Understanding pH Sensor Basics
The Measuring Electrode
An electrode that is specifically made for measuring can be used to determine the pH of an aqueous solution.
The platinum/hydrogen electrode was created in 1897 and has been used to measure hydrogen ion concentrations in aqueous solutions ever since. It still serves as a reference standard for the electrometric measurement of pH and still serves this purpose. A platinum plate or rod that has been platinized, covered in platinum black, and exposed to a flow of hydrogen gas serves as the hydrogen electrode. A silver wire covered in silver chloride serves as the reference electrode. The use of a hydrogen electrode is supported by the following core concept: When a metal rod is immersed in an aqueous solution that contains its own salt, the atoms on its surface can ionize. For instance, when a silver electrode is immersed in silver nitrate, it might get ionized. The negatively charged water molecules will attract the positively charged metal ions on the rod’s surface, leaving the metal of the rod with a negative charge. A potential difference is produced as a result of this charge exchange at the metal and solution phase boundary. The potential also called the galvanic potential, varies with the concentration of ions in the solution. The hydrogen electrode is still used as a reference standard today, in part because the measurements produced by it are so incredibly accurate. The hydrogen pH sensor electrode, on the other hand, has lost its significance since it is difficult and complicated to handle for a number of more pragmatic reasons. Only the antimony electrode has managed to survive out of all the other metal electrodes. Hydrofluoric acid, which is frequently used to etch glass, has no effect on antimony. However, both the precision and the measuring range are severely constrained. Antimony must be treated carefully because it is a material that can lead to cancer in people.
Glass serves as the electrode. It wasn’t until the development of the glass electrode that pH measurement became a simple, dependable tool that could be applied in a range of situations. The most common type of electrode used for pH sensors is glass, which has exceeded all other types of indicator electrodes in recent years. The measurement of an aqueous solution’s pH has become as commonplace as the measurement of temperature and pressure because of the glass electrode’s dependability and accuracy in conjunction with the exceptionally steady electronic amplification. You will learn everything you need to know about the glass electrode’s operation and upkeep from this book, enabling you to successfully use it. Glass is used to create the shaft of a glass electrode, which must have electrical resistance several times greater than that of membrane glass and robust resistance to hot alkaline solutions. Glass is used to construct the shaft of a glass electrode. The part of the pH sensor glass electrode that is pH-sensitive is the hemispherically shaped electrode tip, sometimes referred to as the glass membrane.
The membrane of the pH sensor is attached to the shaft of the electrode and is made of a special glass that reacts to hydrogen ions. A buffer solution known as a pH sensor Cable, which commonly has a pH value of 7, is partially saturated on the glass electrode.
Potassium chloride, or KCl, is added in a specified amount to this internal buffer. A silver wire that has been coated with silver chloride (Ag/AgCl) and is used as a conducting electrode is inserted into the glass electrode all the way down to the internal buffer. An electrochemical circuit is thus created. The core of the coaxial pH sensor cable has been used to make a connection between the Ag/AgCl wire and one of the terminals on a pH sensor meter. Glass Membrane as the Membrane All types of glasses have the ability to produce a potential difference in proportion to the hydrogen ion concentration in aqueous solutions.
A thin gel layer, measuring around 10-4 mm in thickness, is created when the membrane glass of a measuring electrode comes into contact with an aqueous solution. The solution cannot penetrate the glass due to the barrier created by this gel layer. The temperature and pH level of the test solution, together with the quality and composition of the membrane glass, all affect how thick the gel layer will be. When the internal side of the glass membrane comes into contact with the inner buffer, an aqueous solution with a pH of 7, a gel layer is also created on the inside of the glass membrane. H+ ions are continuously exchanged between the gel layers and the H+ ions that are present in the solutions on both sides of the membrane. The outcome of this ion exchange depends on the concentration of H+ ions in one or both solutions. Once an equilibrium between the H+ ions in the solutions and the H+ ions in the gel layers has been reached, the ion exchange will stop if the concentration of hydrogen ions in each solution is the same on both sides of the pH sensor glass membrane. This holds true even if the hydrogen ion concentration in each solution on either side of the glass barrier is the same. As a result, there is no difference in potential between the two sides of the membrane glass and it has the same potential on both sides.
The inner and outer sides of the membrane glass may differ if there is a variation in the concentration of hydrogen ions between the inner buffer and the outer solution. The size of this potential difference will depend on how different the pH of the inner buffer and the outside solution are from one another. For the purpose of determining the membrane’s potential, the membrane must possess some degree of conductivity. This is made feasible by the membrane glass’s alkaline ions’ mobility (most modern membrane glasses contain Li+ ions, whereas older membrane glasses included Na+ ions). Both the thickness of the gel layer and the makeup of the gel has an impact on the glass electrode’s characteristic slope and reaction time. As a result, the gel layer plays a crucial role in the electrode’s overall function.
Without the gel layer, it is difficult to detect pH. The unfortunate truth is that a gel layer doesn’t fully form for one to two days. A measurement electrode must therefore be hydrated (immersed in ordinary, clean tap water) for at least twenty-four hours before it can be utilized. Most manufacturers supply their electrodes with the membrane already hydrated (the membrane is maintained wet with a KCl solution that is enclosed beneath a plastic cap), making the electrode instantly usable.
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