Cleaning of pH Selective Electrodes with Ionophore-doped Fluorous Membranes in NaOH Solution at 90°C

This work demonstrates the remarkable stability of fluorous ion-selective electrode (ISE) membranes by exposing them to cleaning-in-place treatments (CIP) as they are used in many industrial processes. The sensing membranes consisted of Teflon AF2400 plasticized with a linear perfluoropolyether and doped with ionic sites and a H+ ionophore (i.e., tris[3-(perfluorooctyl)propyl]amine, 1, or tris[3-(perfluorooctyl)pentyl]amine, 2). To mimic a typical CIP treatment, the electrodes were repeatedly exposed for 30min to a 3.0% NaOH solution at 90 degrees C (pH12.7). ISE membranes doped with the less strongly H+ binding ionophore 1 started to show reduced potentiometric response slopes and increased resistances after one exposure for 30min to hot 3.0% NaOH solution. No decomposition of the ionic sites and ionophore 1 at 90 degrees C was evident by H-1 NMR spectroscopy, suggesting that the performance of membranes doped with 1 was compromised primarily by leaching of the negatively charged ionic sites along with H+ into the hot caustic solution. In contrast, even after ten exposures to hot 3.0% NaOH for a cumulative 5h at 90 degrees C, the fluorous sensing membranes doped with the more strongly H+ binding ionophore 2 still showed the ability to respond with a theoretical (Nernstian) slope up to pH12. Addition of the fluorophilic electrolyte salt methyltris[3-(perfluorooctyl)propyl]ammonium tetrakis[3,5-bis(perfluorohexyl)phenyl]borate reduced the membrane resistance by an order of magnitude.