Ions, absolute mobility transport number

There is difficulty in defining the absolute mobilities of the constituent ions in a molten salt, since it does not contain fixed particles that could serve as a coordinate reference. Experimental means for measuring external transport numbers or external mobilities are scarce, although the zone electromigration method (layer method) and the improved Hittorf method may be used. In addition, external mobilities in molten salts cannot be easily calculated, even from molecular dynamics simulation. 

It is generally held that even though the law of mass action is not obeyed at ordinary concentrations, it must, for thermodynamic reasons hold for the infinitely dilute solution Even in this case, there is no theoretical reason why d-wddfL should be equal to dvu/dCu The calculation increases, then if the mobility of each ion increases by the same fractional amount the transport number /( 4- v) will remain constant although (u + v), which determines the conductivity, may have altered The probability of the increase being the same fractional amount for both ions is perhaps not great, and if this probability be negligible then constancy in transport number would actually mean constancy in absolute mobility... 

A great difficulty when dealing with electrolytes is to ascribe individual properties to individual ions. Individual thermodynamic properties cannot be determined, only mean ion quantities being measurable. Interionic and ion-solvent interactions are so numerous and important in solution that, except in the most dilute cases, no ion may be regarded as behaving independently of others. On the other hand, there is no doubt that certain dynamic properties such as ion conductances, mobilities and transport numbers may be determined, although values for such properties are not absolute but vary with ion environment. 

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