In Depth Overview on How a pH Sensor Works
The Electrode Used for Measuring
An aqueous solution’s pH can be evaluated with the help of the measurement electrode, which is designed for the purpose.
Since 1897, when it was first invented, the platinum/hydrogen electrode has been used to determine the concentration of hydrogen ions in aqueous solutions. It continues to perform this function today, in addition to acting as a reference standard for the electrometric assessment of pH. The hydrogen electrode is made out of a platinized platinum plate or rod, which is then covered in platinum black and sent through a flow of hydrogen gas. The reference electrode comprises a silver wire that has been coated with silver chloride. The following is the fundamental idea that underpins the utilization of a hydrogen electrode: Ionization of the atoms on the surface of a metal rod can occur when that rod is submerged in an aqueous solution that contains its own salt. For example, a silver electrode can get ionized when it is submerged in silver nitrate. The positively charged metal ions on the rod’s surface will be drawn to the negatively charged water molecules, which will result in the rod’s metal being left with a negative charge. Because of this exchange of charges at the phase boundary between metal and solution, a potential difference is created. The potential, also known as the galvanic potential, is something that changes depending on the ion concentration in the solution. Even in modern times, the hydrogen electrode is used as a reference standard, particularly due to the fact that the results of its measurements are exceedingly precise. On the other hand, for a variety of reasons that are more pragmatic in nature, the hydrogen electrode has lost its significance due to the fact that it is tough and complicated to handle. Out of all the other metal electrodes, only the antimony electrode has managed to endure. Antimony is not affected by hydrofluoric acid, which is an acid that is commonly used to etch glass. The precision and measuring range are both severely restricted, however. Due to the fact that antimony is a substance that can cause cancer in humans, it must be handled with caution.
The Electrode Made of Glass
It wasn’t until the invention of the glass electrode that pH measuring was transformed into a straightforward and dependable tool that could be used in a wide variety of contexts. Glass electrodes have surpassed all other kinds of indicator electrodes for pH measurements in recent years, making them the dominant form of electrode used for these tests. Because of the dependability and accuracy of the glass electrode in combination with extremely stable electronic amplification, the pH measurement of an aqueous solution is now as routine as the measurement of temperature and pressure. This guide will give you the knowledge you need regarding the functionality of the glass electrode as well as its maintenance so that you can successfully apply it. Glass is used to make the shaft of a glass electrode, which must have a strong resistance to hot alkaline solutions and an electrical resistance that is several times higher than that of the membrane glass. A glass electrode comprises glass used to make the shaft. The hemispherically shaped electrode tip, also known as the glass membrane, is the portion of the glass electrode that is sensitive to pH.
The membrane is composed of a unique glass that is sensitive to hydrogen ions, and it is bonded to the shaft of the electrode. The glass electrode is partially saturated with a buffer solution known as pH Cable, which typically has a pH value of 7.
This internal buffer receives an addition of a predetermined quantity of potassium chloride or KCl. As a conducting electrode, a silver wire that has been coated with silver chloride (Ag/AgCl) is placed into the glass electrode all the way down into the internal buffer. This creates an electrochemical circuit. A connection has been made between the Ag/AgCl wire and one of the terminals on a pH meter using the core of the coaxial pH cable. The Membrane Made of Glass The production of a potential difference in relation to the concentration of hydrogen ions in aqueous solutions is a trait that is shared by all different kinds of glasses.
When the membrane glass of a measuring electrode comes into contact with an aqueous solution, a thin gel layer of roughly 10-4 mm thickness is formed between the glass surface and the solution. This gel layer acts as a barrier to prevent the solution from penetrating the glass. The quality and composition of the membrane glass, in addition to the temperature and pH value of the solution being tested, are the factors that determine the thickness of the gel layer. A gel layer is also generated on the interior of the glass membrane as a result of the internal side of the glass membrane coming into touch with the inner buffer, which is an aqueous solution with a pH of 7. On both sides of the membrane, there is a continual exchange of H+ ions that takes place between the gel layers and the H+ ions that are present in the solutions. The amount of H+ ions present in either or both solutions is what determines the outcome of this ion exchange. If the concentration of hydrogen ions in each solution is the same on both sides of the glass membrane, then the ion exchange will cease once an equilibrium has been reached between the H+ ions in the solutions and the H+ ions in the gel layers. This is the case even if the concentration of hydrogen ions in each solution is the same on both sides of the glass membrane. As a result, both sides of the membrane glass have the same potential, and there is no variation in the potential between the two sides.
If there is a difference in the concentration of hydrogen ions between the inner buffer and the outer solution, then there will be a potential difference between the inner and outer sides of the membrane glass. The magnitude of this potential difference will be proportional to the difference in pH that exists between the inner buffer and the outer solution. It is necessary for the membrane itself to have some degree of conductivity in order to be able to measure the potential of the membrane. This is made possible by the mobility of the alkaline ions in the membrane glass (Li+ ions in most glasses made today, whereas older membrane glasses had Na+ ions). The reaction time and the characteristic slope of the glass electrode are both affected by the thickness of the gel layer as well as the composition of the gel. As a result, the gel layer is of paramount significance to the overall performance of the electrode.
A pH measurement is impossible without the gel layer being present. Unfortunately, it takes between one and two days for a gel layer to completely develop. Because of this, a measurement electrode needs to be hydrated (immersed in regular, clean tap water) for a minimum of twenty-four hours before it can be used. The majority of manufacturers ship their electrodes with the membrane already hydrated (the membrane is kept wet with a KCl solution that is contained under a plastic cap), which makes the electrode immediately ready for use.
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