Art. III.—The First-noted Occurrence of Pentathionic Acid in Natural Waters.

By J. S. Maclaurin, D.Sc.

[Read before the Wellington Philosophical Society, 5th October, 1910.]

Pentathionic acid is readily formed by passing hydrogen-sulphide into a solution of sulphur-dioxide, but, though well known to the chemist, it has not previously been found in natural waters. This may be due to the fact that in ordinary analyses of mineral waters no special search is made for such compounds. Some account of the circumstances that led to the discovery of the acid should therefore be of interest.

The water in which it was found was obtained from a lake on White Island. This island, which is the summit of a volcano, lies in the Bay of Plenty, about thirty miles from the mainland. The lake covers an area of approximately 15 acres, and has a mean temperature of about 110° Fahr. The water is of a very unusual character, containing a great variety of salts and an enormous amount of free hydrochloric acid. In examining it for iodine by the well-known potassium-nitrite test, the author noticed the formation of a brown colour, which suggested the presence of a ferrous salt, but on titrating with potassium-permanganate more permanganate solution was used than the iron (previously determined gravimetrically) could require, even if all in the ferrous state, and, judging by the colour of the water, much of the iron was in the ferric state. There was no organic matter to cause this excessive reduction, and on adding decinormal iodine solution no appreciable action took place, showing that the reduction could not be caused by sulphur-dioxide or hydrogen-sulphide. On precipitating with excess of barium-chloride to free from sulphate, filtering off the BaSO₄, and heating with bromine water, a further precipitate of BaSO₄ was formed, showing that some sulphur compound was present. It seemed probable that this was one or more of the polythionic acids—most probably pentathronic acid, because of its greater stability. In order to prove the truth or otherwise of this surmise the water was boiled with mercuric cyanide (Debus, Jour. Chem. Soc., 1888, p. 288). The reaction between mercuric cyanide and pentathionic acid is as follows: Hg(CN)₂ + H₂S₅O₆ = HgS + S₂ + 2SO₃ + 2HCN. If the Hg in the precipitate (HgS + S₂) be determined, the equivalent amount of H in the H₂S₅O₆ can be calculated, and by precipitating the SO₃ in the filtrate as BaSO₄ the oxygen and part of the sulphur can be estimated. The remainder of the sulphur can be determined in the HgS + S₂ precipitate. The results so obtained for H, S, and O approximated roughly to the formula H₂S₅O₆. The oxygen was, however, too high, and it was thought probable that this was due to the oxidizing action of the ferric chloride and hydrogen-chloride present in considerable quantities in the water. Attempts to remove these substances were at first unsuccessful. The iron could not be precipitated by an alkali, since pentathionic acid is at once decomposed in alkaline solution; nor could the hydrogen-chloride be neutralized with a soluble alkali (soda, &c.), since momentary local supersaturation of the acid, with consequent decomposition of pentathionic acid, cannot be avoided. Precipitation of the iron as basic acetate was also tried, but this was not satisfactory. Finally the