Effect of the Nature of a Quaternary Ammonium Salt and the Addition of a Neutral Carrier on Analytical Characteristics of Sulfate-Selective Electrodes

A film polyvinyl chloride sulfate-selective electrode based on the sterically accessible higher quaternary ammonium salt, 3,4,5-tris(dodecyloxy)benzyl(oxyethyl)3trimethylammonium chloride, using 1-bromonapthalene as a plasticizer and heptyl p-trifluoroacetylbenzoate as a solvating agent was developed. The limit of detection of the electrode was 6.7 × 10^?7 M, lifetime was 1 month, and the slope of the electrode function was 27 mV/decade. The electrode is selective in the presence of interfering Cl^?, C₂O₄^?, Br^?, and NO₃^?ions. The interference of carbonate ions was eliminated by maintaining pH at 3.2 ± 0.1. Based on IR spectroscopic and potentiometric studies, it is most likely that the solvation of sulfate ions with heptyl p-trifluoroacetylbenzoate occurs through interaction with the trifluoroacetyl carbonyl carbon atom rather than with hydroxyl groups of the hydrate form.     

Quartz crystal microbalance measurements of kinetic parameters of anodic dissolution of gold in thiocarbamide electrolytes in the presence of sulfide ions

As shown by quartz-crystal microbalance measurements, in the potential range from 0.0 to 0.55 V (NHE), sulfide ions adsorbed on the gold electrode surface accelerate the electrode reaction of anodic dissolution of gold in acidic thiocarbamide solutions. The microbalance determination of kinetic parameters at a constant electrode surface coverage with sulfide ions includes a special procedure developed for the determination of the gold dissolution rate. The conditions (the potential range and the potential scan rate) of independence of the dissolution rate from the diffusion limitations associated with the ligand delivery is determined. Under these conditions, the polarization curve is shown to be linear on semilogarithmic coordinates and correspond to the Tafel equation. In this potential range, the transfer coefficient α and the reaction order with respect to the ligand p are determined at a constant electrode surface coverage θ with adsorbed sulfide ions. It is shown that with the transition from the surface coverage with sulfide ions θ = 0.1 to θ = 0.8, the transfer coefficient α changes from 0.25 to 0.55, the exchange current (i ₀) changes from 10^?5 to 5 × 10^?5 A/cm², and the effective reaction order p with respect to the ligand changes from 0.2 to 1.3. The mentioned changes are associated not only with the acceleration of gold dissolution in the presence of chemisorbed sulfide ions but also with the changeover in the mechanism of this process. Quartz-crystal microbalance data on the gold dissolution rate qualitatively agree with the results of voltammetric measurements of a renewable gold electrode. A possible version of explanation of the catalytic effect of sulfide ion adsorption on the gold dissolution is put forward.     

Anodic dissolution of gold in cyanide solutions containing ions of thallium(I)

The potential dependence of the gold dissolution rate in alkali-cyanide solutions with and without 1.5 × 10^-5 M T1NO₃ is studied using the technique of the electrode surface renewal prior to taking measurements. The thallium ions accelerate the gold dissolution at potentials below 0.2 V and inhibit it at more positive potentials. The effect depends on the time the electrode is in contact with solution before the measurements, the potential scan rate, and concentrations of KCN and KOH. Possible approaches to interpreting the discovered phenomena are suggested.     

Kinetics of silver electrodeposition from thiocarbamide solutions at a regular surface coverage with sulfide ions

Kinetics of silver electrodeposition in the presence of sulfide ions is studied on electrodes renewed by cutting off a thin surface layer, at a controlled time of contact of the “fresh” surface with the electrolyte. Solutions containing 10^?2 M AgNO₃, 0.1 M thiocarbamide, 0.5 M HClO₄, and from 2 × 10^?6 to 1.5 × 10^?5 M Na₂S are studied. It is shown that under the studied conditions, the effect of silver electrodeposition on the surface concentration of sulfide ions is insignificant. As the concentration of sulfide ions in solution and their coverage on the electrode surface θ increase, the cathodic polarization decreases. Tafel curves plotted for θ = const are used in estimating the exchange current i ₀ and the transfer coefficient α. It is shown that α ≈ 0.5 and weakly depends on θ, whereas the exchange current increases with the increase in θ by an approximately linear law from 10^?5 A/cm² at θ ? 0 to 10^?4 A/cm² at θ = 0.43. The obtained data are compared with the results of kinetic studies of silver anodic dissolution in similar solutions.     

Effect of Thallium Ions on Anodic Dissolution of Gold in Alkali–Cyanide Electrolytes at Potentials More Positive Than the NHE Potential

The gold dissolution rate iin solutions containing 0.1 M KOH, 0.1 M KCN, and 2.5 × 10^–7to 1.5 × 10^–5M TlNO₃is studied as a function of potential Eof the electrode whose surface is renewed prior to each experiment, the TlNO₃concentration c, and the time tof the electrode contact with solution. At cexceeding 0.5 × 10^–5M and t 0, the rate is 1.5–2 times that at c= 0. Initial portions of ivs. tcurves in the absence and presence of TlNO₃coincide only at cbelow 10^–6M. Potentiostatic and potentiodynamic measurements show that, at positive E, only small coverages of the electrode surface with thallium are obtained, which make no impact on iat E< 0 and heavily increase it at 0 < E< 0.3 V. The discovered effects are attributed to the formation, during the adsorption of oxidized thallium forms, of dipoles comprising thallium adions and gold atoms. Presumably, the dipoles face the gold with their negative ends and make the potential of zero free charge more negative.     

Catalytic effects of metal submonolayers formed by faradaic adsorption on the anodic oxidation of ascorbic acid at a platinum electrode

The catalytic effects of metal ions on the anodic oxidation of ascorbic acid on a Pt electrode in 1 M HClO₄ were studied by linear sweep voltammetry. The anodic peak due to a two-electron oxidation of ascorbic acid shifts to the negative potential side on the addition of Bi³⁺. This indicates the accelerating effect of Bi³⁺ on the oxidation of ascorbic acid. The presence of other metal ions, such as Pb²⁺, Hg²⁺, Tl⁺, Ag⁺ and Sb³⁺, also exerts similar effects. These metal ions were adsorbed on a Pt electrode at underpotentials and the adsorbed metals (denoted as M_ad) still remain on the electrode surface until the electrode potential goes up to and beyond the peak potential of the oxidation of ascorbic acid. On the other hand, metal ions forming no adsorbed layer on Pt, such as Co²⁺, Zn²⁺, Fe³⁺ and Ni²⁺, exhibit no catalytic effect. These facts suggest that the presence of a M_ad on Pt is essential for the promotion of the anodic oxidation of ascorbic acid. However, there is a difference in the catalytic action among the M_ad, for example, Cu_ad, Cd_ad, In_ad, Sn_ad and Mo_ad display no catalytic action.The catalytic activity depends on the degree of surface coverage by the M_ad. The maximal effect of the M_ad is attained in the submonolayer region. The effects of metal ions were discussed on the basis that the M_ad plays its major role in the removal of the adsorbed ascorbic acid occupying active sites on the electrode surface, and provides effective sites for the activation of adjacent water molecules. Furthermore, from the ¹³C NMR spectra for the oxidation products, the adsorbed water on the M_ad appears to function by promoting the subsequent hydration steps, following the electron-transfer step of ascorbic acid.     

The effect of chemisorbed sulfide ions on the gold electrodeposition from acidic thiocarbamide electrolytes

Introducing sodium sulfide (about 10^?5 M) into acidic thiocarbamide solutions reduces the gold reduction overpotential. The reaction rate passes through a maximum at a potential of 0.1 V. The overpotential depends on the sulfide ion concentration and the time of electrode exposure to solution prior to the beginning of scanning. Transients of potential measured on a renewable gold electrode in thiocarbamide electrolytes containing catalytically active species served as the basis for calculations of the coefficient of trapping of sulfide ions by the growing gold deposit. The kinetics of gold electrodeposition at fixed surface coverages with adsorbed sulfide ions θ is studied. It is shown that at θ = const, the dependence of the reaction rate on the overpotential is described by the Tafel equation. It is shown that with an increase in θ, the effective values of exchange current and transfer coefficient increase from i ₀ ≌ 10^?5 A/cm² and α ≌ 0.25 in pure solutions to α ≌ 0.5 and i ₀ ≌ 10^?4 A/cm² at θ ≥ 0.3 and then remains virtually unchanged. The reaction order decreases in the absolute magnitude, remaining negative. Thus for θ ≌ 0, p _k = ?logi/?logc = ?1, whereas for θ ≥ 0.3, p _k = ?0.3. A possible explanation is proposed for the catalytic effect of the sulfide ion adsorption on the mechanism of the gold reduction from acidic thiocarbamide electrolytes.     

Stripping Voltammetry of Thallium at a Film Mercury Electrode

The electrochemical dissolution of thallium from the surface of a mercury film electrode was studied in different supporting electrolytes. The effects of the concentration of thallium(I), electrolysis potential and time, and the potential sweep rate on anodic voltammograms were studied. The electrode process in solutions containing EDTA, HCl, and ascorbic acid at a potential sweep rate of 0.020 V/s is reversible. A stripping voltammetric procedure was developed for determining thallium(I) in the concentration range 1 × 10^–9 to 5 × 10^–7 M. The detection limit (3, n = 3) is 5 × 10^–10 M.     

A Prussian blue-based reactive electrode (reactrode) for the determination of thallium ions

A reactive electrode (reactrode) made of Prussian blue (PB), graphite and paraffin can be used for a selective determination of thallium ions down to a concentration of 2 · 10^–8 mol 1^–1. The working principle of the reactrode is that thallium ions can be pumped into Prussian blue during alternating oxidation-reduction cycles. After a preconcentration of thallium ions in PB, the voltammetric determination follows as usually in anodic stripping voltammetry, i.e. the thallium ions are reduced to thallium metal which is subsequently oxidized to give the anodic stripping signal. The peculiarity of the Prussian blue-thallium system is that the thallium ions are situated in the holes of the PB matrix. When reduced to metallic thallium, they are substituted by potassium ions. Cd²⁺, Fe³⁺, Zn²⁺, Cu²⁺ and Ni²⁺ do not interfere up to a hundredfold excess, NH₄⁺ does not interfere up to a thousandfold – and Bi³⁺ up to tenfold excess. The interference by Pb²⁺ can be suppressed with EDTA.     

Catalytic Activity of Thallium, Lead, and Bismuth Adatoms in the Gold Dissolution Reaction in Cyanide Solutions: A Comparative Characterization

Quartz microgravimetry is used to determine the ratio between coefficients of mass transfer (1 : 0.54 : 0.48), which characterizes relative values of rates of diffusion of hydroxy complexes of thallium, lead, and bismuth in alkaline solutions. The ratio is used when refining the condition under which on a renewable electrode in solutions containing these ions at the concentration c _i the electrode coverage by relevant adatoms _i with increasing duration of contact of the electrode with solution reaches constant values: (c _Tl t) = (0.54c _Pb t) = (0.48c _Bi t). Measured are i,t curves on a renewable gold electrode at E = const in solutions containing 0.1 M KCN, 0.1 M KOH, 0.01 M KAu(CN)₂, and 8 × 10^–6 M compounds of thallium or 1.5 × 10^–5 M, lead, or 1.6 × 10^–5 M, bismuth. Shown is that at (, E) = const the currents of dissolution of gold in these solutions increase in the series Tl < Pb < Bi, which evidences an increase in this series of the catalytic activity of adatoms of these metals. Shown is that at = const the catalytic action of adatoms of thallium and bismuth has an approximately additive character. The obtained data are analyzed with allowance made for the explanation offered earlier for the catalytic effect of adatoms on the anodic dissolution of gold based on the hypothesis about the shift of the potential of the free zero charge in the negative direction after substituting a metal atom for chemisorbed cyanide ions.     

Electrocatalytic oxidation of nickel amalgam in aqueous solutions of halogen and pseudohalogen ions

The voltammetric oxidation of nickel amalgam from the hanging mercury drop electrode in aqueous solutions of F^?, Cl^?, Br^?, I^?, N₃^?, SCN^?, and ClO₄^? ions have been investigated. Concentrations of these anions were sufficiently low to depress the formation of complexes with nickel(II) in the bulk of the solution.An increase in the rate of anodic oxidation with increase of concentration of anions was observed both without and with correction for the φ₂ potential. This increase is explained as due to a catalytic effect of anions adsorbed on the electrode surface.Using the concept of changes of the activity coefficient of the activated complex it was possible to show that the oxidation of the nickel amalgam in thiocyanates and azides proceeds by the formation of the activated complex with bound SCN^? and N₃^? anions. These complexes form only in the activated state and decompose when products leave the double layer.In chlorides and bromides a similar mechanism is suggested only at larger surface concentration of anions. At lower surface concentration and in iodides the oxidation proceeds by the activated complex with no anions bound to the nickel, only long-range interactions of adsorbed anions with activated complex then exist.The order of these electrode reactions was calculated using the concept of the surface activity.The two-step mechanism of the charge transfer is also discussed.     

Carbonate-Selective Electrodes Based on Higher Quaternary Ammonium Salts with Increased Steric Accessibility of the Exchange Site

A film polyvinyl chloride (33 wt %) carbonate-selective electrode based on a higher quaternary ammonium salt with increased steric accessibility of the exchange site, 3,4,5-tris(dodecyloxy)benzyl( oxyethyl)₃trimethylammonium chloride (5 wt %), was developed using o-nitrophenyl decyl ether (52 wt %) as a plasticizer and heptyl p-trifluoroacetylbenzoate (10 wt %) as a solvating agent. The limit of detection of hydrocarbonate was 1.9 × 10^?9 M, lifetime was 2.5 months, and the slope of the electrode function was 32.3 ± 0.6 mV/decade. The electrode is selective in the presence of interfering SO₄^2-, Cl^?, C₂O₄^2-, Br^?, HPO₄^2-, and NO₃^- ions, which allows its application for the determination of HCO₃^- ions in mineral water.     

Electrical double layer at the mercury-nitromethane solution interface

The differential capacity and the surface charge density curves as a function of the electrode potential for mercury/electrolyte solution in nitromethane interface are presented. For all the systems studied the capacity hump at the anodic potential region is observed. The height and the location of the hump considerably depends on the kind of anion. As a test of specific adsorption of ions in the systems studied the Esin-Markov effect was examined. The results indicated that anions appear to be specifically adsorbed from nitromethane in the order PF₆^?<ClO₄^?<Cl^?<SCN^?.     

A Cobalt Film Electrode for Nitrite Determination in Natural Water

In this study a cobalt film electrodeposited on a copper disk (Ø=3.1 mm) was tested as electrode to measure nitrite ions in raw water. This electrode was able to determine the nitrite ions concentration in nondeaerated synthetic media and in natural water. The electrode reached a detection limit of 0.2 μmol L^?1 and has a linear concentration range of 0.4 to 2 μmol L^?1 NO₂^?. The influence of several ions such as NO₃^?, Cl^?, SO₄^2?, Mg²⁺, HCO₃^? and NH₄⁺ was also tested. The electrode was used to determine the concentration of nitrite ions in a real sample.     

Bifunctional Ion‐Selective Electrode Based on C60‐Cryptand 22 for Silver and Iodine Ions

Fullerence C60‐cryptand 22 was prepared and successfully applied as the electric carrier in the PVC electrode membrane of a bifunctional ion‐selective electrode for cations, e.g., Ag⁺ ions as well as anions, e.g., I^? ions. The bifunctional ion‐selective electrode based on C60‐cryptand 22 can be applied as a Silver (Ag⁺) ion selective electrode with an internal electrode solution of 10^?3 M AgNO₃ in water (pH = 6.3), or as an Iodide (I^?) ion selective electrode with an acidic internal electrode solution of 10^?4 M KI_(aq) (pH = 2) in which the cryptand 22 is protonated, and the C60‐cryptand 22 is changed to C60‐Cryptand22–H⁺ and becomes an anionic electro‐carrier to absorb the I^? ion. The Ag⁺ ion selective electrode based on C60‐cryptand 22 gave a linear response with a near‐Nernstian slope (59.5 mV decade^?1) within the concentration range 10^?1‐10^?3 M Ag⁺_(aq). The Ag⁺ ion electrode exhibited comparatively good selectivity for silver ions, over other transition‐metal ions, alkali and alkaline earth metal ions. The Ag⁺ ion selective electrode with good stability and reproducibility was successfully used for the titration of Ag⁺_(aq) with Cl^? ions. The Iodide (I^?) Ion selective electrode based on protonated C60–cryptand22‐H⁺ also showed a linear response with a nearly Nernstian slope (58.5 mV decade^?1) within 10^?1 ‐ 10^?3 M I^? _(aq) and exhibited good selectivity for I^? ions and had small selectivity coefficients (10^?2–10^?3) for most of other anions, e.g., F^? , OH^?, CH₃COO^?, SO₄^2?, CO₃^2?, CrO₄^2?, Cr₂O₇^2? and PO₄^3? ions.     

The effect of thallium ions on the gold dissolution rate in thiosulfate electrolytes

The effect of TlNO₃ additions in the concentration (c ₁) range from 5 × 10^?6 to 1 × 10^?4 M on the anodic dissolution of gold in sodium thiosulfate solutions with the concentration (c ₂) from 0.005 to 0.2 M is studied by voltammetry on the electrode surface renewed by cutting off a thin metal layer immediately in solution and also by the quartz-crystal microbalance method. For c ₂ = 0.2 M, as c ₁ increases from 5 × 10^?6 to 1 × 10^?4 M, the gold anodic dissolution rate is observed to increase from 0.02 (in the absence of TlNO₃) to 0.75 mA/cm² for c ₁ = 7.5 × 10^?5 M according to a nearly linear law. The dissolution accelerates because the effective values of the transfer coefficient and the exchange current density increase from 0.2 and 4 ??A/cm² (in the absence of TlNO₃ admixtures) to 0.47 and 35 ??A/cm² (for c ₁ = 1 × 10^?4), respectively. Experiments with the renewal of the electrode surface in the course of electrolysis suggest that the gold dissolution is catalyzed in the presence of thallium ions by the adsorption mechanism and also as the result of the mixed kinetics of their adsorption on the electrode surface.     

Kinetics of Anodic Dissolution of Gold in Cyanide Solutions Containing Thallium Ions: Potentiostatic Measurements

Effective reaction order by cyanide ions, p, effective transfer coefficient _ef, and exchange current i ₀of the gold dissolution process are measured at a constant surface coverage by thallium adatoms _Tl. The constancy of _Tlis ensured by constant time tof the electrode's contact with solution after renewal of its surface and before starting the measurements. Solutions containing 2.5 × 10^–6to 10^–5M TlNO₃, 0.1 M KCN, and 0.1 M KOH are studied. With an increase in t, quantities p, _ef, and i ₀increase from values typical for pure KCN solutions (0.17, 0.1, and 10^–5A/dm², respectively) to p 1.1, _ef 0.4, and i ₀ 3 × 10^–4A/dm². The increase in pby nearly unity is interpreted as indicating a change in the limiting stage in the presence of thallium ions: in the latter case, it involves not only adsorbed cyanide ions (as in pure cyanide solutions), but also those located beyond the adsorption layer. A possible mechanism explaining the acceleration of gold dissolution in the presence of thallium adatoms is put forward.     

Solid Membranes of Copper Hriacyanoferrats (III) as Thallium (I) Sensitive Electrode

Abstract

Solid membranes of copper hexacyanoferrate (III) in Areldite are evaluated as thallium (I) sensitive electrode. The membrane electrode gave a linear near Normstian response to thallium (I) ions in the concentration range 10^?1 - 5 × 10^?4 M and can be used to estimate T1 (I) down to 10^?4 M. The responses of the electrode is fast and steady potentials are obtained in less than a minute. The same membrane has been used over a period of six months without any appreciable drift in potential. The electrode can also be used satisfactory in partially non-aqueous media and in presence of a number of interfering ions. It is superior to the existing T1(I) solid membrane electrodes as it can function in alkaline range also.     

Electrochemical Behavior of Chloranil Chemically Modified Carbon Paste Electrode. Application to the Electrocatalytic Determination of Ascorbic Acid

A p‐chloranil modified carbon paste electrode was constructed and the electrochemical behavior of this electrode was studied in the aqueous solution with different pH. From the E_1/2–pH diagram for this compound the values of formal potential E^0' and pK_a of some different redox and acid‐base couples depending on the solution pH were estimated. The diffusion coefficient, D, value for p‐chloranil was estimated 1.5×10^?7 cm² s^?1. It has been shown by direct current cyclic voltammetry and double potential step chronoamperometry, that this p‐chloranil incorporated carbon paste electrode, can catalyze the oxidation of ascorbic acid in the aqueous buffered solution. Under the optimum condition (pH 7.00), the oxidation of ascorbic acid at the surface of such an electrode occurs at a potential about 325 mV less positive than that at an unmodified carbon past electrode. The catalytic oxidation peak currents was linearly dependent on the ascorbic acid concentration and a linear calibration curve was obtained in the range of 7×10^?5 M–4×10^?3 M of ascorbic acid with a correlation coefficient of 0.9998. The limit of detection (3σ) was determined as 3.5×10 ^?5 M. This method was used as simple, selective and precise voltammetric method for determination of ascorbic acid in pharmaceutical preparations.     

Determination of pH value of isoelectric solution and identification of passive iron surface phases using immersion method

Using the method of phase modeling, the pH values of solutions corresponding to the uncharged surface of passive iron and ferric oxide γ-Fe₂O₃ (pH₀) are compared. According to the theory of connected places, the charge of metal oxide surface is determined by the adsorption or desorption of hydrogen ions leading to a change in the potential drop at the oxide/solution interface. Preliminarily passivated iron electrode was washed with twice-distilled water and placed into 0.5 M NaNO₃ solution with various pH values; the variation in the potential (ΔE) with time was studied. The pH₀ value for passive electrode under the open-circuit conditions was determined by the dependence of ΔE on the pH value (pH₀ 6.2 ± 0.1). The pH₀ value was close to that for γ-Fe₂O₃ (pH₀ 6.2), which was determined by the method of potentiometrical titration of oxide suspension in the nitrate solution. The introduction of surface-active ions Ba²⁺ and Cl^? changes the charge of passive iron surface: Ba²⁺ ions increase the electrode potential, while Cl^? ions decrease it. Comparing the pH₀ values for passive electrode and metal oxides, one can identify the composition of passive electrode surface.