120 ACTION OF AQUEOUS AMMONIA ON 



Other. Hg = HgCli!^Hg-|-HgCU. This equilibrium may be 

 expressed according to the law of mass action as 



[Hg] [HgCU] 



[Hg = Hga] 



K. 



Any secondary reaction that will lower the concentration of 

 mercuric chloride will cause further dissociation of the mer- 

 curous chloride until a new point of equilibrium will have been 

 reached. The addition of soluble chlorides such as NH^Cl 

 or NaCl with the formation of M2[HgCl4i has this effect to some 

 extent'^. Ammonia reacts very completely with mercuric 

 chloride to form infusible white precipitate, which is very in- 

 soluble. Therefore when aqua ammonia is added to calomel, 

 it reacts with the- small concentration of mercuric chloride in 

 equilibrium with the calomel. JMore of the calomel then disso- 

 ciates, and, if there is sufficient ammonia present, the reaction 

 proceeds to completion. When the materials are thoroughly 

 dried, the reaction with ammonia is slowed down very con- 

 siderably if not prevented entirely'^. This is probably due 

 to the fact that the presence of water is necessary for the forma- 

 tion of mercuric chloride which, evidently, must be present 

 before the reaction with ammonia takes place. 



Another fact pointing to the dissociation of mercurous 

 chloride preliminary to the reaction with ammonia is that one 

 product formed, namely infusible white precipitate, is identical 

 with that produced by the action of ammonia on mercuric 

 chloride. This last reaction may be expressed by the equation 

 HgCU + 2NH3 = HgNH2Cl-FNH,Cl, but it probably takes place 

 with a number of intermediate steps. 



An attempt to determine the structural formula of in- 

 fusible white precipitate is rather futile so long as its molecular 

 weight is not knowi\. Nevertheless, many such attempts 

 have been made. A provisional formula is often an aid to 

 further work and, for this purpose, that of R. J. Kane's and 

 E. C. Franklin"" is most suitable. Franklin has been able to 

 assign structures in agreement with the empirical formulae 



