INDICATOR (from Lat. indicare, to point out), that which points out or records. In engineering, the word is specifically given to a mechanical device for registering the pressure of the working fluid in an engine cylinder during a stroke of the piston, the record so provided being termed the " indicator diagram " (see STEAM-ENGINE). In chemistry, the word is generically applied to re-agents or chemicals which detect usually small quantities or traces of other substances; it is, however, more customarily restricted to re-agents which show whether a substance or solution is acid, alkaline or neutral, the character being revealed in a definite colour change.
Here we shall only deal with indicators in this last restricted sense. They were first systematically employed in analytical chemistry by Robert Boyle, who used the aqueous extracts of the coloured principles present in red-cabbage, violets and cornflowers. The indicator most in use to-day is litmus (q.v.), whose solution is turned red by an acid, and blue by an alkali. Several synthetic indicators are employed in acidimetry and -alkalimetry. The choice is not altogether arbitrary, for experiments have shown that some are more suitable for acidimetry, while others are only applicable in alkalimetry; moreover, the strength of the acids and bases employed may exert a considerable influence on the behaviour of the indicator.
The following are well-known synthetic indicators: hacmoid, obtained from resorcin and sodium nitrite, resembles litmus. Phenolphthalein, obtained by condensing phenol with phthalic anhydride, is colourless both in acid and in neutral solution, but intensely red in the presence of alkali; the colour change is very sharp with strong bases, but tardy with weak ones, and consequently its use should be restricted to acidimetry when a strong base can be chosen, or to alkalimetry when a strong base is present. a-Naphtholphthalein has also been used (Biochem. Zeit., 1910, p. 381). Methyl orange, which is' the sodium salt of the acid helianthin, obtained by diazotizing sulphanilic acid and coupling with dimethylaniline, is yellow in neutral and alkaline solutions, but red in acid; the change is only sharp with strong acids. Para-nitrophenol, obtained in the direct nitration of phenol, yields a colourless solution in the presence of acids, and an intense yellow with alkalis. Of more recent introduction are: alizarin red, I.W.S. (alizarin monosulphonic acid), claimed by G. E. Knowles (Abst. J.C.S., 1907, ii. 389) to be better than methyl orange in alkalimetry; 3- amino2-methylquinoline, used by O. Stark (ibid. 1907, i. 974) in ammonia estimations; para-nitrobenzeneazo-a-naphthol, shown by J. T. Hewitt (Analyst, 1908, 33, p. 85) to change from purple to yellow when alkalis are titrated with weak acids; para-dimethylaminoazobenzene-ortho-carboxylic acid, proposed by E. Rupp and R. Loose (Ber., 1908, 41, p. 3905) as very serviceable in the estimation of weak bases, such as the alkaloids or centinormal ammonia; the " resorubin " of M. Barberio (Gazzetta, 1907, ii. 577), obtained by acting with nitrous acid on resorcin, which forms a violet, blue or yellow coloration according as the solution is neutral, alkaline or acid. Mention may be made of E. Linder's (J. Soc. Chem. Ind., 1908, 27, p. 485) suggestion to employ metanil yellow, obtained by coupling diazotized meta-aminobenzenesulphonic acid with diphenylamine for distinguishing mineral from organic acids, a violet coloration being produced in the presence of the former.
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 hydrpxidions. 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 io~ 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 lo~ 14 . It follows, therefore, that the acidity or alkalinity of any solution can be expressed both in terms of hydrion or hydrpxidipn 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 5Xio~ 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 hydrox- idion 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 IO~*H- and lo^OH', a result which shows this dye to be the best indicator of true neutrality. Methyl orange responds to between io~ 4 H- and 10 6 H-; para-nitrophenol to between io~ 6 H- and IQ-'H-; and phenolphthalein to between io- 6 OH' and io-OH'. Salm(Z'/. 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; Pels, 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 inconsiderable 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, 0: X u -H^X-O-H^X r -O'+H-, X u and X,, 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.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)