3.14 ION SELECTIVE MEASUREMENTS
Methods of Analysis
Measurements with Ion Selective Electrodes are often carried out with simple and straightforward calibra-
tion techniques, over a linear range of concentration. Concentration can be determined Ion Selective Elec-
trodes in two ways.
Direct potentiometry relates directly to the measured potential for the electrochemical cell to concentration
or activity of a given ion in solution, or its logarithm (only pH). Usually a procedure is followed in which a
calibration graph is prepared, with voltage plotted against activity or concentration. The plot is usually linear
over a range from 1 to 5 pX units, except at higher concentrations where activity and concentration are not
identical. Calibration methods are useful provided the measured solution does not contain interfering ions
which form complexes or precipitates with the test ions, or the measured potential is not affected by other
ions present in the test solution. The activity coefficients of the test and standard solutions must be roughly
similar, which can be achieved by the addition of Ionic Strength Adjustment Buffers (ISAB or TISAB) to
both sample and standard solutions. As the electrode responds to the activity of the uncomplexed analyte ion,
an excess of an inert salt is added to ensure both standards and test solutions have a constant high ionic
strength. This enables activity to be related directly to concentration. Alternatively, standard and sample
addition techniques are used to avoid these sources of error.
Another application of Ion Selective Electrodes for the determination of concentration is a potentiometric
titration, in which changes in potential of the indicator electrode are a function of the volume of the reagent
solution added. The results by this method are usually more precise than those obtained by direct potentiometry.
The potential of an ion selective electrode is measured against the constant fixed potential of a reference
electrode also in contact with the solution.The three most common types of reference electrodes are the
calomel, mercurous sulphate and double junction.
The calomel is the most widely used of the three and contains a saturated potassium chloride filling solution.
Potassium chloride is often used as an electrode fill solution because the mobilities of K
+
and Cl
-
are similar,
thus the junction potential at the interface is relatively small. The mercurous sulphate reference electrode is
similar to a calomel electrode although not as useful due to its high liquid junction potential. It uses a potassium
or sodium sulphate filling solution, so is used when potassium or chloride contamination of the sample is
undesirable. The double junction has an inner chamber containing 3.8M KCl/AgCl, which is insulated from
the sample solution by a second chamber containing a solution to which the ion selective electrode does not
respond.
Double junction reference electrodes are used in sludge or organic/caustic solutions and where potassium or
chloride in the calomel internal reference will interfere with the ion measurement and when interferents are
present. Refer to ion selective electrode manufacturer’s specification for recommended buffer solution.
Changes in temperature have an effect upon the electrode potential and sensitivity of both ion selective
electrode and reference electrodes. The measuring range is often reduced with an increase in temperature,
although this will vary with the individual electrode type. According to the Nernst equation the effect of
temperature is the same for all ion selective electrodes. However, because of the temperature effects of the
different materials used to make an electrode, each type of electrode has a different voltage change with
temperature. There is a theoretical electrode potential which is independent of temperature, although this
value is difficult to determine, and is a characteristic of each individual electrode. Therefore, the best approach
is to ensure that both standard solutions and sample solutions are at the same temperature.
Ionic Strength Adjustment Buffers (ISAB) raise the total ionic strength of the sample and standard solutions
to a level which makes initial variation insignificant. Addition of ISAB also improves electrode sensitivity and
3345/REV A/08-99
24
