smaller than in the absence of methanol. Curve C in Figure 3 is that calculated in the presence of 0.5M of the strong hydrogen bond donor p-bromophenol. The heteroconjugation constants of AHR~and A(HR)-r, Adenoting 3.5-dinitrobenzoate and HR, p-bromophenol are 3.6 X 101 2 and 3.9 X 103 4, respectively, as determined previously (2).The dissociation constant in AN of p-bromophenol is of the order of 10~27, that of 3,5-dinitrobenzoic acid is 10-17 (6), so that negligible proton transfer occurs in the reaction between 3.5-dinitrobenzoate and the phenol. The potentiometric titration curve in the presence of 0.5M p-bromophenol is virtually identical with that of a weak acid in water and the apparent pKdnA. is 3 units smaller than in the absence of the phenol. Use of this effect can be made in the titration of mixtures of acids in AN.In conclusion, acid-base equilibria in AN are generally much more complex than in aprotic protophylic solvents like dimethylsulfoxide. The latter solvents are much to be preferred over AN for the titration of weak acids. On the other hand, AN is one of the best solvents for the titration of weak bases, as will be shown in a subsequent paper.

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