2 Knecht, An Interesting Reaction of Copper Salts. 



of the copper comes out as such, presumably according to 



the scheme : — 



Ti^Og + CuO = 2X10^ + Cu 



but up to the present I have not been able to verify its 

 correctness quantitatively. Failure in this respect has 

 been partly due to the fact that the precipitated copper is 

 so finely divided that it is found impossible to filter it, 

 while on the other hand its intense colour prevents the 

 use of an indicator in titrating back the excess of titanous 

 sulphate. Moreover, even if it were possible to filter the 

 precipitate, another drawback presents itself, inasmuch 

 as the metal is in such a fine state of division that in 

 contact with the air it is rapidly oxydised and is liable to 

 be partly redissolved by the excess of sulphuric acid 

 which must of necessity be present. In using the reaction 

 as a test for small quantities of copper, this phenomenon 

 accounts for the disappearance of the precipitate after the 

 liquid has been left exposed to the air for some length 

 of time. 



With very dilute solutions of copper (one part or less 

 of the metal in 10,000) the reaction with titanous sulphate 

 is not instantaneous, requiring from 2 to 30 minutes for its 

 completion. Under these conditions, it does not in fact 

 show at all at first, but by degrees the solution acquires a 

 dull pink colour, different from that of the titanous sulphate, 

 and only after this the metal makes its appearance. The 

 reaction is best observed by reflected light ; transmitted 

 light is of no use, showing only a bluish-green colour 

 when appreciable quantities of the metal are present, 

 and nothing at all in presence of minute quantities. 

 The reaction can be accelerated by the application of 

 heat, but the precipitated metal is never so bright as when 

 operating on cold solutions. In any case boiling should 

 not be resorted to since this will bring down titanic 



