The ethane-1,2-diol–water solvent system. The dependence of the dissociation constant of picric acid on the temperature and composition of the solvent mixture

The dissociation constants of 2,4,6-trinitrophenol (picric acid) in a set of ethane-1,2-diol–water mixtures have been determined by the conductometric method at temperatures ranging from –10 to 80 °C. The experimental k values exhibit a regular trend, with a relative maximum in the investigated temperature range for all the binary solvent systems, and they are well fitted by an equation of the type ln K=a0+a1T+a2/T+a3 ln T. In order to verify the reliability of Kmax values, a series of correlations was made, on the basis of the above equation, of Harned's theory and of a pKvs. 1/ε plot; all the methods provide results consistent with each other. Furthermore, two empirical equations for the functions K=K(χ) and K=K(T,χ) are proposed in order to try to predict the K values of the studied solute over the whole variation of the investigated T and χ. Both these model equations allow us to extrapolate the K values of picric acid in pure water solution, in which conductometric and potentiometric methods are unable to give consistent values.