Page:Text-book of Electrochemistry.djvu/297

 282 ELECTRO-ANALYSIS. chap.

instance, to determine iron in presence of nickel or cobalt, both metals are completely deposited, and the Weight ascer- tained ; the mixed metal is then dissolved in sulphuric acid, and by titration with potassium permanganate the quantity of iron present is determined.

Zinc, which under ordinary circumstances cannot be separated electro-analytically from the metals of the iron group, can be separated (it deposits first) if we use a potassium cyanide solution. Zinc, iron, nickel, and cobalt can be separated from aluminium and chromium, because these two latter elements are not deposited in the metallic condition.

In potassium cyanide, double oxalate, or sulphuric acid — 3 '6 volts). Silver can be separated from copper in nitric acid solution by using an electromotive force of 1*36 volts, in potassium cyanide solution by using 2*3 — 2*4 volts. Mercury behaves similarly to silver. Copper can be sepa- i-ated from cobalt, and nickel from copper in hot oxalate solution (60°), and from manganese in presence of free oxalic acid (t = 80°). Copper is deposited from sulphuric acid solution when the electromotive force is 1*85 volts, whilst cadmium remains dissolved. Mercury can easily be sepa- i-ated from iron, cobalt, nickel, zinc, or cadmium in nitric acid solution.

Antimony, in presence of arsenic (as arsenic acid, into which form the arsenic is transformed by the current, pro- vided that alkali is present) and tin, are precipitated from a concentrated sodium sulphide solution. Arsenic can best be separated from tin by chemical means. These three metals should first be separated from other metals by ammo- nium sulphide, and the mixture then analysed by electrolysis.

Primary and Secondary Deposition of Metal. — More than forty years ago, Bunsen observed that metals deposited secondarily have a much more even and brighter surface than those which ai'e primarily separated. It has also been observed that primarily evolved hydrogen leaves

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