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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 corrosion control systems in sea water environments.
The corresponding equations can be presented as follows:
or an overall reaction can be written:
This reaction is 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 deposition of the silver-ions). The reaction has been proved to obey these equations in solutions of pH values between 0 and 13.5.
The standard electrode potential E0 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.)
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.
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.
The potential of a silver:silver chloride reference electrode with respect to the standard hydrogen electrode depends on the electrolyte composition.
|Electrode||Potential E0+Elj||Temperature Coef.|
|(V) at 25 °C||(mV/°C) at around 25 °C|
|Ag/AgCl/Sat. KCl||+0.197||-1.01|
|Ag/AgCl/3.5 mol/kg KCl||+0.205||-0.73|
|Ag/AgCl/3.0 mol/kg KCl||+0.210||?|
|Ag/AgCl/1.0 mol/kg KCl||+0.235||+0.25|
|Ag/AgCl/0.6 mol/kg KCl||+0.25|
Notes to the Table: (1) The table data source is, 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:
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.
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:
|°C||V versus SHE at the same temperature|
Bard et al. 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 source also gives the fit to the high-temperature potential, which reproduces the data in the table above:
E0(V) = 0.23735 - 5.3783x10−4t - 2.3728x10−6t2 for 25 < t < 275 °C.
The extrapolation to 300 °C gives E0 of -0.138 V.
Farmer 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- at the elevated temperature:
E0.1 mol/kg KCl(V) = 0.23735 - 5.3783x10−4t - 2.3728x10−6t2 + 2.2671x10−4(t+273) for 25 < t < 275 °C.
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.