Abstract: Silver wire electrodes coated with AgCl, AgBr, AgI, Ag3PO4, Ag2S and AgSCN were evaluated for potentiometric detection of various anions with a flow injection system. A 0.01 M NaClO4 solution was pumped through the system at 4.5 mL min-1 and a small volume of sample solution was injected. The best response and repeatability for several different anions were obtained with silver - AgCl and silver - AgSCN electrodes. The surfaces of several electrodes were examined by electron microscopy and preliminary evaluations indicated that both silver - AgCl and silver - AgSCN electrodes should be satisfactory detectors for flow injection systems or ion chromatography with both isocratic and gradient elution. The silver - AgCl electrode selectively detected Cl-, Br-, I-, SCN- and S2O32- in ion chromatography over a working range of 0.05 to 2 mM.

Abstract: Details are given of a gas diffusion unit (diagram given) with an easily constructed tubular microporous PTFE membrane through which halogen molecules can permeate into an acceptor solution. The sample is introduced via a six-way valve with a variable loop into water as carrier, and this stream is mixed with an oxidizing reagent to generate the molecular halogen (e.g., 10 µM-KMnO4 - 1 M H2SO4 to produce I2 from I-; 0.1 M NaCl - 0.1 M H2SO4 to generate Cl2 from ClO-). After passage through a 10-cm mixing coil, the stream passes through the diffusion unit, where the halogen molecules pass into 0.2 mM NN-diethyl-p-phenylenediamine - 0.1 mM H2SO4 for spectrophotometric detection or (for Cl2) into 0.1 mM acetate buffer of pH 4.8 for potentiometric detection with a coated-wire ion-selective electrode. Conditions are described for the determination of I-, Br-, residual Cl and (by the reduction of I2 to I-, thus affording negative peaks) S2O32-. Microporous polytetrafluoroethylene membrane tubing was used to assemble a newly designed gas diffusion unit and was applied to the permeation of halogens. It was found that halogens such as bromine and iodine, which are not gaseous at room temp., permeated through the membrane tubing; the permeability decreased in the order chlorine > bromine > iodine. The permeated halogens react with N,N-diethyl-p-phenylenediamine to produce colored substances. By coupling this chromogenic reaction with the permeation of halogens, the sensitive and selective determination of halogens and halide ions can be achieved. Iodide and bromide ions were determined by spectrophotometric methods coupled with oxidation by permanganate. Thiosulfate was determined on the basis of the reduction of iodine to iodide. Residual chlorine was determined by a flow injection potentiometric method with a coated-wire ion-selective electrode, and by a flow injection spectrophotometric method.

Abstract: The suppression of the dissolved oxygen reduction signal at Pt and Au electrodes by the adsorption of organic and other non-metal species was used as an indirect measure of adsorbate concentration. This indirect approach, indirect adsorption detection (IAD), was used to detect amines, halides and sulfur compounds following FIA or HPLC. The measurements were carried out using a three electrode system consisting of a circular Pt or Au working electrode (2 mm diameter), a Ag/AgCl reference electrode and a stainless steel counter electrode. A pulsed voltage was applied to the working electrode and O2 reduction was monitored at negative potentials. The detection limits achieved by IAD were similar to those of pulsed electrochemical detection. The detection limit was 50 nM for the methylthiohydantoin derivative of glycine with a detection potential of -200 mV. The negative detection potentials required by IAD for the highest sensitivity also promoted gas production. A detection cell designed to be tolerant to gas production is required to realize the full potential of IAD.

Abstract: The flow injection system consists of an HPLC pump, a sample injection point and a measuring cell coupled to a potentiostat and a pH meter interfaced with a computer for amperometric recording. The cell is a Plexiglas cylinder with bores for inflow and outflow and Ag - AgCl measuring and reference electrodes; the measuring chamber volume is ~50 mm3. The carrier stream (2 mL min-1) is 1 mM NaClO4 - 1 mM HClO4. A single measurement takes ~2 min including pause. Results for determination of 3 to 450 mg L-1 of Cl- in synthetic and real water samples agree well with potentiometric measurements and the average coefficient of variation is 1.5%. Selectivity coefficient are quoted for the Cl- - I-, Cl- - Br-, Cl- - CN- and Cl- - S2- ion pairs and instructions are given for rectilinear and non-rectilinear calibration in the presence of ion matrix effects, by the method of standard addition and dilution (or precipitation with AgNO3).