196 EQUILIBRIUM BETWEEN ELECTROLYTES, chap.

and observed conductivities of water as found by Kohlrausch and Heydweiller is very perfect, as the numbers in the follow- ing table show. The equivalent conductivity (X.) is taken as equal to 340 + 8^.

Specific Coxductiyity (ic) of Water.

��Temperature.

�lO'ic(obeerved).

�KTk (calculated).

�-2^

�+ 4°

�10°

�18°

�26°

�34°

�42°

�50°

��Heat of Neutralisation. — From the description given

above, it follows that the heat of neutralisation must be the

same for all dilute strong acids and bases, independent of the

+ nature of the acid and base, since in all cases only the H and

OH combine to form H2O. This fact, which had been known for a very long time, seemed peculiar until the dissociation theory (1884) ^ gave the key to the explanation.

The development can, however, only be applied to strong acids and bases, because at the dilutions at which we com- monly work the weak acids and bases are only dissociated to a slight extent. For these also, however, the heat of neutralisation can be determined from electrical measurements.

If we investigate a solution of succinic acid, for example, we find that in 0*28-normal solution (the concentration used per cent, at 21*5°. In order, therefore, to compare this acid with the strong acids, we must first supply so much heat as is necessary for the dissociation of the remaining 98*5 per cent.

1 It may not be out of place here to rectify the common belief that the dissociation theory was suggested in 1887. As a matter of fact, it was, in a less perfect form, propounded by Airhenius in his Inaugural Dissertation in 1884.— Tr.

�� �