pH / ION METER
Application note: A04-001A
A Comparison of Ion-Selective Electrode Analysis Methods
Introduction
The measurement of ion concentrations in solution is
a requirement in many industries. The use of an ionselective electrode (ISE) is the technique of choice for
many users as it is portable, relatively inexpensive
and can be performed by operators with only a
minimal amount of training.
A number of analysis methods have been developed
to quantify the analyte concentration of a solution
using an ISE. This application note will investigate
three of these techniques – direct potentiometry,
standard addition and sample addition.
Direct potentiometry is the preferred analysis method
for many ISE users. The technique does not require
quantitative measurements or complex mathematical
calculations to be performed by the operator. It
requires the user to construct a calibration curve of
electrode response versus analyte concentration. The
curve is used to quantify the analyte concentration of
an unknown sample, directly from the electrode
response of the sample.
The standard addition technique calculates the
analyte concentration in an unknown sample by
measuring the change in an electrode’s response
when a small volume of solution, with known
concentration, is added to a large volume of sample.
The sample addition method is used in exactly the
same way as standard addition, except that a small
volume of sample is added to a large volume of
standard.
This application note will compare the results obtained
with these three analysis methods, when determining
the concentration of a prepared fluoride solution. A
discussion of the advantages and disadvantages of
each technique will also be made using observations
from this and other published studies.
Methods
The Measurement of Analyte Concentration
by Direct Potentiometry
Two standard solutions of 100ppm and 10ppm were
prepared from a 1000ppm fluoride standard (part code
0250087) by diluting 25.0ml and 2.5ml respectively to
250.0ml with deionised water.
25.0ml aliquots of the 1000ppm, 100ppm and 10ppm
standards were taken and 5.0ml of ionic strength
adjustment buffer (ISAB - part code 0250107) were
added to each to ensure that the ionic strength of
each sample was consistent . A fluoride combination
ISE (part code 924 305) was used to measure the
response of each solution at 20˚C and a plot of
electrode response vs. Log10 concentration was
plotted.
A 200ppm working sample solution was prepared by
diluting 50.0ml of the 1000ppm standard to 250.0ml
with deionised water. 5.0ml of ISAB were added to
five separate 25.0ml aliquots of the working sample
solution and the electrode response was recorded for
each sample solution. The calibration graph was used
to determine the fluoride concentration of the sample.
The Measurement of Analyte Concentration by
Standard Addition
A 100ppm standard solution was prepared from a
1000ppm fluoride standard (part code 0250087) by
diluting 25.0ml to 250.0ml with deionised water.
A 25.0ml aliquot was taken from each of the 1000ppm
and 100ppm standards and 5.0ml of ISAB (part code
0250107) were added. The electrode response of
each solution was recorded with a fluoride
combination ISE (part code 924 305) and the slope of
the electrode’s response was calculated.
Three 25.0ml aliquots of the 200ppm working sample,
prepared previously, had 5.0ml of ISAB added and the
electrode response recorded. A 3.0ml aliquot of the
1000ppm fluoride standard was added to each sample
and the electrode response was recorded for a
second time.
The Measurement of Analyte Concentration by
Sample Addition
Two standard solutions of 100ppm and 10ppm were
prepared from a 1000ppm fluoride standard (part code
0250087) by diluting 25.0ml and 2.5ml respectively to
250.0ml with deionised water.
A 25.0ml aliquot was taken from the 100ppm and
10ppm standards. Each aliquot had 5.0ml of ISAB
(part code 0250107) added. The electrode response
of each solution was recorded with a fluoride
combination ISE (part code 924 305) and the slope of
the electrode response was calculated.
Three 25.0ml aliquots of the 10ppm standard had
5.0ml of ISAB added and the electrode response of
each solution was recorded. A 3.0ml aliquot of the
200ppm
working
sample
solution,
prepared
previously, was added to each solution and the
electrode response was recorded for a second time.
jenwayhelp@bibby-scientific.com
www.jenway.com
Tel: 01785 810433
Results
The sample concentration values in Table 2 were
calculated using the following equation:
Direct Potentiometry
The electrode response values that were recorded for
each of the standard calibration solutions used in the
direct potentiometry technique are shown in Table.1.
Standard
Concentration
(ppm)
-300
-350
Standard Addition
To obtain the highest level of accuracy, the standard
addition technique requires the electrode response
values of two standard solutions to be measured. If an
estimate of the unknown sample’s concentration can
be made, the concentrations of the two standard
solutions should bracket this estimated value.
-400
Standard
Concentration
(ppm)
-450
y = -56.75x - 328.27
2
R =1
-550
0
0.5
1
1.5
2
2.5
LOG(10) Concentration
+ 328 .27 


The electrode response values of the selected
standard solutions and the slope of a line plot of
electrode response vs. Log10 concentration are shown
in Table 3.
Fluoride ISE Calibration Graph
-500
(mV)
Stable electrode response values were obtained in
less than 60 seconds for all sample and standard
solutions.
Electrode
Response
(mV)
10
-385.2
100
-441.4
1000
-498.7
Slope
-56.75
2
R
1.00
Table 1: The electrode response values of the fluoride
standard solutions.
Electrode Response (mV)
 Electrode Response
Conc. (ppm) = 10^ 
− 56 .75

3
3.5
Figure 1: A graph of the standard electrode response
of the fluoride combination ISE.
The slope, intercept and correlation coefficient were
calculated by performing a linear regression analysis
of the data in Excel. The results are displayed in Table
1 and Figure 1.
The values obtained indicate that the electrode was
functioning correctly.
Five sample solutions had their electrode response
values measured. The recorded values, mean value
and standard deviation are displayed in Table 2.
Sample
Electrode
Response
(mV)
Sample Conc.
(ppm)
1
-460.2
211.2
2
-459.9
208.7
3
-460.1
210.4
4
-459.8
207.8
5
-459.7
207.0
Electrode
Response
(mV)
100
-441.4
1000
-498.7
Slope
-57.3
Table 3: Electrode response and calculated slope
values used in the standard addition procedure.
The slope value is used in the equation to calculate an
unknown sample’s analyte content.
The electrode response values of the three sample
solutions were measured before and after the addition
of a known volume of 1000ppm standard solution. The
recorded values are displayed in Table 4.
Sample
Electrode
Response
E1 (mV)
Electrode
Response
E2 (mV)
Sample
Conc.
(ppm)
1
-460.2
-467.7
205.4
2
-460.3
-467.7
207.9
3
-460.3
-467.8
205.4
Mean
206.2
Std.Dev.
1.4434
Table 4: Electrode response values before and after
standard addition, with the calculated concentration
values.
208.5
Mean
1.2677
Std.Dev.
Table 2: Electrode response values of the sample
solutions with the calculated concentration values.
jenwayhelp@bibby-scientific.com
www.jenway.com
Tel: 01785 810433
The sample concentration values in Table 4 were
calculated using the following equation (1):


Vs
Cs × 

 ( Vu + Vs ) 
Cu =
 ^ (E -E ) m  

Vu
 10
 - 


  ( Vu + Vs )  
2
Sample
Electrode
Response
E1 (mV)
Electrode
Response
E2 (mV)
Sample
Conc.
(ppm)
1
-385.3
-410.3
207.6
2
-385.1
-410.1
206.4
3
-385.4
-410.3
205.1
1
Where:
Cu = concentration of the unknown sample (ppm)
Cs = concentration of the standard (1000ppm).
Vs = volume of standard (3.0ml).
Vu = volume of sample + ISAB (30.0ml).
E1 = electrode potential before addition (mV).
E2 = electrode potential after the addition (mV).
m = the electrode slope (-57.3mV).
Stable electrode response values were obtained in
less than 60 seconds for all solutions.
206.4
Std.Dev.
1.2503
Table 6: Electrode response values before and after
sample addition, with the calculated concentration
values.
The sample concentration values in Table 6 were
calculated using the following equation (1):
 Vs + Vu   ^ (E -E ) m   Vs  
Cu = Cs × 
 -   
 ×  10
  Vu  
 Vs  
2
1
Where:
Sample Addition
To obtain the highest level of accuracy, the sample
addition technique requires the electrode response
values of two standard solutions to be measured. If an
estimate of the unknown sample’s concentration can
be made, the concentrations of the two standard
solutions should bracket the range of concentrations
that will be measured during the analysis.
The electrode response values of the selected
standard solutions and the slope of a line plot of
electrode response versus Log10 concentration are
shown in Table 5.
Standard
Concentration
(ppm)
Mean
Electrode
Response
(mV)
10
-385.2
100
-441.4
Slope
-56.2
Table 5: Electrode response and calculated slope
values used in the sample addition procedure.
The electrode response values of three 10ppm
standard solutions were measured before and after the
addition of a known volume of the 200ppm working
sample solution. The recorded values are displayed in
Table 6.
Cu = concentration of the unknown sample (ppm)
Cs = concentration of the standard (10ppm).
Vs = volume of standard + ISAB (30.0ml).
Vu = volume of sample (3.0ml).
E1 = electrode potential before addition (mV).
E2 = electrode potential after the addition (mV).
m = the electrode slope (-56.2mV).
Stable electrode response values were obtained in
less than 60 seconds for all solutions.
Conclusions
The results from this investigation are summarised in
Table 7.
Technique
Results
(ppm)
Mean
(ppm)
Std.
Dev.
Direct
Potentiometry
211.2, 208.7,
210.4, 207.8,
207.0
208.5
1.2677
Standard
Addition
205.4, 207.9,
205.4
206.2
1.4434
Sample Addition
207.6, 206.4,
205.1
206.4
1.2503
ANOVA (α
α=0.05)
F(calc)
4.1188
F(critical)
4.4590
Table 7: Results and statistics obtained from the three
analysis techniques under test.
jenwayhelp@bibby-scientific.com
www.jenway.com
Tel: 01785 810433
The data collected in this study shows that the
techniques of direct potentiometry, standard addition
and sample addition do not give significantly different
results, when compared at the 95% confidence interval
using two-way ANOVA.
By demonstrating that these techniques produce
statistically equivalent results, when testing a sample
with a simple sample matrix, the decision on which
analytical technique to use for a particular assay can
usually be based on the practical advantages and
disadvantages of each technique.
Calibration curves can also be used repeatedly when
analysts are confident that the probe in use is
performing consistently, allowing preparation times to
be reduced further.
References
(1)
Rundle C. A Beginners Guide to Ion-Selective
Electrode Measurements. 2000, chapter 10b.
(2)
Vesely J, Weiss D, Stulik K. Analysis with Ionselective Electrodes. Ellis Horwood 1978, pg
101 –103.
The techniques of standard and sample addition
benefit from having the electrode immersed in solution
throughout the procedure. This virtually eliminates the
error that is caused by a change in the reference
electrode’s liquid junction potential when an electrode
is moved between different samples. Eliminating this
error can significantly improve the precision of an
assay, particularly in samples with complex or dirty
matrices (2).
The standard and sample addition techniques also
allow the user to determine the slope of the electrode
at a point that is very close to the sample’s
concentration. A standard with a concentration that
lies within the range of the samples under test is
prepared and tested using the same test procedure.
The electrode slope is re-calculated and the new slope
value is used to determine the unknown sample’s
concentration. The main advantage of this fine tuning
is that it allows these techniques to give reliable results
in the non-linear range of the electrode. This may be a
significant advantage when an analyst only has access
to old or worn electrodes (1).
An additional advantage of the sample addition
technique is that it is ideal for applications where only
small amounts of sample are available or where the
sample is dirty or viscous in nature. The practical
advantages of being able to perform measurements
when only a small amount of sample is available are
clear, as these applications would otherwise require
expensive micro-electrodes (2), whilst the ability to
minimise the matrix effects of dirty samples by
dissoluting a small volume of sample in a large volume
of standard, ensures that any interferences in these
types of sample are minimised.
The techniques of standard and sample addition offer
the analyst a number of practical advantages when
compared to direct potentiometry and while the
addition techniques require accurate volumetric
measurements and more complicated mathematical
calculations, the practical advantages should be
thoroughly considered when deciding on which
method to use in new ISE applications.
Direct potentiometry is still however the preferred
method of analysis in many applications as it allows a
wide range of sample concentrations to be analysed
without the planning and preparation steps required in
the standard and sample addition techniques.
jenwayhelp@bibby-scientific.com
www.jenway.com
Tel: 01785 810433