ELECTROMOTIVE FORCES IN PYRIDINE 57 



calomel and allowed to stand in contact with it until saturated. 

 The single potential of the calomel electrode was calculated from 

 the value given by Kichards, 14 the values taken being -j-0.5986 

 volts at 0° and -f-0.6186 volts at 25°. These electrodes were re- 

 newed alternately every two weeks and were found to be repro- 

 ducible to within two-tenths of a millivolt. 



The electromotive forces were measured by means of a Wolff 

 potentiometer in connection with a Leeds-Northrup, "Type IT," 

 wall galvanometer. In aqueous solutions it was easily sensitive 

 to .01 millivolt, but the high resistance in non-aqueous solutions 

 made it almost impossible to detect differences of less than 0.1 

 millivolt, A Cadmium-Weston cell which had been recently 

 standardized against a similar element certified by the Bureau 

 of Standards was used as the standard of reference. It had an 

 electromotive force of 1.01745 volts of 25°. While its tempera- 

 ture coefficient is practically negligible, it was kept at this tem- 

 perature by insulating it in a beaker immersed in the 25°-bath. 



The silver electrodes were prepared according to the method 

 described by Farr. 15 Short pieces of pure silver wire were 

 fused into the ends of glass tubes. To the ends sealed into the 

 glass were soldered copper wires, each the length of the glass 

 tube. The tubes were then filled to within an inch of the top 

 with hard paraffine which prevented the mercury with which 

 the contact was made with the wire leads, from amalgamating 

 the silver. Before being used the electrodes were plated by 

 connecting them in series in a solution of potassium-silver 

 cyanide. After a current of ten milliamperes had been passed 

 for three hours, they were removed, rinsed with distilled water 

 and allowed to stand for forty-eight hours in contact with a 

 button of pure silver under a pure- aqueous solution of silver 

 nitrate. Ten or twelve electrodes were thus prepared. The 

 choice of the electrodes was made in the following manner. They 

 were all grouped in a single cell containing a 0.1 N solution of 

 silver nitrate which was in turn connected with a calomel elec- 

 trode through an intermediate solution of ammonium nitrate. 

 Only those electrodes were chosen which gave an electromotive 

 force varying not more than 0.1 millivolt from 0.3886 volts. 



It was observed early in the work that the electrode potential 

 of a freshly prepared half-cell changes on standing. This 



"Zeit. physik. Chem., 24, 29, 1S97. 

 15 Loc. cit. 



