Preparation of mercury(II) nitrate (mercuric nitrate)
When mercury is heated with an excess of nitric acid, mercury(II) nitrate is formed. Mercury(II) nitrate is exceedingly soluble in water, and it is difficult to crystallize. When a solution of it containing an excess of nitric acid is evaporated, it becomes a thick, heavy syrup, which by further driving off of nitric acid and water becomes a pasty mass, due to formation of small crystals of basic mercury nitrate (Hg(OH)(NO3)). If the materials taken for the preparation of this salt are pure, the product can contain no other foreign matter than an excess of nitric acid; consequently, in view of the difficulty of obtaining good crystals, it is convenient to preserve the mercury(II) nitrate in this pasty condition.
25 grams of mercury are heated with 60 ml 6N (25%) nitric acid until it is all dissolved. The reaction is fully complete when a drop of the solution added in a test tube containing 1 ml of cold water and a drop of dilute hydrochloric acid (or sodium chloride solution) gives no precipitate of mercury(I) chloride. If the reaction is not complete additional 10 ml of concentrated nitric acid are added to the flask and boiled until a precipitate is no longer obtained when tested as above. The solution is poured into a casserole and evaporated over a very small free flame until the liquid has assumed a syrupy consistency and crystals just commence to form on the surface. Then the mass is transferred to a warmed stoppered bottle. Mercury(II) nitrate obtained by described method contains half molecule of water (Hg(NO3)2 · 1/2H2O)
Synthetic inorganic chemistry, by A. A. Blanchard, 233-234, 1936