I. Canals et al., Retention of ionizable compounds on HPLC. 6. pH measurements with the glass electrode in methanol-water mixtures, J CHROMAT A, 911(2), 2001, pp. 191-202
The relationship, delta values, between the two rigorous pH scales, (s)(s)p
H (pH measured in a methanol-water mixture and referred to the same mixture
as standard state) and (S)(W)pH (pH measured in a methanol-water mixture b
ut referred to water as standard state), in several methanol-water mixtures
was determined (delta=(s)(w)pH-(s)(s)pH). delta values were measured using
a combined glass electrode and a wide set of buffer solutions. The results
are consistent with those obtained with the hydrogen electrode. This confi
rms the aptness of the glass electrode to achieve rigorous pH measurements
in methanol-water mixtures. An equation that relates delta and composition
of methanol-water mixtures, and allows delta computation at any composition
by interpolation, is proposed. Therefore, (s)(s)pH can be achieved from th
e experimental (s)(w)pH value and delta at any mobile phase composition. (s
)(s)pH (or (s)(w)pH) values are related to the chromatographic retention of
ionizable compounds through their thermodynamic acid-base constants in the
methanol-water mixture used as mobile phase. These relationships were test
ed for the retention variation of several acids and bases with the pH of th
e mobile phase. Therefore, the optimization of the mobile phase acidity for
any analyte can be easily reached avoiding the disturbances observed when
(w)(w)pH is used. (C) 2001 Elsevier Science B.V. All rights reserved.