106 NON-METALS AND THEIR COMBINATIONS. 



3. Sulphates, sulphur, or any compound containing it, fused on 

 charcoal with sodium carbonate and potassium cyanide by means of 

 a blowpipe, form hepar (chiefly sulphide of an alkali metal), which, 

 when placed upon a silver coin and moistened with dilute hydro- 

 chloric acid, causes a black stain, due to the formation of silver sul- 

 phide. This test is of value in the case of insoluble sulphates, such 

 as barium sulphate and others. 



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

 acid. 



Sulpho-acids. Whilst but two oxides of sulphur exist in the 

 separate state, there are a large number of sulpho-acids known. 

 They are : 



Hydrosulphuric acid, H 2 S. 

 Hyposulphurous acid, H 2 SO 2 . 

 Sulphurous acid, H 2 SO 3 . 

 Sulphuric acid, H 2 SO 4 . 



Pyrosulphuric acid, H 2 S 2 O 7 . 

 Thiosulphuric acid, H 2 S 2 O 3 . 

 Dithionic acid, H 2 S 2 O 6 . 



Trithionic acid, H 2 S 3 O 6 



Tetrathionic acid, H 2 S 4 O 6 . 

 Pentathionic acid, H 2 S 5 O 6 - 



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

 acid, Nordhausen oil of vitrol). 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. 



Thiosulphuric acid, formerly Hyposulphurous acid, H 2 S 2 O 3 , is 

 of interest, because some of its salts are used, as, for instance, sodium 

 thiosulphate, Na 2 S 2 O 3 , the sodium hyposulphite of the U. S. P. The 

 acid itself is not known in the separate state, since it decomposes into 

 sulphur and sulphurous acid when attempts are made to liberate it 

 from its salts. 



Tests for thiosulphates. 

 (A solution of sodium thiosulphate, Na 2 S 2 O 3> may be used ) 



1. Thiosulphates liberate with sulphuric or hydrochloric acid sul- 

 phur dioxide, while sulphur is set free : 



Na 2 S 2 O 3 + H 2 S0 4 = Na 2 S0 4 + H 2 O -f SO 2 -f S. 



