J. H. Reedy — Anodic Potentials of Silver. 281 



Art. XXII. — Anodic Potentials of Silver: I. The Deter- 

 mination of the Reaction Potentials of Silver and their 

 Significance ; by John Henry Reedy. 



(Contributions from the Kent Chemical Laboratory of Yale Univ. — cclxix.) 



It is generally recognized that electrode reactions are con- 

 ditioned by the existence of a more or less definite potential 

 difference between electrode and solution. In general, these 

 reactions consist in (a) the discharge of anions, (b) the forma- 

 tion of cations, or (c) change of valence of ions. The potentials 

 corresponding to these cases are called (a) " deposition " or 

 "discharge" potential, (b) "solution" potential, and (c) 

 "oxidation" (or "reduction") potential. The reactions dis- 

 cussed in this paper belong wholly to the first two cases. 



As it is desirable to have a term to include all of these and 

 to represent, in general, the potential at which a reaction takes 

 place, the expression "reaction potential" will be used.* In 

 all cases this potential is dependent upon temperature and con- 

 centration of the substances or ions involved. When elec- 

 trolytes, such as sulphuric acid with platinum electrodes,f show 

 two or more reaction potentials, it is generally assumed that 

 each represents the beginning of a new reaction. 



The Determination of Reaction Potentials. — Since the 

 reaction potential is the potential difference that must exist 

 between an electrode and a solution in order that the reaction 

 may begin, it can therefore be found by making the metal one 

 of the electrodes of an electrolytic cell, and determining (by 

 comparison with some standard electrode) the potential at 

 which an appreciable current begins to pass. This potential is 

 most accurately measured by the third electrode method,:}: in 

 which no current flows in the subsidiary circuit, thus insuring 

 a constancy of potential for the reference electrode. This 

 method is suitable for all cases, whether the action is reversible 

 or not, and has been used exclusively in this work. 



LeBlanc§ has shown that, for reversible electrodes, the reac- 

 tion potential is the same as the potential shown by the metal 

 when immersed in the solution, and hence may be readily 

 determined by electromotive force measurements. This 

 method, however, can not be used when the action is irrevers- 

 ible, owing to polarization effects. 



* The term " electrode decomposition potential " used by some writers is 

 practically synonymous with " reaction potential," but, in the opinion of 

 the writer, is open to objection. Preferably "decomposition potential" 

 should be limited to the potential drop across the cell required to effect 

 electrolysis. 



fLe Blanc, "Electrochemistry," 1900, p. 307. 



jCf. Bose, Zeitschr. Elektroch., v, 153. 



§ Zeitschr. phys. Chem., xii, 833. 



