310 Van Name and Hill — Solution, of Metals 



subjected to any special tests for purity. Both silver and cop- 

 per retained a bright clean surface in dissolving in the ferric 

 sulphate, but both gave in all cases velocity constants which 

 decreased as the experiment progressed, though this effect was 

 much more marked with silver than with copper. 



For silver, at least, the explanation is clear, since the reac- 

 tion between silver and ferric ion has been shown to be far 

 from complete. Noyes and Brann* found that only about one- 

 fourth of the ferric ion was reduced at 25°. The rate of the 

 chemical reaction proper must therefore decrease with increase 

 in the silver content of the solution, which accounts for the fall- 

 ing constants in our silver experiments. The correctness of 

 this explanation is placed beyond question by Experiment 11, 

 in which silver sulphate, added at the outset, produced a large 

 decrease in the reaction velocity. f 



By analogy, the decrease in the copper constants should be 

 explainable in the same way. Electrochemical evidence tend- 

 ing to confirm this is found in Table VI, where it is shown 

 that a copper electrode becomes more positive toward a solution 

 of ferric sulphate when copper sulphate is added, thus proving 

 that the reaction is to some extent reversible. Silver and cad- 

 mium electrodes show a like behavior. All potentials in the 

 table are referred to the normal calomel electrode as + O560 

 volts, but no corrections have been applied for diffusion poten- 

 tials. The values lay no claim to accuracy, as the concentra- 



Table VI. 

 Metal. Solution. Single Potential 



Silver '/20 m. KFe(S0 4 ) a , X m. H 2 S0 4 , +0937 



" " " 1% " " +0-919 



" " " 5 " " +0-883 



" " " X" " V200 m. Ag«SOi , +0-948 



Copper " " " % " " +0-540 



" " " 1% " " +0-525 



" " " 5 " " +0508 



" " " X" " '/lOO m. CuSO-4, +0-549 



Cadmium " " " X " " -0-205 



" " " " IX " " -0-224 



" " " 5 " " -0-236 



" " " X" " '/lOO m. CdSOi , -0-200 



Silver 0-015m.CrO 3 , X" " +0968 



" " " " 5 " " +0-894 



" " " " " " >/200 m. Ag 2 S0 4 , +0-905 



*Jour. Am. Chem. Soc, xxxiv, 1016, 1912. 



f A part of the silver sulphate added was still undissolved at the start, but 

 dissolved during the course of the experiment. This was no doubt the cause 

 of ihe falling constants. 



