102 



DIVERS AND HAGA. 



excess of iiitrnte rem-nuing, is oxijmercunc Injdrugeu salt. With too 

 Jittle nitrate remaiiiiiig, some oxyiiierciiric sodium salt will be 

 deposited. Mercuroiis nitrat.^ behaves in the main like mercuric nitrate 

 but gives besides a j)recipitate of the metal. 



Mercuric disodiuiii hiiidosiiljHioNaie. — Little more needs to l)e stated 

 concerning the preparation of this salt from the trisodium salt. Here, 

 as in other cases, the mercuric nitrate solution is preferablv to be 

 highly concentrated, because then the excess of nitric acid necessary 

 becomes very small. To form such a solution, moderately concentrated 

 nitric acid should be so far saturated with mercuric oxide that oxyni- 

 trate beg-ins to form, and then decanted from excess of oxide and left 

 to clarify by subsidence. When too much mercuric nitrate has been 

 added to the s<^dium salt, the proportions can be rectified quite suc- 

 cessfully by adding more sodium salt. The mercuric disodium salt 

 can be purified if necessary, liy recrystallisation from hot water. 



Mercuric disodium iinidosulphonate can also be readily prepared 

 from disodium imidosulphonate and mercuric oxide. The two sub- 

 stances may be triturated together in al)Out the right proportions 

 mixed with water, and then warmed witli it. The solution is to be 

 filtered if necessary, and then set aside to crystallise. 



The crvstals of mercuric disodium imidosulphonate are small bril- 

 liant prisms, always separate, quite permanent in the air, and spar- 

 ingly soUible in cold water. The solution has a, neutral reaction. 

 The crystals contain six molecules of water of which only four are lost 

 in a vacuum at common temperatures. 



Heated to 100°, after exposure' in a vacuum desiccator, it loses most 

 of the remaining water, but not all, tor then (and at higher tempera- 

 tures, such as that of 130°, more quickly) it also increases slowly in 

 weight by fixing atmospheric moisture, becoming hydrolysed and 

 thereby strongly a(nd (see effects of heating dipotassium imidosulpho- 



