Gamry reference electrodes

Product archive | Electrochemistry accessories, Gamry Instruments | Topic: Gamry Accessories Electrochemistry

CorrosionElectrochemical impedance spectroscopyElectrochemical InstrumentationQuartz microbalance applications

Choosing the right reference electrode determines the quality of electrochemical measurement results and the electrode’s service life.

The stability of the reference electrode potential and a low impedance of the reference electrode frit are key factors in ensuring the quality of electrochemical measurement results. Conversely, inaccuracies in potential measurement caused by an unstable reference electrode can lead to quantitative and qualitative measurement errors during data acquisition and analysis. Excessively high frit impedance in the range of several kΩ leads to instability in the potentiostat’s sensitive control system.

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Product Description

Ag/AgCl (-42 mV vs. SCE or 199 mV vs. NHE)

This reference electrode is very common in the cathodic corrosion protection of systems in contact with seawater because it is an environmentally friendly alternative to mercury-containing reference electrodes. The Ag/AgCl reference electrode is a relatively simple redox electrode whose stable potential (± 1 mV) is based on the equilibrium reaction between metallic silver (Ag) and silver chloride salt (AgCl). The electrolyte is saturated potassium chloride solution, which affects the reference electrode potential by changing the electrolyte concentration.

Note:

Silver chloride is somewhat soluble in highly concentrated potassium chloride solutions. Therefore, the use of a saturated potassium chloride solution is the starting point to prevent the silver chloride from dissolving. However, the reference electrode can also be filled with low-concentration potassium chloride solutions such as 1 M KCl or even seawater.

Hg/Hg₂Cl₂ (42 mV vs. Ag/AgCl or 241 mV vs. NHE)

This reference electrode is historically the most commonly used in various applications (electrochemical analysis, characterisation and maintenance of batteries, corrosion testing, etc.) and the most universal in the literature. The calomel reference electrode (SCE) filled with saturated potassium chloride solution is based on the redox couple mercury and mercury(I) chloride with ± 1 mV potential stability.

Hg/Hg₂SO₄ (399 mV vs. SCE or 640 mV vs. NHE)

The mercury/mercury sulphate reference electrode is required for chloride-free electrolytes and therefore has an electrolyte filling with a total potassium sulphate solution.

Hg/Hg₂SO₄ (394 mV vs. SCE or 635 mV vs. NHE)

The mercury/mercury sulphate reference electrode is required for chloride-free electrolytes and therefore has an electrolyte filling with 3.8 M sulphuric acid. Suitable for characterising lead-acid batteries (determination of the individual capacities of working and counter electrodes, individual behaviour of the electrodes during deep discharge and polarity reversal, optimisation of the charging and discharging parameters taking into account the half-cell potentials, tracking of different acid concentrations by measuring the diffusion potentials / cell voltages and the determination of defective cells and defective electrodes in a series of cells at the end of their service life).

Hg/HgO (98 mV vs. NHE)

The mercury/mercury oxide reference electrode is ideal for use in alkaline media. Battery development, energy storage research and development, process control and fundamental electrochemical studies will benefit from this soon-to-be industry standard due to its stable and reproducible potential. The potential stability is ± 1 mV and even after temperature increases of 125°C the reference electrode is relatively quickly back in the stationary potential range. The reference electrode is filled with 20 % KOH solution (4.24 M), but can also be requested in any KOH concentration in order to counteract leakage rate effects as closely as possible to the test medium. The reference electrode can of course also be filled with sodium hydroxide.

Ag/AgNO₃ (541 mV [10 mM AgNO₃] or 601 mV [100 mM AgNO₃] vs. NHE

The silver/silver nitrate (Ag/AgNO₃) reference electrode is ideal for use in non-aqueous/organic media. The reference electrode can either be filled with a solution of silver nitrate in acetonitrile or with silver nitrate in an application-specific organic solvent. Depending on the concentration, a potential of either 541 mV (10mM AgNO₃) or 601 mV (100mM AgNO₃) is set compared to the standard hydrogen reference electrode.

Pseudo / quasi-reference electrodes

Sometimes a classic reference electrode is not practical. For example, a glass electrode can easily break in outdoor 3-electrode arrangements. Many users therefore use a metal electrode made of the same material as the working electrode as a reference electrode. These reference electrodes are known as pseudo or quasi-reference electrodes (QRE). In connection with microelectrodes, the current is very small and therefore a 2-electrode setup with a combined counter electrode / QRE can be tolerable. The QRE used is usually silver or platinum as a well-positioned QRE material. In this context, well positioned means that the surface potential changes little when current flows through it.

Users therefore usually assume that particularly corrosion-resistant materials such as Hastelloy are well positioned QRE materials, which is unfortunately not the case. Gamry therefore advises not so much the use of Hastelloy as a pseudo-reference material, but rather recommends mild steel as a much better positioned material for use as a QRE. The use of “ionic liquids” excludes aqueous reference electrodes. Instead, it is suggested to use a silver wire as a reference electrode if possible. The potentials are then subsequently corrected against Ag/Ag⁺, although a substance such as ferrocene can also be dissolved subsequently and subsequently corrected against ferrocene / ferrocenium.

High temperature / pressurised reactor reference electrodes

Other applications that can cause headaches regarding the necessary use of a reference electrode are high temperature and/or high pressure experiments in a pressure reactor or autoclave. Fortunately, there are many variants of reference electrodes for pressurised reactors. The latest variant from 02/2003 is cited in the following publication:

– Simple and Robust External Reference Electrodes for High-Temperature Electrochemical Experiments” from Bosch, Bogaerts, and Zheng in CORROSION, Volume 59, page 163 (2003).

Note:

When using a reference electrode in a pressurised reactor (autoclave), a potentiostat that is free-floating from the mass must be used!

Cu/CuSO₄ (74 mV vs. SCE or 315 mV vs. NHE)

Gamry does not offer Cu/CuSO₄ reference electrodes. Many users use these or similar reference electrodes for outdoor measurements.