pH Measurement

pH

The range of hydrogen ion concentration met with in practice is quite extensive. Normal hydrochloric acid has a hydrogen ion concentration of 1N, while in normal sodium hydroxide the concentration is as low as 0.000, 000, 000.000. 01N. To cover such a wide range as this logarithmic scales are used, or alternatively, powers (indices) and exponents of ten such that the concentration of hydrogen ions in hydrochloric acid is expressed as 10^oN and in sodium hydroxide as 10^-14 N or 1/ 10¹⁴ N. All solutions, which are less acid than hydrochloricacid (N), will therefore have a negative power or exponent, for convenience the negative value is omitted and the hydrogen ion concentration is expressed in terms of what a Danish bio-chemist, called 'pH', p representing power or exponent and H representing Hydrogen ion.

To give an example, the hydrogen ion concentration of pure water is 0.000, 000, 1 or 10^-7 The exponent, or power, of 10 is -7, therefore the pH value of pure water is expressed as +7.

The advantage of this system of expression of hydrogen ion concentration is that, by a series of positive numbers ranging from 1 to 14, all degrees of acidity and alkalinity from 1N acid to 1N alkali can be expressed. pH7 representing water neutrality, all numbers above this will represent alkaline solutions while those below it will represent acidic solutions, each change of 1pH representing a tenfold change of hydrogen ion concentration.

It must be realized that pH meters measure the effective concentration or the activity of the hydrogen ions and not the actual concentration.

Electrode Potentials

To understand the method of measuring pH it is perhaps best to start with an understanding of a simple cell. when a metallic electrode is placed in a solution a redistribution of electrical charges takes place.

Positive ions of the metal enter the solution leaving the electrode negatively charged relative to the solution which will be positively charged. if the solution already contains ions of the metal, there will be a tendency for ions to be deposited on the electrode giving it a positive charge. The electrode eventually reaches an equilibrium potential with respect to the solution, the magnitude and sign of the potential depending on the metallic ion concentration in the solution and the nature of the metal. Zinc has such a strong tendency to form ions, that the metal forms ions in all solutions of its salts, such that it is always negatively charged. Copper, on the other hand, even in the most dilute solution of its salt becomes positively charged.

This difference between Zinc and Copper is largely responsible for the e.m.f. of a Daniell Cell.

Fig.33. Daniell Cell

In the cell shown in fig. 33, when the poles are connected by a wire, sudden differences of potential are possible;

(a) at the junction of the wires with the poles
(b) at the junction of the zinc with its solution
(c) at the junction of the zinc solution with the copper solution
(d) at the junction of the copper with its solution

The e. m. f. of the cell will be the algebraic sum of these potential differences.

Electrodes for routine pH measurement

There are three types of electrodes used in pH measurement

(a) The quinhydrone electrode
(b) The intimony electrode
(c) The glass electrode

The quinhydrone electrode was, until recent years, in extensive use. However, its use had certain disadvantages such as, it necessitated the addition of impurities, into the solution under test, for its functioning.

The antimony electrode has or did supersede the quinhydrone type. It consists of a piece of pure antimony, usually in the form of a rod or button, mounted In an insulated support and immersed in the solution under test.