144 PHYSICAL CHEMISTRY 



the place they occupy in the series of electrolytic solution ten- 

 sions. The ZnCl 2 used was more toxic than it should be, owing 

 probably to hydrolysis, by which the solution became acid. The 

 great toxicity of the CdCl 2 is probably not due entirely to the 

 cadmium ion but partly to the unusually large proportion of un- 

 dissociated molecules which are easily adsorbed. 



In estimating the toxicity of ions, the degree of ionization 

 must be calculated. The addition of large quantities of some 

 electrolytes to solutions of some heavy metals inhibits the ioniza- 

 tion of the latter by the formation of double salts. If Nad is 

 added to HgCl 2 solution, Na 2 HgCl 4 is formed, which is very 

 poorly dissociated by steps, forming: Na' and NaHgCl/, Na' 

 and HgCl/', Hg"' and 4CI'. The number of Hg ions formed 

 is very small and the addition of quantities of NaCl reduces it 

 to a vanishing magnitude. 



The addition of NaCl increases the solubility of HgCl 2 but not 

 the concentration of Hg". If solid HgCl 2 is shaken with water, 

 the salt dissolves until there is an equilibrium between the solid 

 phase and the undissociated molecules in solution. Some of 

 these molecules 'dissociate, thus disturbing the equilibrium, and 

 more of the solid dissolves to restore it, until a second equilib- 

 rium is established between the ions and the molecules. This 



[Hg"]X[Cl'] 



equilibrium is expressed by the equation = a con- 



[HgCl 2 ] 



stant, but since the concentration of the undissociated molecules 

 is constant so long as there is any undissolved salt, the equation 

 becomes [Hg"]X[Cl'] = a constant. If we add Hg (NO s ) 2 , 

 thus increasing the concentration of Hg ions, this solubility 

 product will be exceeded and some Hg" and CI' will combine, 

 the additional HgCl 2 thus formed crystallizing out. If, on the 

 contrary, we add NaCl, the CI' will in the same way lower the 

 solubility of Hg ions, but the Na' combines with the Hg and CI 

 ions forming Na 2 HgCl 4 , thus removing them, more HgCl 2 disso- 

 ciates, and more bichloride dissolves. This process is continued 

 until the solution is saturated with Na 2 HgCl 4 . The concentra- 

 tion of HgQ 2 at equilibrium is the same as before, but the con- 

 centration of Hg ions decreases when the concentration of CI 

 ions increases, since their product is constant. The solution 



