United States
Environmental Protection
Agency
Municipal Environmental Research EPA 600 278106
Laboratory May 1978
Cincinnati OH 45268
Ion Selective
Electrodes in Water
Quality Analysis
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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
8 "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-78-106
May 1978
ION SELECTIVE ELECTRODES IN WATER QUALITY ANALYSIS
by
Robert C. Thurnau
Water Supply Research Division
Municipal Environmental Research Laboratory
Cincinnati, Ohio 45268
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Municipal Environmental Research
Laboratory, U.S. Environmental Protection Agency, and approved for publication.
Mention of trade names or commercial products does not constitute endorsement
or recommendation for use.
ii
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FOREWORD
The Environmental Protection Agency was created because of increasing
public and government concern about the dangers of pollution to the health and
welfare of the American people. Noxious air, foul water, and spoiled land are
tragic testimony to the deterioration of our natural environment. The complex-
ity of that environment and the interplay between its components require a
concentrated and integrated attack on the problem.
Research and development is that necessary first step in problem solution
and it involves defining the problem, measuring its impact, and searching for
solutions. The Municipal Environmental Research Laboratory develops new and
improved technology and systems for the prevention, treatment, and management
of wastewater and solid and hazardous waste pollutant discharges from municipal
and community sources, for the preservation and treatment of public drinking
water supplies, and to minimize the adverse economic, social, health, and
aesthetic effects of pollution. This publication is one of the products of
that research; a most vital communications link between the researcher and the
user community.
The condition of our drinking water has received considerable attention
recently, and we are becoming increasingly more aware that some of the pollu-
tants or low level chemicals might be potentially hazardous to our health.
The Water Supply Research Division is constantly promoting a high standard of
excellence in the quality of water consumed by the Nation. Our research
efforts will help pave the way for advances in the art of drinking water treat-
ment and will combat, minimize and eventually eliminate all hazards to health
derived from drinking improperly treated water.
One of the most effective ways to insure water quality is to maintain a
constant check on its condition. The work presented in this report illustrates
one aspect of continuous water quality examination; namely by ion selective
electrode.
Francis T. Mayo, Director
Municipal Environmental Research
Laboratory
111
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ABSTRACT
The maintenance of water quality whether at the treatment plant or out in
the distribution system is predicated on accurately knowing the condition of
the water at any particular moment. Ion selective electrodes have shown
tremendous potential in the area of continuous water quality analysis, and
were employed by the Water Supply Research Division's Mobile Water Quality
Laboratory to monitor: alkalinity, calcium, chloride, fluoride, hardness,
nitrate, and pH.
The pH and the chloride electrodes were housed in a commercial unit and
linked to the computer with a minimum number of operating problems. The other
parameters required more development and all relied on ionic strength or pH
buffers to swamp out problems of activity and ionic strength. The test
periods were usually about a week in length, and data was presented as to the
reliability and accuracy of the electrodes.
It was found that the electrodes performed quite well, and when compared
to accuracy statistics found in Standard Methods for the Examination of Water
and Wastewater, the electrode methods were in the same region.
iv
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INTRODUCTION
Recently, the news media have kept the public informed about several
important episodes in the controversy over drinking water quality and public
health. A notable instance is the finding of asbestos fibers and/or chlori-
nated organics in some water supplies. As a direct result of receiving this
type of information, the public has become more knowledgable about the
quality of drinking water and its associated problems. Many people became
aware that the water taken for granted for so many years may not be as pure
and wholesome as was once thought.
Some of the chlorinated hydrocarbons, thought to be cancer-causing
agents and, paradoxically, appearing as a result of the disinfection
processchlorination are undesirable in a drinking water system, and
research has been undertaken to eliminate the problem. Another problem
associated with water quality, but without as much public recognition, is the
deterioration of drinking water as it travels through the distribution
system. Consumers commonly think that quality water leaving the treatment
plant will arrive at its destination having the same quality. In a few cases
this assumption is correct, but for many users the water can arrive signifi-
cantly changed in quality. Examples are the dissolution under certain con-
ditions of metals such as lead, iron, zinc, and cadmium as well as the loss of
asbestos fibers from asbestos-cement pipes.
The traditional approach to water quality analysis has involved grab
sampling and transportation to the laboratory for subsequent work. This
approach has the disadvantages associated with grab sampling, namely,
representative samples, necessary preservation, and changes caused by trans-
portation conditions (temperature, sunlight, travel time, etc.). To circum-
vent many of these problems, some groups have installed analytical equipment
on trucks and moved the laboratory into the field for on-site analysis.
However, on-site analysis does not answer the question of the validity of the
results based on one sample. The ideal solution to the problem of sampling
would be to install a field laboratory with the capability for continuous
sampling as well as continuous analysis.
The Water Supply Research Division of the U.S. Environmental Protection
Agency had assembled under contract with the National Sanitation Foundation a
mobile water quality laboratory with the capability for continuous sampling
and analysis. The laboratory is capable of monitoring 19 water quality
parameters and, in most cases, provides a continuous record of the analysis of
that parameter. Some parameters are semi-continuous in that the water sample
is compared with a standard before the final result is recorded. All param-
eters are sampled and analyzed at least twice an hour. The purpose of this
report is to describe certain aspects of this mobile monitor in detail.
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Ion Selective Electrodes
One of the best ways to continuously analyze for a particular substance
is to use an electrochemical sensor, and the best electrochemical sensors are
known as electrodes. There are many types of electrodes, and one of the types
employed on the mobile laboratory was the specific ion electrode. Specific
ion electrodes are a special type of sensor that can detect one type of ion in
a mixture of many and report its concentration. These electrodes are receiv-
ing a great deal of attention in water quality analysis because of this dis-
criminating ability. 2,6,19,38-41 Traditional analysis might require expensive
spectrographic equipment or involved wet chemical analysis, while specific ion
electrodes give rapid, sensitive, economical, continuous and reasonably accu-
rate results. It was these attributes that led to their incorporation into
the analytical systems of the mobile laboratory. The systems employing spe-
cific ion electrodes are: pH, alkalinity, chloride, fluoride, divalent
hardness, calcium, and nitrate.
The operating principle for all specific ion electrodes revolves around
the creation of a classical concentration potential across a permeable mem-
brane, or simply stated, follows the Nernst Equation:
E = E° + ^^ log [A+] Eq (1)
where E equals the equilibrium potential, E° is the standard potential,
2.3 RT is the Nernst Factor (59.16 mv 25°C) , Z is the charge on the
F
diffusing ion. The standard potential developed will depend on the choice of
reference electrode as well as the internal reference solution. Equation (1)
is thus a direct relationship between the activity of an ion and the potential
it developed.
Activity is sometimes a difficult parameter to work with because a
knowledge of the activity coefficient is necessary before the analytical
concentration can be determined. Activity is related to concentration by the
following equation:
A = 6 x c Eq (2)
where A is the activity of the ion, 6 is the activity coefficient and
c = the concentration of the ion being measured. The activity coefficient is
a function of ionic strength and can be determined by a modification of the
Debye Huckel Equation:
where 6 is the activity coefficient, Zi is the charge on the i ion, and u is
the ionic strength. If the value of u is sufficiently high due to the addition
of an inert species, the contribution of ionic strength by the sample will be
negligible. This technique of establishing a constant ionic strength is called
swamping and is used in many specific ion applications. »»»»«»
Constant ionic strength establishes that the activity coefficient will also be
-------
constant. If the activity coefficient is constant, changes in activity will
be directly reflected by changes in concentration of the ion of interest, or
the equilibrium potential will be indicative of the concentration of the
specific ion.
EXPERIMENTAL
The Apparatus and reagents used on the mobile laboratory are listed by
parameter in alphabetical order. All the systems were supplied reagents by a
Technicon II Proportioning Pump and the data collected and processed by a
Texas Instruments 960A Minicomputer. The switching between baseline and
sample was handled by Valcor thru-way miniature "dri-solenoid" valve, series
SV-72, and supervised by the computer.
Alkalinity
Apparatus:
A conventional combination pH electrode, Sargent No. S-30070-10, was used
in conjunction with its flow cell to determine alkalinity. A Radiometer pHM
Research pH meter was used with a 7100 series Hewlett Packard recorder, to
monitor and record the alkalinity.
Reagents:
A buffer solution of 0.05 M Potassium Hydrogen Phthalate (KHP) and 0.019
M HC1 was prepared by mixing 91.8 g KHP and 13.4 m& con HC1 in 9 £ of dis-
tilled water. The pH was adjusted, if necessary, with con HC1 to 3.15.
A baseline solution of 0.95310 g of Na?CO^ was dissolved in one liter of
distilled water. Two hundred milliliters of this solution was diluted to 9 &
and was equivalent to 20 mg/£ alkalinity as CaCO_.
Calcium
Apparatus:
An Orion 93-20 Calcium Selective Ion Electrode was used to determine
calcium hardness. The electrode was used in a custom made flow cell with a
Sargent, S-30080, Miniature Calomel Electrode. An Orion 801A Research pH
Meter monitored the calcium and it was recorded on an Esterline Angus L11025
dual channel recorder.
Reagents:
A buffer solution was prepared in the same manner as described in Stand-
ard Methods p 181 but diluted 38 m£ to 100 m£. The resulting solution had
nearly the same pH, 10.1, but the ionic strength was reduced from 11.4 M to
4.275 M.
An additional Ammonia Chloride-Ammonia Hydroxide buffer was prepared to
have a pH of 9.0 and an ionic strength of 0.60 M.
A baseline solution was prepared by spiking tap water with about 200 mg/£
of calcium expressed as CaCO~.
Chloride
Apparatus:
A Beckman 19521 Silver-Silver Chloride billet type electrode was used in
conjunction with a Beckman 19730 Palladium Junction reference electrode to
determine chloride ion concentrations. The electrodes were housed in a plastic
flow cell located in a Schneider Robot Stream Monitor, and the data recorded
on an Esterline Angus Multipoint Recorder.
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Reagents:
Potassium chloride standards were prepared to have concentrations of 35,
88, and 177 mg/£ chloride. These solutions were made by diluting 0.01 M KC1
and were also used for standardization of the conductivity cell.
Free Fluoride
Apparatus:
An Orion No. 94-09A Fluoride Ion Electrode was used with a Sargent No. S-
30080 Miniature Calomel Electrode to determine free fluoride ion concentra-
tions. A Leeds and Northrup No. 7415 Research pH Meter was used with a Linear
Instruments No. 297 dual channel recorder to observe and record the fluoride
data.
Reagents:
A buffer solution of 8.14 m£ (0.1 M) Acetic Acid, 121.41 g (1.0 M) Sodium
Nitrate were dissolved in 1 Si of distilled water and the pH adjusted, if
necessary, with NaOH. The ionic strength of this buffer was kept about 1.1
M.
A baseline solution was prepared by diluting (l:100)_a commercial stand-
ard for fluoride (0.1 MO to a stock solution of 19 mg/£ F and subsequent
dilution of the stock solution 105.3 m& to 20 £ to obtain a baseline solution
of 0.1 mg/£ fluoride.
Total Fluoride
An Orion No. 94-09A Fluoride Ion Electrode was used in conjunction with a
Sargent No. 30080 Miniature Calomel electrode for the Total Fluoride deter-
mination. An Orion 801A Research pH Meter was used in conjunction with a
Linear Instruments No. 297 dual channel recorder for monitoring and recording
the data.
Reagents:
A buffer solution of 16.29 m£ (0.2 M) Acetic Acid, 85.0 g Sodium Nitrate
(0.7 M) and 84.0 g (0.2 M) Sodium Citrate were dissolved in 1 £ of water and
the pH adjusted, if necessary, with NaOH. The ionic strength of the buffer
was also about 1.1 M.
Baseline:
The same baseline solution used for Free Fluoride was also used for total
fluoride.
Hardness:
An Orion 93-32 Divalent Hardness Specific Ion Electrode was used in
conjunction with a Sargent 30080 Miniature Calomel reference electrode to
determine the hardness of the water samples. A Corning 101 Research pH meter
was used with the Esterline Angus L11025 recorded to monitor and record the
data.
Reagents:
A buffer solution was prepared in the same manner as described in Stand-
ard Methods p 181, but diluted. 18.75 m£ to 100 mJL The resulting solution
had a pH of about 10.1 but the ionic strength was reduced from 11 M to 2.14 M.
The baseline solution used was the same solution used for the calcium
baseline, with total hardness being about 200 mg/£ higher than the tap water.
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£H
Apparatus:
A Beckman 19500 general purpose glass electrode was used in conjunction
with a Beckman 19730 Palladium Junction reference electrode to measure hydro-
gen ion concentration. The electrodes were located in the Schneider Robot
Stream Monitor and the data was collected on a Esterline Angus Multipoint
recorder.
Reagents:
Commercial buffers of pH 7 and 10 were used for the calibration of the
electrode.
Nitrate
An Orion 93-07 Nitrate Ion Electrode was used in conjunction with a
Sargent S-30080 Miniature Calomel single junction electrode. A Corning
research pH meter 112 was used to monitor the nitrate electrode and the data
was collected on a 7100 series Hewlett Packard dual channel recorder.
Reagents:
A nitrate standard of 100 mg/& N was prepared by dissolving 721.8 mg of
anhydrous KNC_ in 1 liter of distilled water. Working standards of 0.1 to 1.0
mg/& N were prepared by dilution.
The baseline was prepared by adding about 1.0 mg/£ N to tap water.
The ionic strength adjuster was prepared by dissolving sodium fluoride in
distilled water to make a solution of 0.006 M.
RESULTS AND DISCUSSION
Alkalinity
Alkalinity is a combination of all the species in water that will accept
protons, and is usually comprised of bicarbonate, carbonate, and hydroxide.
Alkalinity can be expressed by summing all the components in the system and
expressing it as an equivalent of the protons accepted.
[2H+] = [HC03~] + [C03=] + [OH~] Eq (3)
Thomas and Lynch suggest that alkalinity is the capacity of a water to
neutralize acidity up to the equivalence point. Regardless of how it is
stated, alkalinity is an important water quality parameter, and is utilized by
treatment personnel for regulating scaling or corrosion tendencies as well as
pH, taste, and odor control.
A continuous system for monitoring alkalinity might well be patterned
after the concept of the neutralizing capacity of water. If a buffer system
of known hydrogen ion concentration is mixed with a known amount of water
sample, the resulting change in hydrogen ion concentration will be a measure
of the extent to which protons were accepted or simply the alkalinity of the
water. The method used involved the mixing of a potassium hydrogen
phthalate/hydrochloric acid buffer with tap water and the change in hydrogen
ion concentration was determined as alkalinity.
Figure I illustrates the response of the phthalate buffer system to
increases in alkalinity. Over the range of 20 mg/£ to 100 mg/£ alkalinity as
CaCO~, the relationship between pH and alkalinity was linear with a correla-
tion coefficient of 0.997- This indicates that the system will behave in an
accurate and precise manner for alkalinity values up to 100 mg/£.
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3.7
3.6
3.5
3.4
20
40
60
ALKALINITY mg/l AS CACo3
Figure 1. Relationship between pH and alkalinity.
80
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After it was established that a linear relationship existed between
changes in buffering capacity and alkalinity, the electrode was calibrated.
Figure II shows the output from the alkalinity system as a function of alka-
linity. The results of a least squares regression analysis shows a correla-
tion coefficient of 0.998 and indicates a linear area of response between 20
mg/£ and 80 mg/& of alkalinity.
The true test of any ion selective electrode employed in a continuous
monitoring situation revolves around its long term reliability and accuracy.
Therefore, after the electrode was calibrated it was placed on line analyzing
a reservoir of the same tap water for an extended period of time. The elec-
trode was operated for over 100 hours and analyzed about 270 samples. The
results of this test are summarized in Figure III, with statistical informa-
tion summarized in Table 1.
TABLE 1. STATISTICAL RESULTS OF THE ALKALINITY ELECTRODE
Mean 56.1 mg/&
Standard Deviation 1.91
Relative Standard Deviation 3.4 %
Relative Error 1.6 %
Average Titration 55.2 mg/£
Hardness and Calcium
Total hardness and calcium are parameters usually associated with the
aesthetic considerations of drinking water, namely customer acceptance.
However, they are also an important consideration to the water utility because
their treatment could make large changes in their concentration and affect the
corrosion or sealing tendencies, alkalinity, pH, and conductivity. Total
hardness and calcium are., also being considered as a possible agent in cardio
vascular diseases. ' ' Therefore, because of traditional and contemporary
reasons, total hardness and calcium were incorporated as two of the parameters
on the Mobile Laboratory.
Historically, hardness was determined by titration with a standard soap
solution, but the subjectiveness of the endpoint cast some doubts on its
accuracy. The development of EDTA titrations and indicators specific for
hardness and calcium made contemporary analysis easy and accurate. However,
titrations do not yield a continuous record of data, and since it was thought
to be important to have continuous analysis it was decided to employ an ion
selective electrode for divalent hardness and,calcium. Ion selective elec-
trodes have been use^ ^ hardness ' ' ' ' ' and, calcium determina-
tions ' ' ' * ' , but little has been done in applying these electrodes
to continuous analysis. '
The initial work on adapting hardness and calcium ion selective elec-
trodes to continuous analysis involved standardizing the electrodes in dis-
tilled water standards, and directly inserting them into the sample stream.
-------
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ALKALINITY AS mg/l CACo3
Figure 2. Alkalinity calibration curve.
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TIME IN HOURS
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Figure 3. Performance of alkalinity electrode as a function of time.
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The electrodes gave theoretical responses to the standards, but consistently
gave low results on tap water. This agreed with published results,
and it was generally accepted that some degree of ion pair association was
responsible for the lower readings. This problem can be overcome by using the
technique of standard additions, and it was decided to adopt our system for
hardness and calcium to a form of standard additions.
The Laboratory was to have national applicability, so a general system of
standardization and analysis was needed. It was decided that to minimize any
interferences inherent to the local water supply, the local tap water would be
the baseline to which standard additions were made. It was also thought that
ionic strength should be controlled more carefully. The selection of the
appropriate ionic strength adjuster for hardness and calcium was a problem.
Several investigators ' ' have advocated the use of an inert salt with
potassium chloride being one of the more promising. Therefore, a solution of
potassium chloride was prepared to have a concentration of 7.0 x 10 moles/£
and was mixed with the standards. The data in Table 2 show the changes in
ionic strength after the addition of potassium chloride, and any changes in
potential due to ionic strength fluctuations should be drastically reduced.
TABLE 2. CHANGE IN IONIC STRENGTH OF STANDARDS MIXED WITH KC1
Calcium Ionic Strength Ionic Strength of
Concentration of Standards Standards Mixed with KC1
mg/£ moles/£ moles/£
96
144
192
241
288
1.
1.
1.
1.
1.
50 x 10 3
63 x "
74 x "
86 x "
98 x "
1.32 x 10 2
1.33 x "
1.34 x "
1.35 x "
1.36 x "
Figure IV shows the results of the calibration of the calcium electrode
utilizing the potassium chloride ionic strength adjuster. The baseline to
which the solutions were compared was tap water with a calcium hardness of 96
mg/Jl as calcium carbonate. It can be readily seen that there was a great
amount of scatter and a low value of 0.863 for the correlation coefficient of
the least squares regression analysis.
Ruzicka and Tjell suggest an ammonia chloride-ammonium hydroxide buffer
for low levels of calcium in serum, and Standard Methods uses the same kind
of buffer for hardness titrations. The ammonium chloride-ammonium hydroxide
buffer was prepared as stated in Standard Methods and substituted for the
potassium chloride. A hardness electrode was used to evaluate this buffer by
checking the response against ionic strength for a hardness standard of 233
mg/£. The results of the study are summarized in Table 3.
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200
300
500
CALCIUM AS CACo3 mg/l
Figure 4. Calibration curve for the calcium electrode using
7.0 x 10*4M KCI as the ionic strength adjuster.
11
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TABLE 3. HARDNESS ELECTRODE RESPONSE TO IONIC STRENGTH OF BUFFER
Ionic Strength Electrode Response
moles/A mv/mg/£
11.4
10.7
9.5
5.7
2.9
2.1
1.4
0.0015
0.0024
0.0078
0.0084
0.0122
0.0159
0.0214
As the ionic strength was diluted, it was observed that the hardness
electrode response increased. It was not shown, but as sensitivity increased,
so did the background noise. Based on considerations of stability and signal
response, an ionic strength of 2.1 moles/£ was selected as the best buffer for
the hardness electrode. The same type of experiment was conducted for the
calcium electrode and it was found that the optimum concentration was 2.9
moles/£ of ionic strength.
As was the case with the alkalinity electrode, the hardness and calcium
electrodes needed to be evaluated for accuracy and reliability. The electrodes
were calibrated using a standard addition of calcium chloride, but expressed
as calcium carbonate. Figure V shows a calcium calibration for a series of
standards 172, 224, 280, and 324 mg/£ as calcium carbonate compared to a
baseline of 334 mg/£. A least squares regression analysis gave a correlation
coefficient of 0.995 indicating a linear relationship over the range calibra-
ted. Figure VI shows a hardness calibration for a series of standards 216,
270, 309 and 358 mg/£ as calcium carbonate compared to a baseline of 357 mg/£.
A least squares regression analysis gave a correlation coefficient of 0.993
indicating a linear relationship over the range calibrated.
It should be noted here that the concentration of the tap water does not
fall directly within the limits of calibration but must be extrapolated about
50 mg/£. It has been shown that the electrodes were linear in their response
over a range of about 200 mg/£, and others ' have shown them linear over
several decades of concentration, therefore, it was reasonable to assume that
a small extrapolation of the standard curves would be valid for determining
total hardness and calcium.
After the hardness and calcium electrodes were calibrated, they sampled a
large reservoir of tap water continuously for about 100 hours. During this
time hardness and calcium tap water samples were repeatedly sampled about 270
times. During this sampling period the electrodes were periodically restand-
ardized, and thus the data presented in Tables I- and 5 are grouped according
to restandardization.
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100 200 300
CALCIUM AS CACo3 mg/l
Figure 5. Calibration curve for calcium electrode in ammomium chloride - ammonium
hydroxide buffer of 2.9 moles/l ionic strength.
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200 250 300
TOTAL DIVALENT HARDNESS AS CACo3 mg/l
Figure 6. Total hardness calibration curve.
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TABLE 4. STATISTICAL RESULTS OF HARDNESS ELECTRODE
Calibration
Number
1
2
3
4
Mean
143 rug/£
152
167
173
Standard
Deviation
6.82
3.70
3.24
2.29
Rel. Std.
Deviation
4.77 %
2.42 "
1.94 "
1.32 "
Relative
Error
10.63 %
6.17 "
1.2. "
Titration
160 mg/SL
162
165
TABLE 5. STATISTICAL RESULTS OF CALCIUM ELECTRODE
Calibration
Number
1
2
3
4
Mean
117 mg/£
118 "
119 "
119 "
Standard
Deviation
13.00
7.83
4.09
6.86
Rel. Std.
Deviation
11.38 %
6.67 "
3.45 "
5.75 "
Relative
Error
1.03 %
0.26 "
0.42 "
Titration
116 mg/£
117 "
119 "
__
Figure VII shows a graphical summation of the long term study for total
hardness and Figure VIII shows the same thing for calcium.
The ammonium chloride-ammonium hydroxide buffer helped to overcome prob-
lems of ionic strength by allowing the electrodes to respond to calcium and
hardness activities while maintaining constant ionic strength. By swamping
out some of the problems, the electrodes could be run at extended periods of
time with reasonable precision and accuracy. However, there was one problem
area of gradual calcium carbonate buildup that detracted from the method.
Carbonate equilibrium diagrams show that bicarbonate was converted to
carbonate at a pH value of 10.4. The ammonium buffer's pH was 10.1 and it was
thought that this would be sufficiently low to prevent bicarbonate conversion.
However, a gradual buildup of calcium carbonate occurred along the inside of
the system and became a problem especially around constrictions. After a two
day induction period, both systems had to be flushed with 0.1 N HCl for about
a half hour. This made the removal of the electrodes, shut down of the
system, and recalibration necessary before it could be brought back on line.
It was clear that even though the buffer worked satisfactorily a change
in pH was necessary. Utilizing the same chemicals, the pH was lowered to 9.0
and the ionic strength was reduced to 0.60 M. The same procedure of calibra-
15
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12Mid
12N
TIME IN HOURS
12Mid
12N
12M
0
10
20
30
40
50
60
70
Figure 8. Performance of calcium electrode as a function of time.
-------
tion and long term study was applied to the new buffer. However, there was
one important difference in this study because it used online tap water rather
than a large reservoir of the same sample. The results of the study for both
calcium and hardness are shown in* Figure IX. Since only one restandardization
was done during the test the data is broken into two groups that correspond to
each calibration. Table 6 summarizes the statistical results for the pH 9
buffer.
TABLE 6. STATISTICAL RESULTS FOR THE HARDNESS AND CALCIUM ELECTRODE
UTILIZING THE 0.6 M AMMONIUM CHLORIDE-AMMONIUM HYDROXIDE BUFFER
Calibration Electrode Mean Standard Rel. Std. Relative Average
Number mg/£ Deviation Dev. Error Titrated Value
mg/£
1
2
1
2
Hardness 175
165
Calcium 136
134
7.99
2.90
3.80
3.28
4.37 %
1.76 "
2.80 %
2.46 "
1.16 %
2.42 "
6.25 %
4.69 "
173
170
128
128
Studies on the calcium and total hardness electrode indicated that those
parameters could be continuously monitored for extended periods of time util-
izing an ionic strength buffer of ammonium chloride and ammonium hydroxide. Of
the two buffer systems studied, the pH 9 system appeared to be superior.
Nitrate
It has been demonstrated that nitrate concentrations in excess of 45 mg/&
were directly related to a temporary blood disorder in infants called methemo-
globinemia. Nitrates are, therefore, routinely monitored as a health pre-
caution. Historically, except for some well water sources, nitrate concen-
trations have usually been considerably under the toxic level and there wasn't
too much concern. However, changes in farming techniques and irrigation have
resulted in substantially higher nitrate concentrations in the farm runoffs
and eventually higher nitrate concentrations in the source of the drinking
water. Although still under the 45 mg/& level, the elevated nitrate levels
promote algae growth in some reservoirs with the resulting customer complaints.
The result of higher nitrate concentrations was that the water utilities were
interested in monitoring nitrates for their own considerations as well as the
public's welfare.
Wet chemical techniques for nitrate are usually quite accurate, but very
time consuming. The ion selective electrode for nitrate was capable of "
measuring concentrations of 45 mg/£ and below, and was selected for use on the
Laboratory because of its versatility.
18
-------
O>
E 150
o
U
E
150
CALCIUM
.*.
.
' * .
KEY:
+= TITRATED SAMPLES
.= CALCIUM ELECTRODE
= HARDNESS ELECTRODE
HARDNESS
.*
* "»
4PM 12Mid
12N
12Mid 12N
TIME IN HOURS
12Mid
12N
12Mid
20
40
60
80
Figure 9. Performance of calcium and hardness electrodes in 0.6 moles/l ammonium
chloride - ammonium hydroxide buffer as a function of time.
-------
As with the other electrodes, a long term reliability test was conducted
using repeated samples from a large reservoir of tap water. The results of
the test are shown graphically in Figure X and the statistics summarized
according to each calibration in Table 7.
TABLE 7. STATISTICAL RESULTS OF THE NITRATE ELECTRODE
Calibration
Number
1
2
Mean
rag/A N03
3.78
3.62
Standard
Deviation
0.07
0.07
Rel. Std.
Deviation
2.0 %
1.9 %
Relative
Error
6.5 %
9.5 %
Brucine
mg/fc NC3
3.55
4.00
Results of the nitrate electrode study suggest that the method was
precise, but possibly lacking in accuracy. It should be pointed out that
three mg/£ NC_ or about 10 molar was near the lower limit of detection of
52
the electrode. Measurements at these concentrations are subject to more
bias than measurements that fall in the middle of the linear response.
Nitrate concentrations are not a problem until they begin to approach 40 mg/£
and it was thought that the ion selective electrode would perform quite well.
Free Fluoride
Fluoride additions to municipal drinking water supplies are quite common
today because it has been shown that fluorides significantly reduce dental
caries especially in younger children. ' The accepted fluoride dosage for
the beneficial effect is around 1.0 mg/£, and thus any sensor utilized on the
Mobile Laboratory would have to measure accurately at this level.
Crosby, et al. evaluated five spectrophotometric, one titration, and
the specific ion electrode methods for determining fluorides and found the
electrode method surpassed all the other methods withregard to speed, accu-
racy and convenience. Numerous other investigators ~ have utilized the
fluoride ion selective electrode to routinely analyze fluorides in the 1 mg/£
range found in municipal drinking water. Their results have shown fluoride
specific ion electrodes to be accurate, and exhibit linerarily in the desired
concentration areas. Armed with these facts it was determined to utilize the
ion selective electrode as the sensing element for the fluoride determination.
The fluoride electrode was more stable than the calcium and hardness
electrodes because the liquid junction had been replaced by a solid crystal
and problems of pump pulsations didn't figure significantly in the signal.
However, the electrode was still subject to drift, as were others, indicating
the need for frequent standardization. A system similar to the alkalinity,
hardness, and calcium was implemented because it incorporated buffer addition
as well as restandardization and offered greater flexibility. There is
evidence the fluorides can be determined without ionic strength buffers, '
20
-------
5.-
CO
O
o>
E
KEY:
= NITRATE ELECTRODE
*= BRUCINE METHOD
I I I I I I I I I I I I I I I I I I M I I I I I I I I I I I I I I I I 1 I I I
12N
12Mid
12N TIME DAYS
i2Mid
12N
10
20
30
TIME IN HOURS
40
50
Figure 10. Performance of nitrate electrode in 0.006 molar sodium fluoride
as a function of time.
-------
but due to the constraint of traveling throughout the country it was thought
to make it as general as possible and include ionic strength adjustment.
A standard of 0.1 mg/£ fluoride was introduced as the baseline in every
sampling sequence and the tap water was compared to this standard. No problems
were found in switching from standards prepared in distilled water to the
samples of tap water as the ionic strength buffer masked any interferences.
The buffer solution was prepared to have an ionic strength of 1.1 M and the pH
adjusted to about 5.2 with sodium hydroxide.
Figure XI illustrates a calibration curve for free fluorides with the
results being expressed as a difference between the calibration standard and
the baseline standard. Figure XII shows the long term accuracy and reli-
ability of the electrode. During a two day sampling period consisting of 135
samples of the same water the system performed in the following manner:
Mean 0.134 mg/£
Standard Deviation 0.002
Relative STD Deviation 1.65 %
This type of test shows that the fluoride electrode can be employed in a
continuous type of analyses, and yields accurate, dependable results.
Total Fluoride
It is common knowledge the fluoride ion will complex with some metals
that are commonly observed in municipal drinking water, namely iron and
aluminum. Therefore, it follows that there has been discussion as to the
effective concentration of fluoride. To circumvent this problem, a second
fluoride system was installed, and,named total fluoride. The total fluoride
system employs a citrate compound ' ' that acts as a decomplexing agent
for fluoride, and the net result is that the electrode sees all the fluoride
in the sample.
The difference between free and total fluoride ion concentrations can be
very important to treatment plant operators. When concentrations of free and
total are similar, operators know their recovery techniques are working satis-
factorily. Conversely, if total fluoride begins to increase the operators are
alerted to a problem within the treatment system and steps can be taken to
bring the system back in balance.
Figure XIII is a typical calibration curve for total fluoride with the
results again being expressed as the difference between the baseline standard
and the calibration standard. Figure XIV illustrates the results of a reli-
ability test run consisting of 180 samples of the same tap water over a 72
hour period. The results of the test are summarized as:
Mean 0.150 mg/£
Standard Deviation 0.002
Relative Std. Deviation 1.16 %
The data indicate the fluorides, whether free or combined can be suc-
cessfully analyzed using an ion selective electrode.
22
-------
3.8
3.4
LLJ
00
I
LU
3.0
2.6
2.2
1.8
1.4
1.0
I
0.15 0.25
I
0.50
1.00
FREE FLUORIDE mg/l
Figure 11, Free fluoride calibration curve.
23
-------
l-o
-P-
O)
0.17
0.16
0.15
LU
Q
CC
O 0.14
0.12
0.11
0
«
;*» .'.
I
111111 iii i 11111111111 ii 11111111111 n 11111111111|11ii 111
12Mid 12N . 12Mid 12N
10 20 30 40
TIME IN HOURS
Figure 12. Performance of free fluoride electrode as a function of time.
50
-------
UJ
z
-J
LJ
CD
LLJ
_l
0.
S
CO
E
-0.6
-0.8
-1.0
-1.2
1.4
-1.6
-1.8
-2.0
I
1
0.15 0.25 0.50
TOTAL FLUORIDE mg/l
Figure 13. Total fluoride calibration curve.
25
0.90
-------
0.17r-
0.16
0.15
0.14
O)
E
LU
Q
OC
§ 0.13
0.12
0.11
*.*. ... ;.. .*..... . ./.* . ..
».« « .*.«. »...,... » . .
I I I I I I I I I I I \ I I I I I I I I I I I | I I I I I I I I I I I \ I I I I I I I I I I I | I I I I II I I | I I | I | M I I I I I I I | I I I I
12Mid
12N
12Mid
I
12N
12Mid
I
12N
10
20
30
40 50
TIME IN HOURS
60
70
Figure 14. Performance of total fluoride electrode as a function of time.
-------
Chloride
Water quality analysis usually have a cere of parameters that are always
determined and chloride is one of those parameters. Chloride ions are an
indication of the general condition of the water as well as being specifically
related to other parameters. In sufficiently high concentrations, chlorides
can impart an objectionable taste in the water. Being one of the principle
anions in drinking water, chlorides are directly related to conductivity and
ionic strength, and can poison some plant life. The chloride ion has been
linked with the corrosion mechanism of iron and steel, and thusly are of
great interest to utilities preventive maintenance programs.
In the past chlorides have been determined by titration with silver
nitrate, but several alternate methods are available today. The samples and
standards for the chloride determinations were done by the Mercuric Nitrate
method. A specific ion electrode for chloride was employed because of its
ability to analyze continuously.
The chloride electrode had no liquid junction or flexible membrane to
cause problems and was installed in the Schneider Robot Stream Monitor. The
electronics on the Monitor were such that zero chloride corresponded to zero
millivolts output, and 240 mg/£ chloride corresponded to full scale or 1000
millivolts output. The electrode was calibrated accordingly. Figure XV was
calibrated using standards of 25, 50 and 100 mg/£ chloride standards and was
shown to be linear with a correlation coefficient of 0.999 and a slope of 4.14
mg/mg/£. Figure XVI illustrates the electrode's response over an extended
period of time. Again, after the initial calibration, the electrode response
fell about 5 mg/£. The system was recalibrated and performed satisfactorily
for about 50 hours. Table 8 shows the statistical results for the chloride
electrode's operation.
TABLE 8. STATISTICAL RESULTS FOR THE CHLORIDE ELECTRODE
Calibration Mean
Number
1 31.2 mg/£
2 35.3 "
Standard
Deviation
1.69
0.74
Relative Std.
Deviation
5.42 %
2.10 "
Standard
Error
7.11 %
3.64 "
Average
Titration
33.6 mg/£
34.0
The data indicate that the chloride electrode can be used to routinely
monitor chloride ion concentrations.
pH
The oldest and.most reliable electrode in the specific ion family of
electrodes is the one that measures hydrogen ion activity. These electrodes
have been developed for the last thirty years since the early investigators
noticed the sensitivity and selectivity some glasses had for hydrogen ions.
This observation had many ramifications and in fact opened a new area in
potentiometry for study.
27
-------
00
O
~
o>
E
oc
I-
z
UJ
O
z
oc
O
X
u
40 _
35
30
/V
«
*«L
^"-*.
I
25
-H-
I I I I I I 1 I I I I
12N
'I
12Mid
1 '
12N
12Mid
12N
I I I I
12Mid
10
I
20
I I I
30 40 50
TIME IN HOURS
60
I
70
Figure 16. Performance of chloride electrode as a function of time.
-------
500,.
4CO .
C/)
O
300 .
NJ
200 .
100 -
10
CHLORIDE mg/l
Figure 15. Calibration curve for chloride electrode.
-------
The pH measurement associated with water quality cannot be over estimated
as to its importance. The control of pH is the most important variable in
treatment techniques such as coagulation and flocculation. Failure to operate
within specified limits results in lower quality water and wasted chemicals.
Hydrogen ion concentration is important to chlorination operations in that it
influences the equilibrium of the hypochlorous acid and has an effect on the
germicidal action of the chlorine. The pH plays an important role in cor-
rosion and corrosion control by being one of the main indices of the stability
of the water. All in all, pH is one of the most basic data parameters util-
ized on the mobile laboratory.
The pH electrode was installed in the Schneider Robot Stream Monitor in
the same manner as the chloride, and with the same kind of electronics. Full
scale output was 1000 millivolts for a pH of 12 or it could be calibrated
using 83.33 mv/pH unit. Buffers of pH 7 and 10 were used to calibrate the
system and a standard curve is illustrated in Figure XVII.
After the electrode was calibrated it was placed on tap water for an
extended period of time. The results of this test are shown in Figure XVIII.
The statistical results for the pH electrode are summarized in Table 9.
TABLE 9. STATISTICAL RESULTS FOR pH ELECTRODE
Calibration
Number
Mean Standard
Deviation
Rel. Standard
Deviation
Relative
Error
Mean
Lab Electrode
7.983 0.062 0.783 % 0.95 % 7.908
The data indicate that the pH electrode can be successfully employed to
measure hydrogen ion concentrations of water for extended periods of time.
The evaluation of all the ion selective electrodes except fluoride and pH
was done by comparison to a standard method. This presents a problem because
some of the standard methods have a degree of uncertainty associated with
them. A second problem facing electrode users was that of ion activity being
different from concentration. It was impossible to calibrate the electrodes
unless it was assumed that ionic activity equaled the analytical concentra-
tion. Above are two major concerns of ion selective electrode users, but if
the recorded data is compared with results of Standard Method's evaluation the
results are encouraging. Table 10 compares the relative standard deviations
and relative errors of the electrode methods with the appropriate standard
method. Examination of the results in Table 10 one finds the electrode
results close to the standard methods in terms of accuracy and precision.
CONCLUSION
It has been suggested by Dr. James W. Ross Jr. that ion selective elec-
trodes are not highly accurate. To help overcome some of these problems,
buffer additions and standards were added to the sampling cycle. It was shown
that selection of the proper ionic strength adjuster made a significant
30
-------
900..
800
o
>
700
600
I
8
PH
Figure 17. Calibration of pH electrode.
T
9
T
10
-------
to
tsj
8.2_
8.0
7.8
Q. 7.6
7.4
7.2
7.0
.V * * **
. v.V.-- A v.-s .
. . « ..» rt- /-.. «v -*'
* ' -^:>:.-'>-
12N
KEY:
= VAN ELECTRODE
*= LABORATORY ELECTRODE
12Mid
12N
12Mid
12N
12Mid
10
20
30
40
50
60
12N
70
Figure 18. Performance of pH electrode as a function of time.
-------
TABLE 10. COMPARISON OF RELATIVE STANDARD DEVIATIONS AND
RELATIVE ERRORS OF THE STANDARD AND ELECTRODE METHOD
Substance
Determined
Alkalinity
Hardness
pH 10.1
pH 9.0
Calcium
Nitrate
Fluoride
Chloride
pH
Electrode
Rel. Std. Dev.
3.4 %
9.7 "
2.4 "
1.9 "
1.3 "
4.4 "
1.8 "
11.4 "
6.7 "
3.5 "
5.8 "
2.8 "
2.5 "
2.0 "
1.9 "
1.7 "
2.1 "
0.78 "
Method
Rel. Error
1.6 %
10.6 "
6.2 "
1.2 "
1.2 "
2.4 "
1.0 "
0.3 "
0.4 "
6.3 "
4.7 "
6.5 "
9.5 "
3.6 "
1.0 "
Standard Method
Rel. Std. Dev. Rel. Error
2.9 % 0.8 %
9.2 " 1.9 "
5.5 " 6.0 "
15.4 " 4.5 "
3.6 " 0.7 "
3.3 " 2.9 "
difference in the operation of the hardness and calcium electrodes, and
allowed continuous operation.
It was also shown that some of the more common ion selective electrodes
such as hydrogen ion, fluoride ion, nitrate ion and chloride ion could be
operated for prolonged periods of time in an accurate and precise manner.
33
-------
REFERENCES
1. Rook, J. J., "Formation of Haloforms During Chlorination of Natural
Waters." Water Treatment and Examination, 23, 234-43, (1974).
2. Frant, M., "Detecting Pollutants with Chemical-Sensing Electrodes."
Environ. Sci. Technol. 8(3), 224-8, (1974).
3. Light, T.S. "Ion Selective Electrodes." R.A. Durst Ed., National
Bureau of Standards (U.S.) Special Publication 314, 1969.
4. Fleet, B., H. Von Strop, "Analytical Evaluation of a Cyanide-Ion
Selective Membrane Electrode Under Flow Stream Conditions." Anal.
Chem. 43, 1571 (1971)-
5. Lee, T. G. "A System for Continuously Monitoring Hydrogen Chloride
Concentrations in Gaseous Mixtures Using a Chloride Ion-Selective
Electrode." Anal. Chem. 41, 391 (1969).
6. Oliver, R.T., Mannion, R.F., "Ion-Selective Electrodes in Process
Control: Water Hardness Measurement of Ion Exchange Treated Water."
ISA Anal. Instr. (preprint) 1970, 8, VII-3.
7. Ruzicka, J. , Tjell, J.C., "Ion-Selective Electrodes in Continuous-Flow
Analysis Determination of Calcium in Serum." Anal. Chim Acta 47, 475-82
(1969).
8. Zipper, J. J., Fleet, B., Perone, S. P., "Computer-Controlled Monitoring
and Data Reduction for Multiple Ion-Selective Electrodes in a Flowing
System." Anal. Chem. 46_, 2111 (1974).
9. Fleet, B., Ho, A.Y.W., "An Ion-Selective Electrode System for Contin-
uously Monitoring Cyanide Ion Based on a Computerized Gran Plot
Technique" Talanta 20_ 793 (1973).
10. Sekerka, I., Lechner, J., "Simultaneous Determination of Sodium,
Potassium and Ammonium Ions by Automated Direct Potentiometry." Anal.
Letters 7(7) 463 (1974).
11. Van Den Winkel, P., Mertens, J. and Massart, D.L., "Streaming Potentials
in Automatic Potentiometric Systems" Anal. Chem. 4^ (12) 1765 (1974).
12. Mertens, J., Van den Winkel, P., and Massart, D.L., "Use of an Automated
Selective Electrode for the Automatic Determination of Ammonia in Boiler
Feed Waters." Anal. Letters 6^ 81 (1973).
13. "Standard Methods for the Examination of Water and Wastewater."
Thirteenth edition, American Public Health Association Inc., N.Y., 1971,
p 181.
14. Webber, W., Strumn, W., "Mechanism of Hydrogen Ion Buffering in
Natural Waters." JAWWA Vol. 55 (12) 1553-78 (1963).
15. Thomas, J.F.J., and Lynch, J.J,, "Determination of Carbonate Alkalinity
in Natural Waters." JAWWA Vol 52, 259-67 (1960).
16. Crawford, M.D., "Hardness of Drinking Water and Cardiovascular Disease."
Proc. Nutro. Soc. 31:347 (1972).
17. Masironi, R., et al., "Geochemical Environments, Trace Elements and
Cardiovascular Diseases." Bull. WHO 47:139 (1972).
34
-------
18. Crawford, M.D., Gardner, M.J., and Morris, J.N., "Cardiovascular
Diseases and the Mineral Content of Drinking Water." Brit. Med. Bull 27:
1:21 (1971).
19. Sekerka, I., Lechner, J., "Automated Simultaneous Determination of Water
Hardness, Specific Conductance, and pH." Anal. Letters 7(6) 399-408
(1974).
20. Zipkin, I., McClure, F.J., "Fluoride Drinking Waters." U.S. D.H.E.W.
Public Health Service Publication 825:483, (1962).
21. Cooley, W.E., "Applied Research in the Development of Anticaries Denti-
frices." J. Chem. Ed. 47:177(1970).
22. Crosby, N.T., Dennis, A.L., and Stevens, J.G., "An Evaluation of Some
Methods for the Determination of Fluoride in Potable Waters and Other
Aqueous Solutions." Analyst Vol. 93(10), 643-52, (1968).
23. Rechnitz, G.A., "Ion Selective Electrodes." Chemistry and Engineering
News, 45(25), 146, (1967).
24. Frant, M.S., Ross, J.W. "Use of Total Ionic Strength Adjustment Buffer
for Electrode Determination of Fluoride in Water Supplies" Anal. Chem.
40(7), 1169(1968).
25. Sekerka, I., Lechner, J.F., "Automated Determination of Fluoride Ion in
the Parts per Millard Range" Talanta Vol. 29, 1167-72, (1973).
26. Babcock, R.H., and Johnson, K.A., "Selective Ion Electrode System for
Fluoride Analysis." JAWWA, 60, 953, (1968).
27. Harwood, J.E., "The Use of an Ion Selective Electrode for Routine
Fluoride Analysis on Water Samples." Water Research, 3, 273, (1969).
28. Collis, D.E., Diggens, A.A., "The Use of a Fluoride Responsive Electrode
for 'On Line' Analysis of Fluoridated Water Supplies." Water Treatment
and Examination 18_ 192, (1969).
29. Warner, T.B., "Electrode Determination of Fluoride in Ill-Characterized
Natural Waters." Water Research 5_, 459, (1971).
30. Babcock, R.H., "Ion-Selective Electrodes for Quality Measurement and
Control" JAWWA 67_ (1) 26 (1975).
31. Liberti, A., and Mascini, M., "Anion Determination with Ion Selective
Electrodes Using Gran's Plots - Application to Fluoride." Anal. Chem.
41 (4) 676 (1969).
32. Brand, M.J.D., and Rechnitz, G.A., "Computer Approach to Ion-Selective
Electrode Potentiometry by Standard Addition Methods." Anal. Chem.
42 (11) 1172 (1970).
33. Moody, G.J., Oke, P.B., Thomas, John, D.R., "Calcium Sensitive Electrode
Based on a Liquid Ion Exchanger in a Poly(vinyl chloride) Matrix."
Analyst 95 (1136) 910-18 (1970).
34. Ammann, D., Pretsch, E., Simon, W., "Calcium Ion Selective Electrode
Based on a Neutral Carrier." Anal. Letters 5(11), 843-50 (1972).
35. Thompson, M.E., Ross, J.W. "Calcium in Sea Water by Electrode Measure-
ment" Science 154, 1643-44 (1966).
36. Fleet, B., Ryan, T.H., "Investigation of the Factors Affecting the
Response Time of a Calcium Selective Liquid Membrane Electrode." Anal.
Chem. 46(1), 12-15 (1974).
37- Rechnitz, G.A., Hseu, T.M. "Analytical and Biochemical Measurements with
a New, Solid-Membrane Calcium-Selective Electrode." Anal. Chem. 41(1),
111, (1969).
38. McClelland, Nina, Mancy, K.H., "Water Monitoring in Distribution Systems:
A Progress Report" JAWWA 64(12), 795, (1972).
35
-------
39. Sekerka, I., and Lechiver, J. "Automatic Direct Potentiometry: American
Laboratory 8(2) , 45, (1976).
40. Brand, M.I.D., Rechnitz, G., "Computer Approach to Ion Selective Elec-
trode Potentiometry by Standard Addition Methods." Anal. Chem. 42(11),
1172, (1970).
41. Andelman, J.B., "Ion-Selective Electrodes-Theory and Applications in
Water Analysis." J. Water Pollution Control Fed. 40(11), 1844 (1968).
42. Handbook of Analytical Chemistry, Meites, L. editor, McGraw-Hill, first
Edition 1963 Chapter 11.6.
43. Riseman, J. , Measurement of Inorganic Water Pollutants by Specific Ion
Electrode.: American Laboratory 1(7) 32, (1969).
44. Arnaldo, L., Mascini, M., "Anion Determination with Ion Selective Elec-
trodes Using Gran's Plots" Anal. Chem. 41(4), 666, (1969).
45. Orion Research Inc., Instruction Manual of Divalent Cation Electrode,
Model 93-32, (1975).
46. Orion Research Inc., Instruction Manual of Calcium Ion Electrode, Model
93-20 (1975).
47. Burr, R.G., "A Source of Error With the Calcium-Specific Ion Electrode
Solvent Effects in Aqueous Solution" Clinica Chim Acta. 43, 311, (1973).
48. Ross, J.W., "Calcium-Selective Electrode with Liquid Ion Exchanger"
Science 15:1378 (1967).
49. Sekerka, I., Lechner, J.F., "Simultaneous Determination of Total Non-
Carbonate and Carbonate Water Hardness by Direct Potentiometry"
Talanta Vol. 22, 459-63 (1975).
50. Rands, D.G., Stocia, F., "Notes and Comments: The Divalent Selective
Ion Electrode and Water Hardness: JAWWA 68 (6), 309 (1976).
51. Comly, H.H., "Cyanosis in Infants in Well Water: J. Am. Med. Assn. 129:
112-116 (1945).
52. Orion Research Inc., Instruction Manual of Nitrate Ion Electrode. Model
93-07 (1975).
53. Uhlig, H.H., Corrosion and Corrosion Control, 2nd Edition, Wiley & sons,
NYC. p 113 (1971).
54. Durst, R.A. ed. "Ion-Selective Electrodes Symposium Spec. Publ. 314
(National Bureau of Standards, Washington, D.C., 1969).
36
-------
TECHNICAL REPORT DATA
(Please read Instructions on 'he reverse bi-lore i
1 REPORT NO.
EPA-600/2-78-106
2.
4. Tl i LE AND SUBTITLE
ION SELECTIVE ELECTRODES IN WATER QUALITY ANALYSIS
3. RECIPIENT'S ACCESSI ON- NO.
5. REPORT DATE
May 1978 (Issuing Date)
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Robert C. Thurnau
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Water Supply Research Division
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
10. PROGRAM ELEMENT NO.
1CC614, SOS 1, Task 14
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
Municipal Environmental Research LaboratoryCin.,OH
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA/600/14
15. SUPPLEMENTARY NOTES
Project Officer: Robert C. Thurnau Phone: 513/684-4363
16. ABSTRACT The maintenance of water quality whether at the treatment plant or out in
the distribution system is predicated on accurately knowing the condition of the water
at any particular moment. Ion selective electrodes have shown tremendous potential in
the area of continuous water quality analysis, and were employed by the Water Supply
Research Division's Mobile Water Quality Laboratory to monitor: alkalinity, calcium,
chloride, fluoride, hardness, nitrate, and pH.
The pH and the chloride electrodes were housed in a commercial unit and linked to
the computer with a minimum number of operating problems. The other parameters
required more development and all relied on ionic strength or pH buffers to swamp out
problems of activity and ionic strength. The test periods were usually about a week in
length, and data was presented as to the reliability and accuracy of the electrodes.
It was found that the electrodes performed quite well, and when compared to
accuracy statistics found in Standard Methods for the Examination of Water and Waste-
water, the electrode methods were in the same region.
17.
KEY WORDS AND DOCUMENT ANALYSIS
];i. DESCRIPTORS
!
*Electrodes
Water Quality
Chemical Analysis
Water Distribution
13. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
b. IDENTIFIERS/OPEN ENDED TERMS
Water Quality Analysis
Ion Selective Electrodes
Continuous Analysis
Computer Instrument
Interfacing
19. SECURITY CLASS (This Report)
UNCLASSIFIED
20. SECURITY CLASS (This page)
UNCLASSIFIED
c. COSATI Field/Group
13B
21. NO. OF PAGES
41
22. PRICE
EPA Form 2220-1 (9-73) 37
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