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.     

Comparative study of different cyanide-selective air-gap electrodes

The characteristics of four solid-state cyanide-selective air-gap electrodes are described. When a minute volume of dicyanoargentate(I) solution was used as electrolyte, the double-Nernstian response to cyanide concentration was obtained only with the sensors based on silver or silver sulphide. Sensors based on silver iodide or a silver sulphide/iodide mixture displayed the normal Nernstian character shown by cyanide-selective electrodes dipped in solution. A theoretical explanation of the results involves the relevant equilibrium equations and the mass balance.     

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.     

Potentiometric argentimetric method for the successive titration of sulphide and dissolved sulphur in polysulphide solutions

Sulphide sulphur and dissolved sulphur in a polysulphide solution can be successively determined with satisfactory accuracy and reproducibility by potentiometric argentimetry in which a sulphide-selective indicator electrode is used. Before the titration, polysulphide ions need to be converted by an excess of potassium cyanide into thiocyanate and sulphide ions. The excess of cyanide ions is masked with formaldehyde and sulphuric acid, then the solution is made alkaline with ammonia and titrated with silver nitrate till the first end-point is reached (sulphide sulphur). After the acidification of the solution with sulphuric acid, the titration is continued till the second end-point is attained (dissolved sulphur).     

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.     

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.     

Determination of low levels of cyanide with a silver/silver sulphide wire electrode

A silver/silver sulphide electrode is prepred quickly by holding a cleaned silver wire in vapours from molten sulphur. In 1000–10 mg l^?1 cyanide solutions, the electrode exhibits a linear E/log C_CN function which becomes slightly sinusoidal for 10–0.1 mg l^–1 cyanide. The average slope is slightl super-Nerstian (10 mV/decade concentration). The applicability of the electrode is demonstrated for the determinations of microgram quantities of water-soluble cyanide from the Prussian blue pigments which are constituents of externally applied cosmetics. The home-made electrode provides results agreing with those obtained with commercially available electrodes.     

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.     

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.     

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.     

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.     

A potentiometric detector for hydrogen cyanide gas using silver dicyano complex

A sensitive method capable of detecting Hydrogen cyanide gas in atmosphere at its TLV is being presented. This method makes use of two silver electrodes kept in two separate compartments which are in contact with a solution of constant concentration of Silver dicyano complex at a pH 11.5. One of the electrodes used as reference is concealed and the other used for sensing is exposed to the incoming air. In the absence of Hydrogen cyanide gas the potential difference between the two electrodes is zero, but when hydrogen cyanide gas is passed into the cell, the activity of Ag(+) ions nearer to the sensing electrode changes, there by generating a potential difference between the two electrodes. The plot between the potential vs. log of Concentration of Hydrogen cyanide gas is linear, in the concentration range 0.66-42.3 mg/m(3) with a slope nearer to 120mV and regression coefficient around 0.997. The standard deviation is 6% (n=4). Minimum detectable limit is 0.66 mg/m(3). Various concentrations of Silver dicyano complex used gave similar plots.     

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.     

Ion-selective electrodes for gold and silver determination

Some new ion-selective electrodes for silver and gold are described. They are based on the ion-associate species formed by the cyanide, chloride or thiourea complexes of the metals, with hydrophobic anions or cations, as appropriate. The electrodes have been applied to the determination of gold and silver in various technological process solutions in industry.     

Röntgenfluorimetrische Bestimmung von Sulfidspuren im ppb-Bereich in wäßrigen Lösungen nach Austauschreaktion an dünnen Silberhalogenidschichten

Sulphide concentrations in the ppm and ppb range can be determined by X-ray fluorescence spectrometry in aqueous solutions with a volume up to 3 l. The solution to be analysed is passed through a thin layer of a silver halide precipitate whereby the sulphide ions are held back as silver sulphide. The membrane filter with the silver halide deposit is directly used for measuring the X-ray intensity from sulphur. The experimental conditions used include: 2?=23.20°, KAP, X-ray tube with a chromium anode, 50 kV, 40 mA, and a throughflow proportional counter. Interferences of numerous accompanying components were investigated as well as the stability of solutions with very low sulphide concentrations. These solutions can be stabilized sufficiently with ascorbic acid at pH 12, but in the case of natural water only in presence of zinc ions. The method is applicable to sulphide amounts greater than 0.1 μg. The variation coefficient for sulphide amounts of ≈ 10 μg was found to be 2.3%.     

Applications of enzyme-catalysed reactions in trace analysis-II Determination of cyanide, sulphide, and iodine with invertase

Methods are described for the determination of cyanide (10⁻⁸–10⁻⁵M and sulphide (10⁻⁷–10⁻⁵ M) based on the de-inhibitory effect of these ions on invertase inhibited by mercury(II) or by silver. Iodine (0.1–3 μg) may be determined by its inhibition of invertase.     

Liquid-membrane dicyanoargentate-sensitive electrodes based on quaternary ammonium salts

Several quaternary ammonium salts have been synthesized and comparisons made of their efficiency as exchange substrates in dicyanoargentate-sensitive electrodes. The electrode prepared from hexadecyltrioctylammonium dicyanoargentate shows the best performance characteristics and has been studied in more detail. Its Nernstian response range is 10(-1)-10(-4)M, the optimum pH range is 10.6-12.6, and the detection limit is 3 x 10(-5)M. It is suggested that the detection limit of the electrode is controlled by interference from cyanide ions, and the difference between detection limits obtained with the various electrodes is discussed in the light of this. Selectivity coefficients for various interfering ions have been determined and related to the extractability of the ions. The electrode has been applied to determination of silver in cyanide-containing plating solutions.     

Interactions of silver with humates and other species in natural waters

Tracer^110mAg has been used to investigate the speciation of silver in natural waters, which may contain chloride, sulphide or humate ions. Silver chloride or oxide is readily absorbed from waters by many materials, and some may be photochemically reduced to metallic silver. Absorbed silver, silver chloride and silver sulphide may be distinguished by their desorption behaviour. Humates form complexes with silver chloride, silver sulphide and the silver cation, which can be separated from smaller species by gel permeation chromatography.     

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.     

Fundamental considerations on the mechanisms of silver cementation onto zinc particles in the Merril-Crowe process

Studies on the Merrill-Crowe process as applied to silver recovery have shown that one half of the used zinc powder is wasted in water reduction at high cyanide concentrations, while the other half reduces silver ions from the cyanide solution. However, the cementation mechanisms as an electrochemical process taking place on the zinc surface do not explain the split of the electric current resulting from the anodic dissolution of zinc into two equal values. This study demonstrates that the mechanism for silver precipitation at high and low cyanide concentrations differs considerably. At low cyanide concentrations cementation is essentially an electrochemically-controlled process following a shrinking-core behavior. At high cyanide concentrations, the process seems not to be electrochemically controlled. The areas for zinc dissolution and silver deposition are not connected by an electrical-conducting medium and reduction of silver-cyano complex ions takes place by hydrogen adsorbed onto silver growing outward from the cementing zinc particles. The results are based on scanning electron microscopy of solids recovered from cementations in stirred reactors and in situ observations by optical microscopy of the cementation process on the edge of thin zinc disks in cyanide solutions.