Dithionate and polythionates

Review articles have concentrated attention on the classification of these reactions as involving nucleophilic or electrophilic attack on a sulphur atom. Most of the reactions of the former type have been examined under the heading appropriate to the nucleophile involved. Those involving organic substrates are not considered here. 

In acidic solutions, dithionic acid decomposes to sulphate and sulphur dioxide at rates which are readily measured (Table 26) . The reaction is of first order at defined acidity and the rate increases, but not proportionately, with acid concentration. The authors have favoured an interpretation in terms of undissociated H2S2O6. Salt effects and solvent deuterium effects are in general agree- 

DEPENDENCE OF DECOMPOSITION RATE OF DITHIONIC ACID ON IONIC STRENGTH 

This can explain the sulphur-isotope results but needs more detail to be satisfying in kinetic terms. The tetrathionate species is reported to decompose by a path in which hydroxide ion is important, with the stoichiometric equation 

Catalysis by thiosulphate and sulphite through reactions (4) and (5) is important in nearly neutral solutions, viz. 

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Figure 9.3-5 is a Pourbaix. predominant-area, diagram for the S-O-HjO system for a total activity of all dissolved sulftir species of 10 1 ( S — 10" ). The only stable sulfiir species are HSOJ. SO2-. H.S. HS". and elemental sulfur. The formation of elemental sulftir films occurs in acid solutions as indicated. In basic solution, during the oxidation of sulfur-bearing compounds, intermediate metastabte solfor species such as thiosulfate, dithionate. and polythionates form. This is a problem previously described in the Sherrill Gordon process. Under acid conditions, during the dissolution of sulfide minerals, elemental sulfur layers often form but metastable solfor intermediates such as thiosulfate and sulfite are not observed. 

Similar reactions occur with dithionates and polythionates in solution. Final copper residuals are stripped using HjS under modest pressure. Aqueous effluent from copper removal is heated to 490 K (425°F) and... 

We considered oxoacids in some detail in Section 11.4. This Section will now give us the opportunity to revise those rules and apply them to the sulfur oxoacids. With sulfur, remember, we have the added complication that an oxygen atom can be substituted by sulfur, giving a range of thioacids, for example thiosulfuric acid (12.18), dithionic acid (12.19) and polythionic acid (12.20). 

Thiosulphates, e.g. sodium thiosulphate, are oxidisable to polythion-ates by the addition of the requisite quantity of hydrogen peroxide. If the reaction mixture is allowed to become alkaline, dithionate, tetra-thionate and sulphate are produced 2... 

An attractive modification of this method is to treat the neutralised polythionate solution with neutral hydrogen peroxide and a known excess of standard sodium hydroxide solution on the water-bath and to determine the quantity of acid formed by the amount of standard alkali neutralised during the oxidation.3 All polythionates except the dithionates are completely oxidised to sulphate by heating in a closed tube for one hour with iodine and sodium bicarbonate by titrating the excess of iodine the total polythionate present may be estimated.4... 

There are a variety of polythionates, [03S-S -S03], where n can vary from 0 (dithionate) to the 20s. The anions with lower n values are named by reference to the total number of sulfur atoms for example, S3OI is trithionate and 840 is tetrathionate. The structure of the latter is given in Table 17.2 as a representative polythionate. None of the corresponding acids are stable. Selenium analogs of the polythionates in which all the sulfur atoms are replaced with the heavier congener do not exist, but compounds with a general formula [03S-Se -S03], = 2—6, do. 

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