Page:Text-book of Electrochemistry.djvu/290

 during washing and drying. On the other hand, the peroxides of both metals can be produced at the anode in a stable, coherent form. To obtain the best results a matt-platinum basin should be used. When a lead salt is electrolysed, at least 10 per cent, of nitric acid must be added, which - 60^ Z> = 1-5, J? == 2-5 volts). The precipitate must be washed before the current is stopped, and in order to get rid of hydrate water it must be dried at 180 — lOO''. The presence of chlorides or metals precipitable by hydrogen sulphide should be avoided.

In depositing manganese as peroxide, about 10 grams of ammonium acetate and 2 grams of chrome alum are added to 0*7 gram of manganous sulphate, and the electrolysis is amp./sq. dm. (-& = 3 — 5 volts). The chrome alum removes the oxygen sej)arated at the anode, which would otherwise prevent the deposition of a coherent precipitate. After wash- ing, the precipitate, which consists of a hydrated peroxide, is converted into mangano-manganic oxide, Mn304, by heating the platinum basin with the point of a blow-pipe flame. It is advisable also to rewash this residue in order to free it from chromic acid. When other metals are present which would be precipitated on account of the existence of the chromium in the solution, the chrome alum is replaced by 5 - 10 C.C. of alcohol {t = 70^ D = 0-15, E = 12 volts). Good results can only be obtained in the absence of chlorides.

Reduction of Nitric Acid to Ammonia.— Another secondary process which has been used in analysis is the cathodic reduction of nitric acid to ammonia. According to Ulsch (5), the best method of carrying out the reduction is to add to the nitrate solution a known excess of sulphuric acid, and to use a copper wire spiral as cathode, and a platinum wire held in the centre of this as anode. The current density at the cathode, which at the beginning may be about 1*5 amp./ sq. dm., gradually sinks as the amount of acid becomes smaller. In the earlier parts of the process the hydrogen

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