"Heterojunction" ionic diodes based on a Nafion cation conductor and a polymer of intrinsic microporosity (PIM) interfaced at a 6 mm thickness polyethylene-terephthalate (PET) film with 20 mm diameter microhole exhibit rectification effects in cation (K+) flux. When combined with the precipitation reaction of potassium cations with perchlorate anions to give insoluble KClO4 (solubility product ca. 1.05 x 10(-2) M-2 at 25 degrees C) at the PIM | Nafion interface, inversion of the rectification/diode effect occurs and the formerly "open" state changes into a "closed" state due to blocking of ion flow. The localised interfacial precipitation reaction is due to up to three orders of magnitude accumulation of Kthorn at the PIM | Nafion interface and shown to be fast and reversible. The effects of K+/ClO4 concentration and of Na+ interference are considered. The blocking process within the heterojunction ionic diode is suggested to be dynamic/fast and potentially useful as a diode sensor mechanism based on solubility product dependent precipitation. (C) 2017 Elsevier Ltd. All rights reserved.