Single-phase equilibrium in an external force field

3 Single-phase equilibrium in an external force field 

Assume now two locations a and yS a differential distance apart. At equilibrium, the net total force Fu on species i should be zero. For the case of a single external force field we get from the general expression (3.1.50) 

An additional relation has to be used in each phase containing charged species/ions. This is the electroneutrality condition, according to which there should be no net electrical charge at any bulk location (there may be deviations from it at phase boundaries see Helfferich (1962, 1995))  

For an external force field describable by — the onedimensional representation of the above relation along the coordinate direction z is 

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