THIONURIf ACID. 53 



SECTION JX. OF TIIIONURIC ACID. 



This remarkable acid was discovered by Wohler and Liebig 

 during their researches on uric acid in 1838.* 



If we add sulphurous acid to a cold saturated solution of allox- 

 ane, it loses its smell. When to such a solution, containing a 

 slight excess of sulphurous acid, we add as much ammonia as will 

 saturate the acid, heat the mixture, and keep it boiling for a 

 short time, it deposites on cooling a considerable quantity of 

 brilliant quadrangular plates. The best method of preparing 

 this substance on a large scale, is to take sulphate of ammonia 

 previously mixed with an excess of carbonate of ammonia, to 

 add to it a solution of alloxane, to raise the mixture to the boil- 

 ing point, and keep it boiling for half an hour. The salt thus 

 obtained is a combination of thionuric acid and ammonia. When 

 dry, it is in thin plates having a strong pearly lustre, soluble in 

 water and again crystallizable without any other alteration than 

 the assumption of a red colour. At 212 3 its loses its water and 

 becomes rose red. 



Dr Gregory of Aberdeen has given the following process as 

 the easiest for preparing thionurate of ammonia. Take a 

 pretty strong cold solution of alloxan, add to it half its volume 

 of a strong solution of sulphite of ammonia, with a little free am- 

 monia, boil for five minutes. On cooling, a large quantity of 

 thionurate of ammonia is deposited in beautiful silvery scales. 

 They are to be slightly washed and dried by pressure. 



If we raise the aqueous solution of this salt to the boiling 

 temperature, and pour into it a solution of acetate of lead, a gela- 

 tinous precipitate falls, which, on cooling, assumes the form of fine 

 needles, arranged concentrically, and having sometimes a white, 

 sometimes a red colour, This is thionurate of lead. By mixing it 

 with water and passing a current of sulphuretted hydrogen gas 

 through the mixture, the lead is separated, while the acid dis- 

 solves in the water. On evaporating the aqueous solution in a 

 gentle heat, the acid is deposited white and crystalline, though 

 the shape of the crystals cannot be determined. 



Thionuric acid does not absorb moisture from the atmosphere. 

 It has a decidedly sour taste, and reddens vegetable blues. 

 When we boil its aqueous solution, the acid is decomposed, being 



* Ann. de Chiin. et de Phys. Ixviii. 253. 



