Alkali metal ion-selective glass electrodes

If the preference for hydrogen ion exchange shown by lime-soda glasses can be reduced, then other cations will become involved in the ion exchange process and the possibility of an electrode responsive to metallic ions such as sodium and potassium exists. The required effect can be achieved by the introduction of aluminium oxide into the glass, and as shown in Table 15.2, this approach has led to new glass electrodes of great importance to the analyst. 

Li2 0 28%, Cs20 2%, BaO 4%, La203 3%, Si02 63% H+ (alkaline error reduced) 

In all cases some sensitivity to hydrogen ions remains in any potentiometric determination with these modified glass electrodes the hydrogen ion concentration of the solution must be reduced so as to be not more than 1 per cent of the concentration of the ion being determined, and in a solution containing more than one kind of alkali metal cation, some interference will be encountered. 

The construction of these electrodes is exactly similar to that already described for the pH responsive glass electrode. They must of course be used in conjunction with a reference electrode and for this purpose a silver-silver chloride electrode is usually preferred. A double junction reference electrode is often used. The electrode response to the activity of the appropriate cation is given by the usual Nernst equation  

Such an electrode may, however, also show a response to certain other singly charged cations, and when an interfering cation C + is present in the test solution, an equilibrium is established between ions M + in the glass surface in contact with the solution, and the ions C + in the solution  

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