Marcin Palys

Flow-injection catalytic determination of molybdenum with blamperometric detection in a microprocessor-controlled system

Abstract The flow-injection determination of molybdenum(VI) is based on its catalytic effect on the oxidation of iodide by hydrogen peroxide. The triiodide ion formed in this reaction is detected amperometrically in a flow-through cell containing two platinum wire electrodes polarized at 100 mV. After optimization of the measuring conditions, the detection limit is 1.2 μg l−1 Mo(VI) and the linear range extends to 1 mg l−1. Interference of various metal ions and their removal is described. The procedure was tested on the determination of molybdenum(VI) in soil extracts.

Synthetic and crystallographic studies on pyridinophanes

Abstract Eight macrocyclic bisamides and tetramides have been synthesized by reaction of dimethyl pyridine-2,6-dicarboxylate 1 with α,ω-diaminoethers in methanol as a solvent. Relationship between structure of an amine used and a ratio of bisamide to tetramide has been investigated. The X-ray structure of macrocyclic amides 8, 9, 14 and linear amide 16 are reported. They show intramolecular hydrogen bond patterns within macrocyclic cavities involving NPy…NHamide, and in the cases of 9, 14 and 16, neutral molecules of water. Stability constants of selected ligands were measured using voltammetry.

THE SEPARATION OF OVERLAPPING PEAKS IN CYCLIC VOLTAMMETRY BY MEANS OF SEMI-DIFFERENTIAL TRANSFORMATION

A method for extracting single peaks from complex linear sweep and cyclic voltamperograms is presented. Voltamperograms are transformed by means of semidifferentiation, then all undesired peaks are removed from the semiderivative curve and replaced by calculated baselines. The resulting curve is semiintegrated back, giving a voltamperogram with one peak only. Baselines in the semiderivative domain are determined by the least-squares curve-fitting of datapoints from peak border regions, using the equation that describes the semiderivative peak of a reversible electrode process. With this procedure peaks can be removed without assumptions about the mechanism of the underlying electrode reaction. Due to its design, the algorithm presented is suitable for the fully automatic processing of cyclic and linear sweep voltamperograms. Performance of the procedure was checked with generated reversible voltamperograms as well as in real experiments with both reversible and irreversible systems. The smallest distance between two peaks of equal height, for which the described method can yield correct results, has been found to be 110 mV for a reversible one-electron process at 298 K. This procedure can also be applied to the elimination of the cathodic current from the cyclic voltamperogram of a single component in order to get a pure anodic current value, free from cathodic contribution, or vice versa.

Digital simulation of chronopotentiometric and steady-state voltammetric curves at microelectrodes in the presence of a low concentration of supporting electrolyte

A simulation scheme for the calculation of theoretical chronopotentiograms at microelectrodes in solutions containing low amounts of supporting electrolyte is presented. The scheme allows computation of the changes in the concentration profiles of the substrates, products and the supporting electrolyte ions with time. The electrode potentials that are established after reaching the steady-state, together with the appropriate current intensities, can be used for constructing the steady-state voltammograms. The simulation of the mixed diffusional and migrational transport is based on the Crank-Nicolson method with an exponentially expanding time and space grids. The scheme does not impose any limitations on diffusion coefficients and it can be applied both to simple electrode reactions (one reactant-one product) and more complicated reactions under the assumption that the double-layer thickness is small in comparison to the diffusion layer. Five simple types of electrode reactions and an example of a more complicated scheme were considered. The results obtained demonstrate that the dependence of the steady-state limiting current on the support ratio (csupp.el./csubst) depends not only on the charge of the reactant and the product, but also on the diffusion coefficient ratio of the substrate and product. If the difference between diffusion coefficients is large, the predictions based on simpler theories available in literature can become invalid.

Voltammetric investigation of the complexation equilibria in the presence of a low level of supporting electrolyte part 1: Steady-state current-potential curves for inert complexes

The use of microelectrodes for voltammetric investigations of the complexation equilibria at very low concentrations of supporting electrolyte allows the risk of competitive complexation or contamination to be avoided, makes the activities of the species involved closer to their concentrations (which facilitates comparisons with the spectroscopic results) and finally, allows the concentrations of the species to be varied over a broader range. This paper presents the calculations of the steady-state currents for a wide range of complexes that are inert on the experimental time scale, and reports the influence of the concentration of the electroinactive ionic species on the limiting currents. Also, for a number of cases the variation of halfwave potential with the ligand concentration, resulting from changes in the ohmic drop, is given. It is assumed that only one species (the complex or the uncomplexed form) is electroactive; if this is the complex, it may or may not change the number of ligands. The theoretical results were obtained either employing the Myland-Oldham theory extended in this paper or by digital simulation. The results of calculations show that the magnitude of the changes in the steady-state limiting current on complexation depends on the type of complexation equilibrium, the type of the change in the reactant charge number in the electrode process, and the complex formation constant. In a number of situations migrational effects are negligibly small and no special treatment is necessary, despite the lack of supporting electrolyte. In other cases, where migration is significant, the relations between the measured steady-state limiting current and the complex formation constant s are given in the form of fitted equations that can be used to obtain s from appropriate experimental data.

Automatic polarographic elucidation of electrode mechanisms by means of a knowledge-based system. Part 1: Sampled d.c. polarography

A knowledge-based system has been developed for the automatic elucidation of electrochemical mechanisms. The system is based on sampled direct current (or Tast) polarography at a dropping mercury electrode as a technique for collecting experimental information and consists of a general expert system shell for the reasoning process, the specific set of rules and experimental modules. The set of rules allows the elucidation of eight relatively simple electrode reaction mechanisms fully atomatically. The computer system has been validated with chemical systems the electrochemical behaviour of which is well established. All parts of the program are written in FORTH language for Apple II microcomputers. This expert system has an open character and new rules can be added to extend the set of mechanisms that can be determined.

Automatic polarographic elucidation of electrode mechanisms by means of a knowledge-based system. Part 2. Staircase voltammetry, convolution voltammetry and chronocoulometry applied to simple mechanisms

An improved expert system for the automatic elucidation of electrode reaction mechanisms is presented. The system is coupled to the electrochemical instrument, designs the experiment, controls it and collects the results automatically. The approximate logic concept is used to handle situations when inconsistent or conflicting information is obtained. Cyclic staircase voltammetry, chronocoulometry/chronoamperometry and convolution voltammetry are used at this stage of development. In the elucidation process, general information and characteristics of particular reaction steps are delivered, and are finally combined in an answer about the mechanism of the electrode reaction. The present set of rules allows full elucidation of ten simple mechanisms. For more complicated mechanisms, all stages of the elucidation except the final combination of all gathered knowledge, are output. The performance of the expert system was checked with four well-known electrochemical systems. In all cases, the answer about the reaction mechanisms was correct. The typical time for a full elucidation cycle was about 25 min.

Digital simulation of migrational voltammetry. The effect of difference in diffusion coefficients of the reactant and the product of an electrode reaction

A simulation scheme, based on the fast implicit finite difference algorithm has been proposed for the modeling of steady-state voltammograms under conditions of mixed diffusion–migration transport in multi-ion systems. The scheme assumes the single-step reversible process taking place at a hemispherical electrode, and does not impose any limitations on the diffusion coefficients of the species present in the solution. Using this simulation tool the theoretical voltammograms for the charge neutralization-, charge decrease-, and charge increase systems were obtained for a broad range of diffusion coefficients of the reactant and the product, and for different concentrations of supporting electrolyte. For the charge decrease- and charge increase systems, the normalized limiting current plotted versus log(DR/DP) gives a sigmoidal curve. The migrational component in the transport of the electroactive species varies significantly with log(DR/DP), the largest variation being observed for log(DR/DP) close to zero. A decrease in DP decreases the limiting current, while for a large DP the limiting current increases asymptotically to a limiting value. This value is identical to the limiting current observed in the analogous systems with a charge neutralization reaction, which shows that the charge of the product plays no role if the diffusion coefficient of the product approaches infinity. Approximate functions have been constructed to calculate the normalized limiting current (relative error <2%) and the shift of the normalized halfwave potential (error <2 mV) in the range of the DR/DP ratio found in real experimental situations.

Automatic polarographic elucidation of electrode: Part 4. Elucidation of mechanisms of newly synthesized compounds

After validation, the previously described expert system has been employed in the investigation of reduction mechanisms of two metallomacrocylic compounds that have not been electrochemically studied before: a nickel salen derivative and a binapthyl-uranyl salen crown ether. The system has found that on a mercury electrode in DMSO, under experimental conditions (concentration in the range of 5 × 10−4 −2 × 10−3 M, time scale corresponding to scan rates 0.15–8.0 V s−1) the nickel salen derivative undergoes a reversible electron transfer followed by a fast, reversible chemical reaction. Additional experiments performed outside the expert system suggest that another very slow chemical step takes place, either subsequent or parallel, resulting in a product accumulated on the electrode surface. In the chemical processes most probably traces of water, oxygen or other impurities are involved. For binapthyl-uranyl salen crown ether the elucidated mechanism is a fast, reversible electron transfer followed by a weak adsorption of the reduction product (studied concentration range 2 × 10−4 −2 × 10−3 M, time scale corresponding to scan rate 0.11–5.0 V s−1. In both cases the expert system provided results which supported a single hypothesis about the mechanism much more than all others, giving a clear, unambiguous conclusion. The full elucidation cycle took approximately one working day.

Investigation of Complexation of Sodium Cation by Anthracene Crown Ethers

Abstract Formation constants for complexes of sodium and anthracene crown ethers were determined using spectrophotometry and cyclic voltammetry. The complexation properties of the ligands examined depend significantly on alkyl substituents at 9 and 10-positions of the anthracene group and are strongly influenced by the solvent.