Catalytic Prewave Research Articles

The "Presodium" Hydrogen Evolution at the Dropping Mercury Electrode Catalysed by Simple Cysteine Peptides

"Presodium" catalytic currents at dropping mercury electrode of cysteine, cysteinylglycine, γ-glutamylcysteine and reduced glutathione were systematically studied in weakly alkaline solutions. They consist in shifts to positive potentials of current due to reduction of the blank solution, and, under some conditions, also in formation of "catalytic prewaves". The two cases have been qualitatively interpreted as based on a suggested common mechanism, differing by the occurrence of weak catalyst adsorption in the prewave case. The catalytic activity depends on the catalyst itself (concentration, structure, conformation at the electrode, partial protonation/ionisation, partial hydrophobic/hydrophilic character) as well as on the solution (pH, ionic strength, nature and concentration of components).

The parameters of polarographic catalymetry using ligand catalysis

A new approach to the determination of the parameters of polarographic catalymetry using ligand catalysis have been developed. The following parameters were determined for the model electrocatalytic systems Ni 2+ + pyridine and Ni 2+ + pyridine + acetate: (1) quantitative characteristic for the existence of the catalytic prewave; (2) the value of the catalytic effect; (3) the detection limit of the ligand catalyst; (4) quantitative characteristic for the existence of the optimum analytical signal.

Study of the Catalytic Polarographic Pre-Wave of Nickel(II) Induced by Cimettdine

Abstract We carried out a comprehensive study of the Ni(II) pre-wave induced by cimetidine in sodium veronal - nitric acid - sodium nitrate media using various polarographic techniques in order to establish the mechanism of the electrode process. This was found to involve the formation of catalytic complexes of the aquo-Ni(II) and veronal-Ni(II) ions with deprotonated cimetidine adsorbed on the electrode surface, which was followed by reduction of the Ni(II) in the complexes and the consequent release of the catalytic ligand. The pre-peak obtained by differential pulse polarography for the 5 l0−4M Ni(II) - 5 10−3M sodium veronal-HNO3-NaNO3 system (pH 8.5, ionic strength 0.4 M) can be exploited for the quantitative determination of cimetidine over the concentration range 1-10−6 -1 105 M.

Nature of the Ni(II) prewaves at the dropping mercury electrode in the presence of thiourea

Two catalytic prewaves are unambiguously resolved. The catalytic character of both prewaves has been shown using different criteria, although the second prewave has also a small diffusive character. The surface nature of the prewaves is also demonstrated.

Some correlations between the structures of cysteine derivatives and the occurrence of a catalytic hydrogen prewave

This paper presents the structure effects observed in the case of the CHP, giving special emphasis to the complexing properties of the Cys derivatives. For practical reasons, cystine or its derivatives were used in some experiments. However, the actual catalyst is the reduced form, since the disulphide reduction precedes the CHP

Voltammetric Study of the Catalytic Hydrogen Prewave Produced by Cysteine. Evidence of the Homogeneous Proton‐Transfer Reaction

Voltammetric Study of the Catalytic Hydrogen Prewave Produced by Cysteine. Evidence of the Homogeneous Proton‐Transfer Reaction

Polarographic Study of the Catalytic Prewave of Co(II) in the Presence of 5,5‐Dimethyl‐1,3‐Cyclohexanedione Dithiosemicarbazone Analytical Applications

AbstractThe dissolutions containing Co(II) in a Tris‐HCl medium and in the presence of perchlorate of 5,5‐dimethyl‐1,3‐cyclohexanodione dithiosemicarbazone (DYDT‐HClO4) produce a prewave that has been studied by means of classical polarography (DC) and differencial pulse polarography (DPP). The catalytic nature of the prewave has been established and the possible electrode mechanism proposed. The analytical possibilities of the mentioned prewave have also been analyzed in order to determine the DYDT, Hg(II) and Ag(I).

Differential pulse polarographic determination of cephalexin based on the catalytic pre-wave of nickel(II)

A nickel-catalytic pre-peak at −0.72 V vs. Ag/AgCl, is caused by cephalexin in sodium acetate (pH 8.3) medium. It is used to determine 2.4 × 10 −7–6 × 10 −6 M cephalexin; the relative standard deviation is 4% for 0.88 μg ml −1 cephalexin. Other cephalosporins behave similarity; some amino acids can be tolerated in moderate amounts.

Catalytic prewaves of hydrogen and nickel in cysteine-containing acetate buffer

The catalytic hydrogen prewave ( E 1/2 =−1.25 V vs. SCE) produced by cysteine in the presence of nickel ion as well as the catalytic nickel prewave ( E 1/2 =−0.6 V) were studied at pH<7. Polarographic and spectrophotometric data show that nickel complexes with cysteine are involved in the electrode reactions. The two electrode processes studied are closely related, since the zero-valent nickel complex formed during the catalytic nickel discharge acts as catalyst for hydrogen evolution. The reaction mechanism given explains the effects of basic parameters, taking into account the change of the surface concentration of reducible species involved in chemical equilibria, as a function of potential.

Catalytic prewaves of hydrogen and nickel in cysteine-containing acetate buffer

The catalytic hydrogen prewave ( E 1/2=−1.25 V vs. SCE) produced by cysteine in the presence of nickel ion as well as the catalytic nickel prewave ( E 1/2=−0.6 V) were studied at pH<7. Polarographic and spectrophotometric data show that nickel complexes with cysteine are involved in the electrode reactions. The two electrode processes studied are closely related, since the zero-valent nickel complex formed during the catalytic nickel discharge acts as catalyst for hydrogen evolution. The reaction mechanism given explains the effects of basic parameters, taking into account the change of the surface concentration of reducible species involved in chemical equilibria, as a function of potential.

Catalytic prewaves of cobalt(II) in presence of dimethyldithiocarbamate

Two well-defined prewaves of cobalt(II) in the presence of dimethyldithiocarbamate (Me 2DTC) in a medium containing ammonia (borax-NH 4Cl and NH 4OH-NH 4Cl buffer) are observed. The effect of gelatine, Me 2DTC, cobalt(II), ammonium hydroxide and ammonium chloride concentration, mercury reservoir height, buffer capacity, pH and temperature on the prewaves are described. In addition, the characteristics of the current-time and electrocapillary curves are shown. This allows to propose a mechanism where the cobalt(II) prewaves are produced by cobalt(II) complexes with Me 2DTC. In these complexes, the cobalt(II) and Me 2DTC are in the relationship 1:1 and 1:2 for the first and second prewave respectively. A value of the rate constant for the formation of the first prewave catalytic complex of 6.0 x 10 6 lmol −1 s −1 has been found at 20°C.

Polarographic determination of methionine by measurement of the nickel(II) catalytic pre-wave

Methionine yields a nickel(II) catalytic polarographic pre-wave in borate media. This pre-wave was studied as a function of the composition and pH of the borate medium, the nickel(II) concentration, the addition of neutral salts and the methionine concentration. The best system for determining methionine is: 2 × 10–4M nickel(II)-0.06 M disodium tetraborate-nitric acid (pH 8.6). D.c. polarography is suitable for the determination of methionine over the concentration range 1 × 10–5–8 × 10–5M. In differential-pulse polarography the calibration graph is linear over the methionine concentration range 3 × 10–6–55 × 10–6M.

Catalytic pre-wave of nickel(II) in presence of cocarboxylase (disulfide form) in borate medium

A brief study is made of the nickel(II) polarography in borate medium. The nickel(II) in borate medium gives one or two waves depending on its concentration as well as the medium composition. In a borate medium, cocarboxylase in its disulfide form gives a nickel(II) pre-wave. The effects of the cocarboxylase (disulfide form) concentration, nickel(II) concentration and mercury column height on the pre-wave are described. The effect of neutral salts on the pre-wave is studied and it is seen that an increase in the ionic strength is accompanied by a decrease in pre-wave current, being of decreasing effect in the order K + < Na + < Li +. The electrocapillary curves are present, and it is shown that the presence of cocarboxylase (disulfide form) in the solution causes a decrease in the interfacial tension. The results are discussed and a mechanism for the electrode process of the pre-wave is proposed. Using the approximate method for the catalytic currents, nickel(II) and cocarboxylase-thiol are shown to form the catalytic complex in the relationship 1:1.

CATALYTIC PRE-WAVE OF NICKEL(II) AND FORMATION OF NICKEL SULFIDE IN THIOSULFATE SOLUTIONS AT MERCURY ELECTRODES

Abstract It was found that nickel(II) in thiosulfate solutions at mercury electrodes gave a catalytic pre-wave, and NiS was produced in the electroreduction process of nickel(II). The electrode reaction which is proceeded by the formation of Ni(S2O3) from Ni(H2O)62+ is retarded by the adsorption of NiS at higher concentrations of S2O32−. It has been proposed that the formation of NiS is due to the chemical reduction of S2O32− bound to Ni(O)S2O32− by the electroreduced metallic nickel.

Catalytic Hydrogen prewave in the presence of cyteine and nickel oins

Catalytic Hydrogen prewave in the presence of cyteine and nickel oins

Catalytic hydrogen prewave in the presence of cysteine and nickel ions

Catalytic hydrogen prewave in the presence of cysteine and nickel ions

Catalytic hydrogen prewave in the presence of cysteine and nickel ions

Catalytic hydrogen prewave in the presence of cysteine and nickel ions

Catalytic hydrogen prewave in the presence of seleno-cystine and nickel

Catalytic hydrogen prewave in the presence of seleno-cystine and nickel

Décharge catalytique du proton en présence de séléno-cystine et de nickel

It has been found that seleno-cystine, reduced to seleno-cysteine and bound in ligand form to nickel ion, produces catalytic hydrogen discharge in slight acid media. This discharge occurs in the region of a catalytic prewave located at more positive potentials (E1/2=−1.14 V, S.C.E.) compared with the normal wave of H3O+(E1/2=−1,67 V, S.C.E.).

Polarographic Behavior of Nickel(II) in Propionitrile and Benzonitrile in the Presence of Thiocyanate Ions

Abstract The effect of the cation of the supporting electrolyte on the electrode reaction of nickel(II) in propionitrile and benzonitrile solutions containing thiocyanate ion was investigated by DC polarography and i-t curve analysis. In the absence of thiocyanate ion, a propionitrile solution of nickel(II) gives a single irreversible wave in LiClO4 and NaClO4 supporting electrolytes and a double wave in Et4NClO4 and n-Bu4NClO4, and a benzonitrile solution of nickel(II) gives a double wave in alkali metal and quaternary ammonium perchlorates. Nickel(II) in these nitrile solvents in the presence of thiocyanate ion exhibits a catalytic pre-wave in alkali metal perchlorates and a catalytic maximum wave in quaternary ammonium perchlorates. The double wave of nickel(II) in the absence of thiocyanate ion is due to the inhibition of the charge transfer process by the relatively weak adsorption of quaternary ammonium cations and not to the presence of two electroactive species. A mechanism which involves the cyclic regeneration of adsorbed thiocyanate ion for the catalytic pre-wave and the catalytic reduction of thiocyanate ion with the electroreduced metallic nickel for the maximum wave has been presented.