l8 ENERGY CHANGES INVOLVED IN DILUTION OF AMALGAMS. 



desiccator. In the morning suitable portions of the zinc and of pure mercury 

 were weighed out into a small tube provided with a ground-glass stopper. 



Since it proved difficult to combine dry zinc or cadmium with mercury 

 without heating, it became desirable to shake them together under some 

 reagent capable of dissolving the superficial film of oxide. The same pro- 

 cedure was adopted in diluting some of the amalgams with mercury. Di- 

 lute ammonia is suitable for the purpose, because it attacks pure zinc and 

 cadmium but slightly and because the last traces of it are removed in 

 a vacuum. The gas was distilled into pure water until the solution had 

 acquired a strong odor. The reagent thus obtained was poured upon the 

 components of the amalgam in the stoppered tube, and vigorous shaking con- 

 tinued until the process of solution was complete. The analysis of the 

 aqueous solution for dissolved zinc and cadmium was carried out volumet- 

 rically with a solution of potassic ferrocyanide, 9.6 grams of the crystallized 

 salt per liter, a dilute copper sulphate solution being used as an outside 

 indicator. To standardize the ferrocyanide a weighed quantity of electro- 

 lytic zinc was dissolved in dilute sulphuric acid, made up in a graduated 

 flask, and known volumes were titrated under the conditions later prevailing 

 in the analysis of unknown solutions. 



This same ferrocyanide solution produces a very insoluble precipitate in 



solutions of cadmium salts, but the composition is not Cd 2 Fe(CN) 6 , as one 



might expect. 41 The various formulas proposed by different authorities are 



all capable of generalization in the expression Cd a Kj / [Fe(CN) 6 ] 2xv. 



4 



Evidently we are dealing with a mixture or loose molecular compound of 

 Cd 2 Fe(CN) 6 and CdK 2 Fe(CN) 6 in proportions that vary considerably 

 according to concentration effects and temperature. Under the conditions of 

 the work here described, the standard ferrocyanide solution was consistently 

 found by a number of analyses to be equivalent to 0.00299 gram of zinc and 

 0.0029 g" ra m f cadmium per milliliter. 



A dilute solution of ammonia that has been shaken with pure mercury 

 gives no precipitate with potassic ferrocyanide ; therefore it is safe to assume 

 that the zinc or cadmium content was determined by the volume of ferro- 

 cyanide added, minus one final drop, whose size was estimated from 0.0 1 to 

 0.04 cc. according to the volume of the ammonia solution and the intensity 

 of the end-point noted. The estimation was based upon a series of blank 

 experiments. For some very dilute solutions an excess of the ferrocyanide 

 solution was added and the resulting opacity compared with a known sus- 

 pension of a second precipitate. The latter was diluted until equal opacity 

 was reached, and the familiar calculation made. 



Journ. Am. Chem. Soc, 22, 537 (1900). 



