A reference electrode is an

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or a gas). In electrochemical cells, electrodes are essential parts that can consist of a varie ...

that has a stable and well-known

electrode potential An electrode is an electrical conductor used to make contact with a nonmetallic part of a Electronic circuit, circuit (e.g. a semiconductor, an electrolyte, a vacuum or a gas). In electrochemical cells, electrodes are essential parts that can c ...

. The overall chemical reaction taking place in a cell is made up of two independent half-reactions, which describe chemical changes at the two electrodes. To focus on the reaction at the working electrode, the reference electrode is standardized with constant (buffered or saturated)

concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', '' molar concentration'', '' number concentration'', ...

s of each participant of the redox reaction. There are many ways reference electrodes are used. The simplest is when the reference electrode is used as a half-cell to build an

electrochemical cell An electrochemical cell is a device that either generates electrical energy from chemical reactions in a so called galvanic cell, galvanic or voltaic cell, or induces chemical reactions (electrolysis) by applying external electrical energy in an ...

. This allows the

potential Potential generally refers to a currently unrealized ability. The term is used in a wide variety of fields, from physics to the social sciences to indicate things that are in a state where they are able to change in ways ranging from the simple r ...

of the other half cell to be determined. An accurate and practical method to measure an electrode's potential in isolation ( absolute electrode potential) has yet to be developed.

Aqueous reference electrodes

Common reference electrodes and potential with respect to the standard hydrogen electrode (SHE): *

Standard hydrogen electrode In electrochemistry, the standard hydrogen electrode (abbreviated SHE), is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials. Its absolute electrode potential is estimated to be at 25 ° ...

(SHE) (E = 0.000 V) activity of H⁺ = 1 Molar * Normal hydrogen electrode (NHE) (E ≈ 0.000 V) concentration H⁺ = 1 Molar * Reversible hydrogen electrode (RHE) (E = 0.000 V - 0.0591 × pH) at 25 °C * Saturated calomel electrode (SCE) (E = +0.241 V saturated) * Copper–copper(II) sulfate electrode (CSE) (E = +0.314 V) *

Silver chloride electrode A silver chloride electrode is a type of reference electrode, commonly used in Electrochemistry, electrochemical measurements. For environmental reasons it has widely replaced the saturated calomel electrode. For example, it is usually the intern ...

(E = +0.197 V in saturated KCl) *

(E = +0.210 V in 3.0 mol KCl/kg) *

(E = +0.22249 V in 3.0 mol KCl/L) * pH-electrode (in case of pH buffered solutions, see

buffer solution A buffer solution is a solution where the pH does not change significantly on dilution or if an acid or base is added at constant temperature. Its pH changes very little when a small amount of strong acid or base is added to it. Buffer solution ...

) * Palladium-hydrogen electrode * Dynamic hydrogen electrode (DHE) * Mercury-mercurous sulfate electrode (MSE) (E = +0.64 V in sat'd K₂SO₄, E = +0.68 V in 0.5 M H₂SO₄) CopperSulphateElectrode

Nonaqueous reference electrodes

While it is convenient to compare between solvents to qualitatively compare systems, this is not quantitatively meaningful. Much as pK_a are related between solvents, but not the same, so is the case with E°. While the SHE might seem to be a reasonable reference for nonaqueous work as it turns out the platinum is rapidly poisoned by many solvents including acetonitrile causing uncontrolled drifts in potential. Both the SCE and saturated Ag/AgCl are aqueous electrodes based around saturated aqueous solution. While for short periods it may be possible to use such aqueous electrodes as references with nonaqueous solutions the long-term results are not trustworthy. Using aqueous electrodes introduces undefined, variable, and unmeasurable junction potentials to the cell in the form of a liquid-liquid junction as well as different ionic composition between the reference compartment and the rest of the cell. The best argument against using aqueous reference electrodes with nonaqueous systems, as mentioned earlier, is that potentials measured in different solvents are not directly comparable. For instance, the potential for the Fc^0/+ couple is sensitive to solvent.Connelly, N. G., Geiger, W. E., "Chemical Redox Agents for Organometallic Chemistry", Chem. Rev. 1996, 96, 877.Aranzaes, J. R., Daniel, M.-C., Astruc, D. "Metallocenes as references for the determination of redox potentials by cyclic voltammetry. Permethylated iron and cobalt sandwich complexes, inhibition by polyamine dendrimers, and the role of hydroxy-containing ferrocenes", Can. J. Chem., 2006, 84(2), 288-299. doi:10.1139/v05-262 A quasi-reference electrode (QRE) avoids the issues mentioned above. A QRE with

ferrocene Ferrocene is an organometallic chemistry, organometallic compound with the formula . The molecule is a Cyclopentadienyl complex, complex consisting of two Cyclopentadienyl anion, cyclopentadienyl rings sandwiching a central iron atom. It is an o ...

or another internal standard, such as

cobaltocene Cobaltocene, known also as bis(cyclopentadienyl)cobalt(II) or even "bis Cp cobalt", is an organocobalt compound with the formula Co(C5H5)2. It is a dark purple solid that sublimes readily slightly above room temperature. Cobaltocene was discover ...

or decamethylferrocene, referenced back to ferrocene is ideal for nonaqueous work. Since the early 1960s ferrocene has been gaining acceptance as the standard reference for nonaqueous work for a number of reasons, and in 1984, IUPAC recommended ferrocene (0/1+) as a standard redox couple. The preparation of the QRE electrode is simple, allowing for a fresh reference to be prepared with each set of experiments. Since QREs are made fresh, there is also no concern with improper storage or maintenance of the electrode. QREs are also more affordable than other reference electrodes. To make a quasi-reference electrode (QRE): # Insert a piece of silver wire into concentrated HCl then allow the wire to dry on a lint-free cleaning cloth. This forms an insoluble layer of AgCl on the surface of the electrode and gives you an Ag/AgCl wire. Repeat dipping every few months or if the QRE starts to drift. # Obtain a Vycor glass

frit A frit is a ceramic composition that has been fused, quenched, and granulated. Frits form an important part of the batches used in compounding enamels and ceramic glazes; the purpose of this pre-fusion is to render any soluble and/or toxic com ...

(4 mm diameter) and glass tubing of similar diameter. Attach Vycor glass frit to the glass tubing with heat shrink Teflon tubing. # Rinse then fill the clean glass tube with supporting electrolyte solution and insert Ag/AgCl wire. # The

(0/1+) couple should lie around 400 mV versus this Ag/AgCl QRE in an acetonitrile solution. This potential will vary up to 200 mV with specific undefined conditions, thus adding an internal standard such as ferrocene at some point during the experiment is always necessary.

Pseudo reference electrodes

A pseudo reference electrode is a term that is not well defined and borders on having multiple meanings since ''pseudo'' and ''quasi'' are often used interchangeably. They are a class of electrodes named pseudo-reference electrodes because they do not maintain a constant potential but vary predictably with conditions. If the conditions are known, the potential can be calculated and the electrode can be used as a reference. Most electrodes work over a limited range of conditions, such as pH or temperature, outside of this range the electrodes behavior becomes unpredictable. The advantage of a pseudo-reference electrode is that the resulting variation is factored into the system allowing researchers to accurately study systems over a wide range of conditions. Yttria-stabilized zirconia ( YSZ) membrane electrodes were developed with a variety of redox couples, e.g., Ni/NiO. Their potential depends on pH. When the pH value is known, these electrodes can be employed as a reference with notable applications at elevated temperatures.R.W. Bosch, D.Feron, and J.P. Celis, "Electrochemistry in Light Water Reactors", CRC Press, 2007.

Aqueous reference electrodes

Nonaqueous reference electrodes

Pseudo reference electrodes

See also

References

Further reading