Page:Text-book of Electrochemistry.djvu/214

 XII.

��NEUTRALISATION VOLUME.

��acid with one of a base, a change of volume ensues which amounts to 19 c.c, when each solution (dilute) contains a gram-equivalent. This is the neutralisation volume. This regularity, like that of the heat of neutralisation, only holds good for strong acids and bases. For weak electrolytes a correction must be introduced, the dissociation volume, i.e. the change of volume which takes place when a gram- molecule of acid splits up into its ions. This volume can be theoretically derived from the formula developed by Planck (17)—

d-p

where the constant jB has the value 81 8 (atmospheres per square centimetre, see p. 26), 2? denotes the pressure in atmo- spheres, K the dissociation constant of the acid, and A, the dissociation volume in c.c. at the absolute temperature T.

Fanjung {18) investigated the influence of pressure on the dissociation constant of weak acids, and calculated the values of A, from his results. He found that A^ is always negative, which proves that the ions occupy a smaller volume than the undissociated molecules, and from this it follows that the dissociation increases with rising pressure (see p. 99). These results were compared with those calculated from Ostwald's experiments on the increase of volume on neutralisation, and, as the following table shows, a very perfect agreement was found : —

Nectralisatiox Volumes.

��Acid.

��Formic acid. Acetic acid. Propionic acid Butyric acid. Isobutyric acid Lactic acid. Succinic acid Malic acid.

��Calculated by

�Obaerved by

�Fanjung.

�Ostwald.

�c.c.

�c.c.

�10-G

�11-H

�� �