Negligible liquid junction potential assumption

Izutsu et al. studied the compiexing of Na" in acetonitrile solution with various protic and aprotic solvents using an ion-sensitive glass electrode. Parker s assumption of negligible liquid junction potential with an tetraethylammonium picrate salt bridge was adopted and found to be valid, even when water was added. The formation constants increased in the order methanol < H2O < DMF < NJ -dimethylace-tamide DMSO < HMPA. 

As is well known, single ion activity coefficients cannot be defined thermodynamically. In the above discussion, 7Lci = 7Na+ 7ci- has a physical meaning in thermodynamics but the separation between and y i- is carried out based on the assumptions that 7k+ = 7ci- i KCl solutions and the activity coefficient of an ion is the same if the ionic strength is the same. Moreover, the liquid junction potential is assumed to be negligible. However, these ambiguities can be eliminated if we calculate the mean activity coefficient of NaCl in the presence of polyelectrolyte, yNaci- The usefulness of the additivity can be observed in the Donnan membrane equilibrium where y aci is discussed [28]. 

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