Departures from electro neutrality, following Newman, estimate the departures from electro neutrality in the stagnant region between the reservoirs of Problem 24B.2 as follows. First calculate the electric field gradient d2Φ/dz2, where z is the distance measured from reservoir 1 toward reservoir 2, assuming that the salt concentration in g-moles/liter is given by c_s = 1.0 – 0.9z/L where L is the length of the stagnant region. Then put the result into Poisson’s equation. Here ε is the dielectric constant, and F/ε may be taken to be 1.392 x 1016 volt-cm/g-equiv(see Newman4, pp. 74 and 256), which corresponds to a relative dielectric constant of 78.303. For this problem, the summation reduces to (c₊ – c_–).

C, = 1.0 0.9 ? . %3 z,C;