APPLICATIONS OF ISOTOPIC INDICATOKS 



933 



diffusion rate of Pb + "'" from the conductivity of lead chloride, on the 

 assumption that the electrolytic conductivity of the salt is due solely 

 to the transference of charges by the Pb+ + . The diffusion rate of Pb+ + 

 thus calculated is, however, many thousand times larger than the value 

 found experimentally, showing that the chloride ions are almost ex- 



25 24 23 22 



2' 20 19 18 1 

 ^x IC* 



Fig. 1. Diffusion rates of metals in solid lead. 



clusively responsible for the conduction of electricity in solid lead chlo- 

 ride. The above data permit the calculation of the transport number 

 of Pb^^ in solid lead chloride. Not far from the melting point, at 484°, 

 the transport number of Pb++ is found^^^^ to be IQ-^, at 273° IQ-^, 

 and at 90° only IQ-io. By no other method can such small transport 

 numbers be determined in any electrolyte. 



In the case of lead iodide, the diffusion rate of Pb++ calculated from 

 conductivity data, under the assumption that the whole conduction is 

 due to Pb + + , is in good agreement with the measurement of the self- 



