Physico-chemical studies on acidity functions in mixed solvents V Studies on the acidity function H of solutions of LioH, NaOH and KOH in methanol-water mixtures using p-nitroaniline as indicator

ABSTRACT

The limited range of the operational pH[x] from 0 14 is a drawback of this acidity function. The validity of Hammett function H0 in solutions of strong acids in alcohol -water mixtures stands behind the desire to construct a scale of acidity in solutions of strong alkalies in these mixtures which are frequently used. For this purpose, calculations are made to calculate the values of H- from the spectrophoto-metrically determined molar extinction coefficients epsilon,, epsilon A- and epsilon HA of p-nitroaniline in solutions of LiOH, NaOH and KOH in methanol-water mixtures. By plotting log. H- against the reciprocal of the stoichiometric molar concentration C of every alkali solution, straight lines are obtained whose intercepts and slopes are computed by use of the least-squares method. These values facilitate the derivation of the value of H- for any of the solutions studied by interpolation. On the other hand, the plots relating H. with medium composition disclose several maxima and minima the location of which does depend on alkali concentration and identity. This reflects the medium effect on the trend of variation of H- in the different solutions. The basic strength in water and pure methano1 increases according to the order KOH>NaOH> LiOH, whereas in methanol-water mixtures is a composite function of alkali concentrations and medium composition. These findings are explained in terms of variations in s Ivation and medium effect with addition of alcohol to aqueous solution. Hence, the solvation factor[r-p-I]and medium effect of log f[mlH] are estimated for the I M solution of the three alkalies. Besides, the values of the O H- ion concentration in solution, the autoprtolysis constant pK[m] and the parameter Beta are given and the validity of H- is emphasized by plotting the logarithm of the indicator ratio I against H- where linear plots are obtained with unit slope