Flux of a Particular Species

One of the general expressions for the electrical potential gradient can now be inserted into the expression for the flux density of a particular species (Eq. (1.15)) or the corresponding partial current density (Eq. (117)). Since these two sum over chemical potential gradients of charged species, we may conveniently use Eq. (1.19) for our purpose. Inserting this into Eq. (1.15) and rearranging yields  

This is a rather general expression, which we can use to calculate flux densities of one charged species in the company of many other species. However, it reflects the flux density and gradients and properties at a particular point in the membrane. The gradients will adjust according to the varying materials properties so as to maintain a constant flux density everywhere - what we call steady state. In 

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