Trithionous acid, 132

When acids act on thiosulphates, polythionic acids are formed.5 This is explained by reactions (b) and (c). The fact that the amount of hydrogen sulphide liberated is very small in proportion to the amount of polythionic acid formed is attributed 8 to the fact that reaction (a) is that which normally occurs to the greatest extent when acid acts upon thiosulphate the hydrogen sulphide is thus liberated in the presence of a large amount of sulphurous acid and hence rapidly destroyed. Trithionic acid is thus a primary product of thiosulphate decomposition, and in its turn decomposes as explained on p. 212. 

The term polythionic acids covers a group of acids 3 of the general formula HgS Oj, where x may range from 2 to 6, the various members of the group being distinguished by the names dithionic acid, trithionic acid, etc. 

Trithionic Acid, H2S3Og.—Formation.—(1) From Thiosulphates Potassium thiosulphate in concentrated aqueous solution reacts with sulphur dioxide forming potassium trithionate 3 the reaction is sometimes represented as... 

Trithionic acid is found together with sulphuric, sulphurous and thiosulphuric acids in the reaction product from the decomposition of nitrogen sulphide, N4S4, with water. ... 

In the oxidation of alkali sulphides or polysulphides by potassium permanganate solution at the ordinary temperature, trithionic acid has been found amongst the reaction products, in addition to sulphuric acid and sulphur.7... 

Properties.—Trithionic acid is the least stable of the polythionic acids. The aqueous solution of the free acid, which is generally obtained from a cold concentrated solution of the potassium salt by the addition of a suitable acid, such as hydrofluosilicie or perchloric acid, which will remove the metal as a sparingly soluble salt, slowly decomposes, even at the ordinary temperature, with formation of sulphur, sulphur dioxide and sulphuric acid ... 

Oxidising adds such as nitric, chloric and iodic acids induce rapid decomposition of trithionic acid with formation of sulphur and sulphuric acid 2 the presence of other acids, for example hydrochloric, perchloric or dilute sulphuric acid, also hydrogen sulphide, is without any harmful effect.3 Addition of sulphurous acid causes the gradual formation of a mixture of all the polythionic acids. 

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... 

With the free acid only reactions (i) and (iii) occur. The decomposition takes place more readily than in the case of trithionic acid. Alkali sulphides in boiling solution convert tetrathionate into thiosulphate, with liberation of sulphur 2... 

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... 

Dithionic acid. Trithionic acid. Tetrathionic acid. Pentatbionic acid. 

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). 

Trithionic acid, H2S3Os, is also known it is a still more unstable liquid. 

Consider the polythionic acids, H2S 06 = [(H0)(0)2SS 2S(0)2(0H)] (n > 2), which have the common names dithionic acid, trithionic acid, tetrathionic acid, etc. They may be named systematically using additive nomenclature, as shown in Table IR-8.1. For > 3, they may also be named substitutively on the basis of the central (poly)sulfane skeleton, as exemplified below. 

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

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). 

Mild oxidizing agents such as hydrogen peroxide in acid solutions produce tetrathionates and trithionates (17) ... 

Diphenylcyclopropene thione (156) was prepared11S-12°) from 3,3-dichloro-1,2-diphenyl cyclopropene (154) by reaction with thioacetic acid, since transformation of the carbonyl function of diphenyl cyclopropenone with P4S10121 was complicated by ring expansion to the trithione 155122 In a useful recent thioketone synthesis123) 156 was obtained directly from diphenyl cyclopropenone in a quantitative yield by simultaneous treatment with HC1 and H2S. 

Against this latter view is the fact that little or no sulphuric acid is formed unless the mixture is boiled for a long time. In alkaline solution, alkali sulphide and trithionate react to form thiosulphate,1 but alkaline solutions of sulphate and sulphide do not.2 It would appear, therefore, that the correct explanation lies in the reversible equation (6) (p. 195). This receives support from the fact that when lead thiosulphate is boiled with water it yields, in the first instance, lead sulphide and lead trithionate.3 Conversely, lead sulphide on digestion with potassium trithionate yields lead thiosulphate.4 Further, weakly alkaline solutions of sodium thiosulphate itself yield, on boiling, sodium sulphide and sodium trithionate, with only a trace of sulphate if boiled with sodium plumbite, lead sulphide is precipitated and sodium trithionate remains in solution. 

The reaction, of course, will not end there continued action of sulphurous acid results in the hydrolysis of the trithionate, with formation of tetra- and penta-thionates, and ultimately of sulphate and sulphur.4... 

Certain double salts of thiosulphurie acid when heated with water undergo decomposition with production of trithionate 8 thus the sodium-mercurous salt decomposes according to the equation ... 

The trithionates may be quantitatively precipitated by boiling for one hour with a mixture of copper sulphate and barium chloride solutions containing free hydrochloric acid 4... 

This method is especially suited to the trithionates and tetrathionates, and is applicable even in the presence of thiosulphate or sulphite by first titrating the neutral solution with iodine, which indicates the total quantity of sulphite and thiosulphate, and then determining the acidity developed during the iodine titration, from which the amount of sulphite can be calculated. Finally the mercuric chloride reaction is applied to another portion of the original solution and by determination... 

---