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

Abstract

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 30 min to a 3.0 % NaOH solution at 90 °C (pH 12.7). 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 30 min to hot 3.0 % NaOH solution. No decomposition of the ionic sites and ionophore 1 at 90 °C was evident by 1H 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 5 h at 90 °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 pH 12. 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.

Potentiometric Sensing

“…Fluorous liquid membranes are particularly promising because their poor solvent properties can translate into dramatically enhanced selectivity (up to 16 orders of magnitude for an ion-exchange membrane vs 8 orders usually obtained for PVC membranes) and chemical robustness. As an example, Bühlmann’s group recently demonstrated the stability of pH electrodes made of ionophore-doped fluorous membranes (Teflon AF2400) by exposing them to harsh on site cleaning protocols used in many industrial processes . To mimic this type of treatment, the electrodes were repeatedly exposed for 30 min to a 3.0% NaOH solution at 90 °C.…”

“…As an example, Buḧlmann's group recently demonstrated the stability of pH electrodes made of ionophore-doped fluorous membranes (Teflon AF2400) by exposing them to harsh on site cleaning protocols used in many industrial processes. 82 To mimic this type of treatment, the electrodes were repeatedly exposed for 30 min to a 3.0% NaOH solution at 90 °C. The application of fluorous liquids that have to be infused into a porous Teflon disc as a mechanical support to prepare the membrane is only compatible with fluorophilic ionophores and ion exchangers.…”

Remediation of Perfluorooctylsulfonate Contamination by in Situ Sequestration: Direct Monitoring of PFOS Binding to Polyquaternium Polymers

“…PFOS–selective membranes were prepared to contain 1.0 mM of the anion-exchanger salt 1,3-di­(1 H ,1 H ,2 H ,2 H -perfluoro-7-methyloctyl)­imidazolium iodide ( 3 ; to provide permselectivity) and 10 mM of the fluorophilic electrolyte 1,3-di­(1 H ,1 H ,2 H ,2 H -perfluoro-7-methyloctyl)­imidazolium tetrakis­[3,5-bis­(perfluorooctyl)­phenyl]­borate ( 4 ; to reduce membrane resistance) , in perfluoroperhydrophenanthrene as the inert fluorous membrane matrix (see Figure ). The syntheses of 3 and 4 and the preparation of the fluorous-phase PFOS–selective electrodes are described in the Supporting Information.…”

“…31 Preparation of PFOS−Selective Membranes. PFOS− selective membranes were prepared to contain 1.0 mM of the anion-exchanger salt 1,3-di(1H,1H,2H,2H-perfluoro-7methyloctyl)imidazolium iodide (3; to provide permselectivity) 32 membrane resistance) 33,34 in perfluoroperhydrophenanthrene as the inert fluorous membrane matrix (see Figure 10). The syntheses of 3 and 4 and the preparation of the fluorous-phase PFOS−selective electrodes are described in the Supporting Information.…”

Solid-Contact pH Sensor with Covalent Attachment of Ionophores and Ionic Sites to a Poly(decyl methacrylate) Matrix

“…Covalent attachment of sensor components is also attractive when ISEs are exposed to high temperatures or organic solvents either during measurements or cleaning. 63 To assess the ability of ISE membranes to withstand this type of stress, electrodes were exposed to 90 °C heat for 2 h or 10 wt % ethanol solution for 1 day. No diminishment of the response slope and selectivities was observed (see Table S2).…”