Gold complexes thiosulfates

Lengke, M.F., Fleet, M.E., Southam, G. 2006. Morphology of gold nanoparticles synthesised by filamentous cyanobacteria for gold(1)-thiosulfate and gold(lll)-chloride complexes. Langmuir, 22, 2780-2787. 

Lengke, M. and Southam, G. 2006. Bio accumulation of gold by sulfate-reducing bacteria cultured in the presence of gold (I)-thiosulfate complex. Geochimica et Cosmochimica Acta, 70 3646-1. 

The recent literature describes several autocatalytic gold bath formulations which are not based on the gold cyanide complex and which are operated at a near-neutral or acidic pH. These baths have been reviewed recently by one of the present authors [168]. Among these new baths, those based on the thiosulfate gold complex are believed to deserve special attention. 

The baths contain both sulfite and thiosulfate with trivalent gold chloride. What is the form of gold complex that is actually reduced to gold metal ... 

When trivalent gold is mixed with sulfite and thiosulfate, it is reduced to monovalent gold. Kato et al. studied the stoichiometry of the redox reaction and arrived at the conclusion that the predominant gold species in the system is [Au(S203)2], a monovalent gold thiosulfate complex. If the solution contains only sulfite but no thiosulfate, the gold complex takes the form of [Au(S03)2] , a monovalent gold sulfite complex ... 

Lengke M, Fleet ME, Southam G (2006b) Morophology of gold nanoparticles synthesized by filamentous cyanobacteria from gold(l)-thiosulfate and gold(ni)-chloride complexes. Langmuir... 

Indirect evidence for the proposed mechanism was found in the adsorption behavior of the thiosulfate anion and the Au(S203)2 complex at gold surfaces [57,58]. Similar conclusions about the nature of the gold complex and its role as electroactive species in the mixed electrolyte were reached in a voltammetric study of the thiosulfate-sulfite bath at a rotating disk electrode [59]. 

Gold thiosulfate complexes of the form Na2[Au(S202)2] 2H20 [19153-98-1] are prepared by addition of gold trichloride to concentrated sodium thiosulfate solution (89). The gold is completely reduced and some thiosulfate is oxidized to tetrathionate. This complex has been used in the treatment of rheumatoid arthritis. 

The development of more benign alternatives to cyanide for gold-leaching (see Section 9.17.3.1) such as thiourea, thiocyanate, or thiosulfate, which form stable complexes in water has prompted research to identify suitable solvent extractants from these media. Cyanex 301, 302, 272, Ionquest 801, LIX 26, MEHPA, DEHPA, Alamine 300 (Table 5) have been evaluated as extractants for gold or silver from acidic thiourea solutions.347 Whilst the efficacy of Cyanex 301 and 302 was unaffected by the presence of thiourea in the aqueous feed, the loading of the other extractants is severely depressed. Formation of solvated complexes of gold and of an inner-sphere complex of silver has been proposed.347... 

Oxidative leaching with ammonium thiosulfate (Section 9.17.3.1) generates solutions containing a mixture of the mono- and di-thiosulfato complexes, [Au(S203)] and [Au(S203)2]3> Gold can... 

Golddll) chloride or tetrachloroaurates(lll) also form thiosulfate complexes. especially in the presence of Nal. of the form Na dAutS Oibl. in which the gold is monovalenL. 

Koslowsky in 1936 observed that certain gold salts, such as aurous thiocyanate and aurous thiosulfate complexes, are sensitizers, but this discovery remained a trade secret for several years (157-159). Gold salts used alone are relatively poor sensitizers, but in combination with sulfur sensitizers they can increase the sensitivity beyond that obtainable by S-sensitiza-tion alone (160). 

Ammonia complexes.. EflFect of complex formation on solubility. Cyanide complexes. The cyanide process of treating gold and silver ores. Complex halides and other complexes. Sodium thiosulfate as photographic fixer. Hydroxide complexes. Amphoteric hydroxides. Sulfide complexes. Equilibrium expressions for complex formation. Structural chemistry—tetrahedral, octahedral, square complexes. Existence of isomers. 

The cathodic polarization curves were determined for the reduction of gold ions in solutions containing, respectively, sulfite alone, thiosulfate alone, and a mixture of sulfite and thiosulfate. These curves are shown in Fig. 39. Addition of thiosulfate to the solution of gold sulfite complex shifts the polarization curve to the potential range where the thiosulfate complex is reduced. This result shows that the reduction of gold in the sulfite-thiosulfate mixture takes place from the thiosulfate complex. This conclusion is consistent with the difference between the stability constants of the two complexes ... 

The Au(I) sulfite complex is well known to be the source of gold in an electroplating bath. Will the Au(I) sulfite complex in the absence of thiosulfate work as a source of gold for electroless plating ... 

The addition of ammonia stabilizes the Cu(II) catalyst by the formation of stable Cu(II) amine complexes, preventing the formation of Cu(OH)2, which has the potential to passivate the surface of gold (Marsden House, 2006). Ammonia is additionally beneficial because it maintains a high pH in solution, which can help limit the decomposition of thiosulfate. At low pH,thiosulfate becomes unstable and converts into tetrathionate, elemental sulfur and/or sulfide (Baron et al., 2013). 

Gold(i) Complexes. There are several aurous complexes stable in aqueous solution, the most important ones being Au(CN)J, AuCl2 and the thiosulfate species. The cyanide complex is very stable (K — 4x 1028) and is formed when AuCN is treated with an excess of cyanide or, more usually, when gold is treated with an alkali cyanide in presence of air or hydrogen peroxide. Crystalline compounds such as K[Au(CN)2] can be obtained, and the free acid, HAu(CN)2, is isolable by evaporation of its solutions as in other free cy ano acids, a hydrogen-bonded lattice with —CN—H—N C— bonds is formed. 

---