Page:Text-book of Electrochemistry.djvu/274

 XV,

��MAXIMUM POLARISATION.

��These decomposition tensions exceed those of the acids by 0*47 volt on the average. The difference between the decom- position tensions of the chlorides, bromides, and iodides and the corresponding acids is somewhat greater, as can be seen from the foUow^ing values found by Le Blanc : —

�� �Volts.

� �Volifl.

�Sodium chloride. .

�Barium chloride. .

�Potassium chloride. .

�Sodium bromide. .

�Lithium chloride. .

�Potassium bromide. .

�Calcium chloride. .

�Sodium iodide. ..

�Strontium chloride. .

�Potassium iodide. .

�. . 1-U

��The difference amounts to about 0*87 volt for the chlorides, 0*67 volt for the bromides, and 0*61 volt for the iodides when these are compared with normal solutions of the acids. If for comparison we take a j^^-^^ormal solution of hydrochloric acid, the difference for the chlorides would only be 0*49 volt. It is diflScult to give the preference to one or other concentration of the acid on any rational basis. If the difference of the decomposition products between the halogen acids and their salts were the same as between the oxygen acids and their salts, then the difference in the electro- motive force would necessarily be the same in both cases.

Maximum Polarisation. — It was formerly supposed that, with increasing current strength, the value of E rose asymptotically to a maximum. From Jalin's results this conclusion is rendered doubtful ; he found that E increases almost proportionally with log i. The increase, however, takes place so slowly that we may assume for the currents which are used practically that there is a maximum electro- motive force of polarisation; without appreciable error we may take the value of this to be 25 volts for acids and bases, and 3*3 volts for the salts of oxygen acids.

Polarisation by Deposition of Solid Substances.— When a solution of copper sulphate is electrolysed between at the anode, and this latter, by reaction with the water, gives H2SO4 and O2. If the electrolysis be carried to a certain

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