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.
Bibliographical noteFunding Information:
The National Science Foundation (CTS-0428046), National Institutes of Health (1R01 EB005225-01), Welch Foundation (Grant A-1656), and Hamilton (Bonaduz, Switzerland) supported this project. We thank Evan L. Anderson and Xin V. Chen for helpful comments on the manuscript and Markus Jurisch for assistance with the ionophore synthesis.
- Teflon AF
- cleaning in place
- ion-selective electrode