Indicator

Theory of Indicators.—The ionic theory of solutions permitted the formulation of a logical conception of the action of indicators by W. Ostwald which for many years held its ground practically unchallenged; and even now the arguments originally advanced hold good, except for certain qualifications rendered necessary by more recent research. In the language of the ionic theory, an acid solution is one containing free hydrions, and an alkaline solution is one containing free hydroxidions. A neutral solution contains hydrions and hydroxidions in equal concentration; this is a consequence of the fact that pure water itself undergoes a certain dissociation, and several different methods show that in the purest water obtainable the concentration of the free hydrions and hydroxidions is 10−7 at 24°. Moreover, the law of mass-action (see Chemical Action) demands that the product of the concentrations of the hydrions and hydroxidions in any solution is constant at a given temperature, and we see from the above values that this constant is 10−14. It follows, therefore, that the acidity or alkalinity of any solution can be expressed both in terms of hydrion or hydroxidion concentration. Many researches have been directed to classify acid and alkaline solutions according to the concentration of the hydrion. Conductivity determinations show that the maximum concentration of hydrion occurs in 5.8 - N nitric acid, where it has a value of about 2 - N, and the minimum occurs in 6.7 - N potassium hydroxide, where its value is 5 × 10−15, that of the hydroxidion being about 2 - N. These figures apply to a temperature of 24°. Bearing in mind the concentration of the ions in a neutral solution, it is seen that a scheme of seven grades of “neutrality,” differing by successive powers of ten, may be formulated. The concentration of hydrion and hydroxidion in any solution may be determined by several independent methods, and it is therefore a simple matter to prepare solutions of definite ionic concentrations and to test these with the object of obtaining a list of indicators according to their sensitiveness. It is found that litmus responds to concentrations of 10−6H· and 10−6OH′, a result which shows this dye to be the best indicator of true neutrality. Methyl orange responds to between 10−4H· and 10−5H·; para-nitrophenol to between 10−5H· and 10−6H·; and phenolphthalein to between 10−5OH′ and 10−6OH′. Salm (Zeit. Elektrochem., 1904, 10, p. 341) gives a list of twenty-seven indicators classified on this principle. Other papers bearing on this subject are Friedenthal, ibid., p. 113; Salessky, ibid., p. 204; Fels, ibid., p. 208; Scholtz, ibid., p. 549; M. Handa, Ber., 1909, 42, p. 3179.

The actual mechanism by which the indicator changes colour with varying concentrations of hydrion or hydroxidion is now to be considered. Ostwald formulated his ionization theory which assumes the change to be due to the transition of the non-dissociated indicator to the ionized condition, which are necessarily of different colours. On this theory, an indicator must be weakly basic or acid, for if it were a strong acid or base high dissociation would occur when it was in the free state, and there would be no change of colour when the solution was neutralized. Take the case of a weakly acid indicator such as phenolphthalein. The presence of an acid depresses the very slight dissociation of the indicator, and the colour of the solution is that of the non-dissociated molecule. The addition of an alkali, if it be strong, brings about the formation of a salt of phenolphthalein, which is readily ionized, and so reveals the intense red coloration of the anion; a weak base, however, fails to give free ions. An acid indicator of medium strength is methyl orange. When free this substance is ionized and the solution shows an orange colour, due to a mixing of the red of the non-dissociated molecule and the yellow of the ionized molecule. Addition of hydrions lessens the dissociation and the solution assumes the red colour, while a base increases the dissociation and so brings about the yellow colour. If the alkaline solution be titrated with a strong acid, the hydrions present in a very small amount of the acid suffices to reverse the colour; a weak acid, however, must be added in considerable excess of the quantity properly required to neutralize the solution, owing to its weak dissociation. This indicator is therefore only useful when strong acids are being dealt with, while its strongly acid nature renders it serviceable for both strong and weak bases.

It seems, however, that in addition to a change in the ionic condition of an indicator, there are cases where the coloration is associated with tautomeric change. For example, J. T. Hewitt (Analyst, 1908, 33, p. 85) regards phenolphthalein and similar indicators as obeying the following equilibrium in solution,

O : Xu·H ⇄ Xv·O·H ⇄ Xv·O′ + H·,

Xu and Xv, being isomeric. This indicates the presence of two tautomeric forms, one being of a quinonoid structure, and an ionized molecule. A similar view is advanced by A. Hantzsch and F. Hilscher (Ber., 1908, 41, p. 1187) who find that helianthin is quinonoid when solid, whilst in solution there is an equilibrium between an aminoazo- and sulphonic acid-form; on the other hand, the sodium salt, methyl orange, is a sulphonate under both conditions.