J. H. Reedy — Anodic Potentials of Silver. 403 



Table I. Reaction Potentials of Acids. 



Acids Eeaction Potentials 



Sulphuric acid, -5 molar -521 volts 



Nitric acid, l'O molar -520 



Phosphoric acid, "33 molar "521 



Oxalic acid, '5 molar -520 



Acetic acid, l'O molar "522 



Tartaric acid, - 5 molar "521 



This particular potential has been investigated in my preced- 

 ing paper* and was found to be the potential at which 

 silver begins to dissolve, irrespective of the nature of the 

 anions present. For this reason it was given the name 

 " solution potential of silver." Evidently there is a free 

 choice among the acids in Table I, so far as margin for separation 

 is concerned. However, 0*5 molar sulphuric acid was chosen, 

 on account of its good conductivity and the fact that no 

 intermediate solution would be necessary between the cell and 

 the mercurous sulphate electrode. 



Stirring — To prevent concentration polarization and to 

 hasten the electrolysis, it is desirable that the electrolyte 

 should be well stirred. Since, as shown in my previous paper, f 

 the deposition potentials of the halogens appear to be indepen- 

 dent of the nature of the stirring, whether by a current of gas 

 or by other mechanical means, a rotating anode was adopted, 

 as it affords the most effective stirring of the anode solution. 

 Inasmuch as iodides in acid solution are readily oxidized by 

 the air to free iodine, some of which (as shown by experiment) 

 tends to be lost by volatilization, it is evident that the presence 

 of air in the cell must be avoided. On account of its high 

 density, carbon dioxide was used for this purpose. During all 

 experiments a steady stream of the gas was introduced by a 

 tube reaching almost to the surface of the solution, and 

 allowed to escape through the space around the anode stem in 

 the lid of the cell. In this way the space above the solution 

 was kept filled with this gas. 



Deposition Potentials of the Halogens. — Fig. 1 shows the 

 relation between the concentration of the halogen ions and 

 their deposition potentials. It is reproduced from fig. 5 of 

 the preceding article, which has been so modified as to show 

 only those lines which bear on the problem now being con- 

 sidered. As explained before, these lines are the " reaction 

 potential curves " of the halogens, and were obtained by plot- 

 ting the logarithms of the dilutions as abscissae and the reaction 

 potentials as ordinates. The figure accordingly shows in a 



* This Journal [4], xl, 281. flbid. 



