- Silver chloride electrode
A silver chloride electrode is a type of
reference electrode, commonly used in electrochemical measurements. For example, it is usually the internal reference electrode in pH meters. As another example, the silver chloride electrode is the most commonly used reference electrode for testing cathodic protection corrosioncontrol systems in sea waterenvironments.
The electrode functions as a
redox electrodeand the reaction is between the silvermetal (Ag) and its salt — silver chloride(AgCl, also called silver(I) chloride).
The corresponding equations can be presented as follows:: Ag+ + 1e− ↔ Ag0(s): Ag+ + Cl− ↔ AgCl(s)or an overall reaction can be written:: Ag0(s) + Cl− ↔ AgCl(s) + e-
This reaction characterized by fast electrode kinetics, meaning that a sufficiently high current can be passed through the electrode with the 100% efficiency of the redox reaction (dissolution of the metal or cathodic
depositionof the silver-ions). The reaction has been proved to obey these equations in solutions with pH’s of between 0 and 13.5.
Nernst equationbelow shows the dependence of the potential of the silver-silver(I) chloride electrode on the activityor effective concentrationof chloride-ions::
The standard electrode potential E0 against standard hydrogen electrode 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 [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] .)
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 by
electroplatingor by dipping the wire in molten silver chloride.
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 voltmeternegative lead is connected to the test wire. The reference electrode contains potassium chlorideto stabilize the silver chloride concentration.
The potential of a silver:silver chloride reference electrode with respect to the
standard hydrogen electrodedepends on the electrolyte composition. "Notes to the Table: (1) The table data source is ["NACE International CP Specialist Course Manual" [http://www.nace.org] ] , except where a separate reference is given. (2) Elj is the potential of the liquid junction between the given electrolyte and the electrolyte with the activity of chloride of 1 mol/kg."
The electrode has many features making is suitable for use in the field:
* Simple construction
* Inexpensive to manufacture
* Stable potential
* Non-toxic components
They are usually manufactured with saturated potassium chloride electrolyte, but can be used with lower concentrations such as 1 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.
Elevated temperature application
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 [R.S. Greeley, J. Phys. Chemistry, 64, 652, 1960.] :
Bard et al. [A.J. Bard, R. Parson, J. Jordan, "Standard Potentials in Aqueous Solution", Marcel Dekker, Inc., 1985.] give the following correlations for the standard potential of the silver chloride electrode as a function of temperature (where t is temperature in °C):
E0(V) = 0.23695 - 4.8564x10-4t - 3.4205x10-6t2 - 5.869 x 10-9t3 for 0 < t < 95 °C.
The same authors also give the fit to the high-temperature potential, but it appears to contain a typographic error. The corrected fit, which reproduced the data in the table above is: [ [http://www.osti.gov/bridge/servlets/purl/664591-Iaj0FM/webviewable/664591.PDF Information Bridge: DOE Scientific and Technical Information - Sponsored by OSTI ] ]
E0(V) = 0.23735 - 5.3783x10-4t - 2.3728x10-6t2 - 2.2671x10-9(t+273) for 25 < t < 275 °C.
An extrapolation to 300 °C gives E0 of -0.138 V.
Standard hydrogen electrode
Copper-copper(II) sulfate electrode
Cathodic protectionFor 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.
* [http://www.nace.org/ NACE International Website for Corrosion Professionals]
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