Sulfite thiosulfate inhibition

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]. 

Aqueous sodium thiosulfate solutions ate neutral. Under neutral or slightly acidic conditions, decomposition produces sulfite and sulfur. In the presence of air, alkaline solutions decompose to sulfate and sulfide. Dilute solutions can be stabilized by small amounts of sodium sulfite, sodium carbonate, or caustic, and by storage at low temperatures away from air and light. Oxidation is inhibited by Hgl2 (10 Ppm) amyl alcohol (1%), chloroform (0.1%), borax (0.05%), or sodium benzoate (0.1%). 

Figure 5. The metabolism of 3S-S radiolabeled sulfide by boiled and intact isolated Solemya reidi mitochondria. Time zero is prior to addition of mitochondria. When mitochondria are added there is a rapid decrease in sulfide concentration caused partially by dilution of the total volume and partially by binding of sulfide to protein. In the boiled preparation there is very little oxidation of sulfide and no appearance of oxidized products. In the healthy mitochondria the sulfide is rapidly oxidized to thiosulfate. Sulfite and sulfate did not appear as oxidation products. The health of the isolated mitochondria was monitored by oxygen consumption rate in the chamber with succinate as substrate and by the ability to inhibit succinate stimulated respiration with respiratory inhibitors such as cyanide. Error bars represent plus and minus one standard deviation of the mean of three runs.

Thiosulfate is believed to inhibit sulfite oxidation by reacting with free radicals generated in the chain reactions involved in sulfite oxidatirm. The chain reactions are catalyzed by transition metal ions such as Fes, and the use of a chelating agent such as ethyloiediaminete-traacetate (EDTA) to remove the metal ions has been shown to augment the oxidation inhibition properties of thiosulfate (Mailer et al., 1990). 

Mailer, G., Meserole, F. B., and Moser, R. E., 1990, Use of EDTA and Thiosulfate to Inhibit Sulfite Oxidation in Wet Limestone Flue Gas Desulfurization Processes Results of Laboratory-Scale and Bench Scale Testing, EPRI/EPA 1990 SO2 Control Symposium, New Orleans, LA, May 8-11. 

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