Electrochemistry of tungsten

The electrochemical behavior of tungsten in aqueous solutions is very closely linked to two facts the high affinity of tungsten to oxygen and the complexity of tungsten species in aqueous solution. 

In aqueous solution, no oxygen-free tungsten cation exists but only monomeric or dimeric species like The discharge of such a species at the cathode yields an oxidic 

The steady-state potential measured against a saturated calomel electrode (SCE) is negative in alkaline and positive in acidic solution, and even higher positive in the presence of oxidizing agents. Calculated standard potentials from thermodynamic data are around —0.1 V in acidic and around —0.9 V in alkaline solution against NHE. 

The pH dependence (0-10) is linear (about 50 mV/pH). The potential becomes more negative with increasing temperature due to a partial transformation of a metal oxide to a metal electrode (the electrode properties shift from semiconductive to metal conductive). 

In aqueous solution, the following reactions and corresponding potentials are assumed  

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The electrochemistry of Ti2+ in 66.7 m/o AlCl3-NaCl has been investigated wherein the electroactive Ti2+ was prepared by the oxidation of Ti metal with liquid A1C13 [176, 185] and by the electrochemical dissolution of titanium metal [120, 177], The authors of both studies concluded that Ti2+ may be oxidized stepwise to Ti3+ and Ti4+ and that both processes are reversible at platinum and tungsten electrodes. However, anomalous voltammetric behavior at high Ti2+ concentrations (greater than 50 mmol L ) suggests the formation of polymeric Ti2+ species in the melt. The reduction of Ti2+ to the metal was not observed at potentials more positive than that required for aluminum deposition. 

The author was brought up in Hastings, on England s south coast, where he attended a local comprehensive school. Despite this education, he achieved entrance to the University of Exeter to read Chemistry. Having obtained a B.Sc. degree and then a doctorate (in 1989) on the electrochemistry of the viologens, he was awarded a fellowship at the University of Aberdeen to study the electrochromism of thin films of tungsten trioxide. 

The electrosynthesis of hydride complexes directly from molecular hydrogen at atmospheric pressure by reduction of Mo(II) and W(II) tertiary phosphine precursors in moderate yield has been described as also the electrosynthesis of trihydride complexes of these metals by reduction of M(IV) dihydride precursors [101,102]. Hydrogen evolution at the active site of molybdenum nitrogenases [103] is intimately linked with biological nitrogen fixation and the electrochemistry of certain well-defined mononuclear molybdenum and tungsten hydrido species has been discussed in this context [104,105]. 

In the preceding section, the electrochemistry of copper-substituted derivatives shows an example of mixing up of the substituent metal cation and tungsten waves, but this process was accompanied by the deposition of Cu . Analogous merging was studied in detail in the case of ay- and a2"[Fe (0H2)P2Wi706i] , where this complicating feature was absent [89]. 

The electrochemistry of Cd(II) was investigated at different electrodes (GC, polycrystalline tungsten, Pt, Ni) in a basic l-ethyl-3-methylimidazolium chloride/tet-rafluoroborate, at room temperature molten salt [312], and in acidic zinc chloride-l-ethyl-3-methylimidazolium [284]. 

Data on the redox potentials of germylenes, stannylenes, plumbylenes and their complexes are scarce. In fact, only the electrochemistry of dihalogermylenes, dihalostan-nylenes and their complexes with Lewis bases338 as well as with chromium, molybdenum and tungsten pentacarbonyles339 has been studied. 

Xu XH, Hussey CL (1993) The electrochemistry of merctrry at glassy carbon and tungsten electrodes in the aluminum chloride-l-methyl-3-ethyUmidaziohum chloride molten salt. J Electrochem Soc 140 1226-1230... 

No detailed study of the electrochemistry of molybdenum or tungsten sandwiches seems to have been completed. Nevertheless, [M(r/-arene)2] (83, 440) and [MCp( -C7H7)] (441) (M = Mo or W) are readily oxidized by iodine to isolable monocations. 

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