TIN. 363 



Metastannic acid is formed when tin is treated with concentrated 

 nitric acid. It is a white powder insoluble in water and acids, hut 

 seems to have the same composition as stannic acid. It forms salts 

 with alkalies which are entirely different in properties and composition 

 from the stannates, and are known as meta stannates. Two sodium salts 

 are known, Na.^Sn^Oj! and Na 2 Sn 9 O 19 . When the acid is heated, 

 stannic oxide, SnO 2 , is formed. 



Stannous chloride, SnCl 2 (Protochloride of tin). Obtained by 

 dissolving tin in hydrochloric acid by the aid of heat : 

 Sn + 2HC1 = SnCl 2 + 2H. 



Sufficiently evaporated, the solution yields crystals of the composi- 

 tion SnCl 2 .2H 2 O. Stannous chloride is a strong deoxidizing agent, 

 frequently used as a reagent for arsenic, mercury, and gold, which 

 metals are precipitated from their solutions in the metallic state. It 

 is used also in calico printing. 



Stannic chloride, SnCl 4 (Perchloride of tin). Stannous chloride 

 may be converted into stannic chloride either by passing chlorine 

 through its solution or by heating with hydrochloric and nitric acids. 



Tests for tin. 

 (Stannous chloride, SnCl. 2 , and stannic chloride, SnCl 4 , may be used.) 



1 . Add hydrogen sulphide to solution of a stannous salt : brown 

 stannous sulphide is precipitated (Plate V., 7) : 



SnCl 2 + -H 2 S 2HC1 + SnS. 



The precipitate is soluble in yellow ammonium sulphide. 



2. Add hydrogen sulphide to a solution of a stannic salt : yellow 

 stannic sulphide is precipitated (Plate V., 8) : 



SnCl 4 + 4H 2 S = 4HC1 -f SnS 2 . 



The precipitate is soluble in ammonium sulphide. 



3. Sodium or potassium hydroxide added to a stannous salt pro- 

 duces a white precipitate of stannous hydroxide, Sn(OH) 2 . The same 

 reagents added to a stannic salt produce white stannic acid, H 2 SnO 3 . 

 Both precipitates are soluble in excess of the alkali, forming stannite, 

 Na 2 SnO 2 , and stannate, Na 2 SnO 3 . 



Gold, Au = 195.7 (Aurum). Gold occurs in nature chiefly in the 

 free state, generally associated with silver, copper, and possibly with 



