Approximate Method for Sufficiently Dilute Solutions

Approximate Method for Sufficiently Dilute Solutions. In order to use the method, one has to use the Einstein equation 

From this equation and from the definition of transport number, i.e., Eq. (4.236), 

The Einstein equation is exact only at very low concentrations in aqueous solution. As explained in a general way in Section 4.4.6, and in a detailed way for a given system in Section 5.6.6.2, there is usually some deviation—perhaps as much as 20%—between the results of Einstein s equation and experimental fact. Thus, outside very dilute solutions, using the Einstein equation to determine transport numbers is a rough-and-ready method and the results carry a burden of 10%. 

The method is clearly limited to ions that have suitable (fi-or a-emitting) radiotracers. Those ions having radioactive isotopes that emit y radiation are more difficult to measure because the long range and penetrating power of the y radiation make it difficult precisely to determine the position of the radiotracer ions as they spread through a solution. 

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