Determination of weak (2.0-2.5) dissociation constants of non-UV absorbing solutes by capillary electrophoresis

Capillary electrophoresis (CE) has proved to be a fast and convenient method for the determination of the dissociation constants of non-UV absorbing solutes in the acidic pK(A) range (2.0-2.5). The electroosmotic flow was reversed by washing the capillary with 0.2% polybren aqueous solution. A series of background electrolytes was prepared with phenylphosphonic acid (pK(A) = 1.29) and beta-alanine (pK(A) = 3.55) with the same ionic strength and a high buffer capacity in order to improve the repeatability (0.1-0.2%) of the electrophoretic mobility and to determine the values of pK(A) accurately. This procedure was applied to the determination of the dissociation constants of several alkyl-alkylphosphonic acids whose PKA values have not yet been published in the literature. In this work, their dissociation constants have been found to vary between 1.91 and 2.34 for alkyl-methylphosphonic acids and between 2.10 and 2.38 for alkyl-ethylphosphonic acids.