Ionic mobility of the solvated proton and acid–base titration in a four-compartment capillary electrophoresis system

Abstract

Although H+ and OH− are the most common ions in aqueous media, they are not usually observable in capillary electrophoresis (CE) experiments, because of the extensive use of buffer solutions as the background electrolyte. In the present work, we introduce CE equipment designed to allow the determination of such ions in a similar fashion as any other ion. Basically, it consists of a four-compartment piece of equipment for electrolysis-separated experiments (D. P. de Jesus et al., Anal. Chem., 2005, 77, 607). In such a system, the ends of the capillary are placed in two reservoirs, which are connected to two other reservoirs through electrolyte-filled tubes. The electrodes of the high-voltage power source are positioned in these reservoirs. Thus, the electrolysis products are kept away from the inputs of the capillary. The detection was provided by two capacitively coupled contactless conductivity detectors (C4D), each one positioned about 11 cm from the end of the capillary. Two applications were demonstrated: titration-like procedures for nanolitre samples and mobility measurements. Strong and weak acids (pKa < 5), pure or mixtures, could be titrated. The analytical curve is linear from 50 μM up to 10 mM of total dissociable hydrogen (r = 0.99899 for n = 10) in 10-nL samples. By including D2O in the running electrolyte, we could demonstrate how to measure the mixed proton/deuteron mobility. When H2O/D2O (9 : 1 v/v) was used as the solvent, the mobility was 289.6 ± 0.5 × 10−5 cm2 V−1 s−1. Due to the fast conversion of the species, this value is related to the overall behaviour of all isotopologues and isotopomers of the Zundel and Eigen structures, as well as the Stokesian mobility of proton and deuteron. The effect of neutral (o-phenanthroline) and negatively charged (chloroacetate) bases and aprotic solvent (DMSO) over the H+ mobility was also demonstrated.