Physico-chemical studies on acidity functions in mixed solvents IV Studies on the optical absorption characteristics of p-nitroaniline

ABSTRACT

The ultraviolet absorption spectra of p-nitroaniline in the 1 different alkaline solutions support the idea that the indicator reacts with the OH- ions existing in solution by proton loss rather than by hydroxide ion addition. Straight lines are obtained by plotting the molar extinction coefficients epsilon of the partially ionized form of the indicator against the reciprocals of the molar concentrations of the alkali in every solvent, with intercepts equal to the values of the molar extinction coefficient epsilon A- of the completely ionized form of the indicator. The values of the ratio R of the intercept to the slope of each line as well as of the equilibrium constant K['] are derived. The observed variations in K['] may be attributed to changes in the ratio fOR-/ fOH- or the ratio fA-/ fOR. FAH, or in both ratios. Also, a procedure is developed for the determination of the equilibrium constants K'm for the indicator in solutions of Li OH, KOH and NaOH in pure methanol, with the assumption that this constant is a true thermodynamic constant independent of solvent composition. The same procedure is followed for the estimation of the values of the equilibrium constants K' and K'm for the same indicator in solutions of potassium hydroxide in ethanol-water mixtures. The attempt is also made to calculate the equilibrium constant K degree of p nitroaniline in solutions of LiOH, KOH, and NaOH in methanol-water mixtures containing from 10-70 percent by weight of methanol. The calculated values of the constant K degree are subsequently used for the evaluation of the fraction OH-/Cb of the base present as OH- ions