By Sunil Bhardwaj

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Ion selective electrodes are selective towards a particular ion or ions. These electrodes are not specific for a particular ion, but each will possess selectivity for a particular ion or ions.

Glass Electrode: The glass electrode consists of a glass tube ending in a bulb (the glass membrane). The glass from which the electrode is made has the general composition of 22% Na2O, 72% SiO2 and 6% CaO. It has a comparatively high electrical conductance. The bulb contains either a buffer solution of pH = 4 in which a platinum wire is dipped or 0.1 M hydrochloric acid in which a silver wire coated with silver chloride is dipped. The concentration of the solution in the bulb is constant. The potential of the electrode dipped in the bulb and the potential between the solution and the inner surface of the bulb will therefore be constant. Therefore the only potential which can vary depends upon the hydrogen ion concentration of the test solution in which bulb is dipped. The standard potential of the glass electrode is determined by measuring the emf of a cell set up by combining a glass electrode dipping in a buffer solution of known pH (pH = 4) with a suitable reference electrode. The glass electrode must be calibrated every time it is used.

Glass electrodes | 0.1 M HCl | Glass electrode

The glass electrode is supposed to function as a result of ion exchange on the surface of a hydrated glass membrane. The glass membrane from which the electrode is made consists of sodium oxide, Na2O and silica, SiO2 a chemically bonded form. A glass electrode which is new has a surface containing fixed silicate groups with associated sodium ions, i.e., SiO Na. it is an experimentally observed fact that before a glass electrode is used it has to be soaked in water. When this is done, the outer surface of the glass membrane gets hydrated; the inner surface is already hydrated since it is in contact with an aqueous solution. After the outer surface has become hydrated, by soaking in water, the electrode is ready for use. When such electrode is dipped in a solution containing H+ ions, the sodium ions in the hydrated membrane surface are exchanged for hydrogen ions in the solution. The hydrated glass surface has a high affinity for hydrogen ions and the equilibrium constant for this reaction is very large. It is for this reason that even if other exchangeable ions are present in the solution, the exchange of Na+ ions by H+ ions preferentially takes place. In other words, the glass membrane electrode is selective or sensitive to hydrogen ions.

The glass electrode is extensively used as an electrode reversible with respect to hydrogen ions. The potential of the glass electrode is not affected by the presence of oxidizing or reducing agents and the electrode can be used over a wide pH range. Further, the electrode functions remarkably well in physiological systems. No other pH measuring electrode has all these properties. Nowadays, because of its convenience, the glass electrode is almost universally used for pH determinations. It is known that at the point of contact of a glass membrane and a solution in which it is dipped, there arises a difference of potential which is dependent on the activity of hydrogen ions in the solution.

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