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.
The essential experimental conditions are:
I. slight acidity of the solution with respect to hydrochloric acid or sulphuric
acid, since the solubility of the precipitate increases appreciably with decreasing pH;
the presence of a reducing agent, such as sulphurous acid
or ammonium
hydrogensulphite, to reduce copper(II) to copper(I);
a slight excess of ammonium thiocyanate, since a large excess
increases the
solubility of the copper(I) thiocyanate due to the formation of a complex
thiocyanate ion;
the absence of oxidising agents.
The reaction may be represented as:
2Cu2++HSO3-
+H2O = 2Cu++HSO4- +2H +
Cu++SCN- = CuSCN
The precipitate is curdy (compare silver chloride) and is readily coagulated by boiling. It is washed with dilute ammonium thiocyanate solution: a little sulphurous acid or ammonium hydrogensulphite is added to the wash solution to prevent any oxidation of the copper(I) salt.
Weigh out accurately about 0.4 g of the copper salt (Note 1) into a 250 mL beaker, and dissolve it in 50 mL of water. Add a few drops of dilute hydrochloric acid, and then a slight excess (about 20-30 mL are required) of freshly prepared saturated sulphurous acid solution. Alternatively, add 25 mL ammonium hydrogensulphite solution: the latter is prepared by diluting to ten times its volume the commercial concentrated solution, which has a specific gravity of 1.33 and contains about 54 per cent sulphur dioxide. Dilute the cold liquid to 150-200 mL, heat nearly to boiling, and add freshly prepared 10 per cent ammonium thiocyanate solution, slowly and with constant stirring, from a burette until present in slight excess. The precipitate of copper(I) thiocyanate should be white; the mother liquor should be colourless and smell of sulphur dioxide. Allow to stand for two hours, but preferably overnight. Filter through a weighed filtering crucible (sintered glass, or porcelain), and wash the precipitate 10 to 15 times with a cold solution prepared by adding to every 100 mL of water 1 mL of a 10 per cent solution of ammonium thiocyanate and 5-6 drops of saturated sulphurous acid solution, and finally several times with 20 per cent ethanol to remove ammonium thiocyanate. Dry the precipitate to constant weight at 110-120 °C. Weigh as CuSCN.