Pentathionic acid

Pentathionic acid solution-when treated with lead dioxide2 or when merely allowed to undergo spontaneous decomposition, yields tetrathionic acid. 

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.11... 

Pentathionic Acid, HaS508.—This is present in Wackenroder s solution 1 (p. 220), and can be separated by removing the sulphuric acid by treatment with a little barium carbonate, the remaining acid liquid after filtration being capable of concentration to a specific gravity of 1-3 by evaporation on a water-bath and to 1-6 by evaporation in a vacuum at the ordinary temperature.2 The solution contains also tetrathionic acid, which may be eliminated by the method described on p. 219. 

The decomposition of sulphur monochloride by water gives rise to a complex mixture of substances including sulphur and pentathionic acid. The equation... 

Pentathionic acid is formed by the action of sulphur dioxide on a suspension of sulphur in water 1... 

Traces of pentathionic acid are also stated to be found in the condensed liquid from the interaction of steam and sulphur vapour at a red heat,2 and in a mixture of sulphur and water after exposure to atmospheric oxidation.3 In the former ease the pentathionic acid probably results after the high temperature reaction by the interaction of sulphur dioxide and hydrogen sulphide in the condensate. 

Properties.—Like the lower members of the series of thionic acids, free pentathionic acid is known only in aqueous solution a solution of the pure acid is obtained by treating an aqueous solution of the potassium salt with the requisite quantity of tartaric acid for the removal of the potassium in the form of hydrogen tartrate. The solution is denser than water 4 it cannot be concentrated beyond a limit of 50 to 60 per cent, acid without decomposition. 

Hydrogen sulphide slowly decomposes aqueous pentathionic acid, the final state being represented by the equation... 

Whenever sulphur dioxide, water and nascent sulphur meet, for example in the action of water on sulphur chloride, of mineral acids on a thiosulphate, or of hydrogen sulphide on aqueous sulphur dioxide solution, formation of polythionic acids is likely to occur. Dalton 1 in 1812 demonstrated that the last-named reagents gave rise to an acid liquid, a result which was confirmed later by Thomson 2 in 1846 Wackenroder 3 proved the presence of pentathionic acid in the liquid, since which date the aqueous reaction product has been known as Wackenroder s Solution. ... 

The reaction was subsequently investigated by other chemists, particularly Debus,4 who found that the most satisfactory procedure was to treat an almost saturated cold solution of sulphur dioxide repeatedly with hydrogen sulphide on successive days until the sulphur dioxide vras consumed. The resulting liquid contained sulphur in colloidal suspension, free sulphur (see also p. 30), sulphuric acid, a little trithionic acid, tetrathionic add and pentathionic acid, and an add or acids still richer in sulphur—possibly a hexathionic acid.5... 

The composition of Wackenroder s solution varies with the conditions of preparation. When prepared in the manner described, the milky solution can be clarified by careful evaporation at the ordinary temperature in a vacuum, when the suspended sulphur separates and a solution containing mainly pentathionic acid is obtained. 

The observed occurrence of pentathionic acid in natural waters6 is doubtless to be attributed to a reaction of this kind between sulphur dioxide and hydrogen sulphide. 

Raschig 2 considered the formation of pentathionic acid in Wacken-roder s solution to be due to the condensation of five SO-groups arising from the reaction... 

The complex nature of the process of decomposition of these acids and their behaviour on oxidation makes it difficult to decide which of the foregoing formulae are most satisfactory for the tetra- and pentathionic acids. In the case of dithionie and trithionic acids, the formulae given by Blomstrand and Mendeleeff and by Vogel appear to accord best with the general behaviour of the acids. 

The bimolecular nature of sulphur trioxide is cited in support of these views, and the absence of colour in the polythionic acids is mentioned as being opposed to the polysulphide linking of some of the earlier formulae. Nevertheless, the above formulae do not appear to be in agreement with the relative stabilities of the acids, but suggest that the tetrathionic acid is less stable than the more compact trithionic acid, or than the more saturated pentathionic acid (cf. pp. 211, 215). 

H,S,0, Pyroaulphuric acid. H,S,0, Dithionic acid. H,S,0, Tritbionic acid. H,S,0, Tetrathionic acid. H,S,0, Pentathionic acid. 

Two acids with mixed three-membered chains are known, namely selenopentathionic and telluropentathionic acid. They are derived from pentathionic acid by substitution of selenium or tellurium for the middle sulfur atom of the latter, and behave in reactions with nucleophilic reagents as thiosulfates of divalent selenium and tellurium. The first salts of scleno-pentathionic (84) and telluropentathionic (85) acid were isolated in 1949, although the formation reactions in aqueous solutions had been known earlier. Wood (229) has by paper electrophoresis shown that the preparative methods lead to pure products. 

HaSaOj Pyrosulfuric acid. HaSaOa Dithionic acid. HaSaO, Trithionic acid. HaS,0 Tetrathionic acid. HaSsOe Pentathionic acid. 

This results in the formation of colloidal sulfur and pentathionic acid, which is unstable and produces a salt, sodium meto-bisulfite. 

Gay-Lussac and Welter discovered dithionic acid (H2S20e). The name thionic acids , and the modern name dithionous acid for hyposulphurous acid, were proposed by Berzelius. Trithionic acid was discovered by Lan-glois and investigated by Mathieu Plessy, who gave a better method of preparation. Sodium tetrathionate was discovered by Fordos and Gelis. Pentathionic acid was discovered by Wackenroder (see p. 904). 

In an investigation of the action of hydrogen sulphide on sulphur dioxide in presence of water, he suspected the presence in the precipitate of sulphur of a new oxyacid of sulphur which he called hydrosulphurous acid. This was afterwards recognised as pentathionic acid by Wackenroder in 1845, who was unaware of Thomson s work, which, if he had followed it up, would have given him the credit of the discovery. In Thomson s paper On some Compounds of Chromium , he describes the discovery ( about a year and a half ago ) of chromyl chloride ( chlorochromic acid , which he thought was a compound of i atom chromic acid and i atom chlorine ), of chromium chromate, some new chromates and dichromates, and some double and complex compounds of chromium. Some compounds he described were mixtures, and carbonate of chromium and disulphuretted oxide of chromium are non-existent. 

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