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

Abstract

Although H + and OH À 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 fourcompartment 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 (C 4 D), 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 (pK a < 5), pure or mixtures, could be titrated. The analytical curve is linear from 50 mM up to 10 mM of total dissociable hydrogen (r ¼ 0.99899 for n ¼ 10) in 10-nL samples. By including D 2 O in the running electrolyte, we could demonstrate how to measure the mixed proton/deuteron mobility. When H 2 O/D 2 O (9 : 1 v/v) was used as the solvent, the mobility was 289.6 AE 0.5 Â 10 À5 cm 2 V À1 s À1 . 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.

Investigating the formation and the properties of monoalkyl carbonates in aqueous medium using capillary electrophoresis with capacitively coupled contactless conductivity detection

“…The construction of the electrophoresis equipment was described elsewhere [17]. It was equipped with two C 4 Ds [23,24] positioned along the capillary at 10 cm from each end.…”

Automation of an analysis system with microvolume porous membrane sampling, capillary electrophoresis separation and contactless conductivity detection for near-real-time monitoring of traces of low-molecular-weight carboxylic acids in air

“…Therefore, in order to ensure a better long-lasting reproducibility, automated HI of sample was implemented in the proposed system in combination with separated semi-cells for the electrodes, coupled through salt bridges, to avoid contamination of the capillary with electrolysis products. 29,30 The CE-C 4 D equipment formerly devised and built in the laboratory 29 presented two operation modes for HI: transient pressurization of the injection reservoir or reduction in reservoir pressure at the detection end of the capillary. The second mode was preferred here because it avoids complications due to interface pressurization.…”

“…The proposed automatic system has potential for development of near-real-time analysis methods of other gaseous species relevant for indoor, outdoor or even in-process applications, in consonance with the 11 th principle of green chemistry. Additional steps of in-line clean up, separation or pre-concentration can be integrated into the TAS 30 while evolution in microchip CE-C 4 D development 34,35 can lead to the conception of a mTAS suitable for field or remote operation and personal monitoring as well.…”

“…A similar salt bridge was placed at the detector end of the capillary (Figure 1a), so that electrolysis products formed at the electrodes remained far from the capillary ends, thus greatly improving the repeatability of the electropherograms during long lasting operations, as previously demonstrated. 29,30 A polyimide coated silica capillary, of the kind used in CE (50 mm I.D., 20 cm long), was connected in series with the supplying polyethylene tube as a hydrodynamic resistor to drastically restrain the flow rate. A needle valve was added for fine flow rate adjustment.…”

Capillary Electrophoresis Assisted Flow System for In Situ Analysis of Products of Heterogeneous Electrocatalytic and Catalytic Processes: Application to the Oxidation of Glycerol

“…All of the CE separations were performed using homemade CE-C 4 D equipment [15], this time equipped only with a negative high voltage supply (instead of a bipolar one). The CE ground electrode was placed in the outlet tube of the cell to avoid base line drift by migration of electrolysis products into the capillary.…”