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AgCl

Ag

/

Reference electrode

Figure 4.15 Representation of the disposable slide for the measurement of potassium by the Vitros Chemistry System. The difference in potential between the two half-cells, one receiving the sample and the other a reference solution of known potassium ion concentration, is mathematically converted to give the concentration of potassium ions in the sample.

Indicator electrode

Reference electrode

Figure 4.15 Representation of the disposable slide for the measurement of potassium by the Vitros Chemistry System. The difference in potential between the two half-cells, one receiving the sample and the other a reference solution of known potassium ion concentration, is mathematically converted to give the concentration of potassium ions in the sample.

> Total ionic strength adjustment buffets i.TISABsi are used to equalize ionic activity in different solutions.

It is often more convenient to relate the potentiometer reading directly to concentration by adjusting the ionic strength and hence the activity of both the standards and samples to the same value with a large excess of an electrolyte solution which is inert as far as the electrode in use is concerned. Under these conditions the electrode potential is proportional to the concentration of the test ions. The use of such solutions, which are known as TISABs (total ionic strength adjustment buffers), also allows the control of pH and their composition has to be designed for each particular assay and the proportion of buffer to sample must be constant.

Ion-selective electrodes are particularly useful for monitoring the disappearance of an ion during a titration. In many cases it is not necessary to calibrate the instrument because there is often a significant change in the potential at the end-point of a titration. However, some electrodes have a slow response time and care must be taken to ensure that titration is not performed too quickly.

Apart from interference the greatest problem in the use of ion-selective electrodes is that of contamination. Any insoluble material deposited on the surface of the electrode will significantly reduce its sensitivity and oil films or protein deposits must be removed by frequent and thorough washing. It is possible to wipe membranes with soft tissue but they can be easily damaged. Solid-state membranes are more robust but they must not be used in any solution which might react with the membrane material.

Self test questions

Section 4.1

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