Salts and their reactions : a text-book of practical chemistry / by Leonard Dobbin and Hugh Marshall.

  • Doddin, Leonard, 1858-
Date:
1912
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Licence: In copyright

Credit: Salts and their reactions : a text-book of practical chemistry / by Leonard Dobbin and Hugh Marshall. Source: Wellcome Collection.

    54/222 (page 40)
    Since HPO/' becomes so slightly ionised, even in very dilute solution, the action will result in the formation of a considerable proportion of such molecules, even although this requires the breaking up of a considerable proportion of water molecules. There will therefore be a decided accumulation of hydroxyl ions in the solution, and the latter must exhibit a marked alkaline reaction. In the case of sodium acetate, hydrolysis involves the forma- tion of free acetic acid, which, in solution, is ionised far more than water itself is. Hence, of the four compounds represented in an equation-diagram indicating the state of affairs in solution of sodium acetate, water is by far the least ionised, and there is therefore very little tendency to form acetic acid. There is consequently only slight hydrolysis and a comparatively low concentration of hydroxyl ions in the solution, which is therefore not markedly alkaline. In order that a salt of a monobasic acid may yield a strongly alkaline solution by hydrolysis, it is necessary that the ionisation of the free acid should be so slight as to be almost comparable to that of water itself. A good example of such an acid is hydrocyanic acid. The hydrolysis of salts derived from weak bases is explicable in a manner similar to that adopted in the case of salts of weak acids, only here it is the hydroxyl ions derived from the ionisa- tion of the water which become used up to form a less ionised product, and there is therefore an accumulation of hydrogen ions in the solution, which consequently has an acid reaction. In some cases the free base is the product of the hydrolysis ; it is rare, however, that the base is distinctly soluble in water, except in the case of the organic bases corresponding to compound metallic radicals. When the base is insoluble in water, an additional factor is introduced, and the hydrolysis may be carried further than might otherwise have been expected. With salts derived from a poly-acid base hydrolysis very frequently gives rise, not to the base itself, but to a basic salt, which may either be precipitated or remain dissolved. In the latter case there will be a certain amount of ionisation, with formation of a cation containing hydroxyl united to the metal, just as in the corresponding case with a weak acid there is formation of an anion containing hydrogen. In the preceding pages an attempt has been made to give a short and simple statement of the ionisation theory of salt solutions. There are many other interesting applications of the theory, to which it has been impossible to refer in limited space, and the discussion has therefore been confined as much as possible to points which have some bearing on the subsequent work.