508 



Nitric acid gives the insoluble, white meta-stannic acid 

 4HN0 3 + Sn -> H 2 Sn0 3 j + 4NO 2 + H 2 O. 



Uses. Tin plate, used in making "tin" cans, is produced by 

 dipping cleaned sheets of mild steel in melted tin. So long as the 

 layer of tin remains intact, the iron is protected from rusting. 

 But, if the tin layer is damaged, the iron rusts. The iron being 

 the more active metal of the two, the tin acts as a contact agent 

 and actually hastens the rusting. 



Tin is sufficiently valuable to render the detinning of scrap 

 tin plate from can factories, bearing 3 to 5 per cent of tin, a pay- 

 ing process. In the Goldschmidt process the scrap is cleaned, 

 dried, and exposed to dry chlorine, which converts the tin into the 

 liquid stannic chloride SnCl 4 , but leaves the iron unaffected. The 

 chloride is used in mordanting. 



Copper vessels for cooking and brass pins are also coated with 

 tin, to preserve them from the action of air and moisture. Tin 

 pipes are used where lead would be unsafe, as, for example, for 

 beverages. 



Tin enters into many useful alloys, such as bronze (with copper) 

 and solder. 



Compounds of Tin. Stannous chloride SnCl 2 ,2H 2 O, "tin 

 crystals/' and a hydrate of stannic chloride SnCl4,5H 2 are used 

 in mordanting (p. 475). With soda the former gives stannous 

 hydroxide Sn(OH) 2 and the latter stannic acid Sn(OH) 4 , both 

 gelatinous substances which are precipitated in the goods to be 

 dyed. Stannic acid is also sometimes precipitated in flannelette 

 (a material made of cotton) to render it non-inflammable, and 

 always in silk to " weight " it (the increase may be from 25 per 

 cent to 300 per cent or more). 



Stannous sulphide SnS (brown) and stannic sulphide SnS 2 

 (yellow) are precipitated by hydrogen sulphide, even from acid 

 solutions of stannous and stannic salts, respectively. 



