512 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



At o KNO 3 is least soluble, at 100 NaCl ; the action may 

 be driven across completely from L to R by crystallising out 

 these salts alternately at o and at 100. 



In quantitative analysis these balanced actions are used to 

 secure complete precipitation of Ag or Cl as AgCl, Ba or SO 4 

 as BaSO 4 , &c. 



In qualitative analysts the balance of actions between 

 neutral salts may be used to secure a broad separation of those 

 metals which form 



(1) Insoluble chlorides: Ag', Pb", Hg' 



(2) Insoluble sulphides : 



(a] Cu", Pb", Hg", Bi" (*) [Fe'"], [Cr'"], [Al'"] 

 () Sn", Sn"", Sb'", As'" (d) Zn", Mn", Ni", Co". 



(3) Insoluble carbonates : (a) Ca", Sr", Ba" (V) Mg' 



(4) Insoluble platinichlorides : K', NH 4 '. 



In class (i), since hydrochloric acid is one of the strongest 

 acids, it may be used instead of a neutral chloride ; thus in the 

 action 



the nitric acid formed as a second product is not strong enough 

 to redissolve the insoluble chloride or to prevent its precipitation. 

 In class (2), however, the use of sulphuretted hydrogen in place 

 of a neutral sulphide tends to liberate a strong acid ; this 

 prevents the separation of the insoluble sulphides of the metals 

 (2f) and (2rtT), but those of the metals (20) and (zb) are 

 precipitated even in presence of much acid, thus : 



CuSO 4 +H 2 S -> CuS + H 2 SO 4 



(precipitated) 



ZnSO 4 + H 2 S <-ZnS + H 2 SO 4 . 



(dissolves) 



The further separation of the metals (20) and (ib] depends 

 on the fact that the latter dissolve in alkaline sulphides to form 

 soluble sulpho-salts, such as Na 3 AsS 4 , Na 3 SbS 4 , &c. The 

 separation of the metals (2?) and (2(f) depends on the fact that 

 the tervalent hydroxides, Fe(OH) 3 , Cr(OH) 3 , A1(OH) 3 , are ex- 

 tremely weak bases, which do not form carbonates or sulphides, 

 and are therefore precipitated in the free state when soluble 

 carbonates or sulphides are added ; in qualitative analysis they 

 are separated by the addition of a mixture of ammonia and 



