When a metal bar or a foil is immersed in a solution which contains its own ions, for example, Zinc dipped in solution of zinc sulphate, a potential difference is established between the metal and the solution. This potential difference (E) is given by the expression:
Thus, if we know the value of Ea which is the standard electrode potential of the metal used (a constant) and the valency of the ion, E can be determined by combining the electrode with a reference electrode (commonly a saturated Calomel electrode) and measuring the e.m.f. of the resultant cell.
From the value of E, cm + (the concentration of the metal ions) can be calculated.
Thus, potentiometry offers a means of determining the concentration of dilute solutions simply by putting electrodes in it and measuring the potentials. However, environmental samples contain many ion species which have to be separated before such estimations can be performed. Hence potentiometry was considered unsuitable for the purpose.
However, since 1966 several new types of electrodes have been developed which respond rather selectively to various ions. These electrodes are referred to as Ion-selective electrodes. The most important potential developing component of the ion selective electrode is a membrane of some kind which allows selective exchange of ions sought for. Only those ions are exchanged for which the electrode is being used. For example, the oldest ion selective electrode widely used these days for measurement of H+ activity or pH contains a glass membrane which selectively permits exchange of H+ ions only.
The membrane which permits selective exchange of ions may be solid state membrane or liquid ion exchange. The conventional ion selective glass membrane of the glass electrode can be made selective to a number of univalent cations like H, Na, K, NH4, Ag, Li, etc.
This requires appropriate changes in the composition of glass used to make the membrane. Non-glass solid state sensors may be used in place of glass with ionically conducting membrane. These may consist of either a single crystal of various Ag+ or S2- salts dispersed in Ag2S matrix or of mixed type. Ion selective electrodes employing solid state membrane have been developed for the estimation of P, Ag, S, Cu, Cd, Pb, SCN, CN, CI, Br, I etc.
The liquid membrane electrode consists of a concentric tube arrangement in which the inner tube contains aquous reference solution and internal reference electrode. The outer compartment contains inorganic liquid ion exchange reservoir in contact with a hydrophobic membrane. A charged site – the lipophilic ion exchanger molecule selective for the particular ion serve as the mobile transport in the electrodes. These electrodes are employed for the estimation of Ca, Mg, NO3, ClO4 etc.
Ion selective electrodes are extremely useful for in-situ measurements of several cation and anions in natural and waste waters. They are very handy for measurement of Ca2+ Mg2+ ions as well as hardness of natural waters.