[67] This view was communicated by me in 1870 to the Russian Chemical Society.
[68] Dithionic acid, H2S2O6, is distinguished among the thionic acids as containing the least proportion of sulphur. It is also called hyposulphuric acid, because its supposed anhydride, S2O5, contains more O than sulphurous oxide, SO2 or S2O4, and less than sulphuric anhydride, SO3 or S2O6. Dithionic acid, discovered by Gay-Lussac and Welter, is known as a hydrate and as salts, but not as anhydride. The method for preparing dithionic acid usually employed is by the action of finely-powdered manganese dioxide on a solution of sulphurous anhydride. On shaking, the smell of the latter disappears, and the manganese salt of the acid in question passes into solution; MnO2 + 2SO2 = MnS2O6. If the temperature be raised, the dithionate splits up into sulphurous anhydride and manganese sulphate, MnSO4. Generally owing to this a mixture of manganese sulphate and dithionate is obtained in the solution. They may be separated by mixing the solution of the manganese salts with a solution of barium hydroxide, when a precipitate of manganese hydroxide and barium sulphate is obtained. In this manner barium dithionate only is obtained in solution. It is purified by crystallisation, and separates as BaS2O6,2H2O; this is then dissolved in water, and decomposed with the requisite amount of sulphuric acid. Dithionic acid, H2S2O6, then remains in solution. By concentrating the resultant solution under the receiver of an air-pump it is possible to obtain a liquid of sp. gr. 1·347, but it still contains water, and on further evaporation the acid decomposes into sulphuric acid and sulphurous anhydride: H2S2O6 = H2SO4 + SO2. The same decomposition takes place if the solution be slightly heated. Like all the thionic acids, dithionic acid is readily attacked by oxidising agents, and passes into sulphurous acid. No dithionate is able to withstand the action of heat, even when very slight, without giving off sulphurous anhydride: K2S2O6 = K2SO4 + SO2. The alkali dithionates have a neutral reaction (which indicates the energetic nature of the acid) are soluble in water, and in this respect present a certain resemblance to the salts of nitric acid (their anhydrides are: N2O5 and S2O5). Klüss (1888) described many of the salts of dithionic acid.
Langlois, about 1840, obtained a peculiar thionic acid by heating a strong solution of acid potassium sulphite with flowers of sulphur to about 60°, until the disappearance of the yellow coloration first produced by the solution of the sulphur. On cooling, a portion of the sulphur was precipitated, and crystals of a salt of trithionic acid, K2S3O6 (partly mixed with potassium sulphate), separated out. Plessy afterwards showed that the action of sulphurous acid on a thiosulphate also gives sulphur and trithionic acid: 2K2S2O3 + 3SO2 = 2K2S3O6 + S. A mixture of potassium acid sulphite and thiosulphate also gives a trithionate. It is very possible that a reaction of the same kind occurs in the formation of trithionic acid by Langloid's method, because potassium sulphite and sulphur yield potassium thiosulphate. The potassium thiosulphate may also be replaced by potassium sulphide, and on passing sulphurous anhydride through the solution thiosulphate is first formed and then trithionate: 4KHSO3 + K2S + 4SO2 = 3K2S3O6 + 2H2O. The sodium salt is not formed under the same circumstances as the corresponding potassium salt. The sodium salt does not crystallise and is very unstable: the barium salt is, however, more stable. The barium and potassium salts are anhydrous, they give neutral solutions and decompose when ignited, with the evolution of sulphur and sulphurous anhydride, a sulphate being left behind, K2S3O6 = K2SO4 + SO2 + S. If a solution of the potassium salt be decomposed by means of hydrofluosilicic or chloric acid, the insoluble salts of these acids are precipitated and trithionic acid is obtained in solution, which however very easily breaks up on concentration. The addition of salts of copper, mercury, silver, &c., to a solution of a trithionate is followed, either immediately or after a certain time, by the formation of a black precipitate of the sulphides whose formation is due to the decomposition of the trithionic acid with the transference of its sulphur to the metal.
Tetrathionic acid, H2S4O6, in contradistinction to the preceding acids, is much more stable in the free state than in the form of salts. In the latter form it is easily converted into trithionate, with liberation of sulphur. Sodium tetrathionate was obtained by Fordos and Gélis, by the action of iodine on a solution of sodium thiosulphate. The reaction essentially consists in the iodine taking up half the sodium of the thiosulphate, inasmuch as the latter contains Na2S2O3, whilst the tetrathionate contains NaS2O3 or Na2S4O6, so that the reaction is as follows: 2Na2S2O3 + I2 = 2NaI + Na2S4O6. It is evident that tetrathionic acid stands to thiosulphuric acid in exactly the same relation as dithionic acid does to sulphurous acid; for the same amount of the other elements in dithionate, KSO3, and tetrathionate, KS2O3, there is half as much metal as in sulphite, K2SO3, and thiosulphate, K2S2O3. If in the above reaction the sodium thiosulphate be replaced by the lead salt PbS2O3, the sparingly-soluble lead iodide PbI2 and the soluble salt PbS4O6 are obtained. Moreover the lead salt easily gives tetrathionic acid itself (PbSO4 is precipitated). The solution of tetrathionic acid may be evaporated over a water-bath, and afterwards in a vacuum, when it gives a colourless liquid, which has no smell and a very acid reaction. When dilute it may be heated to its boiling-point, but in a concentrated form it decomposes into sulphuric acid, sulphurous anhydride, and sulphur: H2S4O6 = H2SO4 + SO2 + S2.
Pentathionic acid, H2S5O6, also belongs to this series of acids. But little is known concerning it, either as hydrate or in salts. It is formed, together with tetrathionic acid, by the direct action of sulphurous acid on sulphuretted hydrogen in an aqueous solution; a large proportion of sulphur being precipitated at the same time: 5SO2 + 5H2S = H2S5O6 + 5S + 4H2O.
If, as was shown above, the thionic acids are disulphonic acids, they may be obtained, like other sulphonic acids, by means of potassium sulphite and sulphur chloride. Thus Spring demonstrated the formation of potassium trithionate by the action of sulphur dichloride on a strong solution of potassium sulphite: 2KSO3K + SCl2 = S(SO3K)2 + 2KCl. If sulphur chloride be taken, sulphur also is precipitated. The same trithionate is formed by heating a solution of double thiosulphates; for example, of AgKS2O3. Two molecules of the salts then form silver sulphide and potassium trithionate. If the thiosulphate be the potassium silver salt SO3K(AgS), then the structure of the trithionate must necessarily be (SO3K)2S. Previous to Spring's researches, the action of iodine on sodium thiosulphate was an isolated accidentally discovered reaction; he, however, showed its general significance by testing the action of iodine on mixtures of different sulphur compounds. Thus with iodine, I2, the mixture Na2S + Na2SO3 forms 2NaI + Na2S2O3, whilst the mixture Na2S2O3 + Na2SO3 + I2 gives 2NaI + Na2S3O6—that is, trithionic acid stands in the same relation to thiosulphuric acid as the latter does to sulphuretted hydrogen. We adopt the same mode of representation: by replacing one hydrogen in H2S by sulphuryl we obtain thiosulphuric acid, HSO3.HS, and by replacing a second hydrogen in the latter again by sulphuryl we obtain trithionic acid, (HSO3)2S. Furthermore, Spring showed that the action of sodium amalgam on the thionic acids causes reverse reactions to those above indicated for iodine. Thus sodium thiosulphate with Na2 gives Na2S + Na2SO3, and Spring showed that the sodium here is not a simple element taking up sulphur, but itself enters into double decomposition, replacing sulphur; for on taking a potassium salt and acting on it with sodium, KSO3(SK) + NaNa = KSO3Na + (SK)Na. In a similar way sodium dithionate with sodium gives sodium sulphite: (NaSO3)2 + Na2 = 2NaSO3Na; sodium trithionate forms NaSO3Na and NaSO3.SNa, and tetrathionate forms sodium thiosulphate, (NaSO3)S2(NaSO3) + Na2 = 2(NaSO3)(NaS).
In all the oxidised compounds of sulphur we may note the presence of the elements of sulphurous anhydride, SO2, the only product of the combustion of sulphur, and in this sense the compounds of sulphur containing one SO2 are—
while, according to this mode of representation, the thionic acids are—
