INDICATOR (from

See also:

• LAT

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 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 to-60H', a result which shows this dye to be the best indicator of true neutrality . Methyl orange responds to between io-4H• and io 6H•; para-nitrophenol to between io-6H• and io-6H•; and phenolphthalein to between to-'OH' and to-60H' Salm (Zeit . Elektrochent., 1904, 10, P . 341) gives a Iist 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 he 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, 0: X„•HX,•O•H-<=>_X„•O'+H•, X„ 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 .