Note 21 — Chapter XVI · ZINC, CADMIUM, AND MERCURY

To obtain the mercuric salt a large excess of strong sulphuric acid must be taken and strongly heated. With a small quantity of water colourless crystals of HgSO4,H2O may be obtained. An excess of water, especially when heated, forms the basic salt (as in Note 20), HgSO4,2HgO, which corresponds with trihydrated sulphuric acid, SO3 + 3H2O = S(OH)6, with the substitution of H6 by 3Hg, which in mercuric salts is equivalent to H6. Le Chatelier (1888) gives the following ratio between the amounts of equivalents per litre:

HgSO₄ 0·318 0·890 1·80 2·02

SO₃ 0·752 1·42 2·10 2·40

—that is, the relative amount of free acid decreases as the strength of the solution increases.