208 NON-METALS AND THEIR COMBINATIONS. 



Tests for sulphurous acid and sulphites. 

 (Sodium sulphite, Na,SO 3 .7H 2 O, may be used.) 



1. Add sulphur dioxide gas, or a solution of it in water, or a solu- 

 tion of a sulphite to an acidified solution of potassium permanganate. 

 The latter is decolorized, due to its giving up oxygen, which oxidizes 

 the sulphurous acid, thus : 



H,S0 3 + O H 2 S0 4 . 



Make the same experiment with acidified solution of potassium 

 dichromate. The same kind of change takes place, but the decom- 

 position products of the dichromate are green. The reaction will be 

 understood when chromium is studied. 



2. Add a few drops of a solution of sulphurous acid or of a sul- 

 phite to a tube containing some zinc and dilute sulphuric acid 

 (nascent hydrogen). Hydrogen sulphide is liberated, which can be 

 detected by the odor, or a piece of filter-paper, wet with solution of 

 lead acetate, which blackens when held in the mouth of the tube : 



H 2 S0 3 + 6H H 2 S + 3H 2 O. 



3. Add to a few drops of the sulphite solution about 2 c.c. of silver 

 nitrate solution. A white precipitate of silver sulphite is formed, 

 which darkens on heating, due to reduction to metallic silver : 



Ag 2 S0 3 + H 2 2Ag + H 2 S0 4 . 



Silver sulphite is soluble in an excess of the sulphite solution. 



4. A strip of filter-paper, moistened with mercurous nitrate solu- 

 tion, turns black when suspended in sulphur dioxide, due to reduction 

 to metallic mercury : 



2HgN0 3 + 2H 2 + S0 2 = 2Hg + H 2 SO 4 + 2HNO 3 . 



Tests 1 and 4, along with the odor of sulphur dioxide, are usually suf- 

 ficient to recognize sulphurous acid or sulphites. 



Sulphur trioxide, SO 3 (Sulphuric acid anhydride). This is 

 a white, silk-like solid substance, having a powerful affinity for 

 water; it may be obtained by the action of phosphoric oxide on 

 strong sulphuric acid, or by passing sulphur dioxide and oxygen 

 together over heated platinum-sponge. It is now made on the large 

 scale by the latter method for producing fuming sulphuric acid. 



Sulphuric acid, Acidum sulphuricum, H 2 SO 4 , SO 2 (OH) 2 ^ 97.35 



(0 if of vitriol). There is no other acid, and perhaps no other sub- 



