrations of calcium in the natural waters were determined
from atomic absorption spectrophotometric measurements. From the seawater
runs, we ascertained the effect of salinity on the electrode potential and
response times, the stability and accuracy in various water types,
sensitivity to variations in light intensity, and suitability for possible
in situ monitoring in freshwater, estuarine, and seawater environments.
-------
SECTION 4
RESULTS
CALIBRATIONS
Three evaluations were performed with three separate sensor modules.
The first module, one of two supplied with the electrode body, failed after
2.5 months. The second module, also supplied with the electrode, was then
assembled and the tests resumed (Evaluation #1). Toward the end of the
tests, the second module became unstable, and a new replacement module was
ordered. When the new sensor was received, Evaluations #2 and #3 were
completed. The low slopes found in Evaluation #3 indicate that this module
was also beginning to fail, although all three modules were used well before
the expiration date stamped on the module container.
At least seven ca-libration runs were performed during each evaluation:
four in freshwater at 10°C and 25°C, and three in seawater at 35-, 20-, and
5-ppt salinity.The sensors were assembled and soaked in 0.1 M CaCl? for 2
days prior to the tests. All concentrations are given in molarity (M)
defined as the moles of calcium (Ca2+) per liter of solution.
The potential developed by the electrode system was remarkably different
between sensors in the same calcium standard solution at the same temperature.
At 25°C, for example, in a 0.1 M Ca solution, potentials of 64.8, 78.4, and
-175.1 mV were recorded with three separate modules against the same
reference electrode. Obviously, the absolute value of the electrode
potential is very dependent on the individual characteristics of each module
and can only be determined empirically by calibration in standard solutions.
The calibration curves for the Orion Calcium Ion Electrode in fresh-
water at 25°C are shown in Figures 2-4, where the electrode potential (mV)
is plotted versus the logarithm of the Ca2+ concentration. Theoretically,
the electrode response should be a linear function of log M with a slope of
around 27 mV/decade at 25°C. The response of the first module in freshwater,
however, was not a linear function of log M (see Figure 2), and therefore
the electrode potential had to be fitted to the equation:
E = E° + S log M + B (log M)2 (1)
rather than the modified Nernst equation:
E = E° + S log M (2)
where E is the electrode potential in a solution of M in calcium con-
centration, E° is a constant (intercept), S is the slope, and B is an
empirical "deviation" coefficient. The values for E°, S, and B are
given in Table 2 for freshwater and seawater for Evaluations #1 to #3, along
with the residual standard error (RSE) for the curve fits. Since the results
from Evaluation #1 with the second module and from Evaluations #2 & #3 with the
-------
third module appear to be linear with log M, the data for these sensors were
therefore fitted to Equation (2), and the coefficients also tabulated in
Table 2.
In Evaluation #1, the freshwater Runs #1-3 were performed with the first
sensor module, which exhibited a nonlinear response with the log of calcium
concentration (Figure 2). The second module (freshwater Run #4 and seawater
runs in Evaluation #1) and the third module (Evaluation #2) had a linear
response with the log M (Ca2+) as shown in Figures 2 and 3. Although the
sensor response in Evaluation #2 was more linear with concentration than
that in Evaluation #1, the scatter about the fitted curves was significantly
greater (higher RSE values).
In Evaluation #2, the large change in electrode slope observed between
Runs #1 and #2 was considerably reduced. (See Figure 3 and Table 2.) This
may be due to a "stabilizing" effect from the low temperature 10°C run, a
characteristic that has been observed for other liquid ion-exchange
electrodes (Ward, 1978b). In Evaluation #2, Runs #3 and #4 in freshwater
were completed after the seawater measurements to show that the use in sea-
water media did not adversely affect the sensor. However, by the time
Run #5 was completed, the electrode slope had decreased to only 4 mV/decade
and the sensor response began degenerating rapidly. In Evaluation #3, the
electrode slope decreased until, finally, negative values was observed.
Apparently, the life of the sensor modules was somewhat shorter than the
6-months suggested by the manufacturer.
The effect of temperature on the calcium electrode response is shown in
Figures 5 and 6 for Evaluations #1 and #2, respectively. The first module
(Evaluation #1) was temperature sensitive (see Figure 5) in that the
"deviation term," B, changed significantly while the slope term, S, remained
nearly constant. In Evaluation #2, the considerable scatter in the data
points for the first two runs made the comparison of the 25°C data to the
10°C results very difficult (Figure 6). The removal of "bad" points from
Run #2 (error > 3 0) resulted in the curve plotted in Figure 6 in which
Run #2 has a slope similar to Run #1 in freshwater. The coefficients given
in Table 2 reflect the uncorrected calibration curve and the values in
parentheses are the corrected coefficients for which the RSE, by removal of
bad data, was reduced by nearly one-third. The corrected linear curves
indicate a decrease of 3 mV in the slope term between 10°C and 25°C. In
Evaluation #3, the sensor was subjected to a second series of 10°C runs, at
which time the slope term became negative, indicating a defective sensor.
The seawater calibrations for Evaluations #1 and #2 are shown in
Figures 7 and 8 as a function of salinity and calcium concentration.
Although the slope term changed significantly with salinity, both modules
maintained a linear response with the logarithm of concentration. In both
evaluations, the electrode slopes approached the Nernst theoretical valve
as the salinity approached zero, indicating that the addition of a low
ionic strength adjuster (ISA) would result in better measurements in
freshwater. While the electrode slopes in pure water without pre-treatment
were quite low, the values in low ionic strength solutions (5-ppt salinity
7
-------
Water type Run
Freshwater
25°C 1
2
3
4
10°C 1
Seawater
5 ppt
20 ppt
35 ppt
Freshwater
25°C 1
2
3
4
5
10°C 1
Seawater
5 ppt
20 ppt
35 ppt
Freshwater
25°C 1
2
E°
57.4
55.8
59.5
89.0(86
47.6
100.7
87.7
84.4
-1 59. 6(-
-179. 5(-
-177.2
-175.7
-235.9
-170.6
-171.8
-186.3
-199.4
-194.0
-200.2
- — — — ' ' ' "• — " ' - IIT n. T
S B
EVALUATION #1
-35.98 -13.47
-15.18 - 6.15
-14.49 - 6.58
.9) 10.54(8.08) - 6.27
-15.05 - 5.16
19.66
16.59
13.11
EVALUATION #2
156.9) 18.56(18.85)
160.1) 9.28(16.14)
10.16
8.77
4.65
13.54
22.35
17.38
13.14
EVALUATION #3
4.19
-0.96
RSE
0.2
0.4
0.8
1.7(2.3)
0.1
1.2
0.9
0.2
2.5(5.2)
0.7(4.5)
1.9
3.8
1.9
0.8
1.0
0.9
0.4
1.5
3.3
10°C
1 -208.9
-2.55
3.2
-------
seawater) were much closer to the theoretical slopes. It is therefore
recommended that the ionic strength of samples should be adjusted (as
recommended by the manufacturer) to obtain reliable results. The ISA was
not added to the freshwater samples in this study, because we were attempting
to use the sensors directly in various water media without sample
pre-treatment. Although ISA is required for freshwater measurements, the
linear calibration curves in seawater suggest a possible application for
oceanographic purposes where the ionic strength is usually constant and
relatively high.
ACCURACY
The accuracy of the calcium ion electrode was determined in natural
waters at 35-, 20-, and 5-ppt salinity by comparing the values for calcium
concentration obtained with the Orion electrode to the values determined by
chemical analysis on the atomic absorption spectrophotometer and by constant-
composition salinity calculations. The electrode measurements of the cal-
cium activity in the natural water samples were completed immediately after
the sensor was calibrated with four standard synthetic seawater solutions of
known calcium content at the same salinity as the test sample.
The electrode accuracy is given in Table 3 for Evaluations #1 and #2 as
the percent error in calcium concentration, calculated from the electrode
measurements, when compared to the "true" values obtained by the standard
laboratory methods. The results from Evaluation #2 indicate that the
electrode worked well in low salinity water with an accuracy of ^ 6%. Above
5-ppt salinity, however, the measurement error increased substantially to
*> 31%. Similar results were obtained in Evaluation #1, where the lowest
error (^20%) was found at low salinity and the highest error (^48%) at high
salinities. In both evaluations, the measurements in 32-ppt Atlantic Ocean
water were worse than in the IAPSO water, because the electrode calibrations
were sensitive to background salinity variations (see Figures 7 and 8),
which could only partially be corrected. Since the sensors were calibrated
in synthetic seawater solutions, the errors due to activity-concentration
variations in the IAPSO samples should have been very small. The results,
therefore, indicate that the calcium electrode cannot be used in high
salinity samples without sample pre-treatment. As noted by the manufacturer,
large deviations in electrode response can occur from a "salt-extraction
effect" in which some salts in samples of high salt concentrations are
extracted into the electrode membrane. Presumably, the salt extraction
effect should have been eliminated by calibrating the sensor at the salinity
of the sample. However, since this was not the case as found in our study,
a better solution would be simply to dilute the sample down to a salinity of
5 ppt or less. This could easily be done in a monitoring mode in which the
sensor could be calibrated with low salinity standards to give reliable
results. Regardless, of the reason, all the electrode readings in
Evaluation #2 were low by 31% at 20-ppt salinity and higher, indicating that
either the response was somehow suppressed or some part of the available
calcium was tied up. If necessary, a purely mathematical factor of 31%
could be used to compensate the values to the correct total Ca+^ concentra-
tion, resulting in accuracies of better than 8%.
-------
TABLE 3. ACCURACY OF THE ORION CALCIUM ION ELECTRODE IN SEAWATER5
Salinity (ppt) Water type % Errorb
Evaluation II Evaluation #2
35 IAPSO 40.8 31.9
Atlantic Ocean 64.9 38.1
20 IAPSO 29.2 31.3
Atlantic Ocean 32.4 31.3
5 IAPSO 20.5 6.2
Chesapeake Bay 23.1 6.6
aThe accuracy is given as the percentage of the difference in
concentration from the true value.
b% Error = 100 x (observed - true)/true.
RESPONSE TIME
The response times are summarized in three different forms in this
study: Tg5 (internal), Tg5 (external), and T-| (+1 mV). The first two are
the 95% response times and are defined as the time required for the system
output to attain 95% of the asympotic value when subjected to a step input.
The Tgs term is equal to three "time constants" (l-e-1) of a pure exponential
response. The third time response P| , is the time required for the sensor to
reach a value within 1.0 mV (^7% in concentration) of the final electrode
potential.
The electrode potential was monitored continuously for 3 hours in each
test solution, starting from the moment the sensor was immersed. The Tgs
(internal), or Tg5(I), is the time required for the electrode response to
reach 95% of the change between the initial and the final electrode potentials
in one standard solution. The Tgs (external), or Tg5(E), is the time
required for the senor output to reach 95% of the change between the final
potential in two consecutive test solutions at two different concentrations.
All three time responses are summarized in Table 4 and were obtained
simply by averaging all the results in each individual run since the time
responses in freshwater were independent of concentration level or direction
of step-change. The T-J time responses were considered to be the most useful
parameters for evaluating the electrode equilibrium time since they were
usually independent of the magnitude of the concentration step-change if
the change was greater than 8%. All three methods of expressing time
response, however, are used by various investigators for specific reasons.
In seawater, the time response decreased with increasing concentration until
10
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it reached a minimum level of 11, 10, and 5 minutes in 35-, 20-, and 5-ppt
salinity, respectively. The average value in freshwater at 25° C was approx-
imately 32 minutes.
TABLE 4. RESPONSE TIMES FOR THE ORION CALCIUM ION ELECTRODE
Response time
T95(D Tg5(E) T/
Freshwater
25°C - #1
#2
#3
#4
#5
10 °C - #1
Seawater
35-ppt synthetic
natural
20-ppt synthetic
natural
5-ppt synthetic
natural
39.5
47.2
53.6
61.1
47.9
51.2
36.1
30.9
45.0
44.5
31.2
26.6
24.6
31.3
65.7
75.4
53.4
44.6
38.7
52.8
43.2
53.9
27.3
46.1
23.6(49.9)
35.5(65.4)
40.8(49.1)
29.6(66.8)
27.6(— -)
41.7(42.0)
32.2(15.0)
13. 8( )
26.3(28.3)
20.1(— -)
5.4(22.0)
10. 1( )
aValues in parentheses are for Evaluation #1.
The responses of the Orion Calcium Ion Electrodes have been plotted in
Figures 9-15 as "delta mV" versus time (minutes) in freshwater and seawater.
The "delta mV" term is the difference between the initial electrode potential
obtained immediately after immersion in the solution and the potential output
at time T. In this manner, the electrode response at various concentrations
could be easily depicted on one graph.(Bottle numbers in figures correspond
to different levels of concentration.)
The response curves in freshwater for both evaluations illustrate a
characteristic found in many other sensors, namely a rapid response with a
positive overshoot, followed by a slow recovery to a final stable value.
The responses in freshwater of the first and second sensor modules are shown
for Evaluation #1 in Figures 9 and 10 over a 2-hour period, and for the third
replacement module in Figure 11 over a 1-hour period. In most cases, the
electrode potentials never actually leveled off, and a constant drift
(usually negative) developed sometime after 90 minutes for the first two
sensors and after 30 minutes for the third. At 10°C, the first sensors
responded about the same as they did at 25°C (see Figure 12), while the
11
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response of the third module was much slower with greater drift and
instability at 10°C. (See Figure 13.)
In seawater, all the sensors exhibited the overshoot characteristic
similar to the freshwater response curves and had the same problems with
negative drifts as observed in freshwater samples. (See Figures 14 and 15.)
The results with the second sensor at 20-ppt salinity was typical of the
behavior at all salinities in Evaluation #1 and are shown in Figure 14. The
results in seawater with the third sensor (Figure 15) were also similar at
all salinities, except for a slight decrease in the magnitude of the over-
shoot behavior. The response of the third module in 35-ppt salinity sea-
water is shown in Figure 15 and illustrates the peculiar behavior of the
large negative drifts in electrode potential in the most concentrated sea-
water samples (#4). In general, the electrode response in freshwater and
seawater was much slower than expected and suffered from continuous drift
problems.
DRIFT
The drift of the calcium electrode for each calibration run is summar-
ized in Table 5 for five different time ranges. The "short-term stability'1
was calculated as the change in the electrode potential over a 15-minute
period after the sensor had "stabilized," or had been immersed in the same
standard solution for 2 hours. The sample was sealed at all times to pre-
vent concentration changes during the tests, and the results were averaged
over the entire calibration run. The drift over a 1-day period was
determined from the reading taken 3 hours after sensor immersion and the
reading taken the following day in the same solution. The drift over 1 week
was determined from the differences between the calibration runs completed
1 week apart in the same standard solutions.
The long-term stability was measured over a 30-day and 50-day period.
The 30-day drift was calculated from the average difference between two
calibration runs completed 1-month apart in the same standard solutions.
The longer 50-day drift test was performed by continuously monitoring the
electrode response while it was sealed in a seawater sample maintained at a
constant temperature. The long-term drift tests were only performed with
the third sensor (30-d test) and the second sensor (50-d test).
The results in Table 5 indicate that in all the runs the first two
sensors were more stable than the third sensor in the short-term tests.
The daily, weekly, and monthly drift results seem to indicate that most of
the electrode drift occurs the first day, after which it begins to level
off. The 50-day drift test, however, illustrated in Figure 16,shows that the
agreement in electrode potential over a long period of time may simply be
gratuitous. Although the initial and final readings may be quite close
(3.7 mV in this case), the differences occurring during the test can be
high (up to 16.2 mV on day 30). To ensure that the concentration in the
sample had not changed during the tests, the calcium concentration was
measured before and after the test.
12
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TABLE 5. DRIFT OF THE ORION CALCIUM ION ELECTRODE
Time period Water type mV
After 2 hours
FW-25°C - #1
#2
#3
#4
#5
0.5(0.3)b
0.3(0.2)
0.6(0.1)
0.1(0.0)
0.5( — )
FW-10°C - #1 0.8(0.1)
SW 5 ppt 0.3(0.2)
20 ppt 0.5(0.3)
35 ppt 0.6(0.5)
1 day
1 week
1 month
50 days
FW
FW
FW
SW
4.9(4.3)b
6.9(6.0)b
7.5
3.7(16.2)C
a
'Change of 0.2 mV corresponds to an error of 1.5% in
concentration.
Values in parentheses are for Evaluation #1. Other
values are for Evaluation #2.
cWorst case value that occurred on 30th day.
ENVIRONMENTAL EFFECTS
The Orion Calcium Ion Electrode was subjected to variations in ambient
light intensity to determine possible deleterious effects of light attenu-
ation (or variations in "turbidity") on the electrode response, as reported
by some recent studies.
The effects of light intensity and other environmental conditions (flow
and external motion) on the sensors were determined,and the results
summarized in Table 6 for Evaluations #1 and #2.
The tests indicate that the sensors are relatively unaffected by
variations in ambient light and only slightly affected by external motion of
the electrode or connecting wire. The sensors were sensitive, however, to
changes in flow around the sensor module (particularly the third module)
where the deviations in electrode potential reached as high as +1.7 mV.
13
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TABLE 6. ENVIRONMENTAL EFFECTS ON THE ORION CALCIUM SENSOR9
Parameter Evaluation #1 Evaluation #2
Light (low and high) 0.0 0.1
Flow ±0.5 +1.7
External motion ±0.3 JQ.3
aChange in mV after parameter was applied.
14
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REFERENCES
Boyd, J. E. 1976. Performance Evaluation of Guildline Model 8400 Laboratory
Salinometer. NOAA Technical Memorandum NOS 18 (PB 259696), National
Ocean Survey, NOAA, Rockville, Md. 20 pp.
Kester, D. R. 1967. Preparation of Artificial Seawater. Limnol. Oceanogr.
12, 176.
Ward, G. K. 1978a. Test and Evaluation of Potassium Sensors in Fresh- and
Saltwater. NOAA-EPA Interagency Energy/Environment R&D Program Report,
EPA, Washington, D. C. (in press).
Ward, G. K. 1978b. Test and Evaluation of the Orion Divalent Specific Ion
Electrode. NOAA-EPA Interagency Energy/Environment R&D Program Report,
EPA, Washington, D. C. (in press).
-------
BIBLIOGRAPHY
Durst, R. 1969. Ion-Selective Electrodes. NBS Publication 314, Washington,
D.C, 452 pp.
Orion Research, Inc. 1975. Analytical Methods Guide. Cambridge, Mass.
32 pp.
Riley, J. P.,and G. Skirrow. 1975. Chemical Oceanography (Vol. 1).
Academic Press, New York, N.Y. 605 pp.
16
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APPENDIX - INSTRUMENT THEORY
The Orion Calcium Ion Electrode Model #93-20 consists of a replaceable,
pretested sensing module which is activated by screwing the module into the
electrode body. The sensing module contains a gelled internal filling
solution separated from the sample by an organophilic membrane that is
saturated with an organic liquid ion-exchanger contained in a reservoir
surrounding the membrane. (See Figure 1.) When the membrane is saturated
with the organic liquid ion-exchanger, it is selective for calcium ions and
a potential develops across the membrane, the magnitude of which is a func-
tion of the amount of calcium ions present in the sample solution. The
potential is measured against a constant reference potential generated by the
reference electrode. The measured potential (E) corresponds to the level of
calcium ion as described by the Nernst equation:
E = E° + S log A (3)
where E° is a constant potential, A is the calcium ion activity, and S is the
electrode slope equal to the term RT/zF. At 25°C, the theoretical slope is
equal to 29.6 mV/decade, where R is the universal gas constant, T is the
temperature (°K), ^ is the ionic charge (+2 for calcium), and F is Faraday's
constant. It is important to note that the activity (A) is not equal to
concentration in most aqueous solutions.
The activity of an ion can be thought of as the "effective concentration"
of the free ion in solution and is affected by temperature, pressure, ionic
interactions (such as ion complexion or ion-pair formation), and ionic
strength (a chemical parameter related to the number and charge of all the
ions in the solution). The activity (A) is equal to the product of the
ionic "activity coefficient," T+, and the total concentration of the ion, m:
A =Y+ m + (4)
where Y + is a function of ionic strength (I) and various ionic interactions.
Regardless of the electrode's "specificity" for the ion desired, the
sensor response can only be affected by the free ionic activity, not the
total concentration. Since many chemical species in natural waters are
partially "tied up" by complexation with other ions or by adsorption on
particulate matter, a large fraction of some elements may not be detected by
these sensors.
Ideally, the electrode pair would be calibrated with standard solutions
of known activity and the electrode potential would be fitted to the Nernst
equation previously given. In this case, a plot of E (in millivolts), versus
the logarithm of the ionic activity, would have a slope equal to the Nernst
slope (2.3 RT/zF) and the intercept equal to E°. The activity coefficients,
however, can only be determined for salts and not for ionic species. For
example, although theY + for the potassium ion - (K+) can only be estimated
by splitting the Y+ (KCi) into its ionic components according to some
arbitrary convention. One of the most common methods in use today for
splitting activity coefficients into ionic components is the Mclnnes
17
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Convention, by which it is assumed that Y + (K+)=Y-(Cr). Since this is only
an arbitrary convention, it is obvious that the ?+ for an ionic species can-
not be determined absolutely, even in pure water.
In natural waters, the situation is much worse due to the presence of
many other ions in the solution, all of which affect the value ofY+ to a
certain extent. To circumvent this problem, a "working curve" can be
established for sensors in natural waters in which the electrode potential is
determined as a function of the logarithm of concentration of the desired
ion. Since the values ofY+ (ion) are not known, the electrodes must be
calibrated in standard .solutions, which not only contain known concentrations
of the desired ion, but also duplicate the background "matrix effects" of the
ions in the sample. In this situation, the activity of the ion is the same
in both the standard and the sample, and a reliable calibration curve can be
prepared in terms of concentration. An electrode measurement in seawater,
therefore, requires the calibration of the sensor in standard seawater
solutions that contain not only a known concentration of the desired ion,
but also a background matrix which simulates the composition of the samples
as closely as possible.
18
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Figure 1. Sensor module for the Orion #93-20 Calcium Ion Electrode.
19
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82 -r
72 --
62 --
52
42
* RUN #1
RUN #2
o RUN #3
# RUN
-3.0
-2.5
-1.5
-2.0
Log M
Figure 2. Calibration curves for Orion calcium electrode at 25°C in
freshwater - Evaluation #1.
-175 -r-
-1.0
-195 --
-215
* RUN #1
RUN #2
x RUN #3
-• o RUN #4
if RUN #5
Log M
-1.0
Calibration for Orion calcium electrode at 25°C in
freshwater - Evaluation #2.
20
-------
-190 -r
-200
-210
-220
* 10C
+ 10C #2
o 25C
-3.0
-1.0
w
Figure 4. Orion calcium electrode response as a function of
temperature and concentration in freshwater - Evaluation #3.
66 -i-
46
Figure 5.
-1.0
. M
Calcium electrode response as a function of temperature
and concentration in freshwater - Evaluation #1.
21
-------
-175 -r-
-185 --
=£-195 --
-205 --
-215
25C #1
* 10C
o 25C #2
-3.0
-2.5
-1.5
-2.0
Log M
Figure 6. Orion calcium electrode response as a function of
concentration at 10°C and 25°C - Evaluation #2.
80 -r
-1.0
70 --
60
50 --
40
* 35 PPT
20 PPT
o 5 PPT
-3.0
-2.5
-1.0
-2. 0 -1. 5
Log M
Figure 7. Evaluation #1 - Orion calcium electrode response in
35-, 20-, and 5-ppt salinity seawater at 25°G.
-0.5
22
-------
-200 -i-
-210 --
-220 --
-230 --
« 35 PPT
20 PPT
o 5 PPT
-240
Figure 8.
4
-1.0
M
Evaluation #2 - Calcium electrode response as a function of
Ca2+ concentration in 25-, 20-, and 5-ppt salinity seawater.
E
O
-8
-14
-20
-26
Time (min)
Figure 9. Time response of the Orion Calcium Ion Electrode #93-20 in
freshwater at 25°C - Run #1, Evaluation #1.
23
-------
-15
Figure 10
5
90
60
Tim© (min)
Time response of the Orion electrode in freshwater at 25°C
Run 14, Evaluation #1.
-15
0
Figure 11
15
45
30
Time (itiin)
Time response of the Orion calcium electrode in freshwater at
25°C - Run #4, Evaluation #2.
24
-------
-29
0
Figure 12.
6
30
60
Time (min)
90
Electrode response as a' function of time and concentration
in freshwater at 10°C - Evaluation #1.
0
5 -6
-12
-18
0.
15
45
60
Figure 13.
30
Time (min)
Electrode response as a function of concentration and time in
freshwater at 10°C - Evaluation #2.
25
-------
-20
Figure 14
8
15
30
Tim© (win)
45
60
Time response of the Orion calcium electrode in 20-ppt
salinity seawater - Evaluation #1.
5-8
"3
-13
-20
15
45
30
Time (min)
Figure 15. Time response of the Orion calcium electrode in 35-ppt
salinity seawater - Evaluation #2.
60
26
-------
16 -T-
12 --
8 --
0
20 30
Time (days)
40
Figure 16.
Drift in electrode potential at 25°C over a 50-day
period at constant calcium concentration.
27
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