Page:Text-book of Electrochemistry.djvu/208

 completely displaces the other from its salts. He found that the acid which conducts best is the stronger. This suggested to me (in 1884) that the strength of an acid is proportional to its conductivity (7), or, more correctly, to its degree of dissociation (according to our more recent theories). A corresponding connection is shown by bases.

The Dissociation of Water.— In my theoretical dis- cussions (7) of 1884 I regarded water as either a Weak acid or a weak base. If we dissolve alkali salts of weak acids, such as carbonic acid or hydrocyanic acid, in water, the solutions have an alkaline reaction indicating the presence of free alkali. Water, HOH, therefore replaces part of the weak acid HCN from the KCN and form? the compound KOH, just as any recognised weak acid would do. This view has been confirmed by later investigations. Shields (8) found 2*4 per cent, of free alkali in a 4^^ -normal solution of potassium cyanide, and 7*1 per cent, in a sodium carbonate solution of the same concentration, whilst in a 01-normal solution of sodium acetate he found 0008 per cent, of alkali. These deteminations were made by ascertaining the powers of the solutions to eflFect the decomposition of ethyl acetate.

Shields determined the extent of decomposition of an acetate solution into acid and base, the so-called hydrolysis, and from this we can calculate the strength of water (as an acid) compared with that of acetic acid. Since the degree of dissociation of acetic acid is known, it is possible to calculate the degree of dissociation of water. I (9) have calculated this to be 203 X 10"® at 25*^, so that in a litre of water there is 1'113 X 10"*^ dissociated gram-molecule.

In aqueous solution methyl acetate is slowly decomposed into methyl alcohol and acetic acid. In this case it is the hydroxyl ion of the water which is the active agent. By comparing the action of water with that of a solution of sodium hydroxide, Wijs (10) determined the number of dissociated gram-molecules in a litre of water to be 12 x 10"'^ at 25^, which agrees well with the value given above.

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