Page:Text-book of Electrochemistry.djvu/298

 the solution in large .bubbles, whilst in the secondary formation of this, e.g. in the electrolysis of an alkali salt solution with a mercury cathode, a fine cloud of very small bubbles is produced. This peculiarity is supposed to be due to the fact that the substance separates more easily on already present parts of the same substance than on foreign substances, on account of the work done in surface formation. In an analogous way a salt, such as Glauber salt, may be maintained in supersaturated solution, so long as crystals of it are not present, but, if these be added, the salt deposits on the crystals. Consequently, if silver be primarily deposited from silver nitrate solution, the metal appears in a granular crystalline form, because it tends to separate on the already formed crystals of metal. On the other hand, if the silver be deposited secondarily from potassium cyanide solution, the positive ion of this salt, potassium, is primarily separated, and this secondarily precipitates the silver. The silver thrown out of solution in this way will naturally deposit at the spot where the primarily separated potassium was. The potassium has no reason for separating at any particular spot (on the silver, for instance), and therefore the deposit of silver is more uniform, and a smooth film is formed on the electrode. In these cases the current density does not require to be large; indeed, smaller current densities frequently give better results. Thus in silvering with potassium argentocyanide a current density of 0*15 — 0*5 amp./ sq. dm. is used, and in gilding with potassium auricyanide 0*2 — 0"25 amp./sq. dm.

For the reasons given, a secondary deposition is almost always used in electro-plating where the essential is a uniform deposit of the metal ; the double cyanides are used in the cases of silver, gold, and copper, and the ammonium sulphate double salts in the deposition of nickel and iron. Particularly in gilding, the process is often carried out at a high tempera- ture, which aids the secondary deposition.

It is a matter of experience that comparatively small amounts of organic substances, such as alcohol, sugar, or

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