Tadeusz Krogulec

On the formation of sulphides in the electroreduction of thiocyanate complexes of nickel(II) at mercury electrodes

Summary The electroreduction of nickel(II) at the HMDE in thiocyanate solutions has been investigated. Chronovoltammetry, chronopotentiometry, a.c. voltammetry and large scale controlled-potential electrolysis with investigation of products were used in this study. It was found that if nickel exists as the nickel(II)-thiocyanate complex then the reduction proceeds in two parallel ways: the formation of nickel amalgam and the formation of NiS and Ni(CN) 4 2− as a result of reduction of thiocyanate ions. The participation of the second process in the overall electrode reaction is not large, however, due to adsorption of NiS on the electrode surface its influence on the total process is considerable. At full coverage of the electrode by NiS, a strong autoinhibition of the reaction occurs and at more negative potentials deposition of metallic nickel on the electrode surface was observed. It has been shown that the electroreduction of ammonia complexes of nickel(II) in the presence of adsorbed thiocyanates does not lead to their reduction. General suggestions concerning the mechanism of the process studied are presented.

FORMATION OF FeS AND ITS EFFECT ON THE ELECTRODE REACTIONS OF THE Fe(II)/Fe SYSTEM IN THIOCYANATE SOLUTIONS AT MERCURY ELECTRODES

The electrode reactions of the Fe(II)-NCS system occurring at mercury electrodes have been studied by using cyclic voltammetry and large scale electrolysis. The elatroreduction of Fe(H) in that system occurs with a fast transfer of the first electron. The resulting intermediate complex of iron(I) is further reduced to metallic iron and partly decomposes to yield FeS which is adsorbed on the electrode surface. Its adsorption initiates the surface deposition of the iron not wetted by mercury. This deposition was further studied by chronoamperometry. The oxidation of deposited iron, which occurs in two peaks, was also investigated in detail. Also the surface transformation of FeS into HgS as well as the electroreduction of HgS in presence of Fe(H)

A study of the deposition of iron on mercury and glassy carbon electrodes

Abstract The processes of reduction and reoxidation of iron(II) in thiocyanate, chloride and perchlorate media have been investigated by cyclic voltammetry and chronopotentiometry on mercury (HMDE) and glassy carbon (GCE) electrodes and interpreted as follows. On mercury electrodes, iron deposited from aquo and chloride complexes of Fe(II) is wetted by mercury and penetrates into the bulk of the electrode. On the other hand, FeS formed during the electroreduction of thiocyanate complexes of Fe(II), is adsorbed on the electrode surface and makes the wetting of iron by mercury impossible. Consequently, the iron stays on the electrode surface and accelerates the process of hydrogen evolution. This, in turn, leads to alkalization of the solution and to precipitation of Fe(OH) 2 in the pre-electrode layer. Subsequently, both FeS and Fe(OH) 2 catalyze the formation of metallic iron on the mercury surface. FeS and Fe(OH) 2 are also formed in the surface layer during the reduction of thiocyanate complexes of Fe(II) on the GCE. In practice, neither substance participates in the initiation of iron settling on the glassy carbon surface, but their influence on the electrode processes of systems examined on the GCE is significant.

The study of electrode processes of Fe(II)-thiosulphate complexes on mercury electrodes

Abstract The electrode processes occuring in the Fe(II)-S 2 O 2− 3 system on mercury electrode were investigated by cyclic voltammetry, chronopotentiometry, polarography and pulse polarography. Electrocatalytic thiosulphate ion decomposition was observed resulting in FeS formation. Formation of the solid FeS phase on mercury surface lowers the Fe(II) reduction overpotential and makes the wetting of iron by mercury impossible. Consequently, iron is deposited on the electrode surface. It was also found that the FeS formation efficiency in the Fe(II)-S 2 O 2− 3 system is much higher than in the Fe(II)-SCN − system. The high amount of FeS formed in the vicinity of the mercury electrode in solution of high thiosulphate ion concentration inhibits the iron deposition on mercury.

On the electroreduction of thiocyanates bound to Ni(II)

Abstract The electroreduction of Ni(II)—thiocyanate complexes has been studied using large-scale electrolysis, cyclic voltammetry, chronoamperometry and pulse polarography. Electrolyses carried out at 95°C, and at potentials not sufficiently cathodic for nickel amalgam formation demonstrate that there is quantitative electroreduction of these complexes to NiS and Ni(CN)42−. Under such conditions, the voltammetric i-E curves also indicate the electroreduction of the SCN− ligands. Adsorption and desorption of the NiS produced at less negative potentials, as well as its electroreduction to nickel amalgam were studied by pulse polarography. The mechanism of NiS formation is discussed.

The electrode kinetics of nickel in thiocyanate solutions at mercury electrodes

Abstract Using pulse chronocoulometric polarography the kinetic parameters of the Ni(II)/Ni(Hg) system in thiocynate solutions have be determined. Under the conditions of the experiments the influence of electroreduction of SCN− ions resulting in formation of NiS adsorbed on the electrode was minimized. The standard rate constants obtained are of the order of 10−3 cm s−1, several times larger than earlier reported in literature. The Tafel slopes indicate a two-step process with slow transfer of the second electron and NiSCN+ directly participating in the electrode reaction.

Determination of NiS, NiSe and PdS formation orders during electroreduction of thiocyanate, selenocyanate and thiourea complexes of Ni(II) and PD(II) at mercury electrodes

Abstract The electroreduction of Ni(II) and PD(II) thiocyanate complexes, Ni(II) selenocyanate complexes and Ni(II) thiourea complexes has been studied at mercury electrodes. It has been confirmed in each case that the ligands undergo electroreduction, as do the metal ions, resulting in NiS, NiSe and PdS precipitate formation. A.c. chronoamperometry was used to determine the formation order of NiS and NiSe in the Ni(II)-thiourea and Ni(II)-selenocyanate systems. A new method of determining this reaction order was proposed on the basis of quantitative analysis of the electroreduction products obtained during potentiostatic electrolysis at a large mercury electrode. The formation orders of NiS and PdS in the Ni(II)-SCN − and Pd(II)-SCN − systems were determined by this method.

On the mechanism of electroreduction of SCN− ions catalysed by simultaneous electroreduction of nickel(II)

Abstract The mechanism of the electroreduction of Ni(II)-NCS − complexes to NiS and CN − anions was studied using ac voltammetry. By analysing the dependence of the alternating current on time, at constant dc potential, recorded at different concentrations of Ni(II) and SCN − ions, it was concluded that the reaction proceeds through an intermediate [Ni I -SCN-Ni I ] + which further decomposes to NiS and cyanide complexes of Ni(II).

The nature of the catalytic influence of thio- and selenocyanate ions on the reduction process of Fe(II) and Co(II) cations

Abstract Studies have been carried out on the processes of catalytic electroreduction of thio- and selenocyanate complexes of Fe( II ) and Co( II ) on mercury electrodes. It has been established that the decrease in the overpotential of the electroreduction of cations of these metals in the presence of SCN − or SeCN − is due to the formation and adsorption of sulphides or selenides of Fe( II ) or Co( II ) on the electrode surface during their electrode processes. The formation of a solid phase of sulphides or selenides of these metals on the mercury surface lowers the crystallization overpotential considerably and shifts the potential of reducing metal ions towards less negative values.

On the formation and oxidation of a heterogeneous amalgam of iron

Abstract Electrode processes involving the formation of a heterogeneous amalgam of iron during the electroreduction of Fe(II) ions on mercury and the oxidation of the iron amalgam were investigated by cyclic voltammetry, chronoamperometry and cyclic chronopotentiometry. The hanging mercury drop electrode and an iron plate placed on a mercury pool electrode were used in the experiments as working electrodes. It was established that particles of the electrodeposited iron are wetted by mercury and penetrate into the bulk of mercury electrodes forming a suspension. However, during electroreduction of Fe(II) ions from concentrated CaCl2 and Ca(ClO4)2 solutions a small amount of metallic iron remains on the surface of mercury. Oxidation of the heterogeneous amalgam of iron occurs at potentials about 100–150 mV less positive than the onset potential of mercury oxidation in the solutions studied. At these potentials oxidation of mercury may become significant enough to uncover some parts of the iron microcrystals. These iron particles become partially passivated and their surface energy at the Fe/solution interface decreases; consequently, the spreading coefficient of mercury on iron decreases, microcrystals of iron are expelled from mercury and finally oxidized.