Originally i^ublishcd in Z. (iiwiy. Chem. 82, 322 (J 913) 



1. THE SOLUBILITY OF LEAD SULPHIDE 

 AND LEAD CHROMATE 



George Hevesy and Fritz Paneth 

 From the Institute of Radium Research of the Vienna Academy of Sciences 



The fourth decay product of radium emanation, RaD, is known to 

 exhibit all the chemical reactions of lead ; if RaD is mixed with lead 

 or lead salts it cannot be separated from the lead by any chemical or 

 physical method^ and if complete mixing of the two substances has 

 taken place then the same concentration ratio is maintained whatever 

 amount of lead is withdrawn from the solution. Since RaD can be 

 determined in much smaller amounts, owing to its radioactivity, than 

 lead, it may be employed for the qualitative and quantitative estimation 

 of lead to which it has been added ; the RaD is an indicator of the 

 lead. 



The lower limit for the qualitative detectability of lead in its most 

 sensitive microchemical reaction^ (precipitation of K2PbCu(N02)6) 

 amounts to 3 x 10-^ gm ; the limit for quantitative determination lies 

 considerably higher and varies with the particular problem ; for ex- 

 ample, the solubility of lead carbonate could be obtained from deter- 

 minations of the conductance but Kohlrausch^ was able only to make 

 an approximate estimate for lead chromate in this way. With the aid 

 of RaD as a tracer these solubilities can easily be determined directly ; 

 an amount as small as 10"io gm RaD may be measured, by means of an 

 ordinary and not particularly sensitive electroscope, if one is content 

 to measure the /^-radiation of RaE which comes to equilibrium with 

 the RaD after a few weeks. By awaiting the formation of a quantity 

 of RaF sufficient for calculating the equilibrium amount, it is possible 

 to determine quantitatively even IQ-^^ gm of RaD by means of the a- 

 radiation. In radiolead from pitchblende there is about 10"" gm of RaD 

 per gm of lead and thus 1 mgm of radiolead can be detected with the aid 

 of its ^-radiation ; since much smaller orders of magnitude are involved 



^ A review of relevant experiments is c-ontained in the paper by F. Paneth 

 and G. Hevesy in Monatsh. Chem. 42, 1 (1913). 

 2 J. Emich, Lehrhuch d. Mikrochemie p. 80 (1911). 

 3F. KoHLRAUscH, Z. phys. Chem. 64. 159 (1908). 



