Understanding pH Reference Electrodes
It is imperative for accurate pH measurement that a well-defined electrical connection between the medium being measured (the process liquid) and the pH meter is represented by the reference electrode located within the meter. Because the accuracy of the pH measurement is dependent on the reference electrode, reference electrode selection is of critical significance. An ideal reference electrode will not only produce a potential that can be accurately predicted but will also only react in a manner that is dictated by the NERNST equation. In addition to having a low-temperature coefficient and being free of temperature hysteresis, a good and stable reference electrode should have these characteristics. An internal electrode, which is very similar to the measuring electrode, is what makes up a reference electrode. This electrode is then submerged in a certain electrolyte. This electrolyte needs to be in close proximity to the medium being analyzed. Throughout the years, numerous reference systems have been utilized; however, only two of these systems, mercury mercurous chloride (calomel) and silver/silver chloride reference electrodes have been discovered to be dependable with regard to an accurate and stable potential. The silver/silver chloride reference system is the only one utilized by Icon Process Controls. The calomel electrode has high potential stability and a high precision down to a hundredth of a millivolt when it is used at temperatures that are low and stable (maximum 80 degrees Celsius). Nonetheless, the silver/silver chloride electrode has attained the level of practical acceptance required to become the reference system that is utilized the vast majority of the time. It is simple to produce, its potential quickly reaches equilibrium at temperatures ranging from -30 degrees Celsius to pH Cable 135 degrees Celsius, and it is very repeatable. The Ag/AgCl reference electrode maintains its stability and accuracy even when subjected to a wide range of temperature changes and high temperatures of up to 135 degrees Celsius.
An Ag/AgCl reference electrode has an internal electrode that is made up of a silver wire that has a coating of silver chloride and is submerged in a solution of potassium chloride that has a concentration of 3M. This solution is contained within a large chamber that is formed by the glass body of the reference electrode. At the very bottom of this chamber is a liquid junction, also known as a “diaphragm,” which is typically a thin rod made of porous ceramic and serves the purpose of allowing potassium chloride to diffuse or seep into the medium being measured. The silver chloride and silver wire are linked to the pH meter using a coaxial cable so that the electrical circuit can be completed. The Electrode Combination in Question Since 1947, electrode manufacturers have merged the measurement electrode and the reference electrode into a single unit, which is why this type of electrode is referred to as a combination electrode. In modern times, the combination electrode is nearly always utilized in research facilities as well as manufacturing facilities. The utilization of a pH measurement system that includes two distinct electrodes is only recommended in situations in which the life expectancy of the measurement electrode and the reference electrode are significantly dissimilar from one another. Within a combination electrode, the reference electrolyte and the internal reference system are housed within the concentric area that surrounds the measurement electrode. The KCl solution and the measured medium are joined together at a connection that is formed by a diaphragm that is located towards the base of the electrolyte chamber. Due to the fact that the reference electrolyte is a conductive medium, it serves the purpose of acting as a screen for the measurement electrode. Reference electrodes that are comprised of a liquid reference electrolyte are notoriously difficult to maintain due to the fact that their electrolyte levels need to be constantly monitored and replenished. Gel reference electrodes were developed as a result of research conducted to find an electrode assembly that would not require routine maintenance. The gel electrode is an electrode that requires little to no care. The electrolyte chamber used as a point of reference is filled with a viscous gel consisting of 3M KCl electrolyte. Ceramics are the typical material used to create the diaphragm. It is common practice to cover the glass shaft with an outer sleeve made of plastic, or the electrode shaft may be built entirely out of plastic (Epoxy). The gel electrode is most commonly utilized for straightforward measuring applications, such as portable laboratory electrode applications or municipal water treatment electrode applications. This electrode does not require any additional reference electrolyte to be added to it, which helps to cut down on the amount of time spent on maintenance. Gel electrodes, on the other hand, have a lower level of precision and a shorter lifespan when compared to their counterparts that use liquid reference electrolytes. This fact needs to be taken into consideration. When compared to an electrode that is filled with liquid electrolytes, the response time of an electrode that is filled with gel electrolyte is slightly longer.
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