W.E. Van Der Linden

Glassy carbon as electrode material in electro- analytical chemistry

Abstract The review is based on a literature search through Chemical Abstracts and the Science Citation Index. The topics covered are: chemical and electrochemical aspects of the glassy carbon/electrolyte interface for both aqueous solutions and non-aqueous electrolytes; analytical applications of glassy carbon electrodes, including voltammetry, stripping voltammetry, amperometry coulometry, potentiometry and chronopotentiometry; flow-through detectors; chemically modified glassy carbon electrodes; electrosynthesis; and pretreatment techniques.

Dispersion in open tubes and tubes packed with large glass beads : The Single Bead String Reactor

Abstract A semi-quantitative description of the dispersion process in open tubes is presented for conditions prevailing in flow injection analysis. It is shown that uncoupling the two functions of the tube (i.e., mixing and creating a residence time) can be accomplished by making use of a simple mixing device followed by a tube packed with glass beads of relatively large diameter (up to 70% of the i.d. of the tube): this gives a single bead string reactor (s.b.s.r.). With a flow injection system consisting of such an s.b.s.r., longer residence times can be obtained without loss of peak height or loss of sampling frequency.

Transport phenomena in flow injection analysis without chemical reaction

Abstract Dispersion phenomena play a very important role in flow injection analysis. In this paper, physical transport phenomena in flow injection methods are discussed. Three different types of reactor — a straight tube, a helically coiled tube and a new single bead string reactor — are compared. Under similar flow conditions, the dispersion in the single bead string reactor is the lowest. The specific advantages of single bead string reactors are their very simple preparation and maintenance and the good reproducibility of the peaks. It is shown that in open capillary tubes (coiled or not) the Taylor dispersion equation is of very limited use, because the residence times are too short, and because secondary flow occurs in the case of coiled tubes.

Some theoretical aspects of flow injection analysis

Abstract The injection and detection methods in flow injection analysis (f.i.a.) are theoretically described. Theory is developed for a simple flow model based on laminar flow without diffusion. The results lead to the conclusion that exact specifications for injection and detection devices in experimental f.i.a. are necessary. The influence of the sample volume is described in more detail with a systems analysis model of f.i.a. in which the tanks-in-series model is used for transport of the fluid. The theoretical and experimental results are in good agreement, thus the theory developed for sample injection in f.i.a. appears to be valid.

The formation of mixed copper sulfide—silver sulfide membranes for copper(II)-selective electrodes: Part III. The electrode response in the presence of complexing agents

Abstract The behaviour of a copper(II)-selective electrode made of ternary copper/silver sulfide, in the presence of complexing agents is discussed. Potential vs. pH curves indicate that in the case of Trien and Tetren, the electrode response reflects the situation in the bulk solution when both the free ligand and its copper(II) complex are present; when only the ligand is present, a small amount of the complex is probably formed near the surface, by dissolution of the membrane material. Similar results were found with citrate and glycine. In the case of EDTA and NTA, however, the response of the electrode is quite different; the concentrations of free copper(II) measured are higher than those expected from chemical equilibrium calculations for the bulk solution. The curves for solutions with and without the complex coincide and show an anomalous dependence on the sidereaction coefficient, α L(H). No explanation has been found for this behaviour. There is good agreement between stability constants for copper complexes or precipitates of Trien, Tetren, hydroxide and sulfide, obtained with this electrode and those given in the literature. The results indicate that Tetren or Trien should be preferred to EDTA and NTA for the preparation of metal buffer solutions.

Determination of the composition and the stability constants of complexes of mercury (II) with amino acids.

Abstract The complex formation of mercury(II) with twenty of the most important amino acids has been investigated, and the corresponding stability constants are reported. Calculations are based on simultaneous pH and pM measurements of solutions containing the amino acid in excess with respect to mercury. It is shown that most amino acids form mononuclear monoligand complexes as well as mononuclear biligand complexes. A different behaviour is observed for histidine and methionine, for which the complexes become strongly protonated at lower pH values.

The formation of mixed copper(II) sulfide—silver(I) sulfide membranes for copper(II)-selective electrodes

Abstract Several methods for the preparation of mixed copper(II) sulfide—silver(I) sulfide precipitates have been investigated. Pellets of these materials have been tested for their suitability as copper(II)-selective membranes. Homogeneous membranes with satisfactory electrochemical properties can be prepared only from precipitates obtained by addition of the metal salts to sodium sulfide Membrane leakages, limits of detection, calibration curves and titration curves with different types of precipitate are discussed.

Determination of iron(II) and iron(III) by flow injection and amperometric detection with a glassy carbon electrode

Abstract Flow injection analysis can be used for the determination of both iron(II) and iron(III) with an amperometric detector. The flow-through cell contains a glassy carbon electrode. Selection of the appropriate voltammetric technique, choice of the indication potentials, sample size, composition of the carrier stream, etc., are discussed. The limit of determination is about 10 -6 M; the calibration curves are linear in the concentration ranges 10 -3 –10 -5 M for iron(III) and 5 × 10 -4 –10 -5 M for iron(II). To illustrate the potentialities of the proposed method, standard rocks have been analysed.

The formation and properties of mixed lead sulfide—silver sulfide membranes for lead(II)-selective electrodes

Abstract The preparation and behaviour of a lead(II)-selective electrode made of mixed silver sulfide-lead sulfide are discussed and compared with those of a copper(II)-selective electrode. X-ray diffraction showed that hot-pressing of the membrane causes changes in the crystalline structure of the material. Responses to lead(II), sulfide, and hydroxide were measured ; potential-pH curves were determined in solutions containing EDTA, NTA, Tetren or Trien with and without the lead complex. The results are similar to those obtained for the copper-i.s.e. Data are given for titrations, selectivity coefficients, and response times. The response of the lead-i.s.e. to an addition of ligand is faster for Trien than for EDTA. This difference is large for the copper-i.s.e. These response rates may be correlated with potential-pH curves in the presence of ligands.

Behaviour of solid electrodes in normal and differential pulse voltammetric methods

In order to investigate the behaviour of solid electrodes in normal and differential pulse voltammetry, step functions have been applied to the electrochemical cell containing the electrodes to be tested, in the absence of electroactive species. The large residual current observed could be attributed to electrochemical reactions of the electrode material.