Section 7.4.3 considered an ionizable surface immersed in pure water. Thus the surface dissociated into a negative plane and a cloud of positive counterions. Real cells, however, are bathed in a solution of salt, among other things there is an external reservoir of both counterions and negative coions. Section 7.4.3' on page 250 gave a solution for this case, but the math was complicated; here is a simpler, approximate treatment. Instead of solving Equation 7.33 exactly, consider the case where the surface's charge density is small. Then the potential V(0) at the surface will not be very different from the value at infinity, which we took to be zero. (More precisely, the dimensionless combination V is everywhere much smaller than 1.) Approximate the right-hand side of Equation 7.33 by the first two terms of its series expansion in powers of V. The resulting approximate equation is easy to solve. Solve it, and give an interpretation to the quantity lambda_D defined in Equation 7.34.