212 NON-METALS AND THEIR COMBINATIONS. 



Usually this test alone is sufficient for recognition, as all other 

 ordinary barium salts are soluble in hydrochloric or nitric acid. It 

 is very delicate. 



2. When a solution of lead nitrate or acetate is employed, a 

 white precipitate of lead sulphate, PbSO 4 is obtained. This is solu- 

 ble in a solution of ammonium acetate. 



3. Grind together in a mortar a knife-pointful of a sulphate, sul- 

 phur, or any compound containing it, with 5 to 10 times its bulk of 

 sodium carbonate and about 3 times its bulk of potassium cyanide. 

 Place the mixture in a hole in a piece of charcoal and heat with the 

 blow-pipe flame until it is thoroughly fused. The mass now contains 

 yellowish-brown alkali sulphide (hepar), due to reduction of the sul- 

 phate by the hot charcoal and potassium cyanide. The sodium car- 

 bonate serves as &flux, or fusing material. 



Remove the mass, place it upon a silver coin, and moisten it with 

 dilute hydrochloric acid. A black stain of silver sulphide will be 

 formed. This test is of value in the case of insoluble sulphates and 

 sulphides. 



The above procedure is known as the charcoal reduction test, and is 

 one of the steps taken in systematic qualitative analysis. 



Antidotes. Magnesia, sodium carbonate, chalk, and soap, to neutralize 

 the acid. 



Acids of sulphur. While but four oxides of sulphur exist in the 

 separate state, there are a large number of acids containing sulphur, 

 some of which, however, are known only as constituents of the 

 respective salts. The acids are : 



Hyposulphurous acid, H 2 S 2 O 4 . Thiosulphuric acid, H 2 S a O s . 



Sulphurous acid, H 2 SO 3 . Dithiouic acid, H 2 S 2 O 6 . 



Sulphuric acid, H 2 SO 4 . Trithionic acid, H 2 S 3 O 6 . 



Pyre-sulphuric acid, H 2 S 2 O 7 . Tetrathionic acid, H 2 S 4 O 6 . 



Persulphuric acid, H 2 S 2 O 8 . Pentathionic acid, H 2 S 5 O 6 . 



Hydrogen sulphide, H 2 S. 



Pyrosulphuric acid, H 2 S 2 O 7 (Disulphuric acid, fuming sulphuric 

 acid, Nordhausen oil of vitriol). This acid is made by passing sulphur 

 trioxide (obtained by heating ferrous sulphate) into sulphuric acid, 

 when direct combination takes place : 



H 2 S0 4 + S0 3 = H 2 S 2 7 . 



It is a thick, highly corrosive liquid, which gives off dense fumes 

 when exposed to the air, and decomposes readily into sulphur trioxide 

 and sulphuric acid when heated. 



