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



20H' ^0 2 + H,0- 108,800 cal.* 



2Ag + £0 1 >Ag 2 + 5,900 cal.f 



Ag,0 + H,S0 4 .Aq >Ag 5 S0 4 .Aq + H,0 + 14,490 cal.f 



20H' + 2 Ag + H,S0 4 . Aq > 



Ag J S0 4 .Aq + 2H,0 — 88,410 cal. 



Compared with the energy required to effect in this way the 

 solution of two gram equivalents of silver, the heat required to 

 form two gram ions of silver by direct ionization is only 46,600 

 calories.;}: From this it appears that silver most probably 

 passes into solution by ionization, and not by the discharge of 

 the hydroxyl ions of the water, (iii) The direct ionization of 

 silver must be postulated to explain the equilibrium between a 

 silver electrode and Ag - -ions, as in the case of all other elec- 

 trodes of the first class. The necessity in that case should at 

 least count for probability in the case of solution potentials. 



The relation between the "solution potential" and the 

 " electrolytic solution pressure " of silver is yet undetermined. 

 Without doubt, other influences, such as electrostatic attraction, 

 are operative in determining the solution potential. 



In this connection, the energy relations of solutions which 

 show two reaction potentials come up for discussion. In 

 addition to potassium bromate, this behavior was found in 

 dilute solutions of chlorides, bromides, iodides and hydroxides. 

 In the case of the bromate curve, for example, the question 

 presents itself : Since in both reactions the same substance is 

 formed (i.e., silver bromate), why should the potential be 

 different, when the latter is understood to be a measure of 

 the chemical work done? 



The answer is, that the lower potential doubtless represents 

 the work done in depositing a bromate ion of that concentra- 

 tion on silver, while the higher potential represents the work 

 done in forming a silver ion. To be sure, the silver ion is at 

 once precipitated by a bromate ion of the solution, forming 

 the same AgBr0 3 as before. It then follows that the total 

 change in free energy is the same in both cases; but in the 

 latter case the combination of the silver radical and the 

 bromate radical occurs in solution, and not on the electrode. 

 Hence the difference in potential may be explained upon the 

 assumption that the energy of ionic combination appears as 

 heat, and not as electrical energy. 



Concentration Effects in the Light of the Nernst Formula. 

 — Figure 5 shows the influence of concentration on the anodic 



*Ostwald,.Grund. allg. Chem., 1909, p. 309. 

 f Thomsen, Thermoch. Untersuch., Ill, p. 381. 

 % Ostwald, Grand, allg. Chem. 1909, p. 309. 



