Catalytic Prewave Research Articles

The polarographic catalytic pre-waves of nickel(II) in presence of thiocyanate

It is shown theoretically and experimentally that the first catalytic pre-wave in the Ni 2+—NCS − system (the reduction of the [Ni(NCS) 2] complex) at c NCS − ⩽ (1·0−1.·5) × 10 −2 M ( c NCS − ⪢ c Ni 2+ ) and within a wide range of ionic strength I can be described by a simplified kinetic equation based on the assumption of the proximity of the first of the two consecutive chemical stages of the Ni(NCS) 2 formation on the electrode surface to the equilibrium state. The process is controlled by the second stage (Ni(NCS) + + NCS − ▪ Ni(NCS) 2). This conclusion conforms to the variation of k 2 with the variation of I. Both of these stages are of a bulk nature, as indicated by the proximity of k 2 to the rate constant of H 2O exchange in Ni(H 2O) 6 2+, the absence of any effect of the supporting electrolyte cation, or of gelatine, on the rate of the process; the thickness of the reaction layer is ca 10 −5 cm. The second (total) catalytic pre-wave results from the reduction of Ni)NCS) + ads formed from Ni 2+ and NCS − ads, which occurs simultaneous with the first pre-wave process. Therefore, this reaction has a surface nature as confirmed by the rate increase in the (C 2H 5) 4N + < Na + < Li + sequence and with I decrease (the ψ 1-effect was confirmed quantitatively) and also by the inhibition caused by gelatin. The polarographically active Ni(NCS) + ads complex is not identical to the corresponding complex adsorbed from the bulk of the solution: this latter would not be reduced at the pre-wave potentials.

Catalytic polarographic current of nickel(II) in acetonitrile in the presence of thiocyanate ions

The effect of thiocyanate ion on the electrode reaction of nickel(II) in acetonitrile solution was investigated by d.c. polarography, i-t curve analysis, a.c. polarography and controlled potential electrolysis. In the presence of thiocyanate ion, nickel(II) exhibited a catalytic pre-wave in LiClO 4 , NaClO 4 and NH 4 ClO 4 supporting electrolytes, and a catalytic maximum wave in Me 4 NClO 4 , Et 4 NClO 4 and n-Bu 4 NClO 4 . The maximum wave usually separated into two maxima. The characteristics of these maxima were studied as a function of concentration of nickel(II), LiSCN and n-Bu 4 NClO 4 . A mechanism which involves adsorption equilibria between bulk and adsorbed thiocyanate and quaternary ammonium ions, the cyclic regeneration of adsorbed thiocyanate ion for the pre-wave and the catalytic reduction of thiocyanate ion with the electroreduced active metallic nickel for the maximum wave has been presented. The characteristics of the two maxima are discussed on the basis of the difference in character of the adsorption process of thiocyanate and quaternary ammonium ions.

Catalytic polarographic current of nickel(II) in acetonitrile in the presence of thiocyanate ions

The effect of thiocyanate ion on the electrode reaction of nickel(II) in acetonitrile solution was investigated by d.c. polarography, i-t curve analysis, a.c. polarography and controlled potential electrolysis. In the presence of thiocyanate ion, nickel(II) exhibited a catalytic pre-wave in LiClO4, NaClO4 and NH4ClO4 supporting electrolytes, and a catalytic maximum wave in Me4NClO4, Et4NClO4 and n-Bu4NClO4. The maximum wave usually separated into two maxima. The characteristics of these maxima were studied as a function of concentration of nickel(II), LiSCN and n-Bu4NClO4. A mechanism which involves adsorption equilibria between bulk and adsorbed thiocyanate and quaternary ammonium ions, the cyclic regeneration of adsorbed thiocyanate ion for the pre-wave and the catalytic reduction of thiocyanate ion with the electroreduced active metallic nickel for the maximum wave has been presented. The characteristics of the two maxima are discussed on the basis of the difference in character of the adsorption process of thiocyanate and quaternary ammonium ions.

Analytical application of the cobalt(II) catalytic pre-wave in the presence of thiamine in its disulphide form.

The cobalt catalytic pre-wave in the presence of thiamine in its disulphide form, and in a borate medium, was studied as a function of the thiamine (disulphide form) and cobalt(II) concentrations and of the pH of the solution. The effects of temperature, the addition of neutral salts and the presence of surface-active agents on the pre-wave are described. The effect of thiamine (disulphide form) is shown on the electrocapillary curves. The electrode process involves the formation of complexes between cobalt(II) and thiamine, in its thiolic form, adsorbed on the dropping-mercury electrode, and the electrochemical decomposition of these complexes with liberation of the catalytic ligand. The analytical application of this pre-wave was studied.

Catalytic prewave of nickel in the presence of selenocrystine

Catalytic prewave of nickel in the presence of selenocrystine

Catalytic prewave of nickel in the presence of seleno-cystine

In the presence of seleno-cystine we have established the formation of a catalytic pre-wave of nickel for which the half-wave potential is about −0.68 V. The pre-wave is visible for seleno-cystine concentrations of 0.7–30×10 −6 M . The pre-wave is well defined at a ratio of the buffer component of NH 4 Cl/NH 4 OH=0.05 M /0.05 M . Since the half-wave potential is located at −0.68 V and seleno-cystine is reduced to seleno-cysteine at a half-wave potential of −0.6 V, the nickel pre-wave is produced by a complex formed with seleno-cysteine and not with the seleno-cystine. The effects of gelatine, buffer concentration, ionic strength, nickel concentration, pH and reservoir height were studied.

Co (II) catalytic prewave in the presence of thiamine in its disulfide form

Co (II) catalytic prewave in the presence of thiamine in its disulfide form

The Effect of the Weak Completing Nature of NaF, NaN3, and NaC3H3O3Supporting Electrolytes on the Electrocatalysis of Nickel(II) Reduction in the Presence of Organic Amines

Abstract Contrary to the results obtained in essentially noncomplexing supporting electrolyte media (NaClo4, NaN03, NaCl and NaBr), the kinetic limited currents of the catalytic prewaves obtained for the polarographic reduction of Ni(II) in the presence of organic amines (o-phenylenediamine, pyridine, etc.) employing weakly complexing supporting electrolytes (NaF, NaN3, and NaC2H3O2) do not correlate with ⊘2 potentials as predicted by simple double layer theory. However, the limiting current does correlate with the ligand exchange rates for Ni(H2O)5X+ type complexes (where X = F−, N3 − and C2H3O2 −) with an organic ligand. These results lead to a general mechanism for the electrocatalysis mechanism of Ni(II) reduction.

Selective effect of supporting electrolyte on thepolarographic catalytic current of nickel(II) in acetonitrile in the presence of halide ions

Summary An acetonitrile solution of nickel(II) in the presence of halide ion gives a catalytic pre-wave. The pre-wave height was studied as a function of the concentration of nickel(II) and halide ions. The catalytic activity of halide ion for the pre-wave in lithium perchlorate solution decreases in the following order: iodide, bromide, chloride ions; on the other hand, in a quaternary ammonium perchlorate solution it decreases in the order: chloride, bromide, iodide ions. A mechanism which involves adsorption equilibria between bulk and adsorbed halide ion and the cation of the supproting electrolyte, and a complexation reaction of nickel(II) and adsorbed halide ions, is presented. The cation effect on the catalytic polarographic current is discussed on the basis of the selective passing of halide ion through holes in the adsorbed, layer of the cation of the supporting electrolyte.

Selective effect of supporting electrolyte on the polarographic catalytic current of nickel(II) in acetonitrile in the presence of halide ions

An acetonitrile solution of nickel(II) in the presence of halide ion gives a catalytic pre-wave. The pre-wave height was studied as a function of the concentration of nickel(II) and halide ions. The catalytic activity of halide ion for the pre-wave in lithium perchlorate solution decreases in the following order: iodide, bromide, chloride ions; on the other hand, in a quaternary ammonium perchlorate solution it decreases in the order: chloride, bromide, iodide ions. A mechanism which involves adsorption equilibria between bulk and adsorbed halide ion and the cation of the supproting electrolyte, and a complexation reaction of nickel(II) and adsorbed halide ions, is presented. The cation effect on the catalytic polarographic current is discussed on the basis of the selective passing of halide ion through holes in the adsorbed, layer of the cation of the supporting electrolyte.

Effects of the electrical double layer and thiourea adsorption on the polarographic catalytic currents of the nickel(II)-thiourea complex

Effects of the electrical double-layer and the adsorption of thiourea on the catalytic pre-waves of nickel(II) caused by the formation and subsequent reduction of nickel(II)-mono-thiourea complex have been investigated. It has been shown that the kinetic equation is in good agreement with the experimental data over the entire range of thiourea concentrations, providing the thiourea adsorbed is taken into consideration. The quantitative estimation of the influence of the ψ1-potential on kinetics is possible for 1-1 electrolyte concentrations of ≤0.2 M. The rate constant of the interaction between Ni(H2O)6 ion and thiourea adsorbed at a mercury surface, ka=(2−4)×102l mol−1 s−1 (ψ1=0), was found.

Effects of the electrical double layer and thiourea adsorption on the polarographic catalytic currents of the nickel(II)-thiourea complex

Effects of the electrical double-layer and the adsorption of thiourea on the catalytic pre-waves of nickel(II) caused by the formation and subsequent reduction of nickel(II)-mono-thiourea complex have been investigated. It has been shown that the kinetic equation is in good agreement with the experimental data over the entire range of thiourea concentrations, providing the thiourea adsorbed is taken into consideration. The quantitative estimation of the influence of the ψ1-potential on kinetics is possible for 1-1 electrolyte concentrations of ≤0.2 M. The rate constant of the interaction between Ni(H2O)6 ion and thiourea adsorbed at a mercury surface, ka=(2−4)×102l mol−1 s−1 (ψ1=0), was found.

Sensitive amperometric titration of o-phenylenediamine employing catalytic electrode reaction end-point detection

Summary The catalytic prewave of the Ni-opda system has been used for a sensitive amperometric detection of the end-point of the opda titration. Au(III) was chosen as titrant from a large group of possible oxidants, because of its favourable equilibrium and kinetic properties. Three end-points are observed in the titration and the reactions involved were shown to be as follows: Download : Download full-size image Download : Download full-size image The limiting sensitivity for the determination of opda was found to be 1·10−6 M, with a standard deviation of 4.6%. The titration of opda was not feasible in the presence of the other phenylenediamines. The catalytic electrode reactions involving cyclic regeneration of the redox species, are applicable for end-point detection systems for the amperometric titrations of trace concentration of these species, provided titrants can be found that have suitable thermodynamic and kinetic properties in their reaction with the redox species.

Catalytic adsorption prewave in polarograms of solutions of pyridine

Catalytic adsorption prewave in polarograms of solutions of pyridine