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INDICATOR (from Lat. indicare, to poi...

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Originally appearing in Volume V14, Page 483 of the 1911 Encyclopedia Britannica.
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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 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.
End of Article: INDICATOR (from Lat. indicare, to point out)
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