On the mechanism of the bipolar reference electrode

Abstract

Commercial silver/silver chloride (Ag/AgCl) reference electrodes are some of the most commonly used reference electrodes, but they suffer from a number of issues due to their porous frits. Such issues include difficulty miniaturizing, silver and chloride ion leakage, charge screening effects at low ionic strength, frit drying if left unattended in air, and incompatibility with organic solvents. To solve these issues, we recently designed a reference electrode that is leakless in principle by replacing the porous frit with a sealed, conductive wire, where the ends of the wire are exposed to the reference electrode solution and the working electrode solution. We hypothesized that the reference electrode operated like a closed, bipolar electrochemical cell, and we termed the name bipolar reference electrode (BPRE). Here, we provide evidence that the BPRE can either act as a reference electrode by operating through an ion transfer mechanism via leakage through the imperfect seal, or it can act as a highly stable quasi-reference electrode through a bipolar electron transfer mechanism (BPQRE). Finally, we demonstrate the effectiveness of the BPRE in other types of common electrochemical studies, including chronoamperometry, linear sweep voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy.

Figure 1.

Conductivity, ICP-MS, and Flame AAS results of the BPRE leakage tests for a) Cl⁻ and K⁺ and b) Ag⁺ after soaking the BPRE in ultrapure water for 31 days.

Figure 2.

Hypothesized mechanism of maintaining charge neutrality during a measurement of a a) commercial Ag/AgCl reference electrode or b) BPRE. Possible routes for current to flow through a BPRE sealed tip c) schematic and d) simplified circuit diagram. e) Open circuit potential experiments done in 1 mM ferricyanide and 1 mM ferrocyanide in 250 mM KCl using either a commercial Ag/AgCl reference electrode, a platinum BPRE, a fully sealed borosilicate BPRE with no tip, or a BPRE with a quartz tip.

Figure 3.

a) Schematic of experimental set-up, where a cylindrical BPRE is fabricated such that the wire|glass interface of the BPRE is not in contact with the test solution. b) Schematic of a Chipquik or Gorilla BPRE. c) Open circuit potential and e) cyclic voltammetric experiments done in 1 mM ferricyanide and 1 mM ferrocyanide in 250 mM KCl using a commercial reference electrode (red) or a tip-only BPRE (blue). d) Open circuit potential and f) cyclic voltammetric experiments done in 1 mM ferricyanide and 1 mM ferrocyanide in 250 mM KCl using a commercial reference electrode (red), a Chipquik BPRE (purple, dashed), or a Gorilla BPRE (pink, dotted) with a gold macroelectrode as the working electrode.

Figure 4.

a) Schematic of the electrochemical cell used in these experiments. b) open circuit potentiometric or c-e) cyclic voltammetric responses using a gold macroelectrode working electrode. These were taken using either 1) the cell in part a or 2) a conventional 3-electrode cell in 1 vial containing the sample solution of ferri- and ferrocyanide and either a commercial Ag/AgCl reference electrode (red) or a BPRE (blue). Relationship between b) open circuit potential and ln([ferricyanide]/[ferrocyanide]), c) scan rate or d) square root of scan rate and peak current on a gold macroelectrode (green, dashed), and e) [ferricyanide]/[ferrocyanide] and cathodic/anodic current on a gold ultramicroelectrode (yellow, dashed).

Figure 5.

Example open circuit potentiometric results of a gold macroelectrode in a solution of 1:1 ferri:ferrocyanide in 250 mM KCl using a) either three different Chipquik BPQREs (purples) or three different platinum wires (oranges), b) a Chipquik BPQRE or c) platinum wire quasi-reference electrode for repeated 30 minute measurements over 8 hours total.