318 Browning and Howe — Detection of Sulphides, etc. 



amount of acid necessary to effect the complete solution of the 

 barium sulphite and thiosulphate when precipitated with the 

 sulphate as compared with the amount required to prevent the 

 precipitation ; third, the lack of delicacy necessitated by a com- 

 parison of portions of a colored solution in looking for small 

 precipitates. The work to be described was undertaken to 

 overcome these difficulties and to test the accuracy of a modi- 

 fied method. Solutions of potassium sulphite and sodium thio- 

 sulphate were made approximately decinormal and standard- 

 ized in the usual manner against an iodine solution of known 

 value. It was found that by making a solution containing 

 sulphates, sulphites and thiosulphates very faintly acid, the 

 sulphates and thiosulphates were held completely in solution 

 when the barium sulphate was precipitated. The extreme 

 sensitiveness of a thiosulphate to the decomposing action of 

 free hydrochloric acid suggested the possible substitution of 

 acetic acid to hold the sulphites and thiosulphates in solution. 

 This being a weaker acid, we hoped to avoid the decomposi- 

 tion of the thiosulphate into sulphur and sulphurous acid, or at 

 least to delay the decomposing action. The results of these 

 experiments appear in the following table : 



Result. 



No sulphur in 20 minutes. 

 Sulphur in 45 seconds. 

 Sulphur in 15 minutes. 

 No sulphur in 20 minutes. 

 Sulphur in 90 seconds. 

 No sulphur in 20 minutes. 

 Sulphur in 15 minutes. 

 Sulphur in 60 seconds. 

 Sulphur in 30 seconds. 



From these results it would seem that the decomposition of 

 a thiosulphate is more rapid in presence of hydrochloric acid 

 than in presence of a much larger amount of acetic acid. 



Our next experiments were directed toward a determination 

 of the effect of adding stannous chloride to bleach the color 

 of the free iodine and bromine used in the oxidation and of 

 acidifying with acetic acid, before treating with barium chlor- 

 ide. That is to say, the process as we used it, consisted in 

 acidifying the solution to be tested with acetic acid, adding 

 barium chloride, filtering to remove precipitated sulphate 

 (always present in the sulphite), adding iodine to the filtrate 

 until the color was permanent, bleaching with stannous chlor- 











Table I. 





Volume 



Hydrochloric 



Acetic 



Na 2 S 2 3 





cm 3 of 



acid (1 : 



4) 



acid. 



taken. 





water. 



drops. 





drops. 



grm. 



1 



10 



2 





_. 



0-01 



2 



10 



2 





.. 



o-i 



3 



100 



3 









o-i 



4 



10 



_ _ 





8 



o-oi 



5 



10 







8 



o-i 



6 



100 



.. 





10 



o-i 



1 



100 



.. 





10 



0-25 



8 



100 









10 



0-5 



9 



100 



.. 





10 



l-o 



