A potentiometer is the device used to measure the potential difference between a working and a reference electrode, when both are immersed in a solution of which it is desired to determine its acidity or basicity, expressing this as pH.
In this way, potentiometry refers to the analytical method used to determine the concentration of H + ions in a substance that is in solution, through the use of a potentiometer and the two aforementioned electrodes.
In the case of the reference electrode, it has a known, constant and stable potential unlike the working electrode. The potential that develops in this last electrode varies in proportion to the concentration of H + ions found in the solution.
This potential is also greatly influenced by the temperature at which the measurement is carried out.
What is a potentiometer for?
There are a large number of industrial processes – such as drug production, food processing, and water purification – that are highly sensitive to changes in pH levels. For this reason its correct measurement is so important.
As mentioned previously, pH is a parameter used to measure the acidity or alkalinity of a solution that is in the aqueous phase by analyzing the concentration of H + ions in the solution. Then, the pH value is calculated by the following equation:
pH = -log [H + ]
So the potentiometer is used to measure the pH of a substance in solution.
When the potentiometer is connected to the two electrodes immersed in the solution to be analyzed, it detects the potential difference between the working and reference electrode, amplifying this signal and converting it into a pH value using the equation described above.
How does a potentiometer work?
The operation of a potentiometer is based on the mechanism of an electrochemical cell, where H + ions are involved in the chemical reaction of the cell to determine the concentration of these ions in the solution and, in this way, obtain the pH of the same .
When it is desired to measure the pH of a solution by potentiometry, a potentiometer and an electrode are used; the first is the device that determines the pH, while the second is based on the combination of a reference electrode and a measurement electrode that is sensitive to analytes.
In this sense, a circuit is formed through which the electric current flows between the electrodes and the solution, where they perform the function of a battery when they are immersed in the mentioned solution.
In this way, the potentiometer is designed in order to generate a voltage equal to zero (in units of millivolts) when it has a pH equal to seven; that is, neutral.
Likewise, when an increase in potential values is registered (with positive numbers) it means that there are lower pH values, and when a decrease is observed – that is, a growth towards negative numbers – we speak of higher values. pH.
The measuring (or working) electrode consists of a device where the reaction under study (oxidation or reduction) takes place.
Although there are many types, it is generally glass, consisting of a very thin glass membrane that has permeability to the H + ions of the medium in which it is.
By placing this in a solution that has a different pH than the solution present in the cell, a potential difference is generated between the two faces of the membrane and this difference can be recorded by means of a reference electrode.
On the other hand, the reference electrode is a device that has characteristics of stable potential and with a known value, which is normally used as the anode in the electrochemical cell.
An example of this type of electrode is one that is made up of a silver wire, which is coated with silver chloride and immersed in a dilute hydrochloric acid solution, or the reference electrode saturated with calomel, like the one shown in the figure below.
So the potentiometer determines the potential difference that is generated between the electrodes, although only the potential of the working electrode depends on the concentrations of the ionic species.
Calibrating a potentiometer
The calibration of a potentiometer must be carried out through known buffer solutions (also called buffers or buffer solutions), which consist of systems with practically invariable pH that contain a weak substance and its conjugated species.
Each buffer solution has a specific pH, which can be acidic (pH <7), basic (pH> 7) or neutral (pH = 7), and can be commercially purchased already standardized or prepared in the laboratory with certified reagents and through the use of established and validated procedures.
As potentiometers measure the pH value within a range that is considered wide, it must be known whether the analyte has a pH greater or less than seven in order to proceed with its correct calibration.
Thus, for samples whose pH is expected to be basic, it should be calibrated with a buffer solution with a pH equal to seven and another with a higher pH (normally one of pH ten is used).
On the other hand, for samples with an expected acidic pH, it is calibrated with a buffer solution with a pH equal to seven and another with a lower pH (normally one of pH four is used).
Finally, the recalibration of this measuring instrument must be carried out before and after each use, recording its results, including the date and time in which they were carried out and the characteristics of the buffer solutions used to control it.