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An atypical example of the effect of the self-induced convection on the morphology of current-potential responses was described, for the case of the parallel electroreduction of some metal cations (Cd 2+ , Cu 2+ , Zn 2+ and Eu 3+ ) and H + ions at the mercury electrode. Separation of the individual faradaic contributions from the metal ions and hydrogen ions reduction, made with the double potential-step chronocoulometry, and based on the difference between the rates of these processes, allowed us to find that the total faradaic current exhibits negative or positive deviations from the sum of individual faradaic currents. In order to diagnose the origin of this anomaly, the series of experiments for various acid concentrations, electrolysis times and the electrode types/geometries, including the hanging mercury drop and the mercury film electrodes, were performed. The results obtained indicated that the observed anomalies are due not to the surface blocking or latent limiting currents, but due to a rather novel mechanism that involves convective instabilities which accompany the hydrogen evolution reaction. The resulting change of the concentration distribution of all species involved in the electrode process appropriately changes the individual contributions from the faradaic currents. These studies show one more role of convection in electrochemical systems.

The properties of a cobalt amalgam formed during electrochemical reduction of Co(II) ions at mercury electrodes were investigated by cyclic voltammetric and chronocoulometric methods. Experiments were performed in a range of temperatures from −20 to 75°C, and a wide range of Co(II) concentrations, in NaClO 4 solutions of various concentrations. At lower temperatures and low cathodic charges of Co(II) reduction, a homogeneous cobalt amalgam is formed, which undergoes a rather fast deactivation, leading probably to a Co 3 cluster. The cluster is not electroactive in the accessible range of potentials. At higher concentrations of Co(II) and at higher temperatures, on anodic waves corresponding to oxidation of cobalt amalgam and recorded in unbuffered solutions, up to four oxidation steps are observed. On the other hand, in acetic buffer solution (pH 3.6), under chronocoulometric conditions, only one oxidation peak appears. Thus, other oxidation steps could be ascribed to the oxidation of Co(II) to Co(III) hydroxide. The formal potential, the rate constant at this potential, and the Tafel slopes (anodic- αn , and cathodic- βn ) corresponding to the electrode process Co(II)+2e − +Hg=Co(Hg) were determined at 253 K. Determination of these parameters at higher temperatures was impossible, due to fast deactivation of the cobalt amalgam.

The temperature dependences of the four transfer coefficients for cathodic and anodic electrode reactions of two different types of systems, namely Cr(III)/Cr(II) and Cr(II)/Cr(Hg), were determined in 5.0 M NaClO 4 solution. The results were obtained for a wide range of temperatures from −20 to 90°C. The α parameter determined from the log k fh on E dependence corresponding to the Cr 3+ +e − →Cr 2+ electrode process shows the highest dependence on temperature (∂ α /∂ T =1.1±0.3×10 −3 K −1 ), whereas the β parameter corresponding to the Cr 2+ →Cr 3+ +e − electrode process is virtually independent of temperature (∂ β /∂ T =0). The same results were obtained when the transfer coefficients were determined from dependence of the enthalpy and entropy of activation on overpotential. The observed temperature dependence of α is attributed to the changes of the ϕ 2 potential. The transfer coefficients for the Cr 2+/0 electrode reaction also show some dependence on temperature (∂ α /∂ T =8.7±2.0×10 −4 K −1 and ∂ β /∂ T =3.2±1.4×10 −4 K −1 ), which probably results from the influence of the ϕ 2 potential.

Oxidation of a homogeneous chromium amalgam and homogeneous Cr–Zn, Cr–Mn, Cr–Tl, Cr–Pb, and Cr–Cu amalgams were investigated by the double potential-step chronocoulometric method. Experiments conducted with various concentrations of chromium in mercury confirm our earlier results, showing de-activation of the chromium amalgam. In the presence of Zn and Tl, no change in oxidation of the chromium amalgam was observed. In the presence of other metals (Mn, Pb and Cu), oxidation of the chromium amalgam was less efficient than in the absence of these metals. We interpret our experimental data in terms of high-spin Cr atom formation (with electronic configuration 3d 5 4s 1 ), and fast Cr–Cr molecule formation. Since the dissociation energy of Cr 2 is 142±5 kJ mol −1 , it is electrochemically inactive in the potential range investigated. In the case of the Cu–Cr amalgam, de-activation of the chromium amalgam was higher than in the cases of Pb–Cr and Mn–Cr. We interpret this as a result of a higher interaction energy of the Cu–Cr compound, than that of the Cr 2 . From literature data it follows that the dissociation energy of the Cu–Cr compound is, indeed, higher, being ca. 150 kJ mol −1 .

Electroreduction of chloramines (H2NCl, HNCl2 and NCl3) at the platinum electrode was investigated in aqueous solutions of varying pH. The two former compounds are reduced irreversibly and their cathodic peak potentials are pH dependent (Epc (H2NCl)=−0.25 to −0.15 V (pH 7–4), Epc (HNCl2)=0.35–0.45 V (pH 4.5–2.5), at scan rate ν=0.05 V s−1). Trichloramine is reduced at the same potential at which Cl2 is formed, and the whole system is reversible. From the dependence of cathodic peak potentials on pH the protonation equilibrium constants of H2NCl and HNCl2 were determined. They were found to be (pK) 6.4±0.1 and 3.41±0.05, respectively. The extinction coefficients of protonated species are lower than that of non-protonated ones. The formal potential of the electrode reaction: NCl3+4H++6e−=NH4++3Cl− extrapolated to pH 0 equals 1.44 V (pH 0, SHE), thus the Gibbs energy of formation of NCl3, ΔGf=362.1 kJ mol−1.

The equilibrium constant ( K 1 (CrOH + )) of Cr 2+ hydrolysis was determined by the potentiometric method in trifluoromethanesulfonic solutions. Log K 1 is equal to −8.2 ±0.2, and the [OH − ]/[Cr(II)] ratio determined at the end of titration was equal to 2.00 ± 0.02. In chloride and perchlorate media the [OH]/[Cr(II)] ratio is much lower, indicating formation of mixed complexes. Chronocoulometric measurements in weakly acidic solutions of Cr 3+ showed that at pH 3.5–5, the Cr(OH) 2+ complex was reduced in a separate wave, which made possible determination of the first and the second hydrolysis constants of Cr 3+ . The derived p K values were 4.26 and 5.92, respectively. Analysis of the standard rate constant for the Cr(OH) 2+ /Cr(II), OH − redox couple proved that OH − ions have high catalytic influence on the Cr III/II system. The formal potential of the CR(III)X + e → Cr 2+ + X sytem is, in the presence of OH − , more negative than that for other ligands.

CrBr 2+ and CrI 2+ complexes formed during the electrooxidation of Cr 2+ on mercury electrodes in the presence of Br − and I − ions, respectively, aquate with rates ca. 10 3 higher than those measured by the spectrophotometric method in the bulk of the solution. The entropies of activation of these reactions are large and negative, which indicates that the transition states are highly organized. Changes in the H + concentration did not influence the rate of aquation. Since Cr 2+ ions are not oxidized on Pt, Au, glassy carbon or pyrolytic graphite electrodes, this high instability was explained in terms of the adsorption of the transition state on mercury during the inner-sphere electron transfer.

Supersaturated chromium amalgam as concentrated as 1.5 M, has been prepared electrochemically. The resulting amalgam has a mirror-like shine and behaves like plasticine. The stability of supersaturated (up to 10 mM) chromium amalgam was investigated in 1 and 5 M solutions of NaClO4, in the range of −20 to 60°C, using double potential step chronocoulometry (DPSC). The results enabled us to determine the conditions in which the oxidation of Cr(Hg) is diffusion controlled. In the millisecond time scale, this usually occurs at concentrations lower than 1 mM Cr3+ Cr(Hg) decomposes more rapidly at increased temperature and concentration of chromium in the amalgam, and, in the first stage, forms inactive clusters (probably Cr3). The energy of activation of this reaction was determined as Ea = 36.2 kJ mol .

A method for determining the oxygen content of the high-temperature superconductor YBa2Cu3O6.5+x is described. The superconductor is dissolved in 4.4 M hydrobromic acid, forming bromine. The mixture is then diluted with hydrochloric acid to obtain a solution of bromine and Cu(II) in 0.44 M hydrobromic acid and 1.1 M hydrochloric acid. As(III) is added in slight excess of that required to react with the bromine and the unreacted As(III) is determined by titration with potassium bromate. Oxygen does not interfere. The results of this method are in agreement with those of other iodimetric procedures. The dissolution of the superconductor in 4.4 M hydrobromic acid is much faster than in hydrochloric acid, the medium used in one iodimetric technique. YBa2Cu3O6.5+x sintered fibers and powder samples weighing from ca. 0.5 to 200 mg were analyzed for oxygen content.

Hydroxylamine oxidation in acidic media at the rotating platinum, gold, and glassy carbon electrodes was investigated. It was found that the reaction occurred only at the platinum electrode. A single oxidation peak (instead of a wave) was observed at the rotating platinum electrode. Evidence for a kinetic hindrance of the reaction caused by the platinum oxide layer present at the electrode surface was shown. The reaction proceeds via NHOH, NOH, nitrous acid, and the final products are nitrate ions. The process involves hydroxylamine adsorption, which is the reason for the absence of the electrochemical signal in the case of gold and carbon, materials of lower catalytic activity. Hydroxylamine oxidation was accompanied by various homogeneous chemical reactions. The reaction of hydroxylamine with nitrous acid (intermediate product of hydroxylamine oxidation), yielding N 2 O, had a rate constant, estimated on the basis of the ring-disk experiments, equal to (6.4 ± 0.4) 10 4 mol -1 cm 3 s -1 .

Electroreduction of chloramines (mono-, di-, and trichloramine) in I M NH 4 Cl solutions of different pH was investigated at the rotating platinum and gold electrodes. It was found that all chloramines are present in the solution in nonprotonated forms and give well-formed one-step or two-step current-potential waves. The final products of reduction are ammonium (or ammonia) and chloride ions. Monochloramine is reduced in a single two-electron irreversible wave. Hydrazine is not an intermediate in monochloramine reduction. Dichloramine reduction generally proceeds in two two-electron steps (via monochloramine). Below pH 4.3 a kinetic current due to the protonated dichloramine reduction (single four-electron wave) is in force, appearing as an increase of the height of the first step on lowering pH. Due to this process below pH 2.5 only one four-electron reduction wave is observed. Trichloramine reduction occurs in two steps: two-electron trichloramine to dichloramine reduction and four-electron dichloramine reduction. In strong acidic solutions the kinetic current due to the protonated trichloramine reduction has to be taken into account. A reaction mechanism common for all chloramines was proposed with [NXCl.] as an intermediate (X = H 2 , HCl, and Cl 2 for mono-, di-, and trichloramine, respectively). The rate-determining step does not involve proton transfer.

High affinity of thallium(I) ions with respect to a copper hexacyanoferrate (CuHCF) film was found. Interaction between the CuHCF film and thallium(I) ions was investigated using the electrochemical quartz crystal microbalance and cyclic voltammetric techniques. In 0.5 M KNO 3 solution, at submillimolar concentrations of Tl(I) ions, the CuHCF film reversibly transforms, during electrochemical experiments, from the potassium into the thallium form. For slightly higher (several millimoles) Tl(I) concentrations, the electrochemical and gravimetric responses prove that the CuHCF film behaves as the thallium form only. Its formal potential (E f 0 ), calculated from the dependence of the E f 0 on [Tl(I)], is for [Tl(I)] = 1 M, 0.28 V more positive than that found for the potassium form. Experimental results obtained suggest that in both cases, potassium(I) and thallium(I) ions, exchange in interstitial positions takes place. Since the ionic radii and the hydration parameters of both ions are similar, we concluded that this high affinity of thallium(I) ions with respect to the CuHCF film, 250-100 times higher than that of potassium(I) ion, results from chemical interactions. In consequence, the solubility product of a thallium analogue of copper hexacyanoferrate is much smaller than that of the potassium form. For different forms they are equal to (pK values): 37.8 (K 2 Cu 3 [Fe(CN) 6 ] 2 ), 46.3 (Tl 2 Cu 3 [Fe(CN) 6 ] 2 ), and 17.2 (Cu 3 [Fe(CN) 6 ] 2 ).

Donnees sur le processus d'electrode Cr(II)+2e − =Cr(O) en presence d'une electrode de mercure, souvent observe comme 2eme etape de la reduction d'ions Cr(III), entre pH 2 et 4

Oxidation of Cr2+ at mercury electrodes in the presence of S2O2−3, thiosemicarbazide, 2,5-dimercapto-1, 3,4-thiadiazole, N-methylthiourea, thiourea and thioacetamide lead to Cr(III) complexes which feature a chromium(III)-sulphur bond. The equilibrium constants of these complexes are equal to 0.21, 0.14, 0.036, 0.0076, 0.0013 and 0.00015 M−1 for S2O2−3, TSC, DMTDA, NMTU, TU and TAA, respectively. The Cr(III)TU complex synthesized electrochemically aquates spontaneously, and the mechanism of this reaction is the same as that characteristic of other Cr(III)X(3-i)+ complexes. This complex is not adsorbed on the electrode surface.

The electrode process Bi(III)+ 3e = Bi(Hg) has been investigated in weakly acidic, neutral and alkaline solutions by means of amalgam polarography and cyclic voltammetry and chronopotentiometry with the use of hanging amalgam drop electrode. It has been found that in weakly acidic and neutral solutions, oxidation of bismuth amalgam goes through aquocomplexes of bismuth. In the cathodic step on cyclic voltammetry curves four peaks of reduction are observed, with magnitude and shape strongly dependent on pH of the solution. These peaks have been ascribed to the reduction of Bi3+, Bi(OH)+2, Bi(OH)+2 and Bi(OH)3. In alkaline solutions, oxidation of bismuth amalgam leads to Bi(OH)3. In cathodic scan on cyclic voltammetry curves only one peak is observed, which corresponds to Bi(OH)3 reduction. Formal potentials of electrode process and solubility product of Bi(OH)3 have been calculated and discussed. The step-wise nature of the overall electrode reaction has been ascertained and discussed. At higher concentrations of OH− ions, Bi(OH)3 dissolves.

The electrode process Bi(III)+ 3e = Bi(Hg) has been investigated in weakly acidic, neutral and alkaline solutions by means of amalgam polarography and cyclic voltammetry and chronopotentiometry with the use of hanging amalgam drop electrode. It has been found that in weakly acidic and neutral solutions, oxidation of bismuth amalgam goes through aquocomplexes of bismuth. In the cathodic step on cyclic voltammetry curves four peaks of reduction are observed, with magnitude and shape strongly dependent on pH of the solution. These peaks have been ascribed to the reduction of Bi3+, Bi(OH)+2, Bi(OH)+2 and Bi(OH)3. In alkaline solutions, oxidation of bismuth amalgam leads to Bi(OH)3. In cathodic scan on cyclic voltammetry curves only one peak is observed, which corresponds to Bi(OH)3 reduction. Formal potentials of electrode process and solubility product of Bi(OH)3 have been calculated and discussed. The step-wise nature of the overall electrode reaction has been ascertained and discussed. At higher concentrations of OH− ions, Bi(OH)3 dissolves.