Browning and Howe — Detection of Sulphides, etc. 317 



Art. XXXI. — On the Detection of Sulphides, Sulphates, Sul- 

 phites and Thiosulp hates in the presence of each other ; 

 by Philip E. Browning and Ernest Howe.. 



[Contributions from the Kent Chemical Laboratory of Yale University — LXXIV.] 



Some three years ago R. Greig Smith* published a method 

 for the detection of sulphates, sulphites and thiosulphates in 

 the presence of each other, which promised much toward the 

 solution of this most difficult problem. The method may best 

 be described in the author's own language : To a solution of 

 the salts of the above mentioned acids " barium chloride is 

 added in excess, together with a good quantity of ammonium 

 chloride, which, like many salts of ammonium, potassium and 

 calcium, acts as' a flocculent or coagulant, and facilitates the 

 filtration of the barium sulphate. Hydrochloric acid is next 

 added, drop by drop, until it is evident that there is no further 

 solution of barium sulphite and thiosulphate, and that only the 

 sulphate remains undissolved ; the solution is then filtered 

 through a moistened double filter paper, which should be free 

 from ' pin holes.' The filtrate will probably be clear, but if 

 not it should be returned to the filter for a second filtration. 

 When too much thiosulphuric acid is present, the clear filtrate 

 will visibly become clouded, or from being whitish will be- 

 come more opaque; if this occurs the solution should be 

 thrown out, and a fresh portion made more dilute. A solu- 

 tion of iodine is added to half of the filtrate until the color is 

 of a permanent yellow tinge ; a white precipitate indicates the 

 presence of a sulphite which has been oxidized by the iodine 

 to sulphate. In the absence of a decided precipitate traces of 

 sulphite may easily be detected by comparing the treated and 

 untreated halves of the filtrate — a procedure which very often 

 saves a good deal of time, as it is unnecessary to wait until a 

 clear filtrate is obtained. The two halves are mixed, and if 

 the yellow color disappears more iodine is added, the solution 

 filtered and the filtrate divided into two halves as before. 

 With a slight turbidity filtration may be omitted. Bromine 

 water is added to one of the halves when any thiosulphate in 

 the original solution shows itself as a white precipitate of 

 barium sulphate, readily seen on comparing the two test tubes. 

 The thiosulphate is by iodine converted to tetrathionate, which 

 is oxidized by bromine water to sulphate." Three objections 

 to this method as described will readily occur to the reader : 

 first, the readiness with which the thiosulphate is decomposed 

 by free hydrochloric acid ; second, the comparatively large 



* Chem. News, vol. lxxii, 39. 



Am. Jour. Sci.— Fourth Series, Yol. YI, No. 34.— October, 1898. 

 22 



