CORRELATION OF ACID-BASE PROPERTIES OF SOLUTES WITH THE POLARITY PARAMETERS AND OTHER SOLVATOCHROMIC PARAMETERS OF DIOXANE-WATER MIXTURES

The behaviour of water-dioxane mixtures as solvents is studied paying special attention to the correlations which may exist among the solvatochromic parameters of these mixtures, as well as to the correlations among some of these parameters and the acid-base properties of model solutes (whose acidity constants are determined). Values of the polarity parameter-pi, proposed by Kamlet and Taft, are determined from measurements of shifts in lambda-max of 2-nitroanisole, 4-nitroanisole and 4-ethylnitrobenzene, proposed as reference dyes for amphiprotic solvents. The correlations of pi values with some bulk properties and different microscopic parameters of the working solvents are established. Acid dissociation constants for propionic acid, chosen as a model for aliphatic carboxylic acids, and salicylic acid, for both aromatic carboxylic and phenolic hydroxylic acids, in water-dioxane mixtures are determined from e.m.f. data (at a constant electrolyte concentration 0.2 M and at 25-degrees-C). The solvent composition range studied comprises from 10 to 70% dioxane (vol./vol.). Correlations obtained among the values of acid dissociation constants and the solvatochromic parameters for the solvent mixtures can be useful in helping to explain the variation of the acid-base character of solutes when properties of solvent are modified.