IM-ATIXTM BLACK 37 



that of the platinum black. Now if eare is taken that the thickness of 

 the deposits attains the range of the a-radiation in the material, which 

 is 12.8 fi in lead and 30.6 jj, in lead peroxide, then the activity will 

 provide a simple measure of the lead content. Denoting the activity of 

 the platinum black electrode by 8.^, that of the lead peroxide electrode 

 of the same size by 8^^ the density of platinum (21.3) by d^, the density 

 of lead peroxide (8.9) by (h, the range of a-radiation in platinum (12.8 [.i) 

 by i?i, the range of a-radiation in lead peroxide (30.6 [a) by R^, the number 

 of grammes of lead in 1 cm^ of the active lead acetate solution by p and 

 the number of grammes of lead in the 2.5% lead acetate solution by P. 

 then the required lead content of the platinum black, x, expressed as 

 a percentage, is given by 



X = 8^pR^d^{ii\. wt. of Pb)lOO/iS'2/;Pi^jr/i(mol. wt. of PbO.,) 



Therefore p was chosen to differ from P because it had been found 

 preferable to produce the lead peroxide deposit from a solution with 

 a lead content higher than that for the platinum black coating. The lead 

 content of the platinum black, as determined, is found in the Figures in 

 Table 1. It may be seen that the lead content of the platinum black rises 

 sharply with increasing lead acetate content of the platinum chloride 

 solution. 



Tablk 1. — Lkad Content of Platintjm Black as a FrNCTioN 



OF THE Lead Acetate Content of the Electboly.sed 0.2 N HCl 



Solution Containing 3°o PtCl^ 



In order to decide whether the lead found in platinum black is present 

 in solid solution or not we have compared the line distances obtained 

 on Debye-Scherrer diagrams for different platinum black samples with 

 those of pure platinum. The difference, as is evident from Table 2, was 

 shown to be vanishingly small and therefore it must hv assumed that 

 the large majority of the lead occurring in platinum black is not present 

 in the form of a solid solution. The measured line distances underwent 

 a considerable increase when the sample was heated. Thus, the second 

 sample recorded in Table 2 showed, after heating for 16 hr in a vacuum 

 to 500°C, a line distance of 124.7 mm ; aflcr 44 hr heating at 025'^ 

 the distance was 125.2 mm. 



