APPAEATUS 97 



If to one-half of the solution an excess of solution of caustic 

 potash is added and the liquid is heated to boiling, a black 

 precipitate of copper oxide is formed, which is then collected on 

 a paper filter, washed by pouring hot distilled water -over it, 

 then dried completely, and after heating to redness it is weighed. 

 From the known composition of the copper oxide, CuO, the 

 amount of copper is calculated. 



Similarly if a solution of barium chloride is added to the other 

 half of the solution a white precipitate of barium sulphate is 

 produced. If this precipitate is in corresponding fashion collected, 

 washed free from the copper solution, dried, ignited, and weighed, 

 the amount of the sulphion, S0 4 , present in the original solution 

 can be calculated from the weight of barium sulphate obtained in 

 the form of the precipitate, and thus the total weight of copper 

 sulphate in the original solution becomes known. 



In electro-analysis, which in many cases is more rapid and 

 more exact, the metal in a metallic salt is deposited as such by 

 the action of an electric current on a weighed plate or gauze of 

 metal which can be used as the cathode in the process. In 

 modern processes for rapid electro deposition of metals one or 

 other of the electrodes is made to revolve rapidly in the solution 

 under analysis. The advantage of this is obvious from the 

 consideration that in decomposing, say, a solution of a copper 

 salt, the liquid in the neighbourhood of the cathode surface 

 becomes impoverished as the metal is deposited upon it, and 

 unless the liquid is stirred so as continually to bring the metalli- 

 ferous solution into contact with the cathode, the operation 

 occupies a considerable length of time, as the extraction of the 

 last portions of metal is then dependent on convection currents 

 bringing those portions of the solution which still contain metal 

 into contact with the cathode. The electrodes should be as 

 close together as possible, and warming the solution is often an 

 advantage. It appears to be of no importance whether the 

 anode or the cathode be made to revolve, or whether the elec- 

 trodes are stationary, the stirring being accomplished by an 

 independent stirrer. With suitable arrangements the estimation 

 of a metal such as copper, once in solution, can be accomplished 

 in a very short space of time, not exceeding a few minutes. 



The arrangement for the use of rotating electrodes is shown 

 in Fig. 36, p. 68. 



By suitable gradation of the potential used two metals such 



