Which Reference Electrode Should You Use Depending on pH?

Factors such as electrolyte composition, temperature, and pH can affect the stability of a reference electrode potential. In this article, we focus exclusively on the effect of pH.

Using a reference electrode outside its recommended pH range can introduce errors of tens of millivolts, which is more than enough to distort the interpretation of electrochemical results. In the worst-case scenario, the electrode may undergo irreversible degradation.

Indicative pH Stability Ranges for Common Aqueous Reference Electrodes

The figure below provides a comparative overview of the pH ranges in which commonly used aqueous reference electrodes typically exhibit stable behavior. It serves as a visual guide to support the discussion developed in the following sections.

The intervals shown are intended as guidance and reflect pH-related limitations only.

You can explore and compare commercially available reference electrodes corresponding to these pH ranges directly on ElectroSeek and by clicking this link.

What Defines a Good Reference Electrode?

An ideal reference electrode is based on a well-defined redox half-reaction and fulfills four classical criteria:

• A constant and reproducible potential
• Obeys Nernst equation
• Chemical inertness with respect to the medium
• Fast and reversible electrode kinetics

A nearly ideal example is the Standard Hydrogen Electrode (SHE), based on the half-reaction H⁺ + e⁻ ⇌ ½ H₂. By convention, under strictly defined conditions, its potential is fixed at 0 V and it serves as the primary reference against which other redox potentials are tabulated.

However, due to its operational complexity, the SHE is rarely used in routine laboratory work. Instead, secondary reference electrodes are employed, each stable only within specific pH ranges.

Internal Filling Solution and Liquid Junction Potential: Why pH Matters

Most commercial reference electrodes contain an internal filling solution with a fixed composition, separated from the working solution by a porous junction. This architecture ensures that the chemistry defining the electrode potential remains constant.

However, ionic diffusion across the junction is unavoidable and gives rise to two key effects:

• Chemical cross-contamination, which can gradually alter the internal electrolyte composition or release species into the working solution.
• Liquid junction potential (LJP), which originates from differences in ionic mobility across the junction.

These mechanisms explain most practical limitations when reference electrodes are used outside their recommended pH range.

The LJP becomes particularly significant when large pH gradients exist between the internal solution and the external medium. Because H⁺ and OH⁻ are the most mobile ions in aqueous systems, extreme acidic or alkaline conditions can generate junction potentials of tens of millivolts that are difficult to correct reliably.

Scope and Limitations of the Examples Discussed

• Scope. The article focuses exclusively on the effect of pH and is not intended to be exhaustive.
• Electrode selection. Only some reference electrode systems are discussed.
• Electrode type. Only single-junction reference electrodes are considered.
• Excluded design aspects. Effects related to Luggin capillaries, frit materials, double-junction electrodes, pseudo-references, and similar configurations are not addressed.
• Other electrolyte effects. Electrolyte contamination effects other than pH are not considered.
• Practical pH limits. The exact pH range ultimately depends on factors like the internal electrolyte composition of the reference electrode.

Reference Electrodes for Strongly Alkaline Media

Hg/HgO and Ag/Ag₂O

Hg/HgO electrode is well known for its long-term robustness at strongly alkaline media. The Ag/Ag₂O electrode is often used as a mercury-free alternative, although it is generally less stable.

Their use in neutral or acidic media is strongly discouraged:

• The large pH gradient between the alkaline internal solution (KOH or NaOH) and the external medium generates a significant and unstable liquid junction potential.
• From a chemical standpoint, both HgO and Ag₂O dissolve under acidic conditions, destroying the active phase and irreversibly altering the electrode potential. Ag/Ag₂O is especially sensitive and may degrade even at mildly alkaline pH values. HgO typically below pH 4.

These electrodes should therefore be reserved exclusively for clearly alkaline systems.

Reference Electrodes for Acidic Media

Hg/Hg₂SO₄, Ag/Ag₂SO₄, and Cu/CuSO₄

Hg/Hg₂SO₄ is widely used and known due to its robustness under acidic conditions. Ag/Ag₂SO₄ is often chosen as a mercury-free alternative or in applications where chloride ions must be avoided, including pH ranges overlapping with Ag/AgCl. Cu/CuSO₄ electrode is another classical option, traditionally used in cathodic protection and corrosion control systems.

In neutral, mildly acidic, or alkaline media, these electrodes exhibit important limitations. The ingress of OH⁻ can lead to the formation of metal oxides or hydroxides, altering the redox system that defines the reference potential. This effect is most severe for mercury-based electrodes, followed by copper-based systems, and to a lesser extent silver-based ones.

Additionally, the pH gradient between the internal electrolyte and an alkaline external solution produces a significant liquid junction potential. Even in the absence of oxide or hydroxide formation, this alone makes reliable measurements at alkaline pH unfeasible.

Reference Electrodes for Neutral or Moderately Acidic/Alkaline Media

Ag/AgCl and the Calomel Electrode (Hg/Hg₂Cl₂, SCE)

These are the most widely used reference electrodes in general aqueous electrochemistry. Both typically employ KCl as the internal electrolyte and offer excellent long-term stability and reproducibility near neutral pH.

The saturated calomel electrode (SCE) served as a standard reference for decades. Its use has declined due to environmental restrictions associated with mercury, with Ag/AgCl becoming the most common and versatile substitute. Nevertheless, the SCE remains well known for its exceptional robustness and stability when properly maintained.

Both electrodes perform well in neutral solutions and in moderately acidic or basic environments. Under extreme conditions, however, clear limitations arise:

• In strongly alkaline media, OH⁻ can convert AgCl or Hg₂Cl₂ into metal oxides, altering the reference potential.
• In highly acidic solutions, the high mobility of H⁺ relative to K⁺ and Cl⁻ leads to significant liquid junction potentials.

Conclusions

Each reference electrode exhibits a specific pH interval in which its potential is stable and reproducible. Outside this range, errors may arise due to chemical degradation reactions and liquid junction potentials associated with extreme H⁺ or OH⁻ gradients. In many cases, these errors are comparable to or even larger than the electrochemical phenomena under investigation.

Selecting the appropriate reference electrode based on the pH of the working medium is therefore not a minor experimental detail, but a fundamental requirement for obtaining reliable and scientifically defensible electrochemical data.

This article should be understood as a practical guide for initial reference electrode selection. In critical systems, extreme conditions, or applications where potential accuracy is essential, the safest approach is to consult the electrode manufacturer directly or seek specialized technical advice.

On ElectroSeek, you can explore commercially available reference electrodes by type and application, and contact manufacturers directly through our platform if you need to confirm electrode compatibility with your specific experimental system. If you are dealing with non-standard conditions or are unsure about reference electrode selection, feel free to reach out and we will be happy to help.

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