Reference electrode

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Standard hydrogen electrode scheme: 1) Platinized platinum electrode, 2) Hydrogen gas, 3) Acid solution with an activity of H = 1 mol/L, 4) Hydroseal for prevention of oxygen interference, 5) Reservoir via which the second half-element of the galvanic cell should be attached. The connection can be direct, through a narrow tube to reduce mixing, or through a salt bridge, depending on the other electrode and solution. This creates an ionically conductive path to the working electrode of interest. Standard hydrogen electrode 2009-02-06.svg
Standard hydrogen electrode scheme:
1) Platinized platinum electrode,
2) Hydrogen gas,
3) Acid solution with an activity of H = 1 mol/L,
4) Hydroseal for prevention of oxygen interference,
5) Reservoir via which the second half-element of the galvanic cell should be attached. The connection can be direct, through a narrow tube to reduce mixing, or through a salt bridge, depending on the other electrode and solution. This creates an ionically conductive path to the working electrode of interest.

A reference electrode is an electrode that has a stable and well-known electrode potential. 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) concentrations of each participant of the redox reaction. [1]

Contents

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. This allows the potential 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):

Cu-Cu(II) reference electrode CopperSulphateElectrode.png
Cu-Cu(II) reference electrode
Ag-AgCl reference electrode Ag-AgCl Reference Electrode.jpg
Ag-AgCl reference electrode

Nonaqueous reference electrodes

While it is convenient to compare between solvents to qualitatively compare systems, this is not quantitatively meaningful. Much as pKa 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 [3] 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. [4] 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. [5] For instance, the potential for the Fc0/+ couple is sensitive to solvent. [6] [7]

Solvent FormulaE1/2 (V)
(FeCp20/+ vs SCE,
0.1 M NBu4PF6 at 298 K)
Acetonitrile CH3CN0.40, [6] 0.382 [7]
Dichloromethane CH2Cl20.46, [6] 0.475 [7]
Tetrahydrofuran THF0.56, [6] 0.547 [7]
Dimethylformamide DMF0.45, [6] 0.470 [7]
Acetone (CH3)2C=O0.48 [6]
Dimethylsulfoxide DMSO0.435 [7]
Dimethoxyethane DME0.51, [6] 0.580 [7]

A quasi-reference electrode (QRE) avoids the issues mentioned above. A QRE with ferrocene or another internal standard, such as cobaltocene 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. [8] 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):

  1. 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.
  2. Obtain a Vycor glass frit (4 mm diameter) and glass tubing of similar diameter. Attach Vycor glass frit to the glass tubing with heat shrink Teflon tubing.
  3. Rinse then fill the clean glass tube with supporting electrolyte solution and insert Ag/AgCl wire.
  4. The ferrocene (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. [9]

See also

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<span class="mw-page-title-main">Electrochemical cell</span> Electro-chemical device

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Ferrocene is an organometallic compound with the formula Fe(C5H5)2. The molecule is a complex consisting of two cyclopentadienyl rings sandwiching a central iron atom. It is an orange solid with a camphor-like odor that sublimes above room temperature, and is soluble in most organic solvents. It is remarkable for its stability: it is unaffected by air, water, strong bases, and can be heated to 400 °C without decomposition. In oxidizing conditions it can reversibly react with strong acids to form the ferrocenium cation Fe(C5H5)+2. Ferrocene and the ferrocenium cation are sometimes abbreviated as Fc and Fc+ respectively.

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<span class="mw-page-title-main">Electrolytic cell</span> Cell that uses electrical energy to drive a non-spontaneous redox reaction

An electrolytic cell is an electrochemical cell that utilizes an external source of electrical energy to force a chemical reaction that would otherwise not occur. The external energy source is a voltage applied between the cell's two electrodes; an anode and a cathode, which are immersed in an electrolyte solution. This is in contrast to a galvanic cell, which itself is a source of electrical energy and the foundation of a battery. The net reaction taking place in a galvanic cell is a spontaneous reaction, i.e., the Gibbs free energy remains -ve, while the net reaction taking place in an electrolytic cell is the reverse of this spontaneous reaction, i.e., the Gibbs free energy is +ve.

In electrochemistry, standard electrode potential, or , is a measure of the reducing power of any element or compound. The IUPAC "Gold Book" defines it as; "the value of the standard emf of a cell in which molecular hydrogen under standard pressure is oxidized to solvated protons at the left-hand electrode".

In electrochemistry, the standard hydrogen electrode, is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials. Its absolute electrode potential is estimated to be 4.44 ± 0.02 V at 25 °C, but to form a basis for comparison with all other electrochemical reactions, hydrogen's standard electrode potential is declared to be zero volts at any temperature. Potentials of all other electrodes are compared with that of the standard hydrogen electrode at the same temperature.

<span class="mw-page-title-main">Cyclic voltammetry</span> Method of analyzing electrochemical reactions

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Ferrocenium hexafluorophosphate is an organometallic compound with the formula [Fe(C5H5)2]PF6. This salt is composed of the cation [Fe(C5H5)2]+ and the hexafluorophosphate anion (PF−
6). The related tetrafluoroborate is also a popular reagent with similar properties. The ferrocenium cation is often abbreviated Fc+ or Cp2Fe+. The salt is deep blue in color and paramagnetic.

<span class="mw-page-title-main">Rhodocene</span> Organometallic chemical compound

Rhodocene is a chemical compound with the formula [Rh(C5H5)2]. Each molecule contains an atom of rhodium bound between two planar aromatic systems of five carbon atoms known as cyclopentadienyl rings in a sandwich arrangement. It is an organometallic compound as it has (haptic) covalent rhodium–carbon bonds. The [Rh(C5H5)2] radical is found above 150 °C (302 °F) or when trapped by cooling to liquid nitrogen temperatures (−196 °C [−321 °F]). At room temperature, pairs of these radicals join via their cyclopentadienyl rings to form a dimer, a yellow solid.

<span class="mw-page-title-main">Neopolarogram</span>

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<span class="mw-page-title-main">Decamethylferrocene</span> Chemical compound

Decamethylferrocene or bis(pentamethylcyclopentadienyl)iron(II) is a chemical compound with formula Fe(C5(CH3)5)2 or C20H30Fe. It is a sandwich compound, whose molecule has an iron(II) cation Fe2+ attached by coordination bonds between two pentamethylcyclopentadienyl anions (Cp*−, (CH3)5C−5). It can also be viewed as a derivative of ferrocene, with a methyl group replacing each hydrogen atom of its cyclopentadienyl rings. The name and formula are often abbreviated to DmFc, Me10Fc or FeCp*2.

References

  1. ↑ Bard, Allen J.; Faulkner, Larry R. (2000-12-18). Electrochemical Methods: Fundamentals and Applications (2 ed.). Wiley. ISBN   978-0-471-04372-0.
  2. ↑ Bates, R.G. and MacAskill, J.B. (1978). "Standard potential of the silver-silver chloride electrode". Pure & Applied Chemistry, Vol. 50, pp. 1701–1706, http://www.iupac.org/publications/pac/1978/pdf/5011x1701.pdf
  3. ↑ Palibroda, Evelina (Jan 1967). "Note sur l'activation anodique de la surface du métal support de l'électrode à hydrogène". Electroanalytical Chemistry and Interfacial ElectrochemistryElectroanalytical Chemistry and Interfacial Electrochemistry. 15 (15): 92–95. doi:10.1016/0022-0728(67)85013-7.
  4. ↑ Pavlishchuk, Vitaly V.; Anthony W. Addison (January 2000). "Conversion constants for redox potentials measured versus different reference electrodes in acetonitrile solutions at 25°C". Inorganica Chimica Acta. 298 (1): 97–102. doi:10.1016/S0020-1693(99)00407-7.
  5. ↑ Geiger, William E. (2007-11-01). "Organometallic Electrochemistry: Origins, Development, and Future". Organometallics. 26 (24): 5738–5765. doi:10.1021/om700558k.
  6. 1 2 3 4 5 6 7 Connelly, N. G., Geiger, W. E., "Chemical Redox Agents for Organometallic Chemistry", Chem. Rev. 1996, 96, 877.
  7. 1 2 3 4 5 6 7 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
  8. ↑ Gritzner, G.; J. Kuta (1984). "Recommendations on reporting electrode potentials in nonaqueous solvents". Pure Appl. Chem. 56 (4): 461–466. doi: 10.1351/pac198456040461 . Retrieved 2016-09-30.
  9. ↑ R.W. Bosch, D.Feron, and J.P. Celis, "Electrochemistry in Light Water Reactors", CRC Press, 2007.

Further reading

  1. ↑ "Reference Electrodes". NACE International. Retrieved 2020-06-29.
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