Mercury thiocyanate-iron method

The method can be used for the direct determination of the chloride ion content in precipitation samples within the 0.05-5 mg/L range. Chloride ions will replace the thiocyanate ions in undissociated mercury thiocyanate. The thiocyanate ions thus released react with ferric ions to form a dark red iron-thiocyanate complex. 

Determination of copper as copper(I) thiocyanate Discussion. This is an excellent method, since most thiocyanates of other metals are soluble. Separation may thus be effected from bismuth, cadmium, arsenic, antimony, tin, iron, nickel, cobalt, manganese, and zinc. The addition of 2-3 g of tartaric acid is desirable for the prevention of hydrolysis when bismuth, antimony, or tin is present. Excessive amounts of ammonium salts or of the thiocyanate precipitant should be absent, as should also oxidising agents the solution should only be slightly acidic, since the solubility of the precipitate increases with decreasing pH. Lead, mercury, the precious metals, selenium, and tellurium interfere and contaminate the precipitate. 

Mercury(II) thiocyanate method Discussion. This second procedure for the determination of trace amounts of chloride ion depends upon the displacement of thiocyanate ion from mercury(II) thiocyanate by chloride ion in the presence of iron(III) ion a highly coloured iron(III) thiocyanate complex is formed, and the intensity of its colour is proportional to the original chloride ion concentration ... 

Dilute hydrochloric acid hydrocyanic acid, HCN, with an odour reminiscent of bitter almonds, is evolved in the cold. It should be smelled with great caution. A more satisfactory method for identifying hydrocyanic acid consists in converting it into ammonium thiocyanate by allowing the vapour to come into contact with a little ammonium polysulphide on filter paper. The paper may be conveniently placed over the test-tube or dish in which the substance is being treated with the dilute acid. Upon adding a drop of iron(III) chloride solution and a drop of dilute hydrochloric acid to the filter paper, the characteristic red colouration, due to the iron(III) thiocyanate complex, Fe(SCN)3, is obtained (see reaction 6 below). Mercury(II) cyanide is not decomposed by dilute acids. 

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