WHITE PIGMENTS. 27 



(b) ANALYSIS OF LITHOPONE IN TUB PUKSENCE OF FOREIGN SUBSTANCES 



(COPALLE). 



Copalle proposes the following method of analysis : 



(1) SOLUBLE SALTS. 



Wash 2 grams with hot water and determine the nature of the soluble salts. 



(2) MOISTURE. 

 Determine on 2 grams the loss in weight on a steam bath. 



(3) INSOLUBLE. 



Oxidize 1 gram with nitric acid of 40 B. (1.38 specific gravity), at first 

 eold, then hot. Then add hydrochloric acid, evaporate to very small volume, 

 dilute with hot water, filter, ignite the precipitate which represents the barium 

 sulphate corresponding to the total barium. If the insoluble exceeds 66 to 68 

 per cent it is necessary to prove that the excess is not due to the addition of 

 kaolin. 



(4) TOTAL ZINC. 



Determine as oxid by precipitation as carbonate in the filtrate from the insol- 

 uble. When more than traces of iron, alumina, or lime are present, it is best to 

 determine the zinc volumetrically. 



(5) SULPHID OF ZINC. 



Add a slight excess of hydrochloric acid to the filtrate from the zinc car- 

 bonate and determine the sulphur by precipitation in the usual manner. This 

 sulphur multiplied by 3.03, or the weight of barium sulphate (BaSO 4 ) multiplied 

 by 0.4603, gives the zinc sulphid. 



(6) OXID OF ZINC. 



Multiply the weight of the zinc sulphid by 0.835 to obtain the zinc oxid cor- 

 responding to the sulphid. Subtract this from the total zinc oxid and report 

 the remainder as zinc oxid (it may be present as oxid or as carbonate). 



(7) BARIUM CARBONATE. 



Digest 2 grams with boiling dilute hydrochloric acid, dilute with hot water, 

 filter from the insoluble and determine the barium in the filtrate by precipita- 

 tion with sulphuric acid. The weight of the barium sulphate multiplied by 

 0.845 gives the barium soluble in the acid calculated as carbonate. 



(8) BARIUM SULPHATE. 



Subtract the barium sulphate corresponding to the carbonate from the total 

 barium sulphate. 



6. General Scheme of Analysis for a White Pigment. 



The pigment may contain white lead, lead sulphate, zinc oxid, baryta, whiting, 

 kaolin, or other silicates, and, in fact, any mixture of white pigments, except 

 lead compounds and zinc sulphid together, as such a mixture has a tendency to 

 blacken. 



Ann. chim. anal, appl., 1907, 12:62-65. 



