434 The Philippine Journal of Science mi 



dioxide. Fifteen cubic centimeters of a cane-sugar solution, 

 prepared by dissolving 1 part of cane sugar in 3 parts of 

 water, are slowly added. On boiling a short time, the lead 

 dioxide is reduced to lead monoxide and dissolves in the nitric 

 acid. The liquid is filtered through a Gooch crucible, and the 

 insoluble residue is weighed. In red lead of good quality there 

 is usually less than 1 per cent of insoluble matter. 



b. Free litharge. — The free lead monoxide is best determined 

 by the method of Lowe. 9 This method gives excellent results 

 when carried out in the following manner: A 0.5-gram sample 

 is treated with 100 cubic centimeters of 10 per cent lead nitrate 

 solution. The mixture is digested on a water bath about one 

 hour, after which it is boiled ten minutes, diluted to twice its 

 volume with hot water to dissolve basic salts, and filtered through 

 a Gooch crucible. The residue is then weighed. 



c. (PbO z + insoluble matter) and (PbO free + PbO com- 

 bined) . — A 0.5-gram sample is treated with 100 cubic centimeters 

 of 10 per cent nitric acid. After heating on a water bath about 

 one-half hour, all the red lead is decomposed to the black lead 

 dioxide ; the liquid is then filtered through a Gooch crucible, and 

 the residue of lead dioxide and insoluble matter is weighed. The 

 percentage of insoluble matter having already been ascertained, 

 the difference gives the percentage of lead dioxide. 



To the filtrate add 20 cubic centimeters of sulphuric acid (1 

 part concentrated H 2 S0 4 to 5 parts H 2 0), evaporate until white 

 fumes are evolved, cool, and add carefully 40 cubic centimeters 

 of water. Now add 100 cubic centimeters of 75 per cent alcohol, 

 stir, and, after the precipitate has settled, filter through a Gooch 

 crucible, dry at 100°, and weigh. If there is considerable in- 

 soluble matter present, the precipitated lead sulphate is probably 

 contaminated with foreign matter. In this case more accurate 

 results are obtained by dissolving the lead sulphate in ammonium 

 acetate and reprecipitating. 



From the weight of lead sulphate obtained, the percentage of 

 (PbO free + PbO combined) is calculated. Having previously 

 determined the percentage of free lead monoxide, the percentage 

 of combined lead monoxide is obtained by difference. 



The percentage of lead dioxide plus the percentage of combined 

 lead monoxide gives the percentage of red lead present in the 

 original sample. Having obtained the percentage of combined 



'Dingler's polytech. Journ. (1889), 271, 472. 



