Since 1947, pH sensor manufacturers have combined the measuring electrode and the reference electrode into one unit, hence the name combination electrode. Today, the combination electrode is almost exclusively employed in laboratories and industrial applications. Only when the life expectancy is significantly different for the measuring and the reference electrode, is the use of a pH measurement system consisting of two separate electrodes recommended.
In a combination pH electrode the concentric space surrounding the measurement electrode is filled with KCl electrolyte and contains the internal reference system. A porous liquid junction (diaphragm) near the bottom of the electrolyte chamber serves as the pathway between the KCl electrolyte solution and the measured liquid medium. Since the reference electrolyte is a conductive medium, it completes the electrical circuit to the measurement electrode.
Combination pH Electrodes with Gel Electrolyte
Reference electrodes incorporating a liquid electrolyte are maintenance intensive as their liquid level has to be controlled and regularly refilled. The search for a maintenance-free electrode assembly led to the development of the gel reference electrode.
The gel electrode is a low-maintenance electrode. The reference electrolyte chamber is filled with 3 molar KCl electrolyte in viscous gel form. The reference junction is normally made from ceramic.
The gel electrode is frequently used for simple measuring applications, e.g. municipal water treatment or portable laboratory electrode applications. As the gel electrolyte is fixed, the electrode does not need to be topped-off with reference electrolyte – which reduces maintenance time. There are some limitations of the gel reference electrode. The measurement can have reduced accuracy and a shorter pH lifespan than its counterpart with a liquid reference electrolyte. Also the response time of a gel electrode can be slower than a liquid filled electrode.
Combination pH Sensors with Polymer Electrolyte
In the early 1980’s electrode manufacturers introduced the polymer reference electrode. The polymer electrode relies on a reference electrolyte chamber that is completely filled with a semi-solid polymerised plastic material in which the KCl is embedded. Due to the solid nature of the electrolyte, no porous junction is required; therefore the KCl saturated polymer has direct contact with the measured solution. The contact is established through a small aperture, which could either be one or more holes near the bottom of the reference electrolyte chamber or a fissure separating the bottom electrolyte chamber from the measuring electrode.
Since its inception, polymer electrodes have had specialized use. Initially values below 2 pH could not be measured, the temperature limit was 90 °C, and measurements in media containing organic solvents were not possible.
With Hamilton’s new developed Polisolve Plus polymer (patented) these limitations are a thing of the past. The Polisolve Plus polymer reference system allows measurements down to pH 0, and it is resistant to organic solvents. pH measurements in the laboratory or at industrial plants utilizing the Polisolve Plus reference system are reliable and accurate. The Polisolve Plus polymer can be applied to almost every measuring problem, including very dirty, fatty, oily, ion weak, or protein-filled media. Suspended solids do not create junction plugging problems anymore.
Example of a Polymer Electrolyte Combination pH electrode - The Polilyte Plus
Since the KCl saturated polymer isolates the Ag-AgCl reference element, there is no possibility of silver sulphide poisoning in processes that contain H2S or other Sulphide molecules.
The high pressure rating of 10 bar (145 PSIG), its extended temperature rating of up to 130°C and its maintenance-free operation should make the Polisolve Plus reference system always a first choice in harsh industrial pH applications.