70 IOWA ACADEMY OF SCIENCE 



concentrated solution, the electrode potential drops very rapid- 

 ly with the first dilutions, and then decreases almost linearly in 

 the more dilute regions. It will also be observed from the vol- 

 ume-electrode potential curves for any set of binary mixtures 

 and hence for all of the pure solvents as well, that the curves 

 obtained are practically parallel to each other. If they were 

 exactly parallel, it would follow, as was stated by Farr, 20 that, 

 "if the electromotive force at the junction of the two solutions 

 has been entirely eliminated, and since the electromotive force 

 of a concentration cell at a given temperature is proportional 

 to the logarithm of the ratio of the ionic concentrations in the 

 two solutions, it follows that the ratio between the ionic concen- 

 trations for equivalent concentrations of the salt in the separate 

 solvents is constant and independent of the dilution." 



The mean temperature coefficient of the electrode potentials 

 in each solvent has been calculated for both the pure solvents 

 and their mixtures and tabulated at the bottom of the tables for 

 the electrode potentials. All of the temperature coefficients are 

 positive except those in the more dilute solutions in the pure 

 water. In all solutions containing pyridine the temperature 

 coefficients are extremely large. In the water-pyridine series 

 they increase continually from the value in pure water to the 

 value in pure pyridine as the per cent of pyridine is increased. 

 In the ethyl alcohol-pyridine solutions they increase rapidly with 

 the first addition of pyridine, then decrease to practically the 

 value in pure pyridine after fifty per cent of the alcohol is re- 

 placed by the pyridine. In the methyl alcohol-pyridine series the 

 temperature coefficients increase rapidly to a maximum value 

 in the fifty per cent mixture, then decrease slowly to the value 

 in pure pyridine as the proportion of alcohol is diminished. 

 The same relations obtain for binary mixtures of pyridine and 

 ethyl alcohol, except that the maximum occurs in the presence 

 of a smaller proportion of pyridine. 



The heats of ionization are given in Table XIV. They were 

 calculated by substituting the mean temperature coefficients 

 and the electrode potentials for the 0.1 N solutions in equa- 

 tion (5). The heat of ionization in pyridine is very low. In 

 any series of solvent mixtures, the heats of ionization decrease 

 the most rapidly upon the first addition of pyridine to the 

 second solvent, and then more slowly as the per cent of pyri- 



»Loc. cit. 



