SULPHUR. 213 



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

 SO 2 .SH.OH, 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 

 commerce. 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. 



Sulphuric is the most stable acid of sulphur, and all the others have a tend- 

 ency to pass to this acid. It is for this reason that both sulphites and thiosul- 

 phates are good reducing agents. A solution of a thiosulphate, when added 

 to an acidified solution of potassium permanganate or dichromate, acts in the 

 same way as a sulphite does. The essential reaction is : 



Na 2 S 2 O 3 + 4O + H 2 O = Na 2 S0 4 + H 2 S0 4 . 



Thiosulphates also react with the halogen elements in the manner shown by 

 this reaction : 



2Na 2 S 2 O 3 + 21 = Na 2 S 4 6 + 2NaI, 



forming sodium tetrathionate and sodium iodide. This reaction is used for the 

 quantitative estimation of free iodine and in the preparation of so-called de- 

 colorized tincture of iodine. It may also be used for removing iodine stains 

 from the skin or fabrics. 



Most of the thiosulphates are soluble in water, those of barium, lead, and 

 silver being only very sparingly soluble. Alkali thiosulphates have a marked 

 solvent action on many salts that are insoluble in water, forming double thio- 

 sulphates. All thiosulphates are decomposed by acids. 



Tests for thiosulphates. 

 (Use about a 5 per cent, solution of sodium thiosulphate. ) 



1. The solution, upon addition of dilute sulphuric or hydrochloric 

 acid, liberates sulphur dioxide, while sulphur is precipitated more or 

 less rapidly, according to the concentration and temperature. The 

 formation of these two products is characteristic and the test is suffi- 

 cient for recognition of a thiosulphate. The precipitate of sulphur 

 distinguishes it from a sulphite. 



2. Addition of silver nitrate gives a white precipitate of silver 

 thiosulphate, Ag 2 S 2 O 3 , which is immediately dissolved if the sodium 

 thiosulphate is in excess. The precipitate is rather unstable, and 

 decomposes on standing, and more rapidly on heating, giving black 

 silver sulphide and sulphuric acid, 



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



Addition of solution of lead nitrate or acetate to thiosulphate solution 

 gives similar results. Most thiosulphates are unstable, like those of 

 silver and lead. 



