Chapter 1: Understanding the "Reference electrode" role in Electrochemical Measurements

1-1 Why Are Reference Electrodes Used in Electrochemical Measurements?

In general, electrochemical reactions can be carried out using two electrodes: a positive electrode and a negative electrode. In practice, many electrolysis reactions are performed using such two-electrode systems.
However, when a voltage is applied between these two electrodes (the working electrode and the counter electrode), the applied potential is divided between them. As a result, while the potential difference between the two electrodes is known, the exact potential at each individual electrode cannot be determined directly. Furthermore, when electron transfer occurs in the solution due to electrolysis (i.e., when a current I flows), a potential drop (E = IR) occurs due to the resistance R of the solution, making accurate potential measurement difficult.
So how can we accurately know and control the potential of the working electrode where the reaction of interest occurs? In electrochemical measurements and organic electrosynthesis, precise control of the working electrode potential is critically important.

1-2 The Universal Potential Scale "SHE": Structure and Challenges of the Standard Hydrogen Electrode

An electrode that serves as a potential reference is called a reference electrode. Before discussing reference electrodes in detail, let us first review the main functions of an electrochemical instrument known as a potentiostat.

A potentiostat controls the system so that a specified potential (for example, +500 mV) is always applied to the working electrode relative to the reference electrode. Therefore, accurate control of the working electrode potential requires a reference electrode that provides a stable and universal potential.
The "Standard Hydrogen Electrode (SHE)" is proposed as the reference for this purpose. The SHE maintains a universal potential by immersing a platinum wire in a solution of pH = 0 ([aH+] = 1) and constantly bubbling hydrogen gas at 1 atm (Fig. 1). By using the SHE as a reference, the electrode reaction potentials of various substances can be compared against a common standard.
However, SHE is difficult to operate in practice due to the need for continuous hydrogen gas supply and strict safety management, making it unsuitable for routine use.

1-3 Practical Selection of Reference Electrodes: What Are “Secondary Standards”?

Although SHE is theretically the most stable reference, in practical measurements it is common to use secondary standard reference electrodes (non-polarizable electrodes) that have known potentials relative to SHE (Table 1).
In aqueous systems, electrodes that maintain a stable potential through equilibrium between internal oxidized and reduced species—without the use of hydrogen gas—are commonly employed. Typical examples include the silver/silver chloride electrode (Ag/AgCl electrode) and the calomel electrode. In Ag/AgCl electrodes, a reversible redox reaction such as the one shown in Equation 1 occurs at the silver surface. The rightward direction represents oxidation, and the leftward direction represents reduction. Because these reactions occur equally in both directions at equilibrium, the electrode potential remains stable and suitable for use as a reference.

Ag + Cl⁻ ⇄ AgCl + e⁻  (Equation 1)

Calomel electrodes also exhibit stable potentials; however, because they contain mercury, their use has declined due to environmental concerns. As a result, Ag/AgCl electrodes are now widely used.
BAS provides reference electrodes that use polymethylpentene resin holders for resistance to external impact, as well as ceramic liquid junctions (porous ceramics) with improved alkaline resistance. In addition to potassium chloride (KCl), BAS also offers Ag/AgCl electrodes that use sodium chloride (NaCl) as the internal solution.
This lineup allows users to select reference electrodes that maintain long-term stability depending on the measurement environment. For example, when perchloric acid is used in applications such as fuel cell catalyst evaluation, electrodes that use KCl as the internal solution may suffer clogging of the liquid junction due to precipitation of low-solubility potassium perchlorate (KClO₄). In such cases, selecting a NaCl-based Ag/AgCl reference electrode (RE-1BP, aqueous reference electrode) enables stable measurements over extended periods.
There is also a reference electrode similar to SHE called the Reversible Hydrogen Electrode (RHE: Reversible Hydrogen Electrode). Like SHE, RHE uses hydrogen gas, but differs in that the pH of its internal solution is the same as that of the measurement solution. The potential of RHE follows the Nernst equation: at pH = 0 it has the same potential as SHE, but for each increase of one pH unit, the potential shifts by −59 mV relative to SHE. Although this change in potential may seem problematic, it poses no issue because the potential can be calculated accurately from the pH using the Nernst equation. A major advantage of RHE is that, because it uses an internal solution with the same pH as the measurement solution, no liquid junction potential is generated. For reactions involving H⁺, the reaction potential itself depends on pH. For example, when scanning a platinum electrode in the negative direction using an Ag/AgCl reference electrode, water decomposition (hydrogen gas evolution) occurs at relatively positive potentials in acidic solutions, whereas in alkaline solutions it begins at more negative potentials. With RHE, the reference electrode potential shifts according to pH following the Nernst equation, so the potential at which water begins to decompose relative to RHE remains constant regardless of pH. Visualizing this difference can help in understanding the concept.
When measurements are performed using the RHE scale, the potentials for hydrogen evolution and oxygen reduction reactions (ORR) can be readily identified, making it easier to set the potential scan range for cyclic voltammetry (CV).
In the following chapters, in addition to the characteristics of each reference electrode introduced here, we will explain methods for selecting, checking, and maintaining the reference electrodes handled by BAS. ________________________________________
Reference Materials
Figure 1: Schematic illustration of the operation of the Standard Hydrogen Electrode
Figure 2: Schematic diagram of a three-electrode configuration
Electrochemical measurements are performed using a combination of a working electrode, a counter electrode, and a reference electrode.
The potentiostat controls the potential of the working electrode using the reference electrode as the potential standard.


Table 1: Comparison of Representative Reference Electrodes Electrode Composition / Principle Standard Potential (vs. SHE) Main Features Typical Applications SHE (Standard Hydrogen Electrode) Pt + H₂ gas + H⁺ (a = 1) 0.000 V Fundamental reference; difficult to operate Fundamental research, calibration RHE (Reversible Hydrogen Electrode) Pt + H₂ + solution with same pH as sample pH-dependent (−59 mV/pH) No liquid junction potential; useful for pH-dependent systems Analysis of electrode reaction mechanisms Ag/AgCl electrode Ag AgCl KCl (saturated) +0.199 V (sat. KCl) Calomel electrode Hg Hg₂Cl₂ KCl (saturated) +0.244 V NaCl-type Ag/AgCl electrode Ag AgCl NaCl solution +0.196 V (3 M NaCl) Except for SHE, the standard potentials of reference electrodes depend on temperature and concentration. The values shown are representative values at 25 °C.