Gravimetric Determination of Cobalt

In neutral solution, cobalt(lI) salt solutions react with mercury(ll) chloride and ammonium diiocyanate to produce a blue precipitate Co[Hg(SCN)4] which is soluble in acids and in excess of the precipitating solution. Hence it is necessary to control the conditions carefully for quantitative analysis. 

Weigh out accurately about 2.5 g of purest ammonium cobaIt(II) sulphate, dissolve it in water and make up to 250 cm . If during dissolution a precipitate appears, add 

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