Ion Selective Electrodes
An ion selective membrane can be used to form an electrochemical cell whose emf depends on the concentration of that ion.
Before we proceed to an important application of emf measurements, brief mention should be made of a component of the most common electrochemical instrument, the glass electrode. Fig indicates the electrode assembly of a pH meter, which includes a typical glass electrode. The electrode, not the cell, usually consists of the arrangement:
Ag |AgCl (s) | HCl (C = 1) |glass
The importance of the electrodes stems from the fact that when it is placed in a solution of given acidity and the cell is completed by use of another electrode, the emf of the cell appears to depend primarily on the difference in the concentration or activity of the hydrogen ions on either side of the glass.
The glass membrane of the glass electrode separates two different solutions, as does the KCl salt bridge. Unlike the salt bridge, which provides for general ionic conduction across the liquid junction, the glass membrane most often used leads to a cell whose emf is primarily responsive to hydrogen ions. Glasses can be made such that it allow passage of only one type of ion, in this case the hydrogen ion, and therefore the electrode can be constructed to be sensitive to this ion only.
Much of the importance of the glass electrode stems from its lack of response to various oxidizing and reducing agents and to a large variety of various ionic species. Difficulties may occur, however, if the glass electrode is used in solutions of high sodium ion concentration or in solution sufficiently alkaline to attack the glass membrane.
The glass electrode that responds to variations in the hydrogen activity is just one member of a growing list of practical electrode devices known as ion-selective membrane electrodes. The three general types of membranes used are illustrated in the fig. modifications of the glass membrane fo a glass electrode can make the membrane permeable and the electrode responsible t a glass electrode ions. Glass electrodes have been prepared that are sensitive to each of the ions of the alkali metal family and to the other ions such as NH4+, Ag+, and Cu+as well as H+.
Solid state membranes can be represented by the fluoride ion electrode. The membrane material is the sparingly soluble crystalline substance LaF3. Fluoride ions are conducted through the crystals with ease while other ions are rejected. The fluoride ion electrode is highly specific and rugged, and thus is a valuable analytic tool.
When supplemented with a reference electrode, all such electrodes produce a potential that more or less conforms to the equation:
Ø° = const + RT/nF In aM
The net reaction, given the assumption that no junction reaction need be considered, is;
Right electrode; H+ (0.1) + e- ? ½ H2
Left electrode: ½ H2 ? H+ (0.01) + e-
H+ (0.10M) ? H+ (0.01M)
Since the value is zero, the cell emf can be written:
= - 0.05915 log a0.01 MH+/a0.10 MH+