Page:Text-book of Electrochemistry.djvu/273

 258 OXIDATION AND REDUCTION ELEMENTS, chap.

��On the other hand, if the products of the electrolysis are not the same, as is the case with hydrochloric acid (Ha and Cla), hydrobromic acid (Ha and Bra), hydriodic acid (Ha and la), different tensions are obtained : in these three cases, 1*31, 0'94, and 0*52 volt respectively for normal solutions. In the case of some organic acids, such as oxalic acid, trichloracetic acid, etc., deviating results were obtained, because in these instances the secondary processes which take place at the electrodes play an important part.

For these acids, which all show a lower value for E than 1*67 volt, corresponding with the separation of hydrogen and oxygen at the platinum electrodes, the influence of the secondary processes diminishes with rising dilution, and at the same ti me the reduction of the separated oxygen does not take place so readily. The polarisation for a normal solution of oxalic acid is 0*95 volt, whilst for a 0067-normal solu- tion it rises to 1*04 volts. A similar behaviour is exhibited by hydrochloric acid solutions, in which more and more oxygen instead of chlorine is separated as the dilution increases. Thus a normal solution of hydrochloric acid shows the tension of polarisation H = 131 volts, and for a ^^-normal solution, E = 1*69 volts, i.e. as great as for oxygen acids. The oxygen acids show no appreciable change of electro- motive force with dilution, so that B = 1'67 volts both for a normal and for a ^^-normal solution of sulphuric acid.

The alkali salts of the oxygen acids behave somewhat differently. With these the decomposition products are not only Oa and Ha, but base and acid as well, i.e, OH and H ions. Consequently more work is required for the electro- lysis of these than for acids, but about the same for all salts. Using platinum electrodes, Le Blanc found the followiug polarisation electromotive force U: —

��Sodium nitrate. Potassium nitrate Lithium nitrate. Strontium nitrate Calcium nitrate Barium nitrate.

��p-olts.

� �VoltB.

�Sodium sulphate. ..

�Potassium sulphate. .

�Sodium acetate ....

�Ammonium nitrate. ..

�Ammonium sulphate. . ,

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