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| [[Image:Ag-AgCl Reference Electrode wide.jpg|thumb|100px|right|Ag-AgCl reference electrode]]
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| A '''silver chloride electrode''' is a type of [[reference electrode]], commonly used in [[Electrochemistry|electrochemical]] measurements. For example, it is usually the internal reference electrode in [[pH meter]]s. As another example, the silver chloride electrode is the most commonly used reference electrode for testing [[cathodic protection]] [[corrosion]] control systems in [[sea water]] environments.
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| The electrode functions as a [[redox electrode]] and the reaction is between the [[silver]] metal (Ag) and its salt — [[silver chloride]] (AgCl, also called silver(I) chloride).
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| The corresponding equations can be presented as follows:
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| :<math> Ag^+ + e^- \leftrightharpoons Ag(s)</math>
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| :<math> AgCl(s) \leftrightharpoons Ag^+ + Cl^-</math>
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| or an overall reaction can be written:
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| :<math> AgCl(s)+ e^- \leftrightharpoons Ag(s)+ Cl^-</math>
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| This reaction is characterized by fast electrode kinetics, meaning that a sufficiently high [[current (electricity)|current]] can be passed through the electrode with the 100% efficiency of the redox reaction ([[solvation|dissolution]] of the metal or cathodic [[Deposition (chemistry)|deposition]] of the silver-ions). The reaction has been proved to obey these equations in solutions with pH’s of between 0 and 13.5.
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| The [[Nernst equation]] below shows the dependence of the potential of the silver-silver(I) chloride electrode on the [[activity (chemistry)|activity]] or effective [[concentration]] of chloride-ions:
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| :<math>E= E^0 - \frac{RT}{F} \ln a_{Cl^{-}}</math> | |
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| The standard electrode potential E<sup>0</sup> against [[standard hydrogen electrode]] (SHE) is 0.230V ± 10mV. The potential is however very sensitive to traces of bromide ions which make it more negative. (The more exact standard potential given by an IUPAC review paper is 0.22249 V, with a standard deviation of 0.13 mV at 25 °C.<ref>R.G. Bates and J.B. MacAskill, "Standard Potential of the Silver-Silver Chloride Electrode", Pure & Applied Chem., Vol. 50, pp. 1701—1706, http://www.iupac.org/publications/pac/1978/pdf/5011x1701.pdf</ref>)
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| == Applications ==
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| Commercial reference electrodes consist of a plastic tube electrode body. The electrode is a silver wire that is coated with a thin layer of silver chloride, either physically by dipping the wire in molten silver chloride, or chemically by electroplating the wire in concentrated hydrochloric acid.<ref>Detail of Making and Setting up a Microelectrode, University of Denver, http://carbon.cudenver.edu/~bstith/detailelectrode.doc (link is obsolete)</ref>
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| A porous plug on one end allows contact between the field environment with the silver chloride [[electrolyte]]. An insulated lead wire connects the silver rod with measuring instruments. A [[voltmeter]] negative lead is connected to the test wire. The reference electrode contains [[potassium chloride]] to stabilize the silver chloride concentration.
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| The potential of a silver:silver chloride reference electrode with respect to the [[standard hydrogen electrode]] depends on the electrolyte composition.
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| {| class="wikitable"
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| |+'''Reference Electrode Potentials'''
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| |-
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| !Electrode !! Potential E<sup>0</sup>+E<sub>lj</sub> !! Temperature Coef.
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| |-
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| | || ([[volt|V]]) at 25 °C|| (mV/°C) at around 25 °C
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| |-
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| | SHE || 0.000 || 0.000 <ref>{{Citation
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| | doi = 10.1063/1.555839
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| | last = Bratsch
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| | first = Steven G.
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| | title = Standard Electrode Potentials and Temperature Coefficients in Water at 298.15 K
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| | journal = J. Phys. Chem. Ref. Data
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| | volume = 18
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| | issue = 1
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| | pages = 1–21
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| | year = 1989
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| | url = http://www.nist.gov/srd/PDFfiles/jpcrd355.pdf|bibcode = 1989JPCRD..18....1B }}</ref>
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| |-
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| |Ag/AgCl/Sat. KCl || +0.197 || -1.01 {{Citation needed|date=February 2009}}
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| |-
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| |Ag/AgCl/3.5 [[Concentration#Molality|mol/kg]] KCl<ref>D.T. Sawyer, A. Sobkowiak, J.L. Roberts, "Electrochemistry for Chemists", 2nd edition, J. Wiley and Sons Inc., 1995.</ref> || +0.205 || -0.73
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| |-
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| | Ag/AgCl/3.0 [[Concentration#Molality|mol/kg]] KCl || +0.210 || ?
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| |-
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| | Ag/AgCl/1.0 [[Concentration#Molality|mol/kg]] KCl || +0.235 || +0.25 {{Citation needed|date=November 2007}}
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| |-
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| | Ag/AgCl/0.6 [[Concentration#Molality|mol/kg]] KCl || +0.25 ||
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| |-
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| | Ag/AgCl (Seawater) || +0.266 ||
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| |} ''Notes to the Table: (1) The table data source is,<ref>[http://www.nace.org "NACE International CP Specialist Course Manual"]</ref> except where a separate reference is given. (2) E<sub>lj</sub> is the potential of the liquid junction between the given electrolyte and the electrolyte with the activity of chloride of 1 mol/kg.''
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| The electrode has many features making is suitable for use in the field:
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| * Simple construction
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| * Inexpensive to manufacture
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| * Stable potential
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| * Non-toxic components
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| They are usually manufactured with saturated potassium chloride electrolyte, but can be used with lower concentrations such as 1 [[Concentration#Molality|mol/kg]] potassium chloride. As noted above, changing the electrolyte concentration changes the electrode potential. Silver chloride is slightly soluble in strong potassium chloride solutions, so it is sometimes recommended the potassium chloride be saturated with silver chloride to avoid stripping the silver chloride off the silver wire.
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| ==Elevated temperature application==
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| When appropriately constructed, the silver chloride electrode can be used up to 300 °C. The standard potential (i.e., the potential when the chloride activity is 1 mol/kg) of the silver chloride electrode is a function of temperature as follows:<ref>R.S. Greeley, J. Phys. Chemistry, 64, 652, 1960.</ref>
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| {| class="wikitable"
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| |+'''Temperature Dependence of the Standard Potential of the Silver/Silver Chloride Electrode'''
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| |-
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| !Temperature !! Potential E<sup>0</sup>
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| | °C || V versus [[Standard hydrogen electrode|SHE]] at the same temperature
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| | 25 || 0.22233
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| |-
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| | 60 || 0.1968
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| |-
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| | 125 || 0.1330
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| |-
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| | 150 || 0.1032
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| |-
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| | 175 || 0.0708
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| |-
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| | 200 || 0.0348
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| |-
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| | 225 || -0.0051
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| |-
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| | 250 || -0.054
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| |-
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| | 275 || -0.090
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| |}
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| Bard et al.<ref>A.J. Bard, R. Parson, J. Jordan, "Standard Potentials in Aqueous Solution", Marcel Dekker, Inc., 1985.</ref> give the following correlations for the standard potential of the silver chloride electrode as a function of temperature (where t is temperature in °C):
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| E<sup>0</sup>(V) = 0.23695 - 4.8564x10<sup>−4</sup>t - 3.4205x10<sup>−6</sup>t<sup>2</sup> - 5.869 x 10<sup>−9</sup>t<sup>3</sup> for 0 < t < 95 °C.
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| The same source also gives the fit to the high-temperature potential, which reproduces the data in the table above:
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| E<sup>0</sup>(V) = 0.23735 - 5.3783x10<sup>−4</sup>t - 2.3728x10<sup>−6</sup>t<sup>2</sup> for 25 < t < 275 °C.
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| The extrapolation to 300 °C gives E<sup>0</sup> of -0.138 V.
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| Farmer<ref>Joseph Farmer, "Waste Package Degradation Expert Elicitation Panel:
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| Input on the Corrosion of CRM Alloy C-22", Lawrence Livermore National Laboratory, report UCRL-ID-130064 [http://www.osti.gov/bridge/servlets/purl/664591-Iaj0FM/webviewable/664591.PDF Information Bridge: DOE Scientific and Technical Information - Sponsored by OSTI (pdf)]</ref> gives the following correlation for the potential of the silver chloride electrode with 0.1 mol/kg KCl solution, accounting for the activity of Cl<sup>-</sup> at the elevated temperature:
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| E<sup>0.1 mol/kg KCl</sup>(V) = 0.23735 - 5.3783x10<sup>−4</sup>t - 2.3728x10<sup>−6</sup>t<sup>2</sup> + 2.2671x10<sup>−4</sup>(t+273) for 25 < t < 275 °C.
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| == See also ==
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| *[[Reference electrode]]
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| *[[Standard hydrogen electrode]]
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| *[[Copper-copper(II) sulfate electrode]]
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| *[[Cathodic protection]]
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| *[[Electromyography]] (especially electrodes used for surface EMG)
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| For use in soil they are usually manufactured with saturated potassium chloride electrolyte, but can be used with lower concentrations such as 1 M potassium chloride. In seawater or chlorinated potable water they are usually directly immersed with no separate electrolyte. As noted above, changing the electrolyte concentration changes the electrode potential. Silver chloride is slightly soluble in strong potassium chloride solutions, so it is sometimes recommended that the potassium chloride be saturated with silver chloride.
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| == References ==
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| {{reflist}}
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| == External links ==
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| * [http://www.nace.org/ NACE International Website for Corrosion Professionals]
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| {{DEFAULTSORT:Silver Chloride Electrode}}
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| [[Category:Electrodes]]
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| [[ja:基準電極#銀－塩化銀電極]]
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