{the Hydrate of) Sulphuric Acid with Nitric Oxide. 83 



strong dark colouring; the other portion, treated in the same 

 manner, did not exhibit this at all. Had the residue con- 

 tained nitrous acid, nitric acid must have formed on dilution 

 with water, and 1 must have obtained in the boiled liquid a 

 reaction of nitric acid. I have subsequently always tested in 

 this way, and have found it to be the best, when I wished to 

 learn whether nitric oxide or nitrous acid was combined with 

 the sulphuric acid. 



The sulphate of the protoxide of iron is a most excellent 

 reagent for the slightest traces of nitric oxide, nitrous and ni- 

 tric acids ; but it is absolutely essential always to add a con- 

 siderable quantity of pure sulphuric acid, as otherwise small 

 traces might easily be overlooked. 



Nitric oxide and nitrous acid may be very well distinguished 

 in concentrated sulphuric acid from nitric acid, by the addi- 

 tion of a dilute solution of the bichromate of potash, for both 

 reduce the chromic acid to oxide, while they themselves are 

 converted into nitric acid. The liquid is thereby coloured 

 green; it is, however, requisite to add the dilute solution of 

 the bichromate of potash in drops, otherwise the green colour 

 might easily be concealed by the excess of the reagent. Nitric 

 acid cannot naturally act in a reducing manner on the chro- 

 mic acid of the bichromate of potash. A solution of the per- 

 manganate of potash cannot be employed, as this is altered by 

 powerful acids ; but it is a most excellent test, if the sulphuric 

 acid has been previously diluted with about six parts of water. 

 If sulphuric acid be mixed with pure distilled nitric acid, 

 diluted with six parts of water, and, after it has perfectly 

 cooled, some drops of a solution of the permanganate of pot- 

 ash be added to it, the colour is not in the least affected ; but 

 if the residue from the distillation, which, as I have shown to 

 be probable, consists of a solution of the sulphate of nitric 

 oxide in sulphuric acid, be diluted with six parts of water, 

 the solution of the permanganate of potash is decolorated by 

 it. If I heat the diluted liquid for a moment, so that the so- 

 lution of the protosulJ)hate of iron with sulphuric acid still in- 

 dicates the presence of nitric oxide, the solution of the per- 

 manganate of potash is likewise decolorated. If I continue 

 to heat the solution of the protosulphate of iron with sulphuric 

 acid, it no longer occasions any colouring, and a drop of the 

 solution of permanganate of potash is no longer decolorated. 



Wackenroder* advised the addition to the concentrated or 

 diluted sulphuric acid of a solution of the sulphate of the deut- 



• AnncUen dir P/iartnacie, vol. xviii. p. 154. 

 G2 



