(pH and ORP sensors use identical reference designs. For the purpose of brevity only pH will be referred to in this article)
The combination pH probe is comprised of two half cells. The glass measurement electrode is well known as the portion of the pH probe which provides varying voltage that relates to changing pH levels within the liquid. The reference electrode is often the lesser understood half of the probe, but is equally important. Choosing the proper reference for the application can greatly help to ensure better pH measurement accuracy and probe longevity. The reference fulfills two main requirements to make the pH probe functional:
Stable Reference Voltage
The reference half cell generates a stable potential voltage. This voltage is created by an Ag/AgCl element that reacts with a KCl-based electrolyte solution.
The reference relies upon a porous liquid junction (diaphragm) to allow KCl electrolyte to come in contact with the process liquid. The contact of the two conductive liquids provides the electrical pathway which current can flow therefore completing the circuit. The net voltage generated by the sensor can be passed to the pH electronics (transmitter or micro-transmitter in the case of Hamilton Arc intelligent sensors) to be calculated as a pH value based upon the Nernst equation.
There are multiple factors with process pH measurement that can affect the reference half-cell and create errors in the pH measurement:
Certain process liquids cause solid build-up on pH sensor clogging the porous liquid junction. When this occurs, the electrical pathway mentioned above may be broken as the electrolyte within the reference cannot maintain contact with process liquid. Coating and blockage of the liquid junction can occur various ways including chemical induced precipitation reactions between the process liquid, electrolyte, and the Ag/AgCl element, build-up on the junction such as with oils and fats, and even hard, crystalline coatings that block the path of the liquid junction.
Since the liquid junction is porous, not only can electrolyte leach out BUT process liquid can leach into the reference contaminating the electrolyte and potentially reacting with the Ag/AgCl element. When this occurs, the potential voltage generated by the reference half cell can change causing error in the pH measurement.
Some liquids act as solvents and draw out the KCl electrolyte contained in the pH reference more quickly than normal. When this occurs there can be an additional potential voltage created at the liquid junction that can offset the pH reading. In extreme cases, the majority of electrolyte can be withdrawn from the reference leaving it dry or exposing the Ag/AgCl element. A good example of a liquid that acts as a solvent are weak ionic solutions such as ultrapure water.
With this basic understanding of the reference and the challenges created by the process liquid we can now understand some of the designs of the reference half cells how they attempt to solve these issues.
Products: EasyFerm Plus, EasyFerm Bio
Pre-pressurized references use an internal pressure on the electrolyte that is greater than the process liquid pressure. This pressure differential ensures that the electrolyte within the reference only flows one way – out through the diaphragm. This design solves issues with coating and poisoning. The flow through the diaphragm prevents any ingress of process liquid that could poison the sensor as well as prevent coating from occurring due to the flowrate. This design is favored in many pharmaceutical applications such as bioreactors for this reason.
Polymer Gel Reference
Products: Polilyte Plus, Polilyte Pro, Polyplast Pro
Polymer gels such as Hamilton’s Polisolve electrolyte are actually semi-solid plastic-like material that embed the KCl electrolyte within the gel. Since the gel is relatively immobile it is very difficult for process liquid to ingress into the reference and affect the measurement. Also, as the gel is immobile, a large aperture diaphragm (essentially a small hole) can be used to create the electrical pathway needed for the measurement circuit. This diaphragm design is very effective against coating and clogging due to its large size. Polymer gels are commonly used in many industrial applications as they are highly effective preventing poisoning chemicals into the sensor.
The polymer electrolyte used in the Polilyte Plus can easily be seen through the glass sensor body. One unique feature of polymer-based sensors is that they can be mounted in any orientation.
Viscous Electrolyte Reference
Products: MecoTrode, Liq-Glass PG, EasyControl
Viscous KCl based electrolytes use the same ideology as polymer gel references. Any process liquid ingress is slowed down by the higher viscosity of the electrolyte while flow through the diaphragm may still occur. Viscous electrolyte solutions are less likely to dry up or crystallize when compared to liquid electrolyte solutions thus provide a cost effective design for simple pH applications found in water treatment, swimming pools, and laboratory sensor applications.
Products: ChemoTrode, IonoTrode
A flowing reference design relies on a steady flow of liquid electrolyte through the reference and out the diaphragm. This flow is created using either head pressure or pressure from an external source to force out the electrolyte. Additional liquid electrolyte can be added through a port or through an external reservoir so that it can be easily replenished as needed. The flowing reference design is popular commonly used in pure water applications such as demineralizers, ultrafiltration, and boiler water. It also works well for aggressive applications where the process liquid would quickly harm the reference half-cell.
The IonoTrode pH Sensor has a small side-arm tube that allows liquid electrolyte replenishment from an external reservoir