NOAA
EPA
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/ 7-79-059
March 1979
Research and Development
Evaluation of the
Orion Divalent
Specific Ion Electrode
Interagency
Energy/Environment
R&D Program
Report
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EPA-600/7-79-059
March 1979
EVALUATION OF THE ORION DIVALENT SPECIFIC ION ELECTRODE
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.
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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
maintain 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
-------
ABSTRACT
The Orion Divalent Cation Specific Ion Electrode #93-32 was evaluated
for suitability in monitoring or in situ marine applications as a magnesium
ion sensor. The electrode was tested with three separate modules for the
following parameters: accuracy, precision, temperature dependence, short-
and long-term stability, durability, sensitivity to variations in light
intensity and flow conditions, response time as a function of temperature
and concentration, and variability between modules. The "liquid ion-
exchange" sensor was 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 tests and results are
included. Although two of the modules supplied with the electrode failed
prematurely, the third replacement module performed well in all media with
an accuracy of 5% in magnesium concentration when properly calibrated on a
daily basis. The response times were generally longer than expected,
ranging on the average from 3 minutes in seawater to 26 minutes in un-
treated freshwater. The electrode was relatively insensitive (+0.2 mV) to
external wire motion, flow conditions, or variations in light intensity.
IV
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CONTENTS
Foreword . . . . iii
Abstract iy
Figures. vi
Tables yii
Abbreviations and Symbols viii
Acknowledgements ~>x
1. Introduction 1
2. Conclusions 2
3. Experimental Procedure . 3
Instrument Description . 3
Test Procedure and Rationale 4
4. Results 6
Calibrations 6
Accuracy ..... 7
Response Time 9
Drift 11
Environmental Effects 12
References 14
Appendix - Instrument Theory 15
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Number FIGURES
Page
1 Sensor module for the Orion #93-32 Divalent Cation Electrode. . •. .17
2 Calibration curves for Orion magnesium electrode at 25°C in freshwater
Evaluation #1 18
3 Calibration curves for Orion electrode at 25°C in freshwater Evaluation
#2 18
4 Orion electrode response as a function of magnesium concentration and
temperature in freshwater 19
5 Electrode response as a function of magnesium concentration in 35-,
20- and 5-ppt salinity seawater at 25°C 19
6 Electrode response as a function of time (min) and concentration in
freshwater at 25°C - Run #1 20
7 Time response of the Orion electrode in freshwater at 25°C - Run #3.20
8 Time response of the Orion electrode in freshwater at 10°C 21
9 Electrode time response in 35~ppt salinity seawater at 25°C. ... 21
10 Time response of the Orion electrode in 20-ppt seawater 22
11 Time response of the Orion electrode in 5-ppt salinity seawater. . 22
12 Drift of electrode potential at 25°C over a 50-day period 23
VI
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TABLES
Number page
1 Manufacturer's Specifications ..... ............ 3
2 Coefficients for Calibration Curve Fits. ........... ^
3 Accuracy of the Orion Divalent Electrode for Magnesium
Measurements in Seawater ................ .... 9
4 Response Times for the Orion Magnesium Sensor
5 Drift of the Orion Magnesium Sensor
6 Environmental Effects on the Orion Sensor. •
vn
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LIST OF ABBREVIATIONS AND SYMBOLS
ABBREVIATIONS
ACS -- American Chemical Society
cm -- centimeter
h -- hour
IAPSO -- International Association for the Physical Sciences of the Oceans
ISA -- ionic strength adjuster
log -- logarithm
min -- minute
ml -- milliliter
mo -- month
mV -- millivolt
ppt — parts per thousand
RSE — residual standard error
SYMBOLS
A -- activity
Ag -- silver
AgCl -- silver chloride
Ca^+ -- calcium
Cl -- chloride
°C -- degrees Celsius
E -- electrode potential
E° -- electrode constant
F -- Faraday's constant
I — ionic strength
K -- degrees Kelvin
KC1 — potassium chloride
M -- molar ity
m -- total ion concentration
Mg2+ -- magnesium ion
MgCl2 — magnesium chloride
R — universal gas constant
S -- electrode slope
T -- time, temperature
T] -- response time within 1 mV
Tg5 -- 95% response time
z -- ionic change
7 — activity coefficient
4 — delta
°
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
as
In the past few years, interest in ion-selective electrodes as
analytical devices has grown considerably. Their selectivity as well «o
their compactness and simplicity make these potentiometric devices
attractive for laboratory or field use either as in situ probes or as com-
ponents of chemical analysis systems for continuous or discrete water
quality monitoring. By eliminating the need for discrete sampling, these
sensors can provide an ideal method for obtaining truly representative
measurements of the chemical parameters in the water environment. For
in situ measurements, the environment itself becomes the sample, eliminating
not only the contamination from sampling and holding containers, but also
the time lag between sampling and analysis, during which time various
chemical and biological alterations take place.
Presently ion-selective electrodes are available for many chemical
parameters, and several are in use for marine chemical research, effluent
and pollution monitoring, mixing studies, and baseline surveys. Despite
these recent applications, very little work has been done on the performance
and characteristic behavior of these sensors in a medium other than pure
water, i.e., seawater. The accuracy, precision, reliability, and
temperature dependence must be known before they can be used for meaningful
measurements.
The Orion Divalent Cation Specific Ion Electrode can be used to measure
the magnesium content in seawater or freshwater without sample pretreatment,
and therefore lends itself to in situ applications. The concentration of
magnesium is of interest in seawater, because it is a major constituent and
is involved in sedimentation (dolomite) and sound absorption processes (as
magnesium sulfate). In this study the Orion Divalent Cation Electrode
#93-52 was evaluated for measuring the magnesium content in freshwater and
seawater solutions at 10°C and 25°C.
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SECTION 2
CONCLUSIONS
1. The modules which were supplied with the electrode were unstable and
short-lived (~1 mo) despite the fact that both were used before the
"expiration date." The replacement modules, however, performed well and
lasted 3 months. Since the sensors have a short shelf life, they were on
general back order and took over 3 months to obtain.
2. The height of the filling solution had no effect on the readings as
long as it was above the sample solution.
3. The sensor response was a linear function of the logarithm of the
concentration in fresh- and saltwater, with slopes in freshwater within 4%
of the Nernst theoretical value.
4. The temperature drop to 10°C stabilized the electrode considerably,
suggesting that the sensor should be subjected to low temperatures while
soaking as a "break-in" procedure.
5. The sensor response in freshwater was quite slow (average 25 min), while
in seawater the response was much faster (average 3 min). Since the drift
was also much less in the seawater samples, the results indicate that the
addition of an innocuous salt as an ionic strength adjuster (ISA) might be
advantageous for freshwater samples.
6. Although two modules from Orion may have similar basic characteristics
(i.e., response curves, temperature dependence, etc.), the electrode
potential for two separate modules can be significantly different in the
same solution with the same reference electrode.
7. Since the entire electrode body is made of hard plastic, the sensor was
physically durable.
8. Although the calibration curves were fairly linear, the residual
standard error (RSE) factor indicates that at least a three-point calibration
is required to obtain reliable electrode slopes.
9. The sensor may be used for monitoring applications if (a) a daily
calibration is possible (by automated standard addition if necessary), (b)
the slow response time is not prohibitive,and (c) the samples are measured
at the same temperature. When a standard addition method of calibration
is not possible the electrode may be calibrated in solutions with the same
background matrix as the samples, if the general composition of the sample
is known, as in the case of seawater. The sensor is not affected by
variations in light intensity or flow, and accuracies of 5% can be achieved
when the modules are calibrated properly.
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SECTION 3
EXPERIMENTAL PROCEDURE
INSTRUMENT DESCRIPTION
The Orion Divalent Cation Electrode Model 93-52 evaluated in this
study was manufactured by Orion Research, Inc., to replace the older Orion
Wd-3l electrode. The specifications are given in Table 1. An external
reference electrode is required in conjunction with the Orion Sensor to
measure calcium, magnesium, or water hardness (calcium plus magnesium) in
aqueous solutions. The reference electrode used in this study was the
Orion Model 90-01 Single Junction Reference Electrode with the Orion
#90-00-11 filling solution recommended by the manufacturer. The reference
electrode was the "sleeve" type and is described by Hard f!978). The
cation electrode is described in further detail in the Appendix.
Table 1. MANUFACTURER'S SPECIFICATIONS
Concentration range
pH range
Temperature range
Electrode resistance
Reproducibility
Sample
Minimum sample size
Storage
Module life
Size
6 x 10~6 to 1M
5.5 to 11 pH
0° to 50°C
2 to 8 megohms
+ 4%
aqueous solutions only
3 ml in a 50-ml beaker,
0.3 ml in Orion Microsample
Dish (Cat. No. 92-00-14)
store in air
6 months under normal
laboratory conditions
electrode length: 13.5 cm
cable length: 75 cm
cap diameter: 1.2 cm
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The electrode potential was measured with an Orion Model 801A Digital
pH/mV Meter with a range of +_ 1999.9 millivolts (mV) in 0.1 mV increments
and with +_ 0.1 mV repeatability. The meter features high input impedance,
extremely low drift, and an output displayed 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 that
was sealed to eliminate errors due to sample evaporation while submerged
in a well-insulated, non-metallic constant temperature bath. The bath was
constructed to maintain temperature to +_ 0.01°C and to eliminate possible
ground loops or stray electrical interferences, which frequently plague
electrode measurements. The sensor was submersed at a constant 4 cm in the
sample solution, which was stirred with a Teflon star-head magnetic stirring
bar and a Troemner variable 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 prior to each run
to remove possible changes in electrode potential due to temperature
equilibration.
The salinities of the seawater samples were measured with a Guild!ine
Model 8400 Laboratory Salinometer (Autosal) which has been described in
detail by Boyd (1976). The natural seawater samples were analyzed for
magnesium and calcium on a Perkin-Elmer Model 503 Atomic Absorption
Spectrophotometer. The calibration curves and fits were determined by a
least-squares method on a Hewlett-Packard 9825A Programmable Calculator.
The freshwater standard solutions were prepared from Fisher Certified
ACS Reagent Grade magnesium chloride and Fisher Certified Atomic Absorp-
tion Standard Magnesium Reference Solution. The synthetic seawater
solutions were also prepared from Fisher Certified ACS Reagent Grade
chemicals using the formula of Kester (1967). The synthetic seawater
solutions at 20- and 5-ppt salinities were prepared by weight-diluting the
artificial 35-ppt seawater with pure water. The natural seawater samples
were: (1) IAPSO Standard Seawater [Pg6 27/7, 1974 - Cl (ppt) = 19.3675] at
35-ppt salinity, as well as at 20- ana 5-ppt salinity by dilution with
pure water; (2) Atlantic Ocean water (32.2-ppt salinity from a station at
38°40.3'N, 74°20.0'W) at 32- and 20-ppt salinity (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 electrode was evaluated with three different modules (see Fig. 1)
at 10°C and 25°C in five different water types: pure water, synthetic sea-
water, IAPSO Standard Seawater, Atlantic Ocean water, and Chesapeake Bay
water. The pure water tests were performed to observe the 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 com-
position that simulated natural seawater and its matrix effects. The
4
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subsequent electrode measurements in 35-, 20-, and 5-ppt salinity IAPSO
Standard Seawater were made to determine the electrode accuracy after proper
calibration with synthetic seawater and to observe the effect of salinity
variations on the electrode response without relative compositional changes.
Measurements were made in clean Atlantic Ocean water to determine variations
in electrode response between IAPSO Standard Seawater (treated Atlantic Ocean
water) and natural Atlantic Ocean water (untreated). 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
magnesium (Mg2+) activity in standard solutions ranging from 0.05 M to
IxlO"^ M-Mg2+. Two consecutive runs were made at 25°C -- the first in de-
creasing concentration step-changes, the second in increasing concentration
steps. With this procedure the effects of directional concentration steps
could be determined for response times, electrode calibration, and re-
producibility (short-term drift). The electrodes were subsequently cali-
brated at 10°C and then again at 25°C in the same standard solutions as the
other 25°C runs. From these tests, the effects of positive or negative
temperature variations could be measured for response times, calibration
curves, electrode slope, long-term drift (between the two 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 different ambient light intensities; (4)
response changes due to motion of the electrode and wire connections; (5)
variation in electrode potential due to different filling-solution heights
(reference electrode); and (6) electrode response as a function of time.
The calibrations in synthetic seawater were completed by measuring the
magnesium activity in four standard seawater solutions with known amounts of
magnesium and calcium (the Ca^+ was held constant in all four solutions).
Immediately after calibration, values for magnesium activity were obtained
in two different natural water samples at the same salinity as the standards.
Since the relationship of magnesium activity to concentration is not known
accurately in seawater (see Appendix) the results and calibration curves
were calculated in concentration units. This procedure was completed at
35-, 20-, and 5-ppt salinity. The concentrations of the magnesium and
calcium were determined from salinity measurements and atomic absorption
spectrophotometry. 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 organic content, and suitability for possible in situ monitoring in
freshwater, estuarine, and seawater environments.
-------
SECTION 4
RESULTS
CALIBRATIONS
Four evaluations were performed with three separate modules. The first
sensor module, which came with the electrode, had large errors due to serious
drift problems and a sensitivity to flow around the sensor. The performance
steadily declined for 1 month after initial use, at which time the sensor
module was deemed useless. The second sensor module (also provided with the
electrode) was then assembled onto the electrode, and the tests were resumed
(Evaluation #1). There were still some drift problems, and the sensor
barely finished the tests before failing. At this time a new replacement
sensor module was ordered and finally received more than 3 months later.
Evaluations #2 and #3 were completed on this module. All sensors were used
well before the expiration date stamped on the module container. '
Seven calibration runs were performed in 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 MgCl? for 2 days
prior to the tests. All concentrations are given in molarity (M), defined
here as the moles of magnesium (Mg2+) per liter of solution.
The potential developed across the membrane was remarkably different
between each of the three sensors in the same magnesium solution. At 25°C
for example, in a 0.01 M solution, the three sensors recorded potentials of
65.2, -45.5, and 29.8 mV, respectively, against the same reference electrode.
Obviously, the absolute value of the potential in mV is very dependent upon
the individual characteristics of each module and can only be determined
empirically by calibration in standard solutions. The electrode response
(in mV) was plotted as a function of the logarithm of the magnesium con-
centration (M) and fitted to the equation
E = E° + S log M (1)
where E° is the intercept, S is the slope, and E is the electrode potential
in a solution with M concentration of magnesium. The theoretical value for
S for the divalent Mg2+ ion should be 29.6 mV/decade at 25°C. (See Appendix.)
The values for E°, S, and the residual standard error (RSE) for the curve
fits are given in Table 2 for the freshwater and seawater runs in Evaluations
#1 and #2.
The calibration curves for the Orion electrode in freshwater are shown
in Figs. 2-4. Evaluation #1 (Fig. 2) included calibration runs with the
sensor modules supplied with the electrode. Although the sensor did not work
well before the 10°C run, the results from Run #3, completed immediately
after the low temperature run, were relatively good with an electrode slope
of 33.8 mV/decade at 25°C. Freshwater Run #4 was performed after the sea-
water runs and shows that the module was obviously not responding properly
at that time.
6
-------
In Evaluation #2, the new replacement module performed well after the
Initial calibration was completed. (See Fig. 3.) Runs 2, 3, and 4 had very
similar slopes of 28.9, 29.6, and 28.5 mV, respectively, and were quite
close to the theoretical value of 29.6 mV decade. Similar to the first
evaluation, the 10°C-calioration seems to have stabilized the sensor since
Run #3 and Run #4 were very similar (Fig. 3). Although the electrode
response was linear for all the runs in Evaluation #2, the slope did change
somewhat with time, and the Mg2+ sensor would therefore require at least a
three-point calibration to obtain reliable data.
The effect of temperature on the electrode response is shown in Fig. 4
for Evaluation #2. It is apparent that the sensor is temperature-sensitive
and should be calibrated at the same temperature as the sample. For
monitoring purposes, this problem could be eliminated by pulling the sample
through a coil in a temperature exchanger to keep the samples at a constant
temperature when in contact with the sensor head.
The seawater calibrations are shown in Fig. 5 as a function of salinity
and magnesium concentration at 25°C. Although the electrode response was a
linear function of the magnesium concentration (RSE <_0.3), the slope
decreased significantly with increasing salinity, the greatest effect
occurring at a salinity near 35 ppt.
In general, the sensor worked well in all solutions, fresh-and salt-
water, but required careful calibration with standard solutions with a
background matrix and temperature very similar to that of the samples. As
long as these factors are considered, the sensor is suitable for oceano-
graphic application. The modules did not last as long as expected, however,
and the sensors supplied with the electrodes did not perform well.
ACCURACY
The accuracy of the sensor was determined in natural seawater at
35-, 20-, and 5-ppt salinity by comparing the values for magnesium con-
centrations determined from the Orion electrode to the values obtained by
analysis with the atomic absorption spectrophotometer. The electrode
measurement of the unknown Mg2+ concentration in the natural water samples
was completed immediately after the sensor was calibrated with four
standard synthetic seawater solutions with a known Mg2+ content at the same
salinity as the sample. The results are given in Table 3 as the percent
error in the magnesium concentration determined with the electrode, compared
to the "true" values determined with the standard laboratory methods. The
results with the electrode were surprisingly good with an accuracy better
that 5% in all the natural waters from 35- to 5-ppt salinity.
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Table 2. COEFFICIENT FOR THE CALIBRATION CURVES
Water type
Freshwater
25°C #1
#2
#3
#4
10°C #1
Seawater
5 ppt
20 ppt
35 ppt
Freshwater
25°C #1
#2
#3
#4
10°C #1
Seawater
5 ppt
20 ppt
35 ppt
E° fmV)
93.4
88.1
71.1
-43.4
78.3
-64.4
-44.8
-54.3
114.1
70.6
135.5
131.5
118.9
50.1
50.7
45.4
S fmV/deraHe)
Evaluation #1
21.63
18.65
33.82
- 1.43
37.67
- 7.95
12.32
14.85
Evaluation #2
41.81
28.88
29.65
28.45
44.32
18.29
15.56
10.32
RSE
10.3
4.4
2.2
7.3
2.4
1.3
1.7
0.8
2.0
2.6
3,1
1.3
1.0
0.3
0.2
0.2
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Table 3. ACCURACY9 OF THE ORION DIVALENT ELECTRODE FOR MAGNESIUM MEASUREMENTS
Salinity (ppt) Water type % Error % Preclsionb
35
20
5
IAPSO
Atlantic Ocean
IAPSO
Atlantic Ocean
IAPSO
Chesapeake Bay
4.2
4.8
4.8
0.2
4.1
3.9
1.6
0.6
1.0
2.6
4.4
4.7
aThe accuracy is given as the percent of the difference in concentration
from the true value for Evaluation #2.
"The difference between two seawater runs at 25°C, divided by the true
value.
RESPONSE TIME
The response times are given for Evaluation #2 in three different forms
for this study: Tgs (internal), Tgs (external), and TI (j^lmV). The first
two are 95% responses (Tgsj, which are defined as the time required for the
system output to attain 95% of the asymptotic value when subjected to a step
input. The Tgs is equal to three "time constants" (1-e-l) of a pure
exponential response.
The electrode potential was monitored continuously for 3 hours in each
test solution, starting from the moment the sensor was immersed. The
"internal" 95% response time, Tg5(I), is the time required for the electrode
response to reach 95% of the change between the initial value and the final
value in the same solution. The "external" 95% response, Tgs(E), is the
time required for the sensor to each 95% of the change between the final
potential in the current test solution and the final potential in the pre-
ceding test solution at a different concentration.
The +1 mV response time (Ti) is the time required for the electrode to
reach within 1 mV (-7% in concentration units) of the final value. All
three methods of expressing the time response gave similar trends (see
Table 4) and were obtained by averaging all the tests in each individual run
since the response was found to be independent of the concentration. The
TI response time was considered to be the most useful parameter for dis-
cussing response time in this study, because it was recommended by the
International Union of Pure and Applied Chemistry (IUPAC), and because it is
less sensitive to the magnitude of the concentration step change than the
other times,if the concentration change is greater than 7%.
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Table 4. RESPONSE TIMES* FOR THE ORION MAGNESIUM SENSOR
Water type
Time Response (mln)
T95(D
T-
Freshwater
25
10
°C
°C
#1
#2
#3
#4
#1
35
56
32
46
51
.3
.8
.3
.0
.2
14
56
14
48
32
.0
.5
.5
.5
.2
12
42
19
29
29
.0
.4
.0
.7
.8
Sea water
5
20
35
ppt
ppt
ppt
Synthetic
Natural
Synthetic
Natural
Synthetic
Natural
33
16
19
3
17
35
.7
.5
.3
.3
.2
.5
11
21
8
10
13
36
.3
.2
.9
.8
.8
.8
5
<0
3
0
1
<0
.9
.1
.0
.1
.2
.1
i
(0.3)b
v-.-;
(
-------
was considered stable when the reading remained within +0.2 mV over a
30-minute period. Despite the somewhat erratic behavior of the response
times of the electrode in freshwater, the response time in seawater was much
faster, possibly due to the high ionic strength. (See Figs. 9-11.) The
unstable appearance of the sensor response in the seawater graphs (Figs. 9-11)
is due to the necessity of expanding the mV axis to separate the curves for
suitable viewing. As a result, a change of only 0.1 mV, which is not a
significant variation, can be clearly seen on the curves.
DRIFT
The drift of the electrode potential is given in Table 5 for each cali-
bration run and was monitored over five different time ranges. The
short-term stability was calculated from the change in response over a
15-minute period after the sensor had "stabilized" (or had been immersed in
the standard solution for 2 hours). The sample container was sealed to
prevent concentration changes during the tests. The drift over a 1-day
period was determined from the reading taken 3 hours after electrode
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 drift over
1 month was calculated from the average difference between two calibration
runs completed 1 month apart in the same standard solutions. The 50-day
drift test was performed over a 50-day period during which the electrode
response was continuously monitored in a sealed seawater sample at a
constant temperature. The 2-hour, daily, weekly, and monthly drifts are
summarized in Table 5. The 2-hour drift is expressed as the change in
millivolts over a 15-minute period after the electrode was immersed in the
sample for 2 hours, and was averaged over the entire calibration run.
From Table 5, it is apparent that the short-term stability improved
considerably after the 10°C run was completed and the sensor was returned
to 25°C (Runs 3 and 4). While the weekly and monthly drifts were quite
large in freshwater, the sensor was stable to within +3 mV in seawater
(See Table 5 and Fig. 12) for at least 30 days. After 35 days, however,
the sensor response deteriorated until the potential had decreased by
19 mV after 50 days (Fig. 12). The magnesium and calcium concentrations were
measured before and after the test to ensure that the concentrations in the
sample had not changed during the test. The drifts of the electrode in the
natural water samples were found to be the same as that in the synthetic
seawater.
11
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Table 5. DRIFT OF THE ORION MAGNESIUM ELECTRfMF
Time _period
2 hoursb
1 dayb
1 week
1 month
50 days
Water
Freshwater
Freshwater
Seawater
Freshwater
Seawaterc
Freshwater
Freshwater
Seawater
type
-25°C #1
#2
#3
#4
-10°C #1
5 ppt
20 ppt
35 ppt
-25°C
-10°C
-25°C
-25°C
-25°C
-25°C
AmVa
0.5
1.8
0.1
0.1
0.9
0.1
0.1
0.1
1.7
1.7
2.5
11.0
44.3
19.0
aChange of 0.2 mV corresponds to an error of 0.7% in concentration.
Averaged over entire calibration run.
cValues for natural water were the same as those in synthetic seawater.
ENVIRONMENTAL EFFECTS
The Orion electrode was subjected to various light intensities to
determine if light attenuation (or variations in "turbidity") could affect
the electrode potential as some studies have reported (Bates,1973). The
effect of variations in environmental conditions (light, flow, external
motion) on the electrode were determined and are summarized in Table 6. In
Evaluation #1, the electrode sensitivity to light increased over a short
time from +0.2 mV. After the tests were completed, the electrode was found
to be defective. Similar problems were found in the flow and motion tests.
Apparently the module became generally unstable as it began to fail. In
Evaluations #2 and #3 the sensor was unaffected by light changes, flow
variations around the electrode, or external motion of the connecting wires
and electrode body.
12
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Table 6. ENVIRONMENTAL EFFECTS3 ON THE ORION SENSOR
Parameter
Light
Flow
External Motion
Eval. #1
+ 0.2
+ 0.3
+ 0.2
Eval. #2
0.0
+0.2
+0.1
Eval.
0.0
+0.1
+0.1
#3b
Changes in mV due to parameter applied.
Conducted only for environmental effects after Evaluation #2 had been
completed.
13
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REFERENCES
Bates, R. 1973. The Determination of pH - Theory and Practice. Wiley Inter-
Science, N.Y. 479 pp.
Boyd, J.E. 1976. Performance Evaluation of Guildline Model 8400 Laboratory
Salinometer. NOAA Tech. Memo. NOS 18, National Oceanic and Atmospheric
Administration, Washington, D.C. 20 pp.
Durst, R. 1969. Ion-Selective Electrodes. NBS Publication 314, National
Bureau of Standards, Washington, D.C. 452 pp.
Kester, D.R.,et al. 1967. "Preparation of Artificial Seawater". Limnol.
Oceanogr. 12, 176.
Orion Research, Inc. 1975. Analytical Methods Guide. Cambridge, Mass. 32 pp.
Ward, G.K. 1978. 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)
14
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APPENDIX - INSTRUMENT THEORY
The Orion Divalent Cation Electrode #93-32 consists of an electrode
body and a pre-tested sensing module, which is activated by screwing the
module into the electrode body. The module should be good for 6 months and
consists of an organophilic-porous membrane surrounded by a porous plastic
reservoir saturated with liquid ion-exchanger (Fig. 1). The internal aqueous
reference solution is located on the inside of the membrane and is also in
contact with silver/silver chloride (Ag/AgCl) internal reference element.
A process of ion-exchange takes place at the membrane interface between
ions of the "ion-site" salt in the organic phase and the free ions in the
aqueous phase. The selectivity of the sensor depends primarily on the
selectivity of this "ion-exchange" process. Once inside the liquid membrane,
the site and ion move together through the membrane phase. Rejection of un-
wanted ion can be achieved by blocking either the ability of the ion to pass
into the membrane solution or the movement of the ion in the membrane phase.
The liquid phase also solves the problem of ensuring the mobility of the
"sought for" ion since the ion will be free to move by diffusion. The
internal solution contains a fixed activity of the ion to which the membrane
is permeable. When the electrode is placed in a sample solution, there is
a momentary flux of ions across the membrane system in the direction of the
solution containing the lower activity of the mobile ion. Since each ion
carries a charge, an electrical potential is set up that opposes further ion
migration. Eventually, an equilibrium is established in which the potential
across the membrane is exactly that required to prevent further net movement
of ions.
The changes in membrane potential can be measured by making electrical
contact to the internal aqueous solution with the internal reference
element (Ag/AgCl). At the same time, the sample solution is contacted with
a second external reference electrode via a salt bridge. A high input
impedance voltmeter connected across the two electrode leads will indicate a
potential given by the Nernst equation:
E = E° + 2.3 RT log A (Al)
HF
where E is the potential in millivolts (mV) developed by the system, A is
the activity of the ion to which the membrane is permeable, and E° is a
constant potential developed by that particular electrode system. The E°
term depends on the type of reference electrode used, the ionic activity in
the inner solution, and the small potential due to the liquid-liquid
junctions at the salt bridge connections (reference electrode). The 2.3
RT/2F term is called the "Nernst theoretical slope", where R is the uni-
versal gas constant, T is the temperature in °K, F is Faraday's constant, and
2 is the charge on the ion (+2 for magnesium or calcium). The Nernst slope
at 25°C for a divalent ion is 29.6 mV per decade of activity (Durst, 1969).
At this point, it is important to note that ion-selective electrodes re-
spond to changes in ionic activity, not concentration.
15
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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 (i.e., ion complexation 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)
where >+ is a function of ionic strength (I) and ionic interactions. Even
if the electrodes were truly "specific" and responded to the ion desired,
the sensor response would be affected by only the free ionic activity, not
the total concentration. Since many chemical species in natural waters are
complexed with other ions or adsorbed on particulate matter, a large fraction
of the element 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. In a plot of E (mV) versus the logarithm of the ionic activity,
the slope should be equal to the Nernst slope (2.3 RT/zF) and the intercept
equal to E°. The basic problem with this "ideal" situation is that the
activity coefficient can only be determined for salts and not for a single
ionic species. For example, although the T ± for potassium chloride (KC1)
is known in pure water, the 7+ for K+ can only be estimated by somehow
splitting the"** (KC1) into its ionic components with some arbitrary con-
vention. One of the most common methods in use today is the Mclnnes
Convention, in which it is assumed that v+ (K+) ="* - (Cl~). Since this is
only an arbitrary convention, it is obvious that the y+ for an ionic species
cannot be accurately determined, 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 the concentration of the
desired ion. In this case, the value of Y + (ion) is not known and, therefore,
the electrode must be calibrated in solutions which not only contain known
concentrations of the desired ion, but also duplicate the background ''matrix
effects" of the other ions in the sample. Since the activity of the ion is
the same in both the standards and the sample, a reliable calibration can be
prepared in terms of total concentration. An electrode measurement in sea-
water, therefore, requires the calibration of the sensors in standard sea-
water solutions with a known concentration of the desired ion and a back-
ground matrix which simulates the composition of the sample as closely as
possible.
16
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-------
76 -r
51 --
-24 --
-49
Run #1
Run #2
Run #3
Run #4
-3.0
-2.6
-1.8
-2.2
Log M
Figure 2. Calibration curves for the Orion magnesium electrode
in freshwater - Evaluation #1.
102
-1.4
at 25°C
* Run
+ Run #2
o Run #3
• x Run #4
-18
-1.0
Figure 3. Calibration curves at 25°C in freshwater - Evaluation #2.
18
-------
105 -i-
-15
-3.0
Figure 4.
38 -i-
-2.5
-1.5
-1.0
-2.0
Log M
Orion electrode response as a function of magnesium concentra-
tion and temperature in freshwater.
31 --
24 --
17
10
-2.3
-2.0
-1.1
-0.8
-1.7 -1.4
Log M
Figure 5. Electrode response as a function of magnesium concentration
in 35-, 20-, and 5-ppt salinity seawater at 25°C.
19
-------
-15
0 15
Time (min)
Figure 6. Electrode response at a function of time (min) and
concentration in freshwater at 25°C - Run #1.
20 -i-
13 --
o
-1
-15
0
Figure 7.
30 45
Time (min)
Time response of the Orion electrode in freshwater as a
function of magnesium concentration at 25°C - Run #3.
20
60
-------
-19
Figure 8.
30 45
Tim© (min)
Time response of the Orion electrode in freshwater at 10°C.
60
0
o
-P
T—t
.
-1
-2
-3
0
15
45
30
Time (min)
Figure 9. Electrode response as a function of time in 35-ppt salinity
natural and synthetic seawater at 25°C.
21
60
-------
0
o
4>
r—i
-------
-20
10
20 30
Time (days)
40
50
Figure 12. Drift in electrode potential at 25°C over a 50-day period
at constant magnesium concentration in seawater.
23
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