SULPHUR. 105 



point is reached the acid acts upon the lead, wherefore the further 

 concentration is conducted in vessels of glass or platinum, until a 

 specific gravity of 1.84 is obtained. This acid contains about 95 

 per cent, of sulphuric acid ; the remaining 5 per cent, of water can- 

 not be expelled by heat. 



Properties of sulphuric acid. Pure acid has a specific gravity of 

 1.848 ; it is a colorless liquid, of oily consistence, boiling at 338 C- 

 (640 F.). It has a great tendency to combine with water, absorbing 

 it readily from atmospheric air. I Upon mixing sulphuric acid and 

 water, heat is generated in consequence of the combination taking 

 place between the two substances.) To the same tendency of sulphuric 

 acid to combine with water must be ascribed its property of destroy- 

 ing and blackening organic matter. Organic substances generally 

 contain the elements carbon, hydrogen, and oxygen. Sulphuric acid 

 added to such organic substances removes the elements hydrogen and 

 oxygen (or at least a portion of them), combines them into water, 

 with which it unites, leaving behind compounds so rich in carbon 

 that the black color predominates. It is due to this decomposing 

 action of sulphuric acid upon organic matter that traces of the latter 

 color sulphuric acid dark yellow, brown, and, when present in larger 

 quantities, almost black. The poisonous caustic properties are due 

 to the same action. 



Sulphuric acid is a very strong dibasic acid, which expels or dis- 

 places most other acids ; its salts are known as sulphates. 



The sulphuric acid of the U. S. P. should contain not less than 

 92.5 per cent, of H 2 SO 4 , corresponding to a specific gravity of not 

 less than 1.835. 



The diluted sulphuric add, Acidum sulphuricum dilutum, is a mix- 

 ture of 100 parts by weight of acid and 825 parts of water, or of 

 about 60 c.c. of acid and 900 c.c. of water. 



Tests for sulphuric acid and sulphates. 



(Sodium sulphate, Na 2 SO 4 , may be used.) 



1. Barium chloride produces a white precipitate of barium sulphate, 

 insoluble in all acids : 



Na^ -f BaCl 3 = BaSO 4 + 2NaCl. 



2. Soluble lead salts (lead acetate) produce a white precipitate of 

 lead sulphate, slightly soluble in hot concentrated acids and in 

 ammonium acetate. 



