Determination of Acid Dissociation Constant of Degradable Tetrabromophenolphthalein Ethyl Ester by Capillary Zone Electrophoresis

Abstract

An acid dissociation constant of tetrabromophenolphthalein ethyl ester (TBPE) was determined through the measurement of electrophoretic mobility by capillary zone electrophoresis (CZE). Although TBPE is degradable in acidic pH region and it gradually degraded at pH conditions around and below its pKa values in the time scale of the CZE measurement, equilibrium species of interest were detected as a peak-shaped signal with tailing of the degraded species. Changes in electrophoretic mobility of the equilibrium species of TBPE were analyzed at its detectable pH range in the presence of phenol red as a mobility standard. An acid dissociation constant of 3.47 ± 0.06 (pKa, I = 0.01, 25 C, standard error) was determined for TBPE.

Analysis of Ion-association Equilibrium of Precipitable Dipicrylaminate Ion in Aqueous Solution by Capillary Zone Electrophoresis

“…(4); then, the values of μep,DPA and Kass were optimized by a nonlinear least-squares analysis. 16 A GNU-style copyleft program, R (The R Foundation for Statistical Computing, Version 2.15.2), 17,18 was used for the analysis. The Kass values determined with the alkali metal ions are summarized in Table 1.…”

Analysis of Fast and Slow Acid Dissociation Equilibria of 3′,3″,5′,5″-Tetrabromophenolphthalein and Determination of Its Equilibrium Constants by Capillary Zone Electrophoresis

“…The electrophoretic mobility of the analytes was calculated in the ordinary manner. 4 Two or more measurements were made to obtain the effective mobility and/or the peak height of the signals. The electrophoretic mobility and the peak height of TBPP were standardized with those of 1-NS by division.…”

“…1,2 Further, gradually degraded species from the subject substance during the electrophoretic separation is resolved from the equilibrium species of interest. 3,4 The degrading species are observed as a leading or tailing to the subjected CZE signal, even though the signal intensity of the subjected species decreases by the kinetic reaction as side reactions to the objective equilibrium. 3,4 Acid dissociation equilibria of phenolphthalein are thus analyzed by changes in the electrophoretic mobility in CZE, and two steps of acid dissociation constants were determined, even in the presence of a degradation reaction under alkaline pH conditions.…”

“…3,4 Acid dissociation equilibria of phenolphthalein are thus analyzed by changes in the electrophoretic mobility in CZE, and two steps of acid dissociation constants were determined, even in the presence of a degradation reaction under alkaline pH conditions. 3 Analyses of acid dissociation equilibrium are also done with tetrabromophenolphthalein ethyl ester 4 and with labile drugs 5 in the presence of a gradually degrading reaction during electrophoretic separation.…”

Determination of Two-Steps Acid Dissociation Constants of L-Ascorbic Acid by Capillary Zone Electrophoresis

“…Since CZE includes electrophoretic resolution, pK a values of degradable substances have been determined through the measurements of the effective electrophoretic mobility of a target substance [12]. By using the separation characteristics of CZE, acid dissociation constants were successfully determined with alkaline-degradable phenolphthalein [13] and catecholamines [14], acid-degradable tetrabromophenolphthalein ethyl ester [15], pravastatin [16] and hexamethylenetetramine [17], labile drugs [18], photo-degradable haloperidol [19], heat-degradable bupropion [20] and hydrochlorothiazide [21].…”