ELECTRODE POTENTIALS 167 



is also electromotively active, and at high temperatures (over 300°) 

 and by employing a solid electrolyte (glass) instead of an electrolyte 

 solution, such 02-electrode behaves, as Haber^ found, exactly as a 

 OH~-ion jdelding electrode would lead us to expect. In other words, 

 a chain consisiting of a H2- and of an 02-electrodes, both immersed 

 into one and the same Hquid of any composition, the so-called oxy- 

 hydrogen gas chain, has that E.M.F. which can be calculated from 

 the chemical affinity of the reaction of the explosive mixture of 

 oxygen and hydrogen. At lower temperatures, however, the usual 

 oxy-hydrogen gas electrode shows a poorly adjusted potential which 

 is about 0.3 volt too low. Otherwise one could determine 0H~- 

 concentrations with an O2 electrode, just as we determine the H+- 

 concent rat ions with an H2-electrode. But in reahty this is not pos- 

 sible. Even though Barendrecht- claims the opposite, he disagrees 

 with all other investigators. As a cause of this irregular behavior 

 of the 02-electrode it is assumed that in the presence of oxygen the 

 platinum is covered by a thin layer of an oxide; hydrogen, on the 

 other hand, reduces this layer and, therefore, the H2-electrode be- 

 haves correctly from the standpoint of theory. 



50. The electromotive series 



The potential of a metal against a solution is determined by the 

 nature of the electrode metal as well as by the concentration of the 

 corresponding ions in the solution. Up to this point we have assumed 

 the nature of the electrode metal as being constant and we have 

 discussed only the effect of the concentration. Now we shall briefly 

 deal with the effect of the nature of the metal. The various electro- 

 lytic solution tensions of the metals could be characterized by giving 

 that concentration of ions against which the metal shows zero 

 potential. But since the absolute value of single potentials cannot 

 be very accurately determined, it is better to select another way. 

 That potential difference which exists between a metal and a normal 

 solution of its ions is arbitrarily assumed to be standard. Among 

 such standards that of any one metal may be arbitrarily set equal 

 to 0. At the suggestion made by Nernst we designate the potential 



1 F. Haber, Ann. d. Physik [4]. 26, 927 (1908); Idem., Thermodynamics of 

 Technical Gas Reactions. London 1908. 



2 H. P. Barendrecht, Akad. Wetensch. Amsterdam 27, 1113, 1236, 1406; 

 28, 23 (1919). 



