Supporting Information to Avoiding Errors in Electrochemical Measurements: Effect of Frit Material on the Performance of Reference Electrodes with Porous Frit Junctions Maral P.S. Mousavi, Stacey A. Saba, Evan L. Anderson, Marc A. Hillmyer, and Philippe Bühlmann* Departments of Chemistry and Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States * FAX: +1(612) 626-7541, buhlmann@umn.edu Potentiometric Response of Reference Electrodes with Porous Glass Frits S-2 Half Cell Potentials of Reference Electrodes with Porous Polymer Frits S-8 Potential Stability S-14 Scanning Electron Microscopy and Nitrogen Sorption Measurements S-16 References S-16 S- 1
Potentiometric Response of Reference Electrodes with Porous Glass Frits Electrodes with Vycor, CoralPor, and Electro-porous KT glass frits (triplicate of each) were prepared using 3.0 M KCl as the inner filling solution; see Figure S1 for a representative electrode. The potentials were measured with respect to an external free-flow sleeve-junction reference electrode (3.0 M KCl inner filling and bridge electrolyte solution), which did not contain a porous frit (see Figure S2). To evaluate the sample dependence of the potential of reference electrodes with porous glass frits, electrodes were placed in 0.1 M solutions of the electrolyte (KCl, NaNO 3, Na 2 SO 4, and NBu 4 ClO 4 ), and the electrolyte concentration was altered by successive dilutions of the test solution with deionized water, while monitoring the potential. Results of these experiments are shown in Figures S3 S6. Figure S1. An electrode with a porous CoralPor frit, with an inner AgCl/Ag reference electrode, and a 3.0 M KCl inner filling solution. The porous frits were attached to glass tubes (7 cm long, 3 mm outer diameter) using heat shrink tubing. S- 2
Figure S2. A free-flow double-junction AgCl/Ag electrode with a movable ground glass sleeve junction (no porous frit) with 3.0 M KCl bridge electrolyte and reference electrolyte solutions (purchased from Mettler Toledo, Columbus, OH) that was used as external reference electrodes for all potentiometric measurements (A). Zoomed-in view of the sleeve junction of this electrode (B and C). S- 3
Figure S3. Effect of KCl concentration on the potential of reference electrodes with Vycor (A), CoralPor (B), and Electro-porous KT (C) glass frits. The black, red, and blue traces show results for three separate but identically prepared electrodes. S- 4
Figure S4. Effect of NaNO 3 concentration on the potential of reference electrodes with Vycor (A), CoralPor (B), and Electro-porous KT (C) glass frits. The black, red, and blue traces show results for three separate, but identically prepared electrodes. S- 5
Figure S5. Effect of Na 2 SO 4 concentration on the potential of reference electrodes with Vycor (A), CoralPor (B), and Electro-porous KT (C) glass frits. The black, red, and blue traces show results for three separate but identically prepared electrodes. S- 6
p Figure S6. Effect of NBu 4 Cl concentration on the potential of reference electrodes with Vycor (A), CoralPor (B), and Electro-porous KT (C) glass frits. The black, red, and blue traces show results for three separate but identically prepared electrodes. S- 7
Half Cell Potentials of Reference Electrodes with Porous Polymer Frits Electrodes with porous Teflon or polyethylene frits were filled with 3.0 M KCl and equilibrated for at least one week. The potentials were measured with respect to an external free-flow sleeve-junction reference electrode (see Figure S2). To evaluate the sample dependence of the potential of the reference electrodes with porous polymeric frits, electrodes were placed in 0.1 M test solutions of the electrolyte (KCl, NaNO 3, Na 2 SO 4, and NBu 4 ClO 4 ) and the electrolyte concentration was altered by successive dilutions of the test solution with deionized water while monitoring the potential. Responses to ph, KCl, NaNO 3, Na 2 SO 4, and NBu 4 Cl are shown in Figures S7 S11. Figure S7. ph dependence of the potential of reference electrodes with (A) porous polyethylene frits or (B) porous Teflon frits in aqueous solutions with a 0.01 M KCl background. The type of the polymer frit is specified in each panel. The starting solution was 0.01 M HCl (with 0.01 M or 0.3 M background KCl), and the ph was increased by successive additions of aqueous 10.0 M NaOH solution. Measurements were performed in triplicate using identically prepared electrodes. The black, red, and blue traces represent individual electrodes. S- 8
Figure S8. Effect of KCl concentration on the potential of reference electrodes with (A) polyethylene and (B) Teflon frits. Measurements were performed in triplicate using identically prepared electrodes. The black, red, and blue traces represent individual electrodes. S- 9
Figure S9. Effect of NaNO 3 concentration on the potential of reference electrodes with (A) polyethylene and (B) Teflon frits. Measurements were performed in triplicate using identically prepared electrodes. The black, red, and blue traces represent individual electrodes.. S- 10
Figure S10. Effect of Na 2 SO 4 on the potential of reference electrodes with (A) polyethylene and (B) Teflon frits. Measurements were performed in triplicate using identically prepared electrodes. The black, red, and blue traces represent individual electrodes. Figure S11. Effect of NBu 4 Cl concentration on the potential of reference electrodes with (A) polyethylene and (B) Teflon frits. Measurements were performed in triplicate using identically prepared electrodes. The black, red, and blue traces represent individual electrodes. S- 11
To improve the signal stability of the reference electrodes with porous polymeric frits and facilitate wetting of the polymer frits, a stream of air was pointed at the top of the inner filling solution of the electrode. The pressure caused by the air stream ( 2 bar) facilitated filling the pores with 3.0 M KCl, as confirmed by measuring the resistance of the frits (<500 Ω). The improved wetting minimized the electrode-to-electrode variations of the emf and reduced the sample dependence of the reference potential. The effects of the concentration of KCl, NaNO 3, Na 2 SO4, NBu 4 Cl, and ph are shown in Figures S12 S15. In some cases, during the measurement or during the preparation steps, air bubbles got trapped in the polymeric frit, which resulted in changes and a sample dependence of the reference electrode potential. Entrapment of air bubbles resulted in increased resistance of the frits, as confirmed by impedance spectroscopy. Pausing the experiment and applying a stream of air (producing an approximate pressure of 2 bar) on top of the inner filling solution restored the previous condition and improved the performance of the reference electrodes. Figure S12. Effect of KCl concentration on the potential of reference electrodes with (A) polyethylene and (B) Teflon frits. Measurements were performed in triplicate using identically prepared electrodes. The black, red, and blue traces represent individual electrodes. The type of the polymer frit is specified in each panel. Wetting of the frit was facilitated by applying pressure on the inner filling solution, thereby filling the pores of the porous frit. S- 12
Figure S13. Effect of NaNO 3 concentration on the potential of reference electrodes with (A) polyethylene and (B) Teflon frits. Wetting of the frits was facilitated by applying pressure on the inner filling solution, thereby filling the pores of the frit. Measurements were performed in triplicate using identically prepared electrodes. The black, red, and blue traces represent individual electrodes. Figure S14. Effect of Na 2 SO 4 concentration on the potential of reference electrodes with (A) polyethylene and (B) Teflon frits. Wetting of the frit was facilitated by applying pressure on the inner filling solution, thereby filling the pores of the frit. Measurements were performed in triplicate using identically prepared electrodes. The black, red, and blue traces represent individual electrodes. Only two replicates are shown for electrodes with polyethylene frits; the flow of inner filling solution to the sample was so rapid for a third electrode (> 1000 µl/h) that the electrode could not be used within the time frame of the experiment. The large flow was possibly caused by a crack in the frit. S- 13
Figure S15. Effect of NBu 4 Cl concentration on the potential of reference electrodes with (A) polyethylene and (B) Teflon frits. Wetting of the frit was facilitated by applying pressure on the inner filling solution, thereby wetting the pores of the frit. Measurements were performed in triplicate using identically prepared electrodes. The black, red, and blue traces represent individual electrodes. Only two replicates are shown for electrodes with polyethylene frits; the flow of inner filling solution to the sample was so large for a third electrode (> 1000 µl/h) that the electrode could not be used within the time frame of the experiment. The large flow was possibly caused by a crack in the frit. Potential Stability Potentials of reference electrodes with porous Vycor glass, CoralPor glass, Electro-porous KT glass, Teflon, and polyethylene frits (reference solution 3.00 M KCl) were monitored for 48 hours in a temperature-controlled (25 C) 0.10 M KCl solution with respect to an external free-flow sleeve-junction reference electrode (see Figure S2). Potential of electrodes with respect to time are shown in Figure S16. S- 14
Figure S16. Signal stability of reference electrodes with Vycor (A), CoralPor (B), Electro-porous KT (C), Teflon (D), and polyethylene (E) frits in a 0.10 M KCl at 25 C. Measurements were performed in triplicate using identically prepared electrodes. The black, red, and blue traces represent individual electrodes. Only two replicates are shown for electrodes with polyethylene frits; the flow of inner filling solution to the sample was so large for a third electrode (> 1000 µl/h) that the electrode could not be used within the timeframe of the experiment. The large flow was possibly caused by a crack in the frit. S- 15
Scanning Electron Microscopy and Nitrogen Sorption Measurements Scanning electron microscopy micrographs were obtained on a Hitachi S-4700 cold field emission gun scanning electron miscroscope with an accelerating voltage of 3 5 kv. Before imaging, the frits were cryo-fractured and coated with ca. 3 nm of Pt via sputtering using either a VCR Group IBS TM200S Ion Beam Sputterer or a Balzers Union MED 010. Nitrogen sorption isotherms were collected on a Quantachrome Autosorb iq 2 -MP at liquid nitrogen temperature (77 K). Prior to measurement, samples were outgassed at 200 C for 2 h, followed by 50 C for 6 h using a turbomolecular vacuum pump. Brunauer-Emmett-Teller (BET) specific surface areas were obtained from the adsorption branch from P/P 0 = 0.05 0.35. 1 Mesopore size distributions were estimated using a nonlocal density functional theory kernel for nitrogen on silica with cylindrical pores applied to the adsorption branch. 2 Figure S17. Nitrogen sorption isotherms for Vycor (A) and CoralPor (B) glass. Filled circles indicate adsorption and empty circles indicate desorption. References (1) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309-319. (2) Thommes, M.; Smarsly, B.; Groenewolt, M.; Ravikovitch, P. I.; Neimark, A. V. Langmuir 2006, 22, 756-764. S- 16