Page:Text-book of Electrochemistry.djvu/124

 correct results are obtained, whilst deviations amounting to as much as 40 per cent, occur when use is made of the concentrations. It has been found for the majority of re- actions that the velocity increases more quickly than it should do on the assumption that it is proportional to the concentra- tion. The osmotic pressure shows the same behaviour, but a thorough investigation of the connection between these two phenomena has not yet been made.

Action of Neutral Salts. — The specific velocity of

reaction ^ of a 10 per cent, solution of cane sugar which

contains 10 per cent, of invert sugar, is the same as that of a 20 per cent, solution containing no invert sugar (p. 101). But

for a 20 per cent, cane sugar solution ^ is 1*11 ( = oTpf)

times as great as for a 10 per cent, solution when no invert sugar is present The relative speed of reaction is therefore per cent, of invert sugar. It has been found that the addition of 0*4 gram-molecule of sodium chloride increases the speed of inversion by 26 per cent. Other salts exert a similar action. Tammann (lf3) found that a solution which contained cane sugar and copper sulphate had an osmotic pressure greater than the sum of the osmotic pressures of the cane sugar without the copper salt and of the copper sulphate without the sugar. It is therefore probable that the osmotic pressure of the sugar is increased by the presence of foreign substances in the solution.

This gives us a probable explanation of the phenomenon which has been recognised for a long time, namely, that the specific velocity of reaction is increased by the addition of foreign substances (the so-called actio7i of neutral salts, because the salts were first investigated in this connection).

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