A series of anion-selective electrodes of the liquid membrane type has been rigorously examined with respect to selectivity characteristics using three different experimental methods. This evaluation gives, for the first time, an understanding of how selectivity ratios depend on concentration levels and other variables and yields a recommended procedure for the consistent numerical determination of selectivity ratios. The resulting selectivity data are examined in terms of a theory of liquid membrane electrodes.Since the introduction of the liquid membrane electrode selective for calcium in 1967 (/), many new electrodes of this type with selective response to both cations and anions have become available. Evaluative studies (2-6) of these electrodes have demonstrated the Nernstian response and dynamic range of these electrodes, but have also revealed their gradual loss of selectivity and general deterioration with time under extended use.Generally speaking, the selectivities of liquid membrane electrodes for the ion of primary interest with respect to common interfering ions are only moderate (7-8). Never-theles^gfhe electrodes have been successfully employed analytical! < direct potentiometry or potentiometric titrations (3, 5, c , 9-13) under suitable conditions where interfering ions are absent or present in low concentrations.If the full usefulness of ion-selective electrodes of the liquid membrane type is to be realized, reliable methods of establishing their selectivity characteristics and accurate numerical data on selectivities must be made available. Unfortunately, there is little agreement in the literature regarding optimal methods of determining selectivities or, even, on the selectivity ratios of a specified electrode. In part, this difficulty arises from a lack of systematic study and from the tendency to report selectivities under a single, and often arbitrary, set of conditions.In this paper we, therefore, set out to evaluate the selectivities of a series of anion-selective, liquid membrane electrodes

Oxidized and reduced glutathiones (GSSG and GSH) are selectively measured In mixtures at levels as low as 7 M using the silver sulfide membrane electrode. The method Is based upon removal of GSH and any other associated thiol from one aliquot by alkylation with AZ-ethylmalelmlde and oxidation of a second aliquot with Iodine. Both are then reduced with glutathione reductase enzyme and NADPH2 at pH 8 followed by potentlometrlc monitoring of the produced GSH. Samples containing as little as 30 ng/mL of each compound (~5 X 10~8 to 10~7 M) show an average recovery of 98% (standard deviation, 1.8%) without any significant Interference from other sulfur compounds. The activity of glutathione reductase enzyme (0.4-4.0 mlU/mL) Is determined by a reaction with controlled excess concentration of GSSG and NADPH2 and measuring the Initial rate of GSH production.

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When strongly adsorbed or chemisorbed molecules behave ''ideally'' it is a relatively simple process to interpret linear sweep voltammograms and extract thermodynamic and kinetic information. Unfortunately it is rare that the criteria for ideality are met and other models have to be developed from which to extract information about the adsorbed redox system of interest. In the last 15-20 years models have been developed which allow for the influence of such factors as lateral interactions, adsorbed dipoles, a distribution of formal potentials, multiple redox sites, interfacial potential distribution, ion pairing, and acid-base equilibria. These models are reviewed with an emphasis on the underlying assumptions on which the each model was derived. Diagnostic criteria are provided from which it may be determined which of the above phenomena, if any, are effecting the voltammetric response.

Information about the kinetics of reactions can readily be extracted from linear sweep voltammograms of adsorbates which show irreversible behavior if the adsorbates behave ideally. Factors which cause nonideal behavior include lateral interactions between adsorbates, influence of solvent reorganization energies, and kinetic dispersion. The difficulties in extracting information on the kinetics of adsorbed redox systems which behave nonideally are discussed. Diagnostic criteria are provided from which it may be determined which of the above phenomena, if any, are effecting the voltammetric response.

An enzyme-based "electrochemical canary" is described for the detection of cyanide. The sensing system imitates cyanide's site of toxicity in the mitochondria. The terminal sequence of electron transfer in aerobic respiration is mimicked by mediator coupling of tyrosinase catalysis to an electro-chemical system. An enzyme-coupled oxygen electrode is created which is sensitive to selective poisoning. Biocatalytic reduction of oxygen is promoted by electrochemically supplying tyrosinase with electrons. Thus, ferrocyanide is generated at a cathode and mediates the enzymatic reduction of oxygen to water. An enzyme-dependent reductive current can be monitored which is inhibited by cyanide in a concentration-dependent manner. Oxygen depletion in the reaction layer can be minimized by addressing enzyme activity using a potential pulsing routine. Enzyme activity is electrochemically initiated and terminated and the sensor becomes capable of continuous monitoring. Cyanide poisoning of the biological component is reversible, and it can be reused after rinsing. The resulting sensor detects cyanide based on its biological activity rather than its physical or chemical properties.

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Log Q = 2.74 -2.78 Ec, 0.55 > Ec > 0 (4) where Q is the amount of copper deposited in microcoulombs (/uC). The total quantity of copper deposited at 0 V, Q*, is 540 ± 25 pC/cm2 for a reduced or an oxidized electrode. Q* represents the total copper involved in the surface controlled reductions and oxidations, Ri -