178 THE DISSOCIATION THEORY. chap.
deviations from this observed, and these could be attributed to disturbing factors. Since the colour, i.e, the absorption, of a compound sufifers a very considerable change by a comparatively small chemical change, such as the replace- ment of bromine by chlorine, the constancy observed with the salt solutions can hardly be otherwise explained than by assuming that the salt molecules have decomposed into ions. Further, since a spectrum consists of several parts, and the agreement in all parts, is perfect, Ostwald's investi- gation, which covered 4 positive and 13 negative, " coloured " ions, may be mentioned as strong evidence in favour of the dissociation theory.
This is the reason why all salts containing the same ion have the same colour in dilute solution. All nickelous salts in dilute solution are green, all cupric salts blue, all manga- nous salts pink, all ferrous salts green, and all ferric salts colourless. The last-mentioned solution has a yellow colour due to the presence of colloidal ferric hydrate. The ferrous
and ferric salts are differently coloured, because they contain
+ + + + + different ions, namely, Fe and Fe; in the same way the
ferrocyanide and the ferricyanide ions have different colours.
A large number of examples are known, particularly amongst
organic compounds.
The application of indicators in the titration of acids and bases is based on this colour of the ions. Phenolphthalein, which behaves as a weak acid, is colourless in solution, whilst its salts (in solution) possess a brilliant pink colour. The acid is hardly dissociated, and therefore does not show the colour of the anion, for this does not exist in acid solu- tion. In the same way, litmus is a weak acid with a red colour, whilst the anion produced from the dissociation of its salts is blue.
Chemical Properties of the Ions. — In mentioning indicators we have touched upon one of the most important additive properties of salt solutions ; one which is the basis of analytical ohemistry. Let us consider a solution containing
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