Page:Text-book of Electrochemistry.djvu/209

 19+ EQUILIBRIUM BETWEEN ELECTROLYTES, chap.

From the electromotive force of the element H2(Pt) | Base J Acid | Ha (Pt), Ostwald (11) determined the degree of dissociation of water, and found 7 x 10"^ to 7*4 x 10"^.

From the hydrolysis of aniline hydrochloride, Bredig (IJ) found the degree of dissociation 6 x 10"^. These two last values do not agree so well with those found by Wijs and myself.

By repeated distillation of water in a vacuum, Eohlrausch and Heydweiller (IJ) obtained water much purer than any which had previously been prepared, and found that it con- ducted about 20 times worse than the best sample of water distilled in the air. The conductivity at 18° was 386 X lO-*® (expressed in the new units).

Since \x> for OH = 172 and for H it is 314, the number of dissociated gram-molecules per litre at 18° is 0*8 x 10"'. At 25° the value is 105 X 10"^. The degree of dissociation, which is 55*5 times smaller, has therefore the value 1-4 X 10-^ at 18°, and 19 X 10-« at 25^

The agreement between the values found by these different methods for the dissociation of water is extremely good, and Kohlrausch regards this as the best proof of the correct- ness of the dissociation theory. Kohlrausch and Heydweiller give the following numbers for the number of gram-ions of hydrogen (-4) in a litre of water at the various temperatures : —

��Temp. 0°

�10°

�18°

�26°

�34°

�42°

�50°

�2-48.

��Heat of Dissociation of Water.— The influence of temperature on the dissociation of water can be calculated as follows. According to the result arrived at on p. 94, the equation —

^ In ^ _ JUL ~dT liT^

can be applied to the dissociation of water, where K is the dissociation constant, T the absolute temperature, and fi the heat of dissociation. The value of jB is 1*99 cal. (see p. 13)

�� �