IOWA ACADEMY OF SCIENCES. m 



If we call the original concentration of zinc C, and 

 start with a neutral solution the equivalents of ZnS pre- 

 cipitated or the HCl set free at any moment X, — and the 

 specific rates of the reactions from left to right and from 

 right to left, respectively, k and k„ then the rates at any 

 moment will be (C - K)h.k and Ko.k' and equilibrium will 

 be reached when (C - X)h.k = X„.k', where h represents the 

 hydrogen sulphide which is regarded here as constant. 



Before proceeding with the presentation of the data, a 

 few w^ords regarding the quantitative methods used are 

 desirable. It was necessary to determine the total acid in 

 each solution at the beginning, and the metal remaining 

 in the solution at the end of any chosen period. From 

 these data the free acid at the end of any period coukl be 

 calculated. Sulphuric acid was determined as barium 

 sulphate in the usual way, hydrochloric acid volumetric- 

 ally by the method of Volhard, cadmium as the sulphide 

 which was weighed in a Gooch crucible, and zinc by the 

 ferrocyanide method of Lyte. 



The last named method seems not to have attained a 

 use to which its accuracy and rapidity entitles it, and a 

 few Avords in recommendation of it are not deemed inap- 

 propriate. The solution is acidified with hydrochloric 

 acid and titrated with a solution of potassium ferrocyanide 

 which has been standardized by a zinc solution of known 

 strength, the titrations being carried on at about 70°. 

 Uranium acetate is used as an indicator. A few drops 

 may be added to the solution which is colored brown when 

 the ferrocyanide is added in slight excess. It is more 

 exact to bring a drop of the solution under titration and 

 near the end point, in contact with a drop of the indicator 

 on filter paper. A brown line is formed where the two 

 drops flow together. With care titrations are concordant 

 to a few hundredths of a per cent., and the method seems 

 to deserve rank among the standard methods of volumetric 

 analysis. 



The zinc used in this work was a pure distilled specimen 

 from Schuchardt, which dissolved without residue and gave 



