Gold Sulfite Research Articles

Gold Recovery from Thiosulfate Leaching Solution Using Silicon Powder and Electrochemical Monitoring of Its Process

Gold recycling from Waste Electrical and Electronic Equipment (WEEE) has been actively conducted. We previously reported a noble metal recovery process using aqua regia and electroless displacement deposition onto silicon powder [1,2]. Using silicon powder has such advantages as that noble metals and copper are selectively recovered, and sawdust silicon, that is, industrial waste silicon powder is available to use. Recently, ammonium thiosulfate solution has attracted attention as a safe and low-cost solution for gold leaching [3-6]. In this study, we successfully recovered gold from an ammonium thiosulfate solution by using silicon powder [7,8], and tried to monitor the recovering behavior electrochemically. Since the ammonium thiosulfate leaching solution includes copper ions as oxidizing agent, copper is recovered by silicon powder. Thus, the monitoring was conducted in order to determine the suitable timing for efficiently obtaining pure gold during the process. For experimentation, an ammonium thiosulfate solution containing gold(I) sulfite and copper(II) sulfate was prepared (pH = 13). We immersed a gold wire in the solution and measured the rest potential during the recovery process using silicon powder. The potential of gold wire shifted stepwise toward negative direction at each start and end of gold and copper recovery. The results suggest that the efficient recovery of high-purity gold can be achieved by monitoring the potential of gold.[1] K. Fukuda, S. Yae, N. Fukumuro, S. Sakamoto, and H. Matsuda, ECS Trans., 53(19), 69 (2013). DOI: 10.1149/05319.0069ecst[2] K. Fukuda, N. Fukumuro, S. Sakamoto, and S. Yae, ECS Trans., 61(10), 1 (2014). DOI: 10.1149/06110.0001ecst[3] M.G. Aylmore and D.M. Muir, Miner. Eng., 14, 135 (2001). DOI: 10.1016/s0892-6875(00)00172-2[4] H. Arima, T. Fujita, and W. Yen, Mater. Trans., 43, 485 (2002). DOI: 10.2320/matertrans.43.485[5] S. Syed, Hydrometallurgy, 115-116, 30 (2012). DOI: 10.1016/j.hydromet.2011.12.012[6] B. Xu, W. Kong, Q. Li, Y. Yang, T. Jiang, and X. Liu, Metals, 7, 222 (2017). DOI: 10.3390/met7060222[7] Y. Iwai, Y. Takashima, A. Matsumoto, and S. Yae, Abst. Interfinish2020 20th world congress, P-E2-001 (2021).[8] Y. Takashima, A. Matsumoto, Y. Iwai, S. Arita, and S. Yae, J. Surf. Finish. Jpn., 73, 98 (2022). DOI: 10.4139/sfj.73.98

Electroplating-based engineering of plasmonic nanorod metamaterials for biosensing applications

Sensing lower molecular weight in a diluted solution using a label-free biosensor is challenging and requires a miniaturized plasmonic structure, e.g. a vertical Au nanorod (AuNR) array-based metamaterials. The sensitivity of a sensor mainly depends on transducer properties and hence for instance, the AuNR array geometry requires optimization. Physical vapour deposition methods (e.g. sputtering and e-beam evaporation) require a vacuum environment to deposit Au, which is costly, time-consuming, and thickness-limited. On the other hand, chemical deposition, i.e. electroplating deposit higher thickness in less time and at lower cost, becomes an alternative method for Au deposition. In this work, we present a detailed optimization for the electroplating-based fabrication of these metamaterials. We find that slightly acidic (6.0 < pH < 7.0) gold sulfite solution supports immersion deposition, which should be minimized to avoid uncontrolled Au deposition. Immersion deposition leads to plate-like (for smaller radius AuNR) or capped-like, i.e. mushroom (for higher radius AuNR) structure formation. The electroplating time and DC supply are the tuning parameters that decide the geometry of the vertically aligned AuNR array in area-dependent electroplating deposition. This work will have implications for developing plasmonic metamaterial-based sensors.

Survey of P-Block Metal Additives for Superconformal Cu Deposition in an Alkaline Electrolyte

Catalysis of Cu deposition from a near-neutral Cu2+ complexed electrolyte is examined using Bi3+, Pb2+ and Tl+ additives that were selected based on their known ability to accelerate Au deposition in near neutral pH gold sulfite electrolytes. Where appropriate, the ability of these electrolytes to yield superconformal filling of recessed features is also briefly examined. Voltammetry reveals strong acceleration of Cu deposition by Bi3+ additions while indication of superconformal filling accompanied by unusual microstructural transitions are evident in cross-sectioned specimens examined by scanning electron microscopy. Results are discussed in the context of behaviors observed for the same heavy metal additives in gold sulfite electrolytes.

Gold（I）Complexes of Hydantoin Derivatives and Their Properties

Among hydantoin derivatives, a monovalent gold complex was obtained when 1-methylhydantoin（MH）and hydantoin（HY）were used as ligands. These are more beneficial for use in gold plating solutions than trivalent gold complex of 5,5-dimethylhydantoin（DMH）. Moreover, these complexes can be extracted as crystals. Therefore, the chlorine concentration, constituting a major impurity in gold plating for the electronics industry, can be kept low. Each crystal coordinated two hydantoin derivatives to gold and had water of crystallization. The gold-HY complex solution is stable at pH 5-9. Moreover, no change in pH or decomposition occurs even when hydrogen peroxide is added. Higher stability can be expected in the gold plating solution than for gold sulfite.

Insights into the Effects of Chloride ions on Cyanide-Free Gold Electrodeposition

It is very simple and convenient using the commercial chloroauric acid instead of sodium gold sulfite as the main salt in the Au(I) sulfite electrodeposition bath. In this paper, the effects of chloride ions on cyanide-free Au(I) electrodeposition in the gold sulfite bath are carefully investigated. Both UV–vis absorption spectroscopy and visual inspection reveal that chloride ions suppress the disproportionation reaction of Au(I) and improve the bath stability. Cyclic voltammetry indicates that chloride ions promote the cathodic reduction of Au(I). EC-SERS spectra elucidate that in the sodium sulfite solution, SO32− ions firstly adsorb on the gold electrode, then experience a structural transformation (SO32− → S2O52−), and last electroreduce to S2O32− ions (S2O52− → S2O32−) as the potential shifts negatively. In the gold sulfite electrolytes, the cathodic reduction of Au(I) and chloride ions hinder the reduction of S2O52− ions. The gold coatings obtained from the gold sulfite electrolyte containing chloride ions present in fine and pure grains without chlorine and sulfur inclusion.

Synthesis of gold nanostructures using wet chemical deposition in SiO2/Si template

The size and interposition of particles is a key parameter for the practical application of metallic nanostructures which requires the development of a synthesis method with precise control over their parameters. In this work the method for the synthesis of gold nanostructures in the pores of silicon dioxide from a gold sulfite complex and a gold chloride solution via wet chemistry technique was proposed. The influence of deposition parameters, such as deposition temperature and electrolyte composition, on the deposit morphology was studied. It was shown that gold agglomerates were unevenly distributed over the silicon surface at high temperatures and practically uniformly distributed with temperature decrease. Addition of fluoric acid at the deposition stage defines the metal precipitation selectivity into the silicon oxide pores. The peculiarities of gold nanostructures formation mechanism were discussed.

Recovery of Gold Using Silicon Powder from Leaching Solutions

Low cost and efficient recovery of noble metals from urban mines (e.g., used electronic equipments) is important for securing scarce resources. We are developing a noble metal recovery technology utilizing electroless displacement deposition on silicon (Si)1. In this method, noble metals can be recovered by adding Si powder in metal salt solutions containing hydrofluoric acid (HF)2,3. For safety concerns, we are considering the use of highly basic solution instead of toxic HF solution. We have reported the effects of solution pH on the gold recovery from a basic solution4. In the practical applications, gold is recovered from leaching solutions. In this study, we examined the gold recovery from a basic solution simulating a thiosulfate-based leaching solution. 0.20 M (M: mol dm-3) Si powder (Kojundo Chemical Laboratory, 99.99%) with a particle diameter of 45 μm or less was added to a simple basic solution of 1 mM hydrogen tetrachloroaurate (III) solution pH-adjusted with sodium hydroxide, or a simulated leaching solution containing 1 mM sodium gold (I) sulfite, 1 M ammonium thiosulfate, 1 M ammonium hydroxide, and 0.5 mM copper (II) sulfate, which solution pH was at 9.3. The solution was stirred at 200 rpm using a magnetic stirrer and kept at 298 K of temperature. After filtering the solution, the concentrations of the metal in the filtrate were measured by an ICP-AES (Hitachi, SPS7800) and the recovery was determined from the change in concentration before and after the recovery process. Figure 1 shows the recovery of gold from the simple basic solution (Fig. 1a) and the simulated leaching solution (Fig. 1b). The recovering rate form the simple basic solution was increased with solution pH. The long time treatment at the solution pH of 8 for 24h increased the recovery to 100%. Using bare Si powder, which was pretreated with a HF solution for dissolving an oxide layer on Si surface, shortened the treatment time to 45 min for getting 100% of recovery from the solution at the pH of 8. These results indicate that Si oxide layer inhibits gold recovery from the basic solution without fluoride species. The recovery of gold from the simulated leaching solution increased with treatment time and reached 98% or more at the treatment time of 120 min (Fig. 1b). The recovery of copper, which was added as the oxidation aid, was less than 10%. This suggests the possibility that gold is selectively recovered from the leaching solution including less noble metal than gold. ACKNOWLEDGEMENT The present work was partly supported by JSPS KAKENHI (JP26289276). REFERENCES S. Yae, N. Nasu, K. Matsumoto, T. Hagihara, N. Fukumuro, and H. Matsuda, Electrochim. Acta, 53, 35 (2007).K. Fukuda, S. Yae, N. Fukumuro, S. Sakamoto, and H. Matsuda, ECS. Trans., 53(19), 69 (2013).K. Fukuda, N. Fukumuro, S. Sakamoto, and S. Yae, ECS. Trans., 61(10), 1 (2014).K. Fukuda, S. Arita, T. Tsuda, A. Matsumoto, and S. Yae, J. Surf. Finish. Soc. Jpn., 69(6), in press (2018). Figure 1

Fabrication and Preparation of Micro-Coaxial Wires: Electrodeposition of Conductive Shields

We describe the fabrication of micro-coaxial cables with diameters of 25 to 300 micrometers. Draper Laboratory is developing a new microelectronics packaging platform that relies on such microcoax, in place of copper interconnects, for both power and signal distribution among the packaged electronic components. This platform could save design time and fabrication labor over conventional packaging techniques for custom electronics with small production volumes and offers a way to rapid prototype and/or edit circuits. In this work, we focus on the fabrication of microcoax for power distribution. The fabrication process of the metal-dielectric-metal cross-section of the microcoax begins with a commercial metal wire of 18 to 150 micrometer diameter, which is either purchased with a polymer insulation of 1 to 12 micrometer thickness, or is insulated with SiO2 or Al2O3 of 0.1 to 2 micrometer thickness by PECVD or ALD processes. Next, a thin Ti/Au bilayer is applied to the insulated wire by electron beam evaporation and acts both as an adhesive layer and as electrical contact to plate the outer metal shield to its full desired thickness. The metal shield is electroplated using a cyanide-free gold sulfite chemistry that produces a soft satin to bright electrodeposit, meeting Type IIIA of Mil-G-45204 C requirements (i.e., ≥ 99.9 % gold purity and Knoop hardness 90 maximum). Straight DC, temperature of 60 °C, current density of 2 mA/cm2and mild agitation were chosen as parameters for Au electroplating conditions. Gold electrodeposits exhibit very low roughness, good corrosion resistance, excellent solderability, high shine and with evaporated seed metal, good adhesion to the polymer dielectrics. The shield thickness is chosen to give a resistance equal to or less than the resistance of the metal core, e.g., 34 micrometers of plated Au for a 127 micrometer diameter Cu core. The use of the microcoax for packaging requires lengths of microcoax of < 15 mm with the wire tips prepared for electrical connection, i.e., tips where the metal shield ends abruptly leaving the center metal wire exposed for bonding, and so we introduce at periodic intervals along the wire length regions where the outer layers are more easily stripped to expose the metal core. We have accomplished this either by ad hoc placement of a plating-resistant (nail polish or soft-baked photoresist) bead on the seed metal to pattern regions where no shield metal is electrodeposited; or by an evaporation mask during seed metal deposition to pattern regions where neither the metal seed nor the metal shield are deposited. The core metal is then exposed in the beaded or masked regions by wet etching the seed metal, if necessary, and UV laser ablation of the polymer dielectric. In addition to the fabrication processes, we will also discuss the fixturing and handling protocols required for these fine diameter wires. Fixturing is designed to accomplish four criteria: to minimize handling among the various process steps (insulating the wire, e-beam evaporation, electroplating); to pattern the microcoax lengths and abrupt edges to the electrodeposited metal; to make electrical contact to the seed metal during electroplating; and for the necessary volume and yield. We will describe our scale-up from an initial proof-of-concept process that produced 0.1 meters of microcoax of arbitrary segment lengths to a simpler process that produces 2 meters of microcoax of well-defined segment length.

Electrochemical Fabrication of Gold Nanoparticle Catalyst on CNTs for Glucose Fuel Cells

Introduction Polymer electrolyte fuel cells (PEFCs) operate at low temperatures and have fast startup times, making them suitable for automobile and household power applications. However, due to their low operating temperature, PEFCs require a catalyst. In general, platinum is used as the catalyst and carbon black (CB) is used as a support. Carbon nanotubes (CNTs) have recently attracted attention as an alternative to CB due to their large specific surface area, high mechanical strength, and high electrical conductivity, which collectively would improve the durability of PEFCs. In recent years, glucose fuel cells have been attracting attention. When the diameter of gold nanoparticles is less than 10 nm, they have been reported to be effective as catalysts.1)In this study, the fabrication process for a new gold catalyst supported by CNTs as an anode for glucose fuel cells was examined. Experimental (1) Fabrication of Ni/CNT composite films 2) A Watts nickel plating bath (NiSO4·6H2O, NiCl2·6H2O, H3BO3), polyacrylic acid (MW = 5000), brightener (saccharin Na, 1.4-butynediol) and vapor grown carbon fibers (VGCFs) were used as the basic plating bath, CNT dispersant, and CNTs (Showa Denko). Electrodeposition was carried out under galvanostatic conditions. Reverse electrolysis was carried out to removing Ni deposite on the CNTs. The surface morphologies of the films were observed by field-emission scanning electron microscopy (FE-SEM). (2) Reforming of the CNT surface by plasma treatment Plasma treatment with argon was carried out for 5 min at 10, 50, 100, and 150 W on the Ni/CNT composite plating films. The surface morphologies of the plasma-treated samples were observed by FE-SEM, and the CNT defects were evaluated by Raman spectroscopy. (3) Formation of Au nanoparticles A gold sulfite bath (NaAuCl4·2H2O, Na2SO3·7H2O) was used as the plating bath. Electrodeposition of gold on the Ni/CNT films was conducted under galvanostatic conditions. The surface morphology of the plating was observed using FE-SEM and scanning transmission electron microscopy (STEM). Results and discussion (1) Fabrication of Ni/CNT composite films Ni/CNT composite films with smoother surface morphology were formed than without the addition of brightener (Fig. 1(a) and (b)). The reverse electrolysis process was effective at removing Ni deposits on the CNTs (Fig. 1(c)). The reason is thought to be due to the effect of the brightener. Fig. 2 shows a plot of the ratio of drain to gate voltages, ID/IG, vs. the power used during the plasma treatment. ID/IGincreased with increasing power, indicating that the CNT defects increased with increasing power. (2) Reforming of the CNT surface by plasma treatment Fig. 2 shows a plot of the ratio of drain to gate voltages, ID/IG, vs. the power used during the plasma treatment. ID/IGincreased with increasing power, indicating that the CNT defects increased with increasing power. (3) Formation of Au nanoparticles Compared to non-treated CNTs, smaller gold nanoparticles with sizes of about 10 nm were observed on the plasma-treated CNTs. References 1) M. Pasta, L.Hu, F. Mantia, Y. Cui, Electrochemistry Communications, 1981-84 (2012). 2) S. Arai, A. Fujimori, M. Murai, M. Endo, Materials Letters, 62 35-45 (2008). Figure 1

Submicron Electrode Gaps Fabricated by Gold Electrodeposition at Interdigitated Electrodes

Electrodes with submicron gaps are desired for achieving high amplication redoxcycling sensors. In this contribution we report the use of electrodeposition of gold in order todecrease the inter-electrode spacing at interdigitated electrodes. Using this method submicronspacings can be obtained without expensive techniques such as e-beam lithography or focusedion beam milling. Initially, gold interdigitated electrodes with a nger spacing of 2.5 m wererealized by lift-o processing. Using a commercial gold sulphite bath (ECF64D) and 100 mscurrent pulses of -1.78 A, these gold electrodes were plated with an additional gold layer. Asa result, the inter- electrode spacing, as measured using atomic force microscopy and conven-tional microscopy, was reduced to 0.6 m. The achieved gap spacing is limited by electrodeimperfections resulting from the lift-o process. At these imperfections the electrodes becomeshorted. Additional experiments with wet etched electrodes are expected to yield smaller gapspacings

High aspect ratio electrodeposited Ni/Au contacts for GaAs‐based III–V concentrator solar cells

AbstractWe report on a photolithographic and electro‐deposition process that results in an optimized front grid structure for high efficiency multi‐junction III–V concentrator solar cells operating under flux concentrations up to 1000 suns. Two different thick photoresists were investigated to achieve a 6 µm wide grid line with an aspect ratio of 1:1. A positive photoresist, SPR220 manufactured by Rohm and Haas was compared with a negative photoresist, nXT15 manufactured by AZ. A gold sulfite electrolyte was employed to prevent underplating as well as for environmental and safety considerations. An initial layer of nickel was discovered to be necessary to prevent delamination of the fingers during the removal of the contact layer. When deposited on a purpose grown, heavily doped GaAs contact layer, this Ni/Au contact exhibits an acceptable specific contact resistance in the low 10−4 to mid 10−5 Ohm cm2 range along with excellent adhesion without sintering. Copyright © 2014 John Wiley &amp; Sons, Ltd.

A new preparation method of gold nanoparticles by intra-reduction of sodium gold(I) sulfite

Abstract In this study, well-dispersed gold nanoparticles were prepared by using intra-molecular reduction of sodium gold sulfite, without using additional reductants and chloride free. The technical parameters including transformation temperature, pH, and concentration were optimized by the single-factor method as 90 °C, pH 1, and 0.01 mmol/L [Na 3 Au(SO 3 ) 2 ], respectively. The resultant colloidal transmission electron microscopy images (TEM) and UV–vis absorption spectrophotometer spectra were acquired to check their properties, and the results show this kind of colloidal gold is controlled to 6 nm in sizes and has good stability in solution.

Stress in electroplated gold on silicon substrates and its dependence on cathode agitation

The influence of cathode agitation on the residual stress of electroplated gold has been investigated. Using a custom-built plating cell, a periodic, reciprocating motion was applied to silicon substrates that were electroplated with soft gold. A commercially available gold sulfite solution was used to deposit the 0.6μm thick gold films using a current density of 3.0mA/cm2 and a bath temperature of 50°C. By increasing the speed of cathode agitation from 0 to 5cm/s, the magnitude of the compressive stress decreased from −64 to −9MPa. The results suggest that cathode agitation significantly alters the mass transport within the electrolytic cell and can be used as a method of stress control in gold electroplating. This finding is potentially significant for plating applications in microelectronics and microsystems that require precise stress control.

Estimation of degree of lability of gold(I) sulfite and thiocarbamide complexes

The supposition that gold thiocarbamide complexes can be involved in the reduction of electrochemically inactive sulfite complex gold ions as intermediate species is supported by investigating the kinetics of cathodic process in the solution of the following composition: 10−3 M Na3Au(SO3)2, 0.1 M Na2SO3, 0.1M thiocarbamide (TC), and 0.2 M Na2SO4. The study was performed by the potentiodynamic method in the potential scan rate range from 0.05 to 1 V/s using an electrode with mechanically renewed surface. It is found that both Au(SO3)23−, and Au(TC)2+ are labile complexes. This confirms the hypothesis for the mechanism of the catalytic effect of thiocarbamide on the electrode processes involving gold ions in the sulfate solutions, which was proposed earlier.

Gold coated para-aramid yarns through electroless deposition

Abstract The interest in electroconductive textiles has tremendously increased over the last few years. By functionalizing textile surfaces with thin layers of metal, electrical conductivity can be combined with flexibility and pliability. We adopted this approach and deposited a thin layer of soft gold onto polypyrrole- and copper-coated para-aramid yarns in an electroless way. This paper presents the coating method for the gold deposition and presents some initial results on the characterization of the gold layer and the performance of the resulting yarns. A deposition solution was used which contained a gold salt, sulfite and thiosulfate. The pH value and temperature of the solution was optimized to result in a smooth and homogenous gold layer. Performance tests showed that the coated yarns offered an excellent electrical conductivity combined with good mechanical properties and satisfying resistance to washing. In addition, electrochemical impedance measurements revealed that the gold coated yarns are promising electrode materials to measure biomedical signals.

Microbalance study of gold dissolution in alkaline sulfite-thiocarbamide electrolytes

It is shown that gold does not virtually dissolve in alkaline (pH 12.5) solutions containing either thiocarbamide or sodium sulfite. Gold dissolves in alkaline solutions simultaneously containing thiocarbamide (0.1 M) and sodium sulfite (0.5 M). The gold dissolution rate increases with the increase in the contents of thiocarbamide and sodium sulfite. The methods of microbalance and voltammetry are used in studying the mechanism of gold dissolution in a solution containing 0.5 M sodium sulfite, 0.1 M thiocarbamide, and 0.03 M KOH. The found relationships are explained based on the assumption that the gold dissolution in alkaline sulfite-thiocarbamide electrolytes affords gold sulfite complexes.

Properties of ECD Gold Composite with Nanostructured Carbon-Based Materials

Results of electrodeposition of gold matrix composite coatings with carbon-based materials, namely ultradispersed diamond UDD and carbon nanotube CNT, are reported. The effects of the bath load on some representative properties of the composite are specifically addressed. Pure gold and gold composite coatings were prepared from a gold sulfite electrolyte a pH 7.5 and bath load from 5 to 20 g l-1 (UDD bath) and from 0.1 to 5 g l-1 (CNT bath). The codeposition was characterized in terms of carbon content of the coating and by examination of the composite coating morphology. The particles incorporation rate and the concurrent impact on morphology were found to depend on both the bath load and the type of material used. The influence of codeposited material on microindentation hardness, wear behavior and thermal stability was further investigated. Some notable improvements of performance are observed in electrocodeposits compared to the pure matrix coatings. Critical factors affecting the codeposition behavior are highlighted in the light of the reported results.

Gold electrodeposition for microelectronic, optoelectronic and microsystem applications

The electrodeposition of gold is a key technology in the fabrication of many microelectronic, optoelectronic and microsystem devices. In this review, we examine some recent applications, and consider the suitability of various gold plating systems for device fabrication. The properties of gold cyanide and sulfite baths and their limitations are considered first. This is followed by an analysis of the latest generation of non-cyanide baths, including the thiosulfate, sulfite-thiosulfate and ammonium gold systems. Plating baths containing mercaptoalkylsulfonic acid and hydantoin ligands are also briefly discussed. Finally an analysis of the stability of gold plating baths is presented.

Electrochemical fabrication of single-crystalline and polycrystalline Au nanowires: theinfluence of deposition parameters

We report the electrochemical growth of gold nanowires with controlled dimensions andcrystallinity. By systematically varying the deposition conditions, both polycrystalline andsingle-crystalline wires with diameters between 20 and 100 nm are successfully synthesized inetched ion-track membranes. The nanowires are characterized using scanning electronmicroscopy, high resolution transmission electron microscopy, scanning tunnellingmicroscopy and x-ray diffraction. The influence of the deposition parameters, especiallythose of the electrolyte, on the nanowire structure is investigated. Gold sulfite electrolyteslead to polycrystalline structure at the temperatures and voltages employed. In contrast,gold cyanide solution favours the growth of single crystals at temperatures between 50 and65 °C under both direct current and reverse pulse current deposition conditions. Thesingle-crystalline wires possess a [110] preferred orientation.

Speciation Analysis of Au(I) Electroplating Baths Containing Sulfite and Thiosulfate

An analysis of the speciation of gold plating solutions containing sulfite and thiosulfate as complexing agents has been performed. In the case of gold sulfite baths, it was found that the observed stability and electrochemical behavior were inconsistent with the commonly assumed stability constants of for the species. The data are, however, consistent with a much larger stability constant of . In gold plating baths containing sulfite and thiosulfate ligands, the speciation model predicts that the dominant species at neutral or basic are mixed complexes of the form and . The thiosulfate complex is only dominant under acidic conditions. It is shown that the electrodeposition characteristics reported in many studies can be interpreted in terms of the existence of such mixed ligand complexes. Finally, the effect of the stability of various gold plating solutions on colloidal gold formation has been explored. For free-standing solutions, decomposition is more likely to occur via the disproportionation of Au(I) species such as AuOH, , and rather than free aurous ions. The stability of gold baths during electrodeposition is mainly influenced by the formation of dithionite ions which reduce Au(I) complexes to form colloidal gold.