Maclauiun. — Pentdfltioiuc Acid i/i Xafurnl Wafers. 



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



Waters. 



By J. 8. Maclaurin, D.Sc. 



[Bead hefore the Wellington Philosophical Societi/, ■')th October, 1910.] 



Pentathionic acid is readily formed by passing hydrogen-sulphide into a 

 solution of sulphur-dioxide, hut, 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 dis- 

 covery 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 re- 

 quire, 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 

 <-aused by sulphur-dioxide or hydrogen-sulphide. On precipitating with 

 excess of barium-chloride to free from sulphate, filtering off the BaS04, 

 and heating with bromine water, a further precipitate of BaSO^ was formed, 

 showing that some sulphur compound was present. It seemed probable 

 that this was one or more of the polythionic acids — most probably penta- 

 thionic 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 + 1^2850^ = HgS + 83 

 + 2SO3 + 2HCN. If the Hg in the precipitate (HgS -l- 83) be determined^ 

 the equivalent amount of H in the H^SgOg can be calculated, and by precipi- 

 tating the SO3 in the filtrate as BaSO^ the oxygen and part of the sulphur 

 <-an be estimated. The remainder of the sulphur can be determined in 

 the HgS -f- 82 precipitate. The results so obtained for H, 8, and ap- 

 proximated roughly to the formula HgSgOg. 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 decomposi- 

 tion of pentathionic acid, cannot be avoided. Precipitatioii of the iron as 

 basic acetate was also tried, but this was not satisfactory. Finally the 



