56 IOWA ACADEMY OF SCIENCE 



were made up to volume at 25° and, to avoid the possibility of 

 any decomposition occurring in the solutions, all electrometric 

 measurements were made on the same day. 



Mixtures. — The solvent mixtures were made up on a percent- 

 age basis by weight, the weights of the separate components be- 

 ing accurate to 0.1 gram per liter. 



APPARATUS. 



The apparatus used in this work was the same as that used 

 by Farr. 12 The constant temperature baths consisted of large 

 deep metal boxes inclosed within larger wooden boxes, the space 

 between being filled with insulating material. The 0°-bath was 

 obtained by clean finely crushed ice moistened with distilled 

 water. The water in the 25°-bath was kept in rapid circulation 

 by a mechanical stirrer. It was electrically heated and main- 

 tained at 25°+ .01 by an electrically controlled temperature regu- 

 lator. 



Seven half-cells and two calomel electrodes were used in this 

 investigation. Each half-cell was fitted with a stop-cock in the 

 connecting tube. These were always kept closed except when 

 measurements were being made. Loose plugs of filter paper in- 

 serted in the ends of the connecting tubes practically eliminated 

 any possible diffusion potential even when the stop-cocks were 

 momentarily opened for potential readings. The middle vessel 

 was so arranged that the connecting tubes of all the cells could 

 be inserted through tight-fitting rubber stoppers. With this 

 arrangement the solutions were not unduly exposed to the air 

 and the measurements could be made on any combination by 

 simply changing the wire leads and opening the stop-cocks in 

 the connecting tubes. A normal aqueous solution of ammonium 

 nitrate was used in the middle vessel. It was assumed that 

 this solution eliminates the diffusion potential. 13 



The calomel electrodes were prepared in the following manner. 

 In the bottom of the electrode vessel was placed a large globule 

 of pure mercury. This was next covered by a calomel paste pre- 

 pared by intimately mixing calomel and mercury moistened with 

 0.1 N solution of potassium chloride. Over this was placed a solu- 

 tion of the 0.1 N potassium chloride which had been shaken with 



12 Loc. cit. 



13 Ostwald-Luther : Messungen, 3d Ed., p. 448. 



