94 ADVENTURES IX EADIOISOTOPE RESEARCH 



phases. It is necessary to know the rate of this exchange because if it is 

 very large the (ThB)C'l2 molecules in the solid phase will exchange 

 directly with tlie Ph( U molecules of the solution and thus simulate 

 direct dissolution. 



An answer to this question would be possible if, for example, a 

 saturated lead chloride solution could be shaken with solid PbClg, the 

 lead atoms of which were numbered or characterized in any other way 

 without affecting their chemical properties, by finding in which phase 

 the numbered atoms then existed. vSuch ideally labelled lead atoms are 

 the radioactive isotopes (ThB, RaB, AcB, RaD, etc.). We need only 

 add, for example, {ThB)(N03)2 to the solution of lead nitrate, precipitate 

 the Pb— ThB mixture as chloride and thus to obtain a "coloured" lead 

 chloride. If 1 mgm of PbCIg were originally associated with one relative 

 unit the detection of this relative unit in the saturated solution would 

 allow the inference that 1 mgm of the lead atoms originally in the solid 

 phase had then been transferred into the saturated solution, or vice versa. 



The application of radioactive indicators thus serves to permit tracing 

 of the exchange of atoms of the same kind between two phases ; the 

 "colouring" of the labelling atoms is a purel}^ radioactive property and, 

 although their mass is different from that of the labelled isotope (they 

 have different atomic weights), their chemical reactions, with which 

 we are concerned, are still the same^. 



To determine the velocity of exchange between the solid lead chloride 

 and its saturated solution, 250 mgm of ThB-labelled PbClg was shaken 

 with 25 cm^ of a saturated solution of pure lead chloride in a thermostat 

 at 20°C. After 24 hr the mean value of ten experiments showed that 

 1.2 mgm, or ^2 V^^ ^^^^ of the lead chloride originally in the solid phase 

 had entered the saturated solution ; after 48 hr the value was about 

 '/4 per cent. The lead chloride solution was prepared by cooling an 

 originally slightly supersaturated solution in the thermostat and was 

 therefore fully saturated ; the possible sources of error all tended to 

 yield high values for exchange and the above value should therefore 

 be regarded as an upper limit only. 



The velocity of exchange depends very markedly on the mechanical 

 consistency of the solid phase, and this also applies to the velocity of 

 solubility-. It is thus imperative to compare these two quantities under 

 the same experimental conditions. For example, on shaking 250 mgm 



1 These ideas do not apply strictly to diffusion processes or, therefore, to the 

 exchange of atoms in the same phase, because the velocity of diffusion is dependent 

 on the mass ; this dependence is very slight, however. Considered from the stand- 

 point of diffusion only those atoms which are both isotopic with and of tlie same 

 mass as those under study are really ideal indicators. It appears, however, that 

 UY may be such an ideal indicator for UXj. 



