Silver chloride electrode
Encyclopedia
Reference electrode
A reference electrode is an electrode which has a stable and well-known electrode potential. The high stability of the electrode potential is usually reached by employing a redox system with constant concentrations of each participants of the redox reaction.There are many ways reference...
, commonly used in electrochemical
Electrochemistry
Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...
measurements. For example, it is usually the internal reference electrode in pH meter
PH meter
A pH meter is an electronic instrument used for measuring the pH of a liquid...
s. As another example, the silver chloride electrode is the most commonly used reference electrode for testing cathodic protection
Cathodic protection
Cathodic protection is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. The simplest method to apply CP is by connecting the metal to be protected with another more easily corroded "sacrificial metal" to act as the anode of the...
corrosion
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
control systems in sea water environments.
The electrode functions as a redox electrode and the reaction is between the silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
metal (Ag) and its salt — silver chloride
Silver chloride
Silver chloride is a chemical compound with the chemical formula AgCl. This white crystalline solid is well known for its low solubility in water . Upon illumination or heating, silver chloride converts to silver , which is signalled by greyish or purplish coloration to some samples...
(AgCl, also called silver(I) chloride).
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
Solvation
Solvation, also sometimes called dissolution, is the process of attraction and association of molecules of a solvent with molecules or ions of a solute...
of the metal or cathodic deposition
Deposition (chemistry)
In chemistry, deposition is the settling of particles or sediment from a solution, suspension and mixture or vapor onto a pre-existing surface...
of the silver-ions). The reaction has been proved to obey these equations in solutions with pH’s of between 0 and 13.5.
The Nernst equation
Nernst equation
In electrochemistry, the Nernst equation is an equation that can be used to determine the equilibrium reduction potential of a half-cell in an electrochemical cell. It can also be used to determine the total voltage for a full electrochemical cell...
below shows the dependence of the potential of the silver-silver(I) chloride electrode on the activity
Activity (chemistry)
In chemical thermodynamics, activity is a measure of the “effective concentration” of a species in a mixture, meaning that the species' chemical potential depends on the activity of a real solution in the same way that it would depend on concentration for an ideal solution.By convention, activity...
or effective concentration
Concentration
In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
of chloride-ions:
The standard electrode potential E0 against standard hydrogen electrode
Standard hydrogen electrode
The standard hydrogen electrode , is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials...
(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.)
Applications
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
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
. An insulated lead wire connects the silver rod with measuring instruments. A voltmeter
Voltmeter
A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an analog to...
negative lead is connected to the test wire. The reference electrode contains potassium chloride
Potassium chloride
The chemical compound potassium chloride is a metal halide salt composed of potassium and chlorine. In its pure state, it is odorless and has a white or colorless vitreous crystal appearance, with a crystal structure that cleaves easily in three directions. Potassium chloride crystals are...
to stabilize the silver chloride concentration.
The potential of a silver:silver chloride reference electrode with respect to the standard hydrogen electrode
Standard hydrogen electrode
The standard hydrogen electrode , is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials...
depends on the electrolyte composition.
| Electrode | Potential E0+Elj | Temperature Coef. |
|---|---|---|
| (V Volt The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the... ) at 25 °C |
(mV/°C) at around 25 °C | |
| SHE | 0.000 | 0.000 |
| 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 | |
| Ag/AgCl (Seawater) | +0.266 |
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:| Temperature | Potential E0 |
|---|---|
| °C | V versus SHE Standard hydrogen electrode The standard hydrogen electrode , is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials... at the same temperature |
| 25 | 0.22233 |
| 60 | 0.1968 |
| 125 | 0.1330 |
| 150 | 0.1032 |
| 175 | 0.0708 |
| 200 | 0.0348 |
| 225 | -0.0051 |
| 250 | -0.054 |
| 275 | -0.090 |
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.
See also
- Reference electrodeReference electrodeA reference electrode is an electrode which has a stable and well-known electrode potential. The high stability of the electrode potential is usually reached by employing a redox system with constant concentrations of each participants of the redox reaction.There are many ways reference...
- Standard hydrogen electrodeStandard hydrogen electrodeThe standard hydrogen electrode , is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials...
- Copper-copper(II) sulfate electrodeCopper-copper(II) sulfate electrodeThe Copper-copper sulfate electrode is a reference electrode of the first kind, based on the redox reaction with participation of the metal and its salt - copper sulfate....
- Cathodic protectionCathodic protectionCathodic protection is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. The simplest method to apply CP is by connecting the metal to be protected with another more easily corroded "sacrificial metal" to act as the anode of the...
- ElectromyographyElectromyographyElectromyography is a technique for evaluating and recording the electrical activity produced by skeletal muscles. EMG is performed using an instrument called an electromyograph, to produce a record called an electromyogram. An electromyograph detects the electrical potential generated by muscle...
(especially electrodes used for surface EMG)
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.