The combination pH sensor is comprised of two half cells. The glass measurement electrode is well known as the portion of the pH sensor which provides varying voltage that relates to changing pH levels within the liquid. The reference electrode is often the lesser understood half of the sensor, but is equally important. Choosing the proper reference for the application can greatly help to ensure better pH measurement accuracy and sensor longevity. The reference fulfills two main requirements to make the pH sensor 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.
Electrical Path –The reference relies upon a porous 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 meter to be calculated as a pH value based upon the Nernst equation.
There are multiple factors with pH measurement that can affect the reference and create errors in the pH measurement.
Coating – Certain process liquids cause solid build-up on pH sensor clogging the porous diaphragm. 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 diaphragm can occur various ways including chemical induced precipitation reactions between the process liquid, electrolyte, and the Ag/AgCl element, build-up on the diaphragm such as with oils and fats, and even hard, crystalline coatings that block the diaphragm.
Poisoning – Since the diaphragm 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.
Leaching – Some liquids act as solvents and draw out the KCl electrolyte contained in the reference more quickly than normal. When this occurs there can be an additional potential voltage created at the diaphragm 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 In extreme cases, the majority of electrolyte can be withdrawn from the reference leaving it dry or exposing the Ag/AgCl element.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.
Reference Design Features
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.
Polymer Gel Electrolyte
Products: Polilyte Lab, FillTrode, FlaTrode, Double Pore, Polyplast,
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. Sensors using polymer gels are typically not refillable due to the viscous nature of the electrolyte.
Products: BioTrode, FoodTrode, TipTrode
Protein precipitation can occur when the proteins in the liquid solution encounter free Ag+ ions. These ions may be present in the pH sensor reference due to the AgCl element within the reference half-cell. The end result is typically clogging of the diaphragm which can degrade the pH measurement. Protelyte electrolyte is specially formulated to immobilize the mobility of these ions thus preventing precipitation from forming. Sensors used in food and dairy applications, Tris buffer, and related liquids with high protein concentrations should use Protelyte electrolyte.