432 The Philippine Journal of Science i9is 



such a factor would make it a somewhat undesirable method. A 

 more practical method will be described later. 



Minium is regarded as composed of lead dioxide combined with 

 two molecules of lead monoxide, Pb0 2 . 2PbO, and a compound of 

 the formula Pb 4 B would consist of one molecule of lead dioxide 

 combined with three molecules of lead monoxide, Pb0 2 . 3PbO ; 

 in other words, both compounds may be considered as composed 

 of Pb0 2 plus combined PbO. 



If red lead consist of a mixture of various oxides such as 

 Pb0 2 . 2PbO, Pb0 2 . 3PbO, and free PbO, we can easily determine 

 the various constituents in the mixture as follows: 



Decompose the complex oxides with dilute nitric acid. Sep- 

 arate and weigh the resultant black lead dioxide, and calculate 

 the lead monoxide from the lead present in the filtrate. The 

 latter result gives the free plus combined lead monoxide. In 

 a separate sample determine free lead monoxide by leaching 

 with a suitable solvent, and compute the lead monoxide by 

 difference. This method of separation is clearly shown in the 

 following diagram: 



Commercial red lead, 

 precipitated by dilute HNO3. 



Total PbO determined as Pb. 

 - PbO dissolved with Pb (NOsla solution. 



The percentage of lead dioxide plus the percentage of combined 

 monoxide gives the percentage of red lead. 



Knowing the percentage of combined lead monoxide, we can 

 calculate the actual amount of each of the oxides Pb 3 4 and 

 Pb 4 5 in the following manner: 



Suppose an analysis of a mixture of the oxides Pb 3 4 and 

 Pb 4 5 shows 70.25 per cent combined PbO. Since 



Pb.Oc contains 73.68 per cent combined PbO and 

 Pb 3 Oi contains 65.11 per cent combined PbO, 



the percentage of each constituent in the sample may be cal- 

 culated as follows: 



Per cent Pb s O«+per cent Pb,O*=100 



(0.6511xper cent Pb,0*) + (0.7368xper cent Pb^O.) =70.25, 



which by computation shows 40 per cent Pb s 4 and 60 per cent 

 Pb 4 B , respectively. 



