A study of the mechanism of response of liquid ion exchanger calcium selective electrodes: Part I. Zero current potential of commercial and modified electrodes

A study of the response mechanism of liquid ion exchanger calcium selective electrodes has been undertaken using three different ion exchangers viz. calcium didecyl phosphate, calcium dihydrogen didecyl phosphate in dioctyl phenyl phosphonate and the Orion 922002 solution. The influence of interfering ions has been taken into consideration and reasons given for the past unsuccessful attempts to interpret their effects.     

A study of the mechanism of response of liquid ion exchanger calcium selective electrodes: Part II. Conductimetric investigation of the ionic dissociation in liquid ion exchangers

Measurements of conductivities of didecyl phosphate salts in DOPP have been undertaken using a dielectric constant determination cell. For concentrations greater than 10^?4M these salts are highly associated. Dissociated constants have been determined for Li and Na DDP in DOPP in the range concentration where ion pairs are formed (C<10^?4 M). The values are identical for the two salts studied. The presence of water and inorganic salts enhances the conductivity of ion exchanger solutions. Aggregation under micellar form or even as a pseudo-liquid crystal is discussed.     

A study of the behaviour of solid-state membrane electrodes: Part III. A model for the response time

A model is proposed to explain the response time of solid-state membrane electrodes, in the range of potential—time curves corresponding to seconds or minutes. Several causes of slow response time are considered. In the range of milliseconds to seconds, diffusion seems to be the rate-determining factor, but for times exceeding about 10s, dissolution or crystallization and charge transfer at the membrane seem to be the predominant factors. A mathematical model is derived. The principle is applicable to any solid-state membrane electrode and takes into account the change in response time with changes in the nature or concentration of the test ion and in the surface electrochemical properties of the membrane.     

Potentiometric response and mechanism of anionic recognition of heterocalixarene-based ion selective electrodes

The ion selective electrode (ISE)-based potentiometric approach is shown to be an effective means of characterizing the anion recognition sites in the molecular receptor calix[2]pyridino[2]pyrrole (CPP). In particular, potentiometric pH-measurements involving the use of experimental PVC-membranes based on CPP revealed the existence of both mono- and diprotonated forms of the receptor under readily accessible conditions. Based on these analyses, apparent surface protonation constants for this heterocalixarene were found to lie between 8.5-8.9 (pK_B1) and 3.3-3.8 (pK_B2). CPP was found to interact with targeted anionic analytes based on both coulombic and hydrogen bond interactions, as inferred from varying the kinds of ionic sites present within the membrane phase. Potentiometric selectivity studies revealed that CPP preferred “Y-shaped” anions (e.g. acetate, lactate, benzoate) over spherical anions (e.g. fluoride and chloride), fluoride over chloride within the set of spherical anions, and the ortho-isomer over the corresponding meta- and para-isomers in the case of hydroxybenzoate (salicylate and congeners). In the context of this study, the advantages of potentiometric determinations of acetylsalicylic acid using optimized PVC-membranes based on CPP relative to more conventional PVC-membrane ISEs based on traditional anion exchanger were also demonstrated.     

A study of the behaviour of solid-state membrane electrodes: Part IV. Experimental studies of the response time

Some ot the factors governing the response time of the fluoride-selective and chloride-selective electrodes have been investigated. The results of E—t and conductance measurements indicate that dissolution and charge-transfer processes control the ion-seiective electrode kinetics. The temperature coefficients of the rate constants for dissolution of the chloride electrode membrane, determined by E—t curves and conductimetric measurements, are similar. The results, and the fact that the dissolution is a second-order process, indicate that surface processes play the predominant role in electrode kinetics near equilibrium.     

Influence of the nature of liquid anion exchanger on the selectivity of anion selective electrodes

Selectivity coefficients of ion-selective electrodes on the basis of liquid anion exchangers, a quaternary phosphonium salt, and 20 higher quaternary ammonium and other nitrogen-containing salts differing in the length of hydrocarbon radicals, the nature of the quaternary nitrogen atom, and the presence of polar groups near to the exchange center with respect to 17 mineral, organic, and metal complex anions are presented. The effect of the nature of the anion-exchanger on the anion selectivity is discussed.     

The behaviour of an electrochemical detector used in liquid chromatography and continuous flow voltammetry: Part III. Potential dependent current

The potential dependence of the steady-state current at a channel electrode has been calculated approximately. Six regions of behavior can be described from a consideration of the characteristic times of diffusion, fluid flow and electrochemical reaction. Wave shapes and half-wave potential shifts are as expected for amperometric cells under all values of the heterogeneous reaction rate constant, and for reversible electrochemical reactions in a coulometric cell. The wave adopts a peculiar shape for irreversible reactions at a coulometric electrode. This is caused by the inapplicability of the concept of mass transfer control in these systems.     

Aroylthioureas: new organic ionophores for heavy metal ion selective electrodes. A nuclear magnetic resonance study

The 1H and 13C NMR spectra of four series of 1-aroylthiourea derivatives in DMSO-d6 are reported. The NH signals for 3-alkyl substituted aroylthioureas are identified by their multiplicity and by homonuclear irradiation experiments. Correlation analyzes are made for NH, CO and CS signals in order to determine the best way to modulate the nucleophilic character of the CS group, as thioureas are well-known ionophore groups. Almost all 1,3-substituted thioureas (Series 2-4) show the reported chelated structure with the exception of those with CF3, CN and NO2 groups. Pyridine group promote a different equilibrium in solution. The fragment -CO-NH- transmits poorly the electronic effects of substituents in the aroylgroup.     

Dynamic theory of the alternating current response of semiconductor electrodes under illumination

A photoinduced admittance enhancement has been observed on n-GaAs and n-GaP electrodes in the potential range between flatband and stationary photocurrent onset. In order to provide a theoretical evaluation, the alternating current response of a semiconductor electrode under illumination has been investigated on the basis of non-equilibrium treatment of the carrier balance in the semiconductor and of the interfacial charge transfer kinetics. Superposition of an irreversible stationary and small-amplitude periodic rate has been treated for the following cases of charge transfer at the interface: (a) one-step electrochemical process; (b) two-step electrochemical process including an adsorbed intermediate and partial charge transfers; (c) parallel couple of one-step electrochemical and partial charge transfer chemisorption process. Empirical criteria for preference of charge transfer over surface recombination have been considered. In connection with the present development, the general equivalent circuit of a semiconductor electrode has been briefly derived from the dynamical charge balance. The theoretical approach of the stationary photocurrent-voltage curve has been discussed and refined.     

Electrochemical behaviour of organotin compounds: Part III. Dialkyltin mercaptocarboxylates

Electrochemical behaviour of di-n-octyltin dithioglycolate has been investigated in acetate buffer (pH 7.0) containing 80% (v/v) ethanol. It has been found to give two well-defined polarographic waves, one of which is anodic and involves two-electron oxidation of the mercaptide. The other wave has been found to be cathodic and is due to one-electron reduction of the dioctyltin compound to a free radical. Strong adsorption of the compound at the DME was indicated by depression in its drop time curve. A mechanism of the electrode process is postulated and a method for detection of these compounds down to p.p.m. level has been developed.     

季铵盐结构与高氯酸根离子选择电极性能关系的研究

本文报道了由七种长链季铵盐制得的高氯酸根离子选择电极,测定了这些电极的Nernst响应范围和检测下限,并研究了各种阴离子的干扰。通过大量实验测得,随着季铵盐分子中取代烷基碳链的增长,电极电势响应范围增宽、检测下限降低、抗外部阴离子干扰能力增强。若取代烷基碳链过长,则季铵盐纯化困难,在膜相中溶解度降低,不适于电极的研制。根据用上述七种选择电极在相同水相条件下测得的数据表明,阴离子的干扰次序为:PA~->ClO_4~->BF_4~->SCN~->I~->ClO_3~->N_3~->N_3~->CN~-～Br~->H_2PO_4~->F~->Cl~-。这些数据对阴离子选择电极研制与相转移催化反应研究均有现实意义。     

Hydrodynamic voltammetry at channel electrodes: Part III. Theory of kinetic currents

A theory of kinetic currents at a channel electrode is developed without any assumption on the magnitude of the rate constant of the preceding chemical reaction. An approximate equation with reasonable accuracy which relates the limiting current to the rate constant, flow velocity and the electrode geometries, is presented. The validity of the equation derived from the concept of the thin reaction layer is also discussed.     

Nitrate and ammonium ion-selective electrodes as sensors : Part II. Assay of nitrate ion and nitrate and nitrite reductases in stationary solutions and under flow-stream conditions

Nitrate ion-selective electrodes were successfully applied both in stationary solutions and in flow streams as new analytical techniques for the assay of nitrate reductase activity. An ammonium-selective electrode was similarly used to measure the increase in ammonium ion concentration during the enzymatic reduction of nitrate to ammonium. Preliminary studies on a true enzyme electrode for assay of nitrate ion are discussed.     

Onset of natural convection in the electrochemical cell with horizontal electrodes under non-steady-state conditions: A numerical study

The disturbance of mechanical equilibrium of stagnant electrolyte containing three types of ions and the onset of natural convection in the electrochemical cell with plane horizontal electrodes under the non-steady-state conditions are studied theoretically. The Navier-Stokes equations in the Boussinesq approximation, the equations of ionic transfer of electrolyte components, which is caused by diffusion, convection, and migration, and the electroneutrality condition were used as the mathematical model of the process. Using the numerical simulation, the regularities of the onset of natural convection of electrolyte solution with the formation of convective cells are studied. The effect of transport properties of solution on the critical time of onset of convection and on the time before the current starts to increase due to the onset of natural convection, on the sizes and shape of convective cells and the mass-transfer rate is investigated by the example of cathodic deposition of metal at the limiting current.     

Repetitive determination of calcium ion and regeneration of a chromogenic reagent using chlorophosphonazo III and an ion exchanger in a circulatory flow injection system.

The spectrophotometric determination of Ca2+ with chlorophosphonazo III (CPN) has been carried out by a circulatory flow injection (FI) method. A cation-exchange mini-column for the on-line regeneration of the main reagent was incorporated in this FT system, allowing a repetitive determination of Ca2+. A solution of 4.0 x 10(-5) M CPN in a 0.05 M acetate buffer (pH 5.0) in a single reservoir (50 ml) was continuously circulated at a constant flow rate of 1.5 ml min(-1). Into the stream, an aliquot (20 microl) of a sample containing Ca2+ was quickly injected by means of a 6-way valve. The complex formed was monitored spectrophotometrically at 670 nm in the flow system. Then, the stream passed through a cation-exchange column, which was introduced after the flow-through cell. A successful ligand-exchange reaction of Ca2+ between the CPN reagent and a cation exchanger, as well as a simultaneous regeneration of the free reagent took place. The stream then returned to the reservoir. The regeneration and recycling of the CPN reagent allowed as many as 300 repetitive determinations of 2.5 mg l(-1) Ca2+ solutions with the same 50 ml circulating solution.     

The surface morphology of ion-selective membrane electrodes: Part 3. Studies on the Lead Ion-selective Membrane

Studies of the changes in the surface state of the ion-selective membrane and in the performance of the silver sulphide-lead sulphide (2:1) precipitate-based electrode are described. A scanning electron microscope was used to examine the surface of the electrode membrane. The results indicate that passivation of the lead-selective electrode caused by chemical and electrochemical oxidation is due to transformation of the lead sulphide at the membrane surface. The electrode surface becomes depleted in lead sulphide; lead sulphate crystals are formed during chemical oxidation with hydrogen peroxide, and lead oxide crystals during anodic oxidation.     

Evaluation of experimental conditions on the response of fluoride and cyanide selective electrodes in flow injection potentiometry

Summary The response of fluoride and cyanide selective electrodes has been investigated under steady-state and flow injection measuring conditions. Response time measurements indicate that at lower concentration levels of these ions the surface condition for both sensors is very critical; the choice of flow cell geometry, however, is of minor importance. An acetate buffer without CDTA is proposed for fluoride determinations because of better long term stability of response time of the fluoride ISE. Problems associated with the corrosion of the AgI-membrane when applied in a flow cell with wall-jet configuration are discussed.Dedicated to Prof. Dr. G. Schulze on the occasion of his 60th birthday