Salt analysis-1

$Hg(NO_{3})_{2} + H_{2}SO_{4} \longrightarrow HgSO_{4} + 2HNO_{3}$

Mercuric sulphate is unstable in an aqueous solution,

$3HgSO_{4} + 2H_{2}O \longrightarrow Hg_{3}SO_{6} + 2H_{2}SO_{4}$

The compound compound $Hg_{3}SO_{6}$ is called mercuric subsulphate and precipitates as a yellow solid.

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On combining both the reactions we get,

$Hg(NO_{3})_{2} + \frac{1}{3}H_{2}SO_{4} + \frac{2}{3}H_{2}O \longrightarrow \frac{1}{3}Hg_{3}SO_{6} + 2HNO_{3}$

We also know that, during hydrolysis of a sparingly soluble salt like, $BaSO_{4}$ the following equilibrium persists:

$BaSO_{4} + 2H_{2}O \leftrightharpoons Ba(OH)_{2} + H_{2}SO_{4}$

So the precipitation reaction is initiated by the consumption of a small quantity of sulphate ions. Le Chatelier's principle dictates that the equilibrium will shift to the right producing more sulphate ions, this is because the pre-existing sulphate ions were involved in another reaction. This cycle of sulphate production and consumption persists until almost all of the sulphate ions are precipitated as Mercuric subsulphate.