Oxoacids and Corresponding Salts

Thiosulfuric acid, H2S2O3, cannot be prepared as the free acid in aqueous solution, but the thiosulfate ion can be made by boiling elemental sulfur with aqueous sulfite solutions, as represented in Equation (17.17)  

The use of sodium thiosulfate as the hypo in black-and-white photography is described in Chapter 6 (p. 132). It is also a moderately strong reducing agent and is routinely used to determine the amount of iodine produced in various quantitative analytical procedures generally referred to as iodimetry. The reaction of thiosulfate with iodine is shown in Equation (17.18)  

Tetrathionate, 8405 , is a polythionate, which is described at the end of this section. 

The free dithionous acid, H2S2O4, is not known, but its salts (sulfur oxidation state = +3) can be prepared by reducing sulfites (S = +4) with zinc dust, as shown in Equation (17.19)  

The structure of the dithionite anion, S2O4 (see Table 17.2), is noteworthy for (1) the longest S-S bond (2.39 A) of any catenated S-S compound and (2) its playground swing structure that puts the lone pairs on each sulfur atom in close proximity to each other. Dithionite solutions are rapid and good reducing agents in basic solution. Its standard reduction potential with respect to sulfite in basic solution is —1.12 V. 

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