The potentials of a half cell depend on the concentration of an ion of interest since it reacts by gaining or losing electrons. In this way, it is possible to determine its concentration using these potentials. However, their values are not absolute but relative to the standard hydrogen electrode (SHE).
By convention, the potentials of the hydrogen electrodes (above) are equal to 0V. The problem is that these electrodes are difficult to manipulate, so they are not practical to install in every analysis. This is where the reference electrodes come in, much simpler, and whose potentials are different from 0V.
Some of the reference electrodes, such as the Calomel Saturated Electrode (ESC), can be built in the same laboratory prior to testing. Also, they are openly available on the market with their proper specifications, and the risks of contaminating the analyte sample are minimal.
Reference Electrode Characteristics
Consider the two-electrode system above. One is the indicator (gray), where the reaction of interest takes place, and the other is the reference indicator (gold). This electrochemical cell will have a potential E, which will be equal to the difference between the potentials of both electrodes (Ei-Er). Therefore, any variation in E must be due solely to the potential of the indicating electrode.
From the above, it is concluded that the reference electrode has the following characteristics:
-It has a stable potential over time and is reproducible. That is, it should be the same no matter where or how many times the measurements are made.
-Its chemical composition is constant and insensitive, so it hardly undergoes changes when it interacts with the analyte solution or is immersed in an aqueous medium.
-It is easy to build and assemble, unlike the standard hydrogen electrode, which requires the use of a hydrogen tank and there is a risk of explosions when hydrogen mixes with the air.
Reference electrode function
The potentials of the reference electrodes are measured against the standard hydrogen electrode, so their values are different from 0V. Thanks to these hydrogen electrodes it is possible to construct tables of standard reduction potentials Eº, which appear in the indexes of chemistry texts for endless redox reactions.
However, in practice, you don’t go around charging a hydrogen electrode everywhere. Instead, reference electrodes are used, which are much simpler and safer to handle.
As its potentials are constant and stable, the readings of the potential of the electrochemical cell will be due exclusively to the indicator electrode; that is, to variations in the concentration of the analyte in question.
Thus, the function of the reference electrode is to determine the potential of any reaction in one of the half cells of the electrochemical system. If its potentials were not stable, then quantitative determinations of the analyte would not be reliable. All potentiometric or voltammetric analyzes must specify which reference electrode they used to report their results.
Examples of reference electrodes
There are several reference electrode designs available on the market. Some are used for very specific determinations. However, there are two electrodes among all of them that stand out thanks to their simplicity and practicality: the calomel electrode, and the silver-silver chloride electrode, both very recurrent in many potentiometric and voltammetric analyzes.
The upper image shows the conventional parts of a saturated calomel electrode, ECS. It is said to be saturated because it contains a saturated KCl solution (blue), which also has undissolved KCl crystals.
The internal column, traversed by a platinum wire, is filled with mercury, which in turn is brought into contact with a paste of calomel, Hg 2 Cl 2 or mercury (I) chloride, and the saturated solution of KCl. Thus, in this electrode, the concentration of Cl – ions is important since their value modifies the potential of the electrode.
The reaction that takes place is as follows:
Hg 2 Cl 2 (s) + 2e – ⇌ 2Hg (l) + 2Cl –
And it has an Eº of 0.241 V.
At the lower end of the ECS is a porous frit, which is electrically contacted with the analyte solution and allows controlled Cl – ion exchange. Otherwise, there would be chloride contamination in the sample, and also the electrode would lose its precious saturated KCl solution.
This electrode is used for pH measurements and in general for any analyte whose concentration can be determined in an aqueous medium.
Silver-silver chloride electrode
Similar to the calomel electrode, the silver-silver chloride electrode consists of a silver wire coated with silver chloride, and also immersed in a solution saturated by a mixture of KCl and AgCl.
All its parts are protected inside a plastic tube, which has a porous opening through which it establishes electrical contact with the solution.
The reaction that defines its potential, Eº = 0.197 V, is the following:
AgCl (s) + e – ⇌ Ag (s) + Cl –
This electrode has greater uses and preferences than calomel, which is due to the negative impact that mercury has on the environment. On the other hand, silver-silver chloride electrodes are simpler, safer, and cheaper to build in the laboratory.
We say then that the silver-silver chloride electrode replaces the calomel electrode in many of its applications (potentiometric titrations, corrosion tests, electrochemical analysis, etc.).
It is also one of the preferred options in biology and medicine, especially with regard to electrocardiography and electroencephalography, since silver and its salt are relatively toxic substances for the body.