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Atomistry » Sulphur » Compounds » Tetrathionic Acid | ||
Atomistry » Sulphur » Compounds » Tetrathionic Acid » |
Tetrathionic Acid, H2S4O6Formation
Properties
The free acid is known only in the form of its aqueous solution, which is without colour or odour.
From the strongly acid taste, the heat of neutralisation with dilute sodium hydroxide and the electrical conductivity of the solution, it appears that tetrathionic acid is a fairly strong acid, comparable with dithionic acid in this respect. The heat of formation is given by the equation: H2 + 4S + 3O2 + Aq. = H2S4O6,Aq. + 260.8 Calories. It is the most stable of the polythionic acids, and a dilute aqueous solution can be heated to boiling without decomposition, although the concentrated solution undergoes decomposition giving sulphurous and sulphuric acids together with sulphur. Mineral acids, excluding those of decided oxidising or reducing character, do not induce decomposition of the cold solutions. Alkalis, however, cause the formation of a mixture of thiosulphate and trithionate: 4K2S4O6 + 6KOH = 5K2S2O3 + 2K2S3O6 + 3H2O, whilst in hot solution some sulphide may also be produced. With sodium carbonate the reaction is 4Na2S4O6 + 4Na2CO3 = 6Na2S2O3 + Na2S3O6 + Na2SO4 + 4CO2. Sulphur dioxide abstracts sulphur from aqueous tetrathionic acid giving trithionic acid, the sulphur remaining in the solution and converting part of the tetrathionic acid into pentathionic acid, so that the final solution contains all three acids. Hydrogen sulphide in excess causes gradual decomposition, sulphur and pentathionic acid being the chief products. In neutral solution the reaction is more rapid and appears to take the following course:
K2S4O6 + K2S = 2K2S2O8 + S. Freshly precipitated lead sulphide also induces the decomposition of tetrathionic acid. Vigorous oxidising agents, such as chlorine and bromine, convert tetrathionic acid into sulphuric acid, whilst reducing agents, for example hydrogen, especially in contact with platinum, and sodium or potassium amalgam, act on the tetrathionates with formation of thiosulphates, K2S4O6 + 2Na = K2S2O3 + Na2S2O3, the reaction being capable of proceeding further, with the production of some sulphide. |
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