414 Allen, Crenshaw, and Merwin — 



with hydrogen sulphide.* Solutions which contained at the 

 start 2 per cent of sulphuric acid were always partially reduced 

 to sulphur after being heated to 300° for one day, while 

 solutions which contained from 5 per cent to 10 per cent sul- 

 phuric acid were further changed to sulphur dioxide under 

 similar conditions. 



C. Distillation of dilute sulphuric acid. — The acidity of 

 the solutions was still more seriously affected by the distillation 

 of acid from the inside to the outside tube. The results of 



Table VI. 

 Shoiving the distillation of dilute sulphuric acid at 300°. 



Inside tube Outside tube 



H 2 S0 4 

 H 2 S0 4 Loss from; taken (to Gain in H 2 S0 4 



H 2 S0 4 from Total H 2 S0 4 inside j decompose H 2 S0 4 outside decomposed 



taken ZnS0 4 H 2 S0 4 found tube NH 4 CNS) found tube by H 2 S 



2-061 g. 0-136 g. 2-197 g. 1-599 g. 0'598g. i 0-677 g. 1-199 g. 0-522 g. 0'076 g. 

 1-603 none 1-603 1-140 0-463 ; 0677 0940 0*263 0-200 



2-061 0-118 2-179 1-516 0-663 ' 0*677 1-086 0409 0-254 



Table YI will make this plain. The quantities of acid taken 

 (Columns 1 and 6) were determined by measuring out the 

 required volume of acid of known strength. The zinc being 

 practically all precipitated, in the experiments quoted, its 

 equivalent in sulphuric acid is added to 1 (Column 2) to give 

 the total acid taken in the inside tube- (Column 3). The acid 

 left in the inside platinum tube was determined by titration of 

 the filtrate from zinc sulphide. It should be stated here that 

 it was ascertained by blank experiment that no sulphuric acid 

 was formed from the thiocyanate in the outside tube. The 

 amount of sulphuric acid in Column 7 was determined by pre- 

 cipitation with barium chloride. A comparison of the results 

 in Column 7, with determinations of the free acid by direct 

 titration, showed that the latter were always lower because 

 presumably, the acid had been partly neutralized by the bases 

 in the glass, which is always considerably attacked under these 

 conditions (300°). The figures in the last column refer to the 

 differences between the losses from the inside tube and the 

 gains in the outside tube. Some of the acid, as we have 

 already learned, is reduced to sulphur and sulphurous acid, a 

 fact which accounts for this. 



*See p. 396. 



