Page:Text-book of Electrochemistry.djvu/300

 The temperature exerts no appreciable influence on primary processes taking place with organic substances, as Tafel {11) has proved in the case of the electrolytic reduc- tion of caflfein and other difficultly reducible substances.

In many cases it is sought to favour the secondary pro- cess, and for this too low a temperature must not be chosen. Thus, in electro-gilding (with potassium auricyanide) it is recommended that the bath be kept at 70-75°, and in the deposition of nickel from ammonium nickel sulphate the temperature should be from SO'' to 90^ according to circum- stances. It has further been found that the optimum temperature for the preparation of iodoform from a solution of potassium iodide and dilute alcohol containing sodium carbonate, is about 60° ; hydriodic acid and carbon dioxide are also formed.

In the analogous preparation of chloral from potaissium chloride and alcohol the temperature must be raised to 100°. Many other organic electrolytic processes, of which Elbs {12) has studied a large number, proceed best at com- paratively high temperatures.

Occasionally several secondary processes take place simultaneously, e.g, in the electrolysis of potassium chloride solution with a platinum anode. Chlorine is primarily separated at the anode, and this gives rise to a secondary formation of hypochlorite, chlorate, and oxygen. The hypo- <jhlorite is formed in largest quantity at low temperature, the chlorate and oxygen particularly at high temperature.

The relative quantities of the different electrolytic pro- ducts can therefore be regulated by altering the temperature.

As a rule the secondary process is favoured by stirring the liquid as well as by elevating the temperature. The ♦eflfect of stirring is to bring fresh quantities of the unionised substances into contact with the ions primarily separated at the electrodes, and thus aid the secondary action. Since the introduction of heat always causes a stirring in the liquid of the bath, the secondary process is helped both by the stirring .and by the rise of temperature.

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