This method is based upon the fact that cobalt(II) in almost neutral solution forms a blue complex salt Co [ Hg (SCN )4 ] with a reagent prepared by dissolving 1 mole of mercury (II) chloride and 5 moles of ammonium thiocyanate in water. The precipitate is sparingly soluble in water, soluble in acids and in a large excess of the reagent, soluble in diethyl ether, chloroform, and carbon tetrachloride, and sparingly soluble in absolute ethanol. It may be dried at 100-110 °C. The following elements interfere: copper, cadmium, zinc, iron(II), iron(III), nickel, manganese(II), bismuth, silver and mercury(II); iron(III) may be rendered innocuous by the addition of phosphate.
The almost neutral sample solution may conveniently contain 35-40 mg of cobalt in 25 mL and be free from the interfering elements mentioned above. Add, with constant stirring, 4.8 mL of the 4.5 % mercury(II) chloride solution followed by 5.2 mL of the 6% ammonium thiocyanate. Do not scratch the sides of the beaker with the stirring rod. A dark blue precipitate forms after stirring for 1-3 minutes; continue the stirring for a further 2-3 minutes and allow to stand for 2 hours at room temperature. Collect the precipitate in a weighed sintered-glass (porosity No. 4) or porcelain filtering crucible; use the filtrate to assist the transfer of any residual precipitate in the beaker. Wash the precipitate with 2-3 mL of a dilute solution of the precipitating reagent (prepared by adding 1.0 mL each of solutions 4.5 % mercury(II) chloride) and 6% ammonium thiocyanate to 100 mL of water) and finally with 5 mL ice-cold water. Dry at 100 °C. Weigh as Co[Hg(SCN)4].