Studies on the analytical applications of the hydroxamic acids t-3 require a knowledge of their ionisation constants. In recent years many analogous compounds have been prepared by the introduction of various substituent groups in different positions in the hope of developing reagents with superior analytical properties. This empirical approach, although sometimes inescapable, is generally wasteful and it is desirable to determine physico-chemical properties of the reagents and their metal complexes, in particular the ionisation constants, which could lead to a better understanding of the causes of selectivity and the sensitivity of relevant analytical reactions.
N-phenyl-m-methylbenzohydroxamic acid is a useful reagent for the gravimetric determination of metal ions 4. Data on its ionisation constants should be of importance in various analytical procedures and useful in the elucidation of the structure of the resulting metal complexes. With this in view the present communication reports on the thermodynamic ionisation constants, pKa, of N-phenyl-mmethylbenzohydroxamic acid in 10, 20, 30, 40, 45 and 50 vol ~ dioxane-water media at 25ยฐ and 35ยฐC.
The pK, have been determined by a pH titration method using glass and saturated calomel electrodes in cells with liquid junction potential. Activity coefficients needed in this work have been interpolated from the data of Harned and Owen s on the mean molal activity coefficient, yยฑ, of univalent ions (HCI) in dioxane-water media at the desired temperature (25 ยฐ or 35ยฐC) and at various ionic strengths, mยฑ, assuming that these activity coefficients will be the same for N-phenylmethylbenzohydroxamic acid. The empirical corrections -log[H +] = B + log Uยฐn + log yยฑ
(1)
for "medium effect" have been applied as suggested by Van Uitert and Haas 6'7. The ionisation of hydroxamic acid in aqueous solution gives hydrogen ion and hydroxamate ion and the equilibrium constant may be: Ka(aq.) -~-([H + ] [At-I/[HAl) (YH+ YAlYHA)