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When performing electrochemical experiments in non-aqueous media, finding the ideal reference electrode can be challenging.  Here, we will describe constructing and using such a non-aqueous reference electrode.  We offer kits, based on silver electrode materials, in which users can develop their own non-aqueous reference electrode.

A good and dependable reference electrode provides a stable potential and is not prone to environmental factors. Ideal reference electrodes have the following characteristics:

1. A reversible reference redox pair (fast electron transfer rate)
2. Good contact between the redox pair
3. A stable liquid junction potential that is unaffected by temperature or local chemical composition around the frit

In practice, all reference electrodes have unstable liquid junction potentials that are affected by temperature and local chemical composition near the frit.  For example, the silver chloride reference electrode, whose shorthand electrochemical reaction is written as Ag|AgCl|KCl (saturated), is the most widely used practical reference electrode.  It is as stable and reliable as the saturated calomel reference electrode (SCE, whose shorthand electrochemical reaction is written as Hg|Hg₂Cl₂|KCl (saturated), but not as toxic.  Ag/AgCl and SCE reference electrodes suffer only from an unstable liquid junction potential affected by temperature and local chemical composition near the frit.  In aqueous solutions, changes in the liquid junction potential produce a drift on the order of few millivolts over a period of one year because of the high K⁺ and Cl^– concentrations; however, when switching to non-aqueous systems, the liquid junction potential can be rather large.  At the interface of two solvents, for example at the aqueous|non-aqueous interface or the interface of two different non-aqueous solvents, it can be hundreds of millivolts.

When the stable Ag/AgCl reference electrode is used for non-aqueous systems, the electrolyte inside the Ag/AgCl reference electrode compartment (saturated KCl in water or a polar organic solvent) is often different than the main electrolyte (a salt in non-aqueous solvent).  Since the reference electrode is placed in close proximity to the working electrode to reduce Ohmic drop, any leakage or mixing of the two electrolytes causes an unwanted response at the working electrode.  Therefore, to avoid contamination of the main electrolyte by the reference electrolyte, a frit is used; frits slow down electrolyte mixing times.  Even with the use of a frit, reference electrodes used for non-aqueous systems may encounter the following problems:

• Contamination of the external electrolyte with water, the filling solution solvent. Despite using a frit, it is still possible for internal reference electrode filling solution to diffuse to the external electrolyte solution.
• Frit pore plugging. Due to the insolubility of KCl in organic solvents, plugged pores are a common reference electrode problem. The use of a reference electrode with plugged pores will often cause potentiostat issues.  For example, plugged pores allow external electromagnetic fields to cause interference, resulting in noisy data. In other words, the impedance (across the frit interface) of the reference electrode increases when pores are plugged. In extreme cases, a completely blocked reference electrode will result in the loss of potentiostat control because it loses its reference point.
• Reference electrode potential drift due to the liquid junction potential across the frit interface.

To alleviate the effects of these common problems, several methods have been proposed, each of which has its own shortcomings (see Table 1).

To illustrate, a silver/silver nitrate reference electrode, whose shorthand reaction is written as Ag|AgNO₃ in CH₃CN (10 mM)|frit, can be used as an alternative redox pair reference electrode.  The silver/silver nitrate redox couple is similar to a primary redox standard; however, the silver/silver nitrate reference electrode suffers from certain instabilities.  For example, in the presence of oxygen, Ag₂O forms and disrupts the Ag/Ag⁺ redox pair, and Ag⁺ can leak into the main chamber, affecting the electrochemistry that a researcher is investigating.

Secondly, one can combine the silver/silver nitrate reference electrode with a double junction (or salt bridge) to minimize Ag⁺ leakage, but an increase in electrode impedance will result, possibly leading to reference electrode instability over time.

The third modification, a pseudo reference electrode, consists of a pure, freshly polished silver wire. The silver pseudo reference electrode functions because a natural oxide layer forms on the silver wire, creating a redox reaction whose shorthand notation is Ag|Ag₂O|frit. As alluded to above, the silver/silver oxide pseudo reference electrode does not have a stable and reproducible redox potential and must therefore be calibrated using an internal standard. Ferrocene is one of the most common internal standards due to its solubility in non-aqueous solvents as well as its highly reversible and well-behaved kinetics. Users will often add solid ferrocene to their electrochemical cell and find the potential at which its redox character is clearly observed. Then, all other measurements are made vs. ferrocene (whose redox potential is known and reported in the literature). Of note, if ferrocene redox peaks overlap with the analyte redox peaks, a different internal standard must be used. Additionally, changes in the chemical composition of the main electrolyte may occur as electrochemical reactions occur, causing a reference potential drift. To prevent this potential drift, it is important to isolate the silver wire with a fritted tube.

This application will focus on reference electrodes created using a silver wire.  Pine Research offers Ag pseudo reference electrodes, standard size (9.5 mm OD, see Figure 1)  and LowProfile size (3.5 mm OD, see Figure 2).   The following discussion applies to the both electrode sizes and the following list describes the features of either size:

1. A glass tube with a mounted frit
2. A silver wire
3. An air-tight way to mount the silver wire
4. Electrolyte solution

Figure 1: Standard Size (9 mm OD) Ag Non-Aqueous Reference Electrode Kit

Figure 2: LowProfile Ag Non-Aqueous Reference Electrode Kit

This application looks to explore the stability across non-aqueous reference electrodes with varying frit composition, glass tube size disparity, and electrolyte filling solution.  Specifically, two reference electrodes have been reviewed and discussed in this document: (A) LowProfile Glass Frit Ag Pseudo Reference Electrode (Part Numbers LowProfile Glass Frit Ag Pseudo Reference Electrode (93 mm) and LowProfile Glass Frit Ag Pseudo Reference Electrode (62 mm)); (B) Standard Size Ag Pseudo Reference Electrode with ceramic frit (Part Number AKREF0033).  Several variations of electrode and filling solutions have been investigated and reported in this document (see Table 2).

While large drifts (1 mV/min) are certainly due to the instability of a reference electrode, the origin of small drifts (less than 0.2 mV/min) observed in both acetonitrile and dichloromethane is not clear.  It cannot be excluded that ferrocene redox chemistry undergoes subtle changes as repetitive CV scans are performed.  In other words, the drift could be due to a shift in the oxidation potential caused by changes in surface electrochemistry or the surface state of the electrode.  In addition, the oxidation product ferrocenium is less stable in organic solvent; a thin film of ferrocenium side reaction products can block the electrode surface, causing changes to the peak position.

The most stable non-aqueous reference electrode is Ag|AgNO₃ (10 mM) in 100 mM NBu₄PF₆ in CH₃CN|frit with a drift rate of less than 0.1 mV/s.  It is recommended that a non-aqueous reference electrode be calibrated daily using a known redox pair such as ferrocene in a standard solution (1-5 mM ferrocene in 100 mM NBu₄PF₆ in CH₃CN). While the silver/silver nitrate reference electrode is the most stable, researchers must exercise caution when using it.  Since silver ions are soluble in acetonitrile, they can pass through the frit and contaminate the bulk solution.  To minimize silver ion leakage, a double junction can be used.  Nevertheless, in certain applications like the study of hydrogen evolution catalysis, even a small amount of silver ion contamination shifts the acid reduction potential, convoluting data obtained for the catalyst of interest^1-2. In these instances, it is better to use a pseudo reference electrode.  A pseudo reference electrode will not contaminate the bulk electrolyte solution.  It does, however, experience greater potential drift.  To account for this, the pseudo reference electrode should be allowed to equilibrate for at least an hour before use and referenced to an internal standard like ferrocene.  If possible, the redox peaks of ferrocene should be included on each cyclic voltammogram taken, or measured directly after a linear sweep experiment.