247 



A Modified Method for the Determination of Lead Peroxide 



IN Red Lead. 



By a. R. Nees akd O. W. Brown. 



Two general methods are used for determining how near commercial 

 red lead corresponds to the fornuila rb304. One method depends upon dis- 

 solving the free litharge from the sample and assuming the residue to be 

 j)ure PbsOi. Other methods depend upon the determination of the per 

 cent, of PbO., in the material, and calculating from this value the per cent, 

 of Pb.Oi in the sample. 



Mr. E. E. Dunlap (J. Am. Chem. Soc. 80, p. Gil) has proposed a 

 method of determining the free litharge in red lead. He states that by 

 digesting a sample of conunercial red lead in a boiling dilute solution of 

 lead acetate, all of the free litharge is dissolved and that the material re- 

 maining corresponds to formula I'bjO^. This method is employed in many 

 conunercial laboratories. However, the writers have not obtained accord- 

 ant results when it is used, because the amount of litharge dissolved by 

 the lead acetate solution depends upon the length of time the sample 

 i5 digested. The analysis of a single sample by this method gave 

 results of 4.71% to S.8% litharge when Mr. Dunlap's directions were care- 

 fully followed, and the time of digestion was varied from ten to thirty 

 minutes. 



For accurate results the writers believe that it is necessary to use 

 some of the methods for the determination of PbO,. A number of metliods 

 have been described in the literature and most of them have been tested 

 in this laboratory. 



The method of Lux (Treadwell and Hall's Quantitative Analysis, p. 

 4[/l) is based upon the fact that oxalic acid is oxidized by PbO, in dilute 

 nitric acid solution. Our experience with this method is that it usually 

 gives high results and that they are not concordant. A series of deter- 

 minations on the same sample gave results varying between 35.1% and 

 31.54%. 



The method of Diehl as modified by Topf (Treadwell and Hall, p. 531) 

 was also tried. This method depends upon the fact that potassium iodide 



