Other ion-sensitive electrodes

The alkaline error of glass membrane electrodes has been exploited in the development of electrodes sensitive to sodium and potassium ions. By analogy with the pH scale we have 

K representing the proportion of the total potential due to constants of the system. Changes in potential, due to changes in a, originate in the outer 

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The unique property of ion-sensitive electrodes is a membrane between the test solution and the electrode sensor that develops an electrical potential, or voltage, in response to a change in the concentration of a single ion. The pH electrode, for example, is shown schematically in Fig. 10.5. Other ion-sensitive electrodes differ in the composition of the membrane and in the salts necessary to develop the potential. In the pH electrode, a silver wire coated with AgCl dips into an HC1 solution. The HCI solution is separated from the test solution by a membrane of special glass, usually a lithium silicate. Differences in H+ activity across this glass membrane cause a difference in electrical potential, which can be measured by a sensitive potentiometer. 

Other ion-sensitive electrodes are not nearly as effective in screening out interfering ions. Selectivity ratios are as low as 1, meaning that the electrode is as sensitive to the interfering ion as to the test ion. Such measurements are valid only when the concentrations of interfering ions are considerably lower than that of the test ion. Ion-sensitive membranes are being continually improved and hold considerable promise for soil chemical analysis. 

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