1496 Journal of Applied Microscopy 



avoid their volatilization. Convert into chlorides by evaporating repeatedly 

 with hydrochloric acid. When perfectly dry, the flame is moved nearer and 

 nearer the substance and a glass slide bearing on its upper side a drop of 

 water** is held directly over the substance being tested. If a sublimate results 

 it may consist of ZnCl2, CdC^, BiClg, PbCU, FeCl.^, CuCU, and perhaps 

 HgClg. Any arsenic, antimony and probably all the mercury which might have 

 been present will have been driven off in the ignition of the nitrates to oxides. 



Dissolve the sublimate in a drop of water and test with ammonium mercuric 

 sulphocyanate. In the event of the sublimate being complex, add water to 

 remove the bismuth. Decant. Remove the lead by sulphuric acid. Add 

 sodium hydroxide, cadmium and iron are precipitated and zinc passes into solu- 

 tion. Dissolve the precipitate in dilute acid and test for cadmium. To the 

 sodium hydroxide solution add ammonium carbonate and examine the prepara- 

 tion for the double carbonate of zinc and sodium. 



When much copper is present it is always best to first remove it by placing a 

 drop of a solution of the substance on metallic iron, thus causing it to separate. 



B. The oxides of zinc and cadmium can be sublimed on charcoal before the 

 blowpipe. Make a slight depression in the carbon, place in it a moderate quan- 

 tity of the material, moisten with water and heat gently till dry, then strongly 

 with the oxidizing flame, holding the coal at an angle. Zinc oxide forms a coat- 

 ing on the charcoal, yellow while hot, pure white when cold. Cadmium oxide 

 yields a brown variegated coating. The film on the charcoal is removed by 

 carefully scraping with the spatula, transferred to a slide, heated with dilute acid 

 and the clear liquid drawn off from the residue of carbon. The solution is then 

 tested for zinc and cadmium. 



In addition to zinc and cadmium, films on charcoal are given by antimony, 

 tin, bismuth, lead, and rarely by arsenic and mercury. 



Removal of Phosphates. 



If phosphates are present in the substance to be tested, it is often necessary 

 to first remove them before any reliable tests can be obtained for magnesium 

 and glucinum. One of the simplest methods of procedure is as follows : 



Add strong nitric acid, then a few small strips of thin, pure tin foil or a little 

 powdered tin. Boil until all the tin has been converted into the oxide and 

 phosphate ; draw off or filter, and repeat the process until no test for phosphoric 

 acid is obtained with ammonium molybdate. The solution is now evaporated to 

 dryness, and if arsenates are also present, the residue is ignited to drive off any 

 arsenous acid which may have resulted from any reducing action of the metallic 

 tin. The residue is extracted with water acidified with nitric acid and tested as 

 given below. 



The removal of phosphoric acid by metallic tin is simple, expeditious and 

 satisfactory. Only a few cautions are necessary. The nitric acid must be 

 strong acid, the size of the drop, and the amount of tin added must correspond 



** This Journal, III, pp. 857-858. 



