430 The Philippine Journal of Science ms 



Some of the most accurate methods for estimating the purity 

 of red lead are based on the direct estimation, either volumetri- 

 cally or gravimetrically, of the lead dioxide thus formed. 



The method of Lux, for instance, consists in merely determin- 

 ing the amount of black lead dioxide (Pb0 2 ) and calculating the 

 percentage of minium (Pb s 4 ) . 



Probably one of the best general methods which has been sug- 

 gested for determining the purity of red lead is that outlined by 

 Baucher. 4 



This consists in determining insoluble impurities, lead dioxide 

 (Pb0 2 ), and free litharge (PbO), and calculating by difference 

 the combined lead monoxide. In this method the free litharge 

 is determined by leaching with a solution of lead acetate, and 

 consequently the results obtained are not especially accurate. 



That these various methods do not give very accurate results is 

 brought out in the following analytical data, in which the per- 

 centage of red lead was determined by treating the sample with 

 nitric acid, filtering off the insoluble lead dioxide, drying, weigh- 

 ing, and calculating as minium. 







Sample A. 









Per cent. 





Insoluble matter 





none 





Red lead (Pb,0«) 





77.44 



(equivalent to 

 27.02 per cent 

 PbO,) 



Free litharge (PbO) 





18.54 





Not accounted for 





3.99 







100.00 





77.44 per cent Pb 3 (X 



is equivalent to 



70.47 per cent Pb 



18.54 per cent PbO is 



equivalent 



to 



17.21 per cent Pb 



Total Pb calculated 



87.68 per cent 



Pb by direct determination 





91.37 per cent 



Pb not accounted for 







3.69 per cent 



The figures show a wide discrepancy between the total lead 

 as calculated and the actual lead content. Soluble lead salts 

 were shown to be absent by leaching the sample with hot water 

 and attempting to precipitate the lead as sulphate. Since the 

 above results were obtained in closely agreeing duplicate deter- 

 minations, it would seem that the error of 3.66 per cent in the 

 calculated total lead content was due to errors in method both 

 in analysis and calculation. Probably the actual percentage of 



4 Petit Monit. Pharm., 47, 2651. 



