404 



TABLE 519. 

 CONTACT ^VOLTA) POTENTIALS- 



There has been considerable controversy over the reality and nature of the contact differences of potential between 

 two metals. At present, due to the studies of Lan^muir, there is a decided tendency to believe that this Volta differ- 

 ence of potential is an intrinsic property of metals closely allied to the phenomena just given in Tables 516 to 518 and 

 that the discrepancies among different observers have been caused by the same disturbing surface conditions. The 

 following values of the contact potentials with silver and the relative photo-sensitiveness of a few of the metals are 

 from Henning, Phys. Rev. 4, 228, 1914. The values are for freshly cut surfaces in vacuo. Freshly cut surfaces are 

 more electro-positive and grow more electro-negative with age. That the observed initial velocities of emission of 

 electrons from freshly cut surfaces are nearly the same for all metals suggests that the more electro-positive a metal is 

 the greater the actual velocity of emission of electrons from its surface. 



From the equation w = RT logCY^/Ng), where w is the work necessary per gram-molecule when electrons pass 

 through a surface barrier separating concentrations N A and Ng of electrons, it can be shown (Langmuir, Tr. Am. 

 Eletroch. Soc. 29, 142, 1916, el seq.) that the Volta potential difference between two metals should be 



n - f 2 = j; {W2 - in + RT \og(N A /N B )} = w *~ Wl = </> 2 - </>! 



(see Table 517 for significance of symbols), since the number of free electrons in different metals per unit volume is so 

 nearly the same that RT log (N A /N g) may be neglected. The contact potentials may thus be calculated from photo- 

 electric phenomena (see Table 517 for references). They are independent of the temperature. The following table 

 gives a summary of values of <j> in volts obtained from the various phenomena where an electron is torn from the attrac- 

 tion of some surface. In the case of ionization potentials the work necessary to take an electron from an atom of metal 

 vapor is only approximately equal to that needed to separate it from a solid metal surface. 



(a) THE ELECTRON AFFINITY OF THE ELEMENTS, IN VOLTS. 



(b) It should not be assumed that all the emf of an electrolytic cell is contact emf. Its emf varies with the elec- 

 trolyte, whereas the contact emf is an intrinsic property of a metal. There must be an emf between the two electrodes 

 of such a cell dependent upon the concentration of the electrolyte used. The following table gives in its first line the 

 electrode potential e^ of the corresponding metals (in solutions of their salts containing normal ion concentration) on 

 assumption of no contact emf at the junction of the metals. The second line, $ e h 3.7 volts, gives an idea of the 

 electrode potentials (arbitrary zero) exclusive of contact emf. 



SMITHSONIAN TABLES. 



