Zeolite Modified Carbon Paste Electrode Research Articles

The Methylviologen-Doped Zeolite Modified Electrode as a New Detector for Suppressor Free Ion Chromatography

A methylviologen-doped zeolite modified carbon paste electrode was applied to the amperometric detection of alkali metal cations in ion chromatography. The detection principle is based on the ion exchange of methylviologen by the target cationic analyte, followed by the reduction of methylviologen at the electrode surface at an operating potential of −0.85 V (vs. SCE). When using a solution of tetrabutylammonium bromide as the mobile phase, the background response was very low because of the size exclusion of tetrabutylammonium ions from the zeolite framework, and the modified electrode was only sensitive to non size-excluded cations. In a large volume wall-jet cell configuration, the sensor allowed the analysis of non reducible non size-excluded cations after their separation on an ion exchange column, in both the presence and the absence of dissolved oxygen. Use of a mobile phase with cations too large to be detected by the zeolite modified electrode did not require any further suppressor technique.

Factors affecting the analytical applications of zeolite-modified electrodes preconcentration of electroactive species

The influence of various experimental parameters on the electrochemical response of zeolite-modified electrodes (ZMEs) have been studied using ion-exchange voltammetry, with regard to their application as amperometric sensors. In order to cover a large range of parameters, the investigations were carried out using three zeolites (A, X and Y), selected for their different cation-exchange capacities and ion-sieving properties; four electroactive species (silver(I), copper(II), dopamine and methylviologen) chosen on the basis of their size, charge and inorganic or organic nature; and two kinds of ZMEs: the zeolite-modified carbon-paste electrode (ZMCPE) and the zeolite monograin layers coated on glassy carbon electrode (ZMGCE). Compared to the corresponding unmodified electrodes, improvements in sensitivity were observed when using ZMEs, due to analyte accumulation at the electrode's surface by ion exchange in zeolite particles. The concentration process, and the subsequent voltammetric response of ZMEs, were found to be strongly affected by the accessibility of the analytes to the ion-exchanging sites and their mobility within the zeolite framework. Good correlation was found between results obtained in ion-exchange voltammetry and conventional ion-exchange characterization. The advantage of these fundamental studies for electroanalytical purposes was exemplified for the determination of copper(II) ions, using both ZMCPE and ZMGCE, zeolites A, X and Y, and the following analytical scheme: “preconcentration/voltammetric detection”.

The methyl viologen incorporated zeolite modified carbon paste electrode—part 2. Ion exchange and electron transfer mechanism in aqueous medium

An investigation of ion exchange between methyl viologen ions (MV ++) and sodium ions (Na +) in zeolite Y has been completed in order to describe the electron transfer mechanism operative in a zeolite modified carbon paste electrode (ZMCPE). Preliminary work needed the characterization of the Linde Y-54 zeolite batch used and the search for equilibrium conditions. The ion exchange isotherm for the MV ++ exchange for Na + demonstrates the great selectivity for the large organic cation and a maximum capacity of 1.0 × 10 −3 mole MV ++ per gram of zeolite, corresponding to an occupation of about 64% of the total exchanging sites. The use of zeolite containing different amounts of methyl viologen has emphasized that the selectivity is greatly influenced by the activity coefficients of the exchanging species within the zeolite framework. This property of the ion exchange equilibrium allowed to determine the electron transfer mechanism that is operative for the cathodic reduction of MV ++ initially exchanged in a ZMCPE. In aqueous solution, MV ++ is reduced after being exchanged for the electrolyte cation. Finally, a structural model of the electrode has been proposed. It is in good agreement with experimental results and takes into account all the electrochemical behaviour of the ZMCPE and the advocated electron transfer mechanism.

The methyl viologen incorporated zeolite modified carbon paste electrode—part 1. Electrochemical behaviour in aqueous media. Effects of supporting electrolyte and immersion time

Methyl viologen (MV ++) is incorporated into a zeolite modified carbon paste electrode (ZMCPE). The electrochemical behaviour of MV ++ is examined in aqueous media with various supporting electrolytes (NaCl, NaBr and several tetraalkylammonium bromides) by cyclic voltammetry, chronoamperometry and chronocoulometry. Quantitative in situ determination of MV ++ leaching from paste into the external solution is performed using square wave voltammetry. The thickness of the ZMCPE involved in the electrochemical reaction of MV ++ has been found to increase with experiment time. This was not the case with unmodified carbon paste. The diffusion coefficient of free MV ++ within a ZMCPE is calculated to be about 8 × 10 −11 cm 2 s −1. This value increases (3 × 10 −10 cm 2 s −1) when the paste is equilibrated in high purity water for several hours before being used to obtain the “hydrated” form of a ZMCPE.

Simplex optimization of electroreduction of oxygen mediated by methyl viologen supported on zeolite-modified carbon paste electrode

By using the methyl viologen—oxygen system as a model, zeolites Y and A were shown to be promising as supports for electrocatalysts, and perhaps as sites for preconcentration of substrates as well. Simplex optimization was used to reach a maximum catalytic peak current for the reduction of oxygen using a modified electrode consisting of carbon paste mixed with the sodium form of zeolite Y. The electrochemistry was carried out in aqueous lithium perchlorate solution buffered at pH 7.00. There were four parameters varied to give a maximum peak current due to oxygen reduction: 1. concentration of methyl viologen in solution; 2. percentage (by mass) of Y zeolite mixed into carbon paste electrode; 3. time of exposure of zeolite-modified electrode to viologen solution prior to voltammetric experiment; 4. duration of oxygenation of solution prior to experiment. Peak current for reduction of oxygen showed a maximum with respect to parameters 1 and 2. Peak currents increased then leveled off as parameters 3 and 4 were increased. The optimal response was obtained when carbon paste electrode containing 49 wt% zeolite Y was immersed in a 32 mM solution of methyl viologen in aqueous electrolyte solution saturated with oxygen. An equilibration period of 46 min was required to achieve an optimal degree of ion exchange of methyl viologen cation radical into the zeolite prior to the electrochemical experiment. In addition to optimizing peak current, the simplex method allowed conclusions to be drawn about the interrelationships among parameters in the electrochemical system.