APPLICATIONS OF ISOTOPIC INDICATORS 929 



fraction obtained in the course of these operations, we must conclude 

 that 1/1000 mgm of the lead atoms present in the lead nitrate we started 

 from, are now present in the fraction. 



Radium D cannot be separated from lead, but it can easily be ob- 

 tained in the pure form from lead-free radium salt samples or from 

 radium emanation, since radium D is formed in the course of the dis- 

 integration of these radioactive bodies and can readily be separated 

 from them. At that time, the Vienna Institute of Radium Research 

 had more radium and radium emanation at its disposal than any other 

 institution. This fact induced me, late in 1912, to start work at the 

 Vienna Institute in collaboration with Dr. Paneth, assistant at that 

 Institute, who himself had made very extensive and abortive trials to 

 separate radium D from lead. The first application of labelled lead^^^ 

 was the determination of the solubility of some very slightly soluble 

 lead compounds such as lead chromate and sulphide. Labelled lead 

 chromate was obtained by adding a solution of 100,000 relative units of 

 radium D to lead nitrate containing 10 mgm of lead and converting the 

 nitrate thus labelled into chromate. 



After the saturated solution of this compound had been held at the 

 desired temperature in a thermostat for a sufficient time, its compo- 

 sition was ascertained by evaporating a few cubic centimetres to dryness 

 and measuring the activity of the almost invisible residue in the electro- 

 scope. From the number of units of radium D found, the amount of 

 lead was calculated, one unit corresponding to 10~^ grams of lead ; 

 finally, the solubility of the lead chromate in moles per litre (2 • 10"") 

 w^as computed. 



The radioactive method is extremely simple, having the advantage 

 that the presence of foreign ions in no way interferes with the measure- 

 ments. The method may be applied without difficulty, for example, in 

 determining the solubility of lead sulphate in the presence of calcium 

 sulphate. 



Simultaneously with the said experiments, we used labelled lead and 

 labelled bismuth (the radioactive bismuth isotope radium E can easily 

 by obtained from radium, radium emanation, or radio-lead) in an in- 

 vestigation of the manner in which unweighable amounts of metals are 

 precipitated during electrolysis.^^' The application of the well-known 

 Nernst formula 



RT , c 



e = logn ~ 



n C 



was extended to concentrations of 10"^ N and even lower. 



On the basis of Nernst's theory, we should expect an interchange 

 to take place between the metal of the electrodes and the ions in solution. 



59 HevesT 



