Thiosulfate baths

How does ascorbate compare with thiourea Are there other reducing agents that may work as well in a thiosulfate bath ... 

As far as the authors are aware, no detailed investigations have been performed to elucidate the overall mechanism of gold deposition from thiosulfate baths. Based... 

From a practical viewpoint, the significance of the thiosulfate bath lies in the fact that it is a noncyanide, near-neutral pH system, and that it is not sensitive to nickel... 

A new class of cathodes has been developed by Tavares and Trasatti [45] and Iwakura et al. [46] for hydrogen evolution. In these electrodes, Ru02 particles were co-deposited with Ni onto smooth or rough Ni supports from Ni baths of different compositions, expediently named Watts, chloride, and thiosulfate baths. The cathodes were electrochemically characterized, and the results showed that the enhanced electrocatalytic activity of the cathodes was mainly ascribable to the increase of active sites or the content of RUO2 particles. The performance of these new developed electrodes was evaluated for the use in a chlor-alkali electrolysis in a laboratory and industrial scale. 

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]. 

Mixed Sulfite-Thiosulfate Bath for Soft Gold Electrodeposition (Concentration in mol h unless Otherwise Noted)... 

The primary component of a fixation bath, thiosulfate, tends to decompose in acidic environments according to the following reaction ... 

This equihbrium explains the stabilization of thiosulfate solutions using sulfite or bisulfite as one of the components of acid photographic fixing baths. 

Sodium thiosulfate is still used in chrome leather tanning as a reducing agent in two-bath processes to reduce dichromate (hexavalent chromium) to chrome alum (trivalent chromium) (see Leather). 

Peracid Analysis. Peracid concentrations can be measured in a product or in the bath by use of a standard iodide / thiosulfate titration (60). With preformed peracids or peracids formed via perhydrolysis care must be exercised to minimize the interference of hydrogen peroxide, present intentionally as a component of the perhydrolysis reaction or as a result of the hydrolysis of the peracid (87,88) as shown in equation 18. 

A mixture of 17 g of the methiodide and 32 ml of a 40 % aqueous potassium hydroxide solution is heated with stirring in a flask fitted with a condenser. The heating bath should be kept at 125-130°, and the heating should be continued for 5 hours. The cooled reaction mixture is then diluted with 30 ml of water and washed twice with 25-ml portions of ether. The aqueous layer is cautiously acidified in the cold with concentrated hydrochloric acid to a pH of about 2 and then extracted five times with 25-ml portions of ether. The combined extracts are washed twice with 10% sodium thiosulfate solution and are dried (magnesium sulfate). Removal of the solvent followed by distillation affords about 3 g of 4-cyclooctene-l-carboxylic acid, bp 125-12671-1 mm. The product may solidify and may be recrystallized by dissolution in a minimum amount of pentane followed by cooling in a Dry-Ice bath. After rapid filtration, the collected solid has mp 34-35°. 

A 250-mL, two-necked, round-bottomed flask equipped with a magnetic stirbar, thermometer, and a reflux condenser fitted with a rubber septum and balloon of argon is charged with a solution of methyltrioxorhenium (MTO) (0.013 g, 0.05 mmol, 0.1% mol equiv) in 100 mL of methanol (Note 1). Urea hydrogen peroxide (UHP) (14.3 g, 152 mmol) is added (Notes 1, 2, 3, 4), the flask is cooled in an ice bath, and dibenzylamine (9.7 mL, 50.7 mmol) is then added dropwise via syringe over 10 min (Notes 1, 5). After completion of the addition, the ice bath is removed and the mixture is stirred at room temperature (Note 6). A white precipitate forms after approximately 5 min (Note 7) and the yellow color disappears within 20 min (Note 8). Another four portions of MTO (0.1% mol equiv, 0.013 g each) are added at 30-min intervals (2.5 hr total reaction time). After each addition, the reaction mixture develops a yellow color, which then disappears only after the last addition does the mixture remain pale yellow (Note 9). The reaction flask is cooled in an ice bath and solid sodium thiosulfate pentahydrate (12.6 g, 50.7 mmol) is added in portions over 20 min in order to destroy excess hydrogen peroxide (Note 10). The cooled solution is stirred for 1 hr further, at which point a KI paper assay indicates that the excess oxidant has been completely consumed. The solution is decanted into a 500-mL flask to remove small amounts of undissolved thiosulfate. The solid is washed with 50 mL of MeOH and the methanol extract is added to the reaction solution which is then concentrated under reduced pressure by rotary evaporation. Dichloromethane (250 mL) is added to the residue and the urea is removed by filtration through cotton and celite. Concentration of the filtrate affords 10.3 g (97%) of the nitrone as a yellow solid (Note 11). 

A method to circumvent the problem of chalcogen excess in the solid is to employ low oxidation state precursors in solution, so that the above collateral reactions will not be in favor thermodynamically. Complexation strategies have been used for this purpose [1, 2]. The most established procedure utilizes thiosulfate or selenosulfate ions in aqueous alkaline solutions, as sulfur and selenium precursors, respectively (there is no analogue telluro-complex). The mechanism of deposition in such solutions has been demonstrated primarily from the viewpoint of chemical rather than electrochemical processes (see Sect. 3.3.1). Facts about the (electro)chemistry of thiosulfate will be addressed in following sections for sulfide compounds (mainly CdS). Well documented is the specific redox and solution chemistry involved in the formulation of selenosulfate plating baths and related deposition results [11, 12]. It is convenient to consider some elements of this chemistry in the present section. 

Generally, the experimental results on electrodeposition of CdS in acidic solutions of thiosulfate have implied that CdS growth does not involve underpotential deposition of the less noble element (Cd), as would be required by the theoretical treatments of compound semiconductor electrodeposition. Hence, a fundamental difference exists between CdS and the other two cadmium chalcogenides, CdSe and CdTe, for which the UPD model has been fairly successful. Besides, in the present case, colloidal sulfur is generated in the bulk of solution, giving rise to homogeneous precipitation of CdS in the vessel, so that it is quite difficult to obtain a film with an ordered structure. The same is true for the common chemical bath CdS deposition methods. 

Pulse plating techniques with symmetric or asymmetric waves have been employed for improving the deposition of CdS in acidic aqueous baths of cadmium sulfate and thiosulfate precursors [46, 47], A better control of sulfur incorporation in the deposits was reported. 

Reducing bath = 1 g potassium iodide and 1 g sodium thiosulfate in 30 mL absolute ethanol and 20 mL distilled H20 add 0.5 mL 2NHC1 (make reducing bath fresh daily)... 

C. Hydrolysia. Into a three-necked, 5-L, round-bottomed flask equipped with a mechanical stirrer, thermometer, and 500-mL addition funnel with gas Inlet Is placed the above concentrate and 2 L of methanol. The solution is cooled to -5° to 0°C by means of an ice-salt bath. A precooled (0°C) solution containing 220 g of 87.5% potassium hydroxide in 400 raL of water is added dropwise at such a rate as to keep the reaction temperature below 10°C. The reaction mixture is stirred for an additional 2 hr at 0°C and for 1 hr at room temperature prior to the addition of 100 mL of glacial acetic acid. Solid sodium carbonate Is added to bring the pH to 8 and the solution Is filtered through Cellte. Concentration of the filtrate at 35°C and reduced pressure affords about 1 L of a dark liquid. The liquid is diluted with 2 L of water and extracted with petroleum ether (6 x 600 ml). The combined extracts are washed with aqueous sodium thiosulfate solution (800 mL) and dried (magnesium sulfate). Concentration at 30°C affords a clear red liquid (occasionally a yellow solid) which is almost pure internal diester (Note 12). 

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