Ion-selective polymeric membranes are integrated in electrodes for measuring concentrations of specific ions. The Nernst equation (Eq. 1) allows the ion activity of an unknown sample, a_out, to be calculated from activity of a known internal reference solution, a_in, and the potential across the membrane, E_m­.

(Equation 1)

R, T, z and F are the gas constant, temperature, charge of the analyte ion and Faraday’s constant, respectively.

While this logarithmic form of the Nernst equation has practical applications in analytical chemistry, the equation can also be written in the exponential form (Eq. 2).

(Equation 2)

This suggests an alternative application where a potential E_app drives ions across the membrane until the ratio of activities satisfies Equation 2. In drug delivery, for example, this provides a method to quantitatively transport a specific drug through a membrane. The delivery rate scales with the current applied across the membrane and dissipates when an equilibrium state is reached. At this point, the activity ratio provides a membrane potential, E_m, equal and opposite to the applied potential, E_app.

We have developed a device to monitor ion flow through a quaternary ammonium gel polystyrene membrane. Applying a potential across this anion exchange membrane resulted in the quantitative delivery of nitrate ions until the activities verified Equation 2. This suggests that dosage-controlled and selective drug delivery can be obtained using this membrane.