THE VELOC'ITV OF DISSOLUTION OF MOLECULAK LAYERS 



95 



of the same lead chloricU^ 44 per cent had already dissolved in 1 hr, 

 representing 44 per cent of the saturation concentration since the liquid 

 volume amounted to 25 cm^ and 250 mgni are soluble in tliis volume. 



It is seen, therefore, that tlie velocity of exchange belween the 1 wo 

 phases is small compared Avitli the velocity of solubility. Il will be seen 

 later that in the case of a molec^ular layer the exchange velocity becomes 

 much larger when expressed on a percentage basis but is still smaller 

 than the velocity of solubility, and thus a diminution of the velocity 

 of dissolution of a substance can still be detected by the presence of its 

 isotope in the solution. 



To this end we have compared the amounts of ThB collected on 

 quartz which dissolved in water and in a saturated PbClg solution, under 

 the same conditions, in 1 hr. There w^as only a small difference since in 

 the first case 79 per cent dissolved while in the second the value was 

 75 per cent. 



Shorter experimental times were then chosen and thus the presence 

 of lead ions in the solution had a very considerable effect on the velocity 

 of solubility of ThB. These experiments were performed with the same 

 apparatus used for obtaining the values recorded in Table 1. 



TaULI-; 4. — ExpERIMENT.JiI. TIME 60 SGC 



Bismuth isotope ThC 

 Lead isotope ThB 



.Vmount ilissolved 



in 10 - 3 N HXO., 



(%) 



Amount dissolved in 



10—' N HXOj s:iturated 



with Pb(X03)2 



(%) 



fi4 

 67 



(■-'; 



The presence of lead ions in the solution diminishes the velocity of 

 solubility of the lead isotope but not that of the bismuth isotope. It is 

 a fortunate circumstance that ThB and ThC are simultaneously present 

 in the same place on the quartz surface, and when, in spite of this, the 

 velocity of solubility of only one is affected, this means that there is 

 a specific effect due to the addition of the appropriate element ; for 

 example, glycerol, which has no selective effect but which increases the 

 viscosity of the solution, affects the velocity of solubility (Table 1) of 

 ThB and ThC equally. 



In order to ascertain whether a small concentration of lead ions 

 affects the amounts of ThB and ThC dissolved, w^e have performed 

 experiments in 10~3N HNO3 solutions which were also 10~^N in lead. 

 The time of experiment was 40 sec and the arrangement was different 

 from that described above. Th(> ratio of the amounts of l^b and Bi 

 dissolved was found to be : 



