246 



s. HADA. 



Potassium hypophospliite is also preferable to the barium salt, for 

 when the latter is used the precipitate is liable to contain barium, 

 apparently as nitrate. 



Mercurous nitrate, which must be frw fi-om nitrous acid, is best 

 prepared by dissolving- mercuric oxide to saturation in nitric acid and 

 shakin«' the solution violently with metallic mercury for a few 

 minutes, for in this way the mercuric salt is quickly and completely 

 changed to jnercunuis salt. 



As the wbite precipitate obtained by adding the potassium hy- 

 pophosphite to the excess of mereuroas nitrate is slowly decomposed 

 bv its mother- liquor, it must be (piiekly ;-emoved and drainerl on a 

 tile without pi-evions washing. 



Mercurous nitrate hypophosj)hite is unstable, but when dry it 

 onlv slowlv decomposes, becoming grey in the course of some days. 

 It is a white micaceous powder, slightly soluble in water, I)y which it 

 is soon decomposed with separation of mercury. Its composition is 

 expressed by the formula — 



HgH,?02,HgN03,H20 

 It loses its water in a vaeiuun desiccator with scarcely any further 

 decomposition for some time. Heated, it turns slightly grey above 

 90^ and exjDlodes a little above lOO* yielding mercury and nitrous 

 vapours. Any quantity of it can be exploded at the common tem- 

 ])erature by touching it with a hot wire. 



With hydrochloric acid, it first gives mercurous chloi'ide and then 

 metallic mercury. With cold dilute nitric acid it yields metallic 

 inercury, while hot strong nitric acid dissolves it completely with 

 escape of nitrous lûmes. Sodium chloride converts it into mercurous 

 chloride and sodium hypophosphite, which only very slowly react to 

 give metallic mercury. Potassium hydroxide blackens it by forma- 

 tion pi'obably of mercurous oxide at first. 



