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Abstract
The electrochemical behaviour of thiourea (TU) on gold in acid solutions is investigated in the range −0.2 to 1.7 V (versus standard hydrogen electrode) by conventional and triangularly potential modulated voltammetry, rest potential ( E rest) measurements, rotating disk and rotating ring-disk techniques. The value of E rest is determined by an electrochemical reaction involving formamidine disulphide (FDS), adsorbed TU and soluble [Au(TU) 2] + species. For TU concentrations ( c TU) in the range 1⩽ c TU⩽50 mM, E rest decreases linearly with log c TU with a slope of approximately −0.090 V decade −1, while in the range 0.1⩽ c TU⩽0.25 mM, it approaches −0.120 V decade −1. Conventional and triangularly modulated voltammetry data indicate the quasi-reversible adsorption of TU occurring in the range −0.2⩽ E⩽0.1 V, and the electro-oxidation of TU to FDS in the range 0.2⩽ E⩽0.9 V, occurring simultaneously with the electrodissolution of gold yielding soluble [Au(TU) 2] + species. Both the electro-oxidation of TU and the electrodissolution of gold are under mass transfer control influenced by adsorbate formation. Different adsorbates from TU are produced depending on the adsorption potential and time. The electro-oxidation of these adsorbates in the range 0.9–1.7 V yields sulphate, carbon dioxide and CN residues. Reaction products are consistent with previous STM and FTIRS data. Comparative electrochemical data from FDS- and sulphide-containing solutions are also presented. Reaction pathways in which the participation of different adsorbates is considered, are discussed.
Highlights
In recent years, interest in the electrochemical processes at different metal j thiourea (TU)-containing aqueous solution interfaces has increased both for the understanding of fundamental aspects of electrochemical interfaces involving sulphur-containing molecules on single crystal metal surfaces [1,2] and for the determination of the properties of TU as an additive in different technological applications
It has been shown that the electro-oxidation of TU on gold in acid solutions takes place in the potential range of both gold immunity and gold corrosion [4], the relative contributions of the TU and gold electro-oxidation reactions depend on both the TU concentration and the range of applied potential
This work deals with the electrochemical processes at gold j TU-containing and gold j formamidine disulphide (FDS)-containing aqueous acid interfaces over the range of potentials related to the thermodynamic stability of water
Summary
Interest in the electrochemical processes at different metal j thiourea (TU)-containing aqueous solution interfaces has increased both for the understanding of fundamental aspects of electrochemical interfaces involving sulphur-containing molecules on single crystal metal surfaces [1,2] and for the determination of the properties of TU as an additive in different technological applications. The electrodissolution of gold has been studied in aqueous TU solutions containing either acid [18] or alkali [10] For the latter, the reaction is partially hindered due to passivation of gold by elemental sulphur from the irreversible decomposition of TU. Soluble TU-gold complexes are formed in aqueous TU solutions containing ferric ions as the oxidant [8]. This work deals with the electrochemical processes at gold j TU-containing and gold j FDS-containing aqueous acid interfaces over the range of potentials related to the thermodynamic stability of water. Results allowed us to distinguish different electroadsorption and electrochemical reactions of TU and FDS on gold in acid, as well as gold complex formation. A pathway for TU electro-oxidation on gold in acid and its role in the gold electrodissolution process can be advanced
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