TRANSACTIONS OF THE SECTIONS. 



53 



Table III. 



In comparing the results contained in this Table with those of the previous ones, 

 it will be noticed that zinc is more easily precipitated from its combination with chlo- 

 rine, and in presence of an excess of hj'drochloric acid, than when it is combined with 

 sulphui-ic acid. Still, in either case, and even in presence of a very large excess of 

 acid, zinc is precipitated, and in many cases completely. 



Before undertaking a series of experiments to discover a new method of separating 

 quantitatively zinc and copper, the author thought it advisable to examine the various 

 processes which have been proposed of late years, and these are the results : — 



He first made a series of experiments with a process which has been recommended 

 by Messrs. Rivot and Bouquet, and which consists in adding an excess of ammonia 

 to an acid liquor containing the above tveo metals, and then adding caustic potash in 

 slight excess. The liquor is to be heated to 158° Fahr. until the whole of the ammo- 

 nia is expelled, the copper being thrown down in the state of black oxide, whilst the 

 oxide of zinc remains in solution ; but Mr. Calvert has always found, even in employing 

 diluted liquors and a very slight excess of potash, that a certain proportion of hydrate 

 of oxide of zinc, dissolved in the caustic potash, was dehydrated, became insoluble, and 

 precipitated with the oxide of copper, thereby increasing its relative proportion, and 

 rendering the results incorrect. 



The two methods having failed in his hands, although he had taken all the 

 necessary precautions recommended to carry out those processes successively, he next 

 had recourse to the methods proposed by M. Flajolot. The first consists in adding 

 to a boiling solution of zinc and copper, rendered slightly acid by sulphuric acid, 

 hyposulphite of soda, until no more black pvotosulphuret of copper precipitates, filter- 

 ing, and determining the copper by oxidizing the sulphuret with nitric acid in the 

 usual way, and throwing down the copper. The zinc is precipitated with carbonate 

 of soda. The second process given by this chemist consists in estimating the copper 

 by precipitating it in the state of protoiodide by a solution of iodine in sulphurous 

 acid*. 



Both these processes of M. Flajolot gave very satisfactory results, and can be 

 adopted when a complete analysis of an alloy of zinc and copper is required ; but as these 

 methods require too much time when rapid analyses are desired, the author next tried 

 M. Pelouze's method, which consists in rendering the liquor containing salts of zino 

 and copper alkaline with an excess of ammonia, and pouring very gradually into it a 

 standard solution of monosulphuret of sodium, which first precipitates all the copper 

 as black sulphuret, leaving the zinc in solution. As this latter metal yields a white 

 sulphuret, it is easy to ascertain when all the copper is precipitated. This method is 



* For further details see ' Chemist,' vol. i. p. 411. 



