Gold complexes sulfites

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 ... 

Various processes for alloy and pure gold deposits are available. Solutions are based on sulfite-gold complexes and arsenic additives and operate at a pH of 8.5 to 10.0. A decision to use this process is based primarily on the need for uniformity (leveling), hardness (180 Knoop), purity, reflectivity, and ductUity. PC board use is Umited to body plating, since wear characteristics of... 

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]. 

Reaction of Na2S03 with [Au(OH)4] gives [Au(S03)4]5- and the salt Na5[Au(S03)4]-5H20 has been isolated.51 This contains S-bonded sulfite, and can be reduced by excess sulfite to [Au(S03)2]3-. Selenite complexes of gold(III) are O-bonded, however.525... 

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 ... 

Better results can be obtained for the electroless deposition (ELD) in solutions. In this case the protective ligands can probably be dissolved in the liquid phase and thus diffuse away from the seeds. Of the several deposition baths considered, each consisting of a soluble gold salt and reducing reagent, only the one based on the sulfite complex of Au and hydroxylamine resulted in dense and relatively thick (up to 800 nm) gold layers. The reason for this behaviour remains unclear but there are hints that complex reactions are taking place in the autocatalytic deposition process. 

Similarly, there are no reliable data on the standard reduction potential for the gold sulfite complex, for which values in the range 0-0.4 were reported [25], However, assuming for the stability constant the previously quoted value of 26.8 [23], the standard reduction potential of the sulfite complex can be calculated as 0.11 V, which is in excellent agreement with the formal potential of the sulfite complex at a mercury amalgam electrode, namely, 0.116 V, as measured by Baltrunas et al. [26]. 

The gold thiosulfate complex Au(S203)2 has a fairly high stability constant (see Table 112), and a wide pH range of thermodynamic stability. Despite these characteristics, a gold thiosulfate electrolyte has never been successfully developed because of the limited stability of the ligand. In lact, at neutral or add pH, thiosuUate is unstable towards decomposition in colloidal sulfur and sulfite [27] ... 

The reduction of the gold sulfite complex by dithionite is also responsible for the instability of the sulfite electrolyte at pH below about 9.0, a well-known drawback of this type of bath. The role of dithionite in the decomposition of the sulfite complex with the formation of colloidal gold has been recently discussed by Green [21,24],... 

Early studies on the electroless deposition of gold from a sulfite-thiosulfate bath suggested that the active species was either the thiosulfate complex [54] or a mixed sulfite-thiosulfate complex [55]. In another study [53], the influence of sulfite addition on the cathodic reduction of the thiosulfate complex was shown to cause a negative shift of the reduction potential which was tentatively explained as being due to the formation of a mixed complex or to surface inhibition through sulfite adsorption at the electrode. This polarization effect was confirmed in a later study [56]. 

The electrochemical reaction of sulphur-oxygen compounds - Part 1. A review of literature on the electrochemical properties of sulphur/sulphur-oxygen compounds, Electrochim. Acta, 1992, 37, 2775. Pulsed potentiostatic deposition of gold from solutions of the Au(I) sulfite complex, J. Horkans and L. T. 

Aqueous Solution of Monovalent Gold and Ammonium Sulfite Complex, Process for the Preparation Thereof and Electrolytic Bath Obtained Therefrom for the Plating of Gold or Gold Alloys, P. Laude, E. Marka, and Z. Morrens, U.S. Patent 4,192,723 (1980). 

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