RICHARDS AND ARCHIBALD. — CONCENTRATED SOLUTIONS. 359 



nearly as the present means can determine, the reaction which we have 

 been studying is to be thus represented in its simplest form : — 



Hg 2 Cl 2 + 2C1' = Hg + HgCl 4 ". 



It is of interest to classify the equilibrium under consideration accord- 

 ing to the Phase Rule of Willard Gibbs. Looked at from this point of 

 view, we may speak of the system as consisting of four components, — 

 water, soluble salt, mercury, and mercuric chloride. It is clear, therefore, 

 that when we have together the four phases, — mercury, mercurous 

 chloride, solution, and vapor, — at a fixed temperature, a single condition 

 of freedom remains to be fixed in order to fix the system. The concen- 

 tration of the ionized chlorine seems to supply this sixth (n + 2d) con- 

 dition, determining the fixed points in the tables. 



At the seven times normal point the concentration of the mercury dis- 

 solved seems to attain almost a constancy, being no longer increased by 

 further addition of soluble electrolyte. According to the Phase Rule, 

 such a phenomenon might be caused by the appearance of a new phase. 

 This new phase would of course be one which would remove hydro- 

 chloric acid from the solution - } hence its presence or absence is easily 

 discovered. 



As a matter of fact, we found that after continued shaking with 

 calomel, hydrochloric acid having an original concentration of 9.22 

 normal was reduced only to 9.20 normal. This is quite too small a 

 difference to be due to the formation of a new phase ; it must be ascribed 

 either to adsorption by the calomel or to analytical error. 



Hence the constancy of mercury dissolved is to be ascribed to con- 

 ditions within the solution, and not to the appearance of a new 

 phase. 



Since the reaction seems to be effected primarily by the action of the 

 chloride ion, it might be used to determine the concentration of the 

 chloride ion, — or in the corresponding cases, that of the bromide or 

 iodide ion. Especially would the case be applicable to the ionized 

 chlorine because here the amount of mercury dissolved is too small to 

 affect seriously other equilibria existing in the solution. Of course, with 

 very dilute solutions the solubility of mercurous chloride itself would 

 have to be taken into account. 



This tendency of mercuric chloride to add to the chloride ion is 

 a highly interesting circumstance. Other similar phenomena are being 

 more and more frequently reported.* The tendency of cadmium to form 



* Cushman, Zeitschr. fur anal. Chem., 34, 3G8 (1895). 



