THE APPLICATION OF RADIOACTIVE RECOTI.TX DIFFUSIOX MEASIREMEXTS 137 



Tabic 3. — Self-Diffttsion of Lead Ions in Phi, 



(q-Particle AIethou) 



Th(» (liilusion constant of lead ions in Pbl, can also he calculated ironi 

 the electrolytic conductance of this compound and the transport number. 

 At 390°C, for example, the calculated diffusion constant is 0.9x10 •> 

 cm2 day~i while the recoil measurement and a -ray measurement yield 

 0.9 X 10"^ and 2.2 x 10-^, respectively. The behaviour at low tempera- 

 tures, where the mobihty of the iodide ions controls the conductanee, 

 is discussed in the subsequent paper. 



Summary 



The veloeitA- of diffusion of leail ions in lead chloride and iodide has been 

 measured by making use of radioactive recoil. The values obtained are DpbOij = 

 = 1.06 x 10"e-38'i2o/iJ?'and£)pi,i, = 3.43 X 10* e-^^'Ooo/^?' . The diffusion velocity 

 values for the lead ion in lead iodide were confirmed by other methods. The high 

 value for the heat of relaxation of lead ions in lead chloride (3S,120 cal/mole) 

 cixplains Tubandt's result, viz. that in lead chloride the chloride ions whose 

 heat of relaxation amounts only to 11,180 cal are practically the only mobile 

 ions. The transport number of the lead ions in lead chloride at 270" is cal- 

 culated to be 10~5. 



The velocity of diffusion of lead ions in lead iodide calculated from the electro- 

 lytic condu<*tance and from the transport numbers at 290°C as determined by 

 TuBANDT, is in good agreement with our experimental value. 



