Abstract: A device is disclosed for determining ion activity in liquid solutions by the use of electrodes and a capillary bridge which promotes ionic migration between the electrodes. The bridge is formed of a porous material layer into which liquid solutions are absorbed, a bottom nonporous layer between the electrodes and the porous material, and a top nonporous layer on the other side of the porous material. Preferably, the three layers are apertured at each electrode to provide wells for receiving drops of liquid solutions, and the top layer is hydrophobic to retain the solution drops in the region of the apertures.