Determination of nanogram amounts of sulphide by release of radioactive iodide

Two variants of a technique for determination of ng-amounts of sulphide ions in liquid samples are presented. They are based on the replacement of radioactively labelled iodide from silver iodide by sulphide. In the first variant, suitable for small sample volumes, the labelled silver iodide is fixed on a filter paper disc which is then shaken with the solution to be analysed until equilibrium is attained. In the second variant, suitable for sample volumes up to 300 ml, the sample solution is passed through a filter paper disc carrying labelled silver iodide or through a labelled silver iodide precipitate. The amount of sulphide is determined from the activity of the released radioiodide by comparison with standards which have been processed in the same way. The method is applicable to sulphide amounts greater than 5 ng and concentrations greater than 0.2 ppM. The interference by many common accompanying anions and cations has been investigated.     

Surface studies on precipitate-based cyanide electrodes

Silver iodide-based electrodes allow indirect measurements of cyanide. Potentiometric investigations and theoretical studies have suggested that a corrosion process is responsible for this cyanide response. Surface analytical methods of providing information at different depths are used to investigate mixed membranes of silver iodide/silver sulphide and pure silver iodide membranes. The results prove that in the surface corrosion process the iodide content of the mixed membrane surface decreases. Further, the membrane loses silver sulphide particles from its surface. Finally, a layer enriched with readsorbed iodide is formed on the outermost surface of the membrane. The composition of the surface layer depends on pH and buffer capacity because of the different fluxes of ions observed in the surface layer.     

Standard potential values of “cyanide-sensitive” silver iodide and silver sulphide ion-selective electrodes

It is shown that the equation for the potentials of semiconductive sulphide membranes given by Sato 25 years ago is valid also for the calculation of E⁰ values of “cyanide-sensitive” ion-selective electrodes. The only requirement for such an approach to the problem is information on the activities of relevant species in both the solid and the liquid phases. In this way, the E⁰ value given by Pungor and Toth for the silver iodide membrane in cyanide solution was confirmed, and E⁰ for silver sulphide in cyanide solution was evaluated for the first time.     

Behaviour of piezoelectric quartz crystals in solutions with application to the determination of iodide

The frequency of oscillation of a piezoelectric quartz crystal immersed in solution changes with the temperature of the solution and with the ambient temperature of the oscillator, especially where the latter is transistorized. The frequency is also affected by the specific gravity, viscosity and specific conductivity of the solution. When all the properties are maintained constant, iodide present in the solution electrodeposits on the silver electrodes of a crystal to produce a reproducible change in frequency which allows iodide to be determined in the range 0.5–7 μM. Thiosulphate, cyanide, sulphide, Fe(III), Hg(II) and Ag interfere, but procedures for preventing their interference are given.     

Enhancement of sensors selectivity by gas-diffusion separation : Part 1. Flow-injection potentiometric determination of cyanide with a metallic silver-wire electrode

The potentiometric response of a metallic silver-wire electrode in the presence of silver ion complexing agents is theoretically derived on the basis of the Nernst equation. The cyanide response is shown to be in good agreement with the theory. The analytical utility of this inherently non-selective sensor is demonstrated by its application in gas-diffusion flow-injection analysis. By making use of a membrane barrier that prevents other than gaseous species from passing through, the almost specific determination of cyanide becomes feasible. Gaseous interferents (i.e., hydrogen sulphide, sulphur dioxide and nitrogen oxides) are chemically converted prior to entering the gas-diffusion unit. The apparent selectivity coefficients thus obtained are significantly better than those reported for common cyanide-selective electrodes.     

Determination of Low Cyanide Levels Using a Deposited-on-Wire Silver/Silver Selenide Electrode

Because of extreme toxicity, cyanide is important in water quality control,and cyanide-selective electrode method is one of the best methods for fast determination of cyanide content. In this paper, a silver/silver selenide electrode is prepared quickly by holding a silver wire in vapours of molten selenide. The electrodes were tested in following steps.     

Highly sensitive determination of iodide by ion chromatography with amperometric detection at a silver-based carbon paste electrode

A silver-based solid carbon paste electrode was developed for use as a detector in ion chromatography (IC) for the sensitive determination of iodide in real samples. Micro- and nano-particles of silver were investigated for the fabrication of different electrodes. The iodide assay was based on IC with amperometric detection (IC-AD) at a silver composite electrode polarized at +0.080 V versus Ag/AgCl. Free iodide and organoiodide compounds were studied. The detection process was characterized by studying the redox behavior of iodide ions at both silver and silver composite electrodes by cyclic voltammetry (CV). The presence of iodide ions in solution was found to considerably facilitate metallic silver oxidation, with response currents directly related to iodide concentration. The calibration curve at the selected silver carbon paste electrode was linear in the concentration range comprised between 0.635 microg/L and 63.5 microg/L iodide. The relative standard deviation (R.S.D.) for successive injections was below 3% for all iodide standard solutions investigated. The limit of detection (LOD) was 0.47 microg/L (3.7 nmol/L) for an injection volume of 20 microL, i.e. 74 fmol injected. The IC-AD method was successfully applied to the determination of iodide in complex real samples such as table salts, sea products and iodide bound drug compounds. The analytical accuracy was verified by the assay of iodide in milk powder from an iodide certified reference material (CRM) Community Bureau of Reference (BCR) 150.     

Ion-selective carbon-paste electrodes for halides and silver(I) ions

The behaviour of a simple type of ion-selective electrode for halogens and silver has been studied. The electrode consists of a plastic body filled with carbon paste, the surface of which can be easily renewed. The paste composition is based on carbon-nujol (5:1, w/v) or carbon-paraffin wax (3:1,w/w) containing a prepared mixture of silver halide-silver sulphide (1–30%). The electrodes have low ohmic resistance and show a rapid Nernstian response (within 2–5 mV) for halide and silver ions down to 5·10^-5M chloride, 1·10^-5M bromide and 5·10^-7M iodide with the respective electrodes. Ions forming very stable complexes with halide or silver and those having strong oxidizing or reducing action interfere.     

Side effects in measurements of selectivity coefficients of solid state ion selective electrodes

The influence of various factors such as solubility, the oxidation of the membrane, and the contamination of the solution, on the experimental values of the selectivity coefficients of solid state sulphide ion selective electrodes is discussed. A new method for the evaluation of very small selectivity coefficients, based on the addition of reagents forming complexes or insoluble salts with the main ion, is proposed. By means of this method, selectivity coefficients for silver, copper, cadmium and lead ion selective electrodes have been determined, which are in far better agreement with thermodynamic values than those described in the literature.     

The response of halide ion-selective electrodes to redox systems : A comparison between silver/silver halide electrodes and silver sulphide- based halide ion-selective electrodes

Silver/silver chloride and bromide electrodes, prepared by anodizing ordinary silver electrodes, and the corresponding ion-selective electrodes based on silver sulphide, were tested for their susceptibility towards redox systems. It proved that the latter type of electrode responded significantly to strong oxidants. In contrast, the silver/silver halide types were highly resistant to redox interference provided that the silver halide layer was free from open pores. This could be achieved by generation of sufficiently thick layers and by selection of suitable current densities during electrodeposition (<20 mA cm^-2). The interrelation between the conditions of silver chloride film generation and redox resistance of the resulting electrodes is described in detail.     

The behaviour of mixed metal sulphide/silver suphide sensing materials in solutions of silver ions

Hydrophobized graphite electrodes activated with CuS/Ag₂S can be used to measure silver ion activities as effectively as silver sulphide electrodes, whereas electrodes activatd with PbS/Ag₂S or CdS/Ag₂S are useless for this purpose. The CuS/Ag₂S electrode is also suitable for use in potentiometric titrations involving species that are not contained in the sensing material, but are capable of producing precipitates with silver or sulphide ions.     

Ion-selective electrodes based on silver sulphide

A series of electrodes with membranes of silver sulphide have been prepared. Their standard potentials and their response to sulphide and silver ions are measured and compared with those of a single-crystal silver sulphide electrode. The selectivity parameters against heavy metal ions and halides, as well as cyanide, are discussed. It is concluded that only interferences from cyanide ions and mercury(II) ions are detrimental to the practical use of the electrodes for measuring pAg and pS.     

Some Characteristics of Ion-Selective Electrode Membrane Surfaces

Abstract

Some studies of the features of surfaces of PVC based liquid ion-exchanger and neutral carrier membranes, and of solid-state membranes of ion-selective electrodes are reviewed with respect to the application of X-ray fluorescence (XRF), Auger spectroscopy and scanning electron microscopy (SEM), Among the membranes discussed are those responding to (i) calcium for organophosphate type membranes, (ii) barium and polyalkoxylate non-ionic surfactants for PVC membranes based on complexes of barium with polyalkoxylate s, (iii) copper(II) for solid-state membranes of mixed copper(II) sulphide and silver sulphide, and (iv) sulphide and thiols for membranes of silver sulphide.     

Sequential flow-injection determination of cyanide and weak metal—cyanide complexes with flow-through heterogeneous membrane electrodes

Free cyanide and cyanide present in weak complexes are determined by using two flow- through silver iodide/silver sulphide electrodes with an intervening gas diffusion unit. Under optimal conditions, the linear range is 10^?5?10^?3 mol dm^?3 cyanide, and the relative standard deviations are ca. 2%, with a sampling rate of 20 h^?1. Total cyanide can be determined in the presence of Zn(II), Cu(II) and Cd(II) but results are low with Ni(II), Co(II) or Fe(III) present. Sulphide and thiocyanate must be absent.     

The behaviour of the silver sulphide precipitate-based ion-selective electrode in the low concentration range

The anomalous behavior of the precipitate-based silver sulphide ion-selective electrode in the low concentration range of silver and sulphide ions is described. The excess of silver ion at the electrode surface, caused by adsorption or oxidation, is responsible for the deviations from the ideal Nernstian response. The adsorption/desorption processes were studied in a microcell by using combined potentiometric and atomic absorption measurements. The oxidation—reduction processes were studied by using polarized electrodes.     

A galvanic solid-state sensor for monitoring ozone and nitrogen dioxide

The sensor is based on silver and platinum electrodes with an intervening silver iodide disk as a solid electrolyte. The small disk (13 mm diameter) is easily made by pressing (7000 kg cm^-2) silver powder, silver iodide and a platinum gauze in layers in a die. The detector cell containing the disk is thermostated at 120 ± 0.1°C. When sample gas at 30 ml min^-1 impinges on the platinum cathode, the current flowing in the external circuit is linearly related to the concentration of ozone and/or nitrogen dioxide up to 0.5 ppm from the detection limit of 0.5 ppb for ozone or 1 ppb for nitrogen dioxide.     

Silver/silver iodide electrodes of the second kind as sensors for cyanide

Silver/silver iodide electrodes of the second kind prepared by electrolytic coating of silver electrodes are advantageous as sensors for cyanide. They are inexpensive and exhibit characteristics similar to those of commercially available ion-selective electrodes. They can be regenerated easily when necessary; this eliminates the lifetime problems generally inherent in iodide electrodes used as cyanide sensors. The preparation, properties and selectivity characteristics of the second-kind iodide electrode are described.     

Ion-selective chalcogenide electrodes for a number of cations

Ion-selective chalcogenide disc electrodes have been developed which are responsive to cations such as silver, lead, chromium(III), nickel, cobalt(II), cadmium, zinc, copper(II) and manganese(II) ions. Each was prepared by using the corresponding metal chalcogenide with silver sulphide. An electrode was assembled with both a compacted and a sintered disc. The sintered electrodes were more sensitive and stable than the compacted ones. Response to silver ion was 59.5 mV pAg , to lead, nickel, cadmium, zinc and copper(II) 29.5 mV pM and to chromium(III) 20 mV pM . Cobalt(II) and manganese(II) electrodes had a non-Nernstian response of 25 mV pM . Both selenides and tellurides can be used for potentiometric determination, but the manganese(II) electrode serves as an analytical tool only when the disc consists of manganese(II) telluride and silver sulphide.     

Oxidative removal of interferences in flow-injection potentiometric determination of chloride

Direct potentiometric determination of chloride in a flow-injection system can be performed in the presence of excess bromide, iodide, sulphide and cyanide, when potassium bromate in nitric acid is used as the carrier solution. The hydrodynamics and temperature of such a system have been examined and various oxidants and indicating electrodes investigated. The analysis can be performed at a maximum rate of 120 samples per hour.     

Potentiometric studies on organic compounds containing sulphur with a sulphide ion-selective membrane electrode

Phenylthiourea and N,N-diphenylthiourea can be determined in the concentration range 10¹–10^?3M by potentiometric titration with silver nitrate solution with a sulphide ion-selective membrane electrode. The influence of the alkali and acid concentration on the course of the reactions with the silver nitrate titrant was studied. When phenylthiourea is titrated in the presence of 0.1 M sodium hydroxide, silver sulphide is precipitated, and the phenylcyanamide formed simultaneously reacts further with silver nitrate to form a silver phenylcyanamide precipitate. When N,N-diphenylthiourea is titrated under similar conditions, silver sulphide is again formed, but no cyanamide-type compound can be formed owing to the presence of the second phenyl group. In 1 M sodium hydroxide both compounds studied react with two equivalents of silver to give silver sulphide and phenylurea or diphenylurea, respectively. In the presence of nitric acid both compounds react similarly with silver nitrate. Two molecules of the compounds react with one molecule of silver nitrate to give a white precipitate.