Acid-base diodes in a hydrogel medium. Nonlinear response for mono- and divalent contaminating ions: theory and experiments

Small hydrogel cylinders connecting aqueous solutions of a strong acid and a strong base show a current-voltage characteristic similar to that of a se miconductor diode. This characteristic is affected by adding a salt either to the acid or to the base and the response is highly nonlinear close to ce rtain critical salt concentrations. Theoretical characteristics derived fro m the Nernst-Planck equations agree qualitatively with the experimental obs ervations for moderate salt concentrations. Applying higher salt concentrat ions, however, deviations can be observed which are similar in character fo r both mono- and divalent contaminating ions. The results suggest that the deviation from the theoretical behaviour is not due to ion specific propert ies, but rather it is caused by the hydrogel medium itself.