202 OUTLINES OF PHYSICAL CHEMISTEY
5. For organic acids, Ostwald has found a regularity which may be made use of for determining their basicity and even their molecular size. The equivalent conduc- tivity of the sodium salt of a monobasic acid increases by ten units when we pass from a solution of 32 litres dilution to one of 1,024 'litres dilution. For a dibasic acid the increase is twenty units ; for a tribasic acid thirty units and so on (always for the same increase of dilution).
Many other regularities have been noticed, but I shall only call attention to
Kohlrausch's Law (1876)
Kohlrausch has found that the molecular conductivity of neutral salts l is composed additively of two factors, one of which depends solely on the nature of the base, and the other on the nature of the acid. This law, it may be noticed, exactly resembles the law of thermo-neutrality, and is in direct correlation with certain of Hittorfs con- siderations (1851).
If in an electrolysis the positive and the negative ions migrate at the same speed, the fall in the concentration of the solution is the same at both electrodes. But as this is not generally the case, Hittorf suggested that the ions migrate at different speeds, as is indicated in the diagram, fig. 49.
The black points represent one ion, the white circles the other. Under the influence of a current the black ions move to the left, the white to the right, with this difference, that to pass from molecule to molecule the black ions move twice as fast as the white ions. The top row represents the condition of the solution before electrolysis, the follow- ing rows successive states during the electrolysis. The vertical line a b divides the original condition into two equal parts. At the beginning there are seven black and seven white ions on each side of this line. The last
1 In sufficiently dilute solution.
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