Note : ”Diaphragm“ is Hamilton terminology for the porous liquid junction found in the reference electrode. These two terms will be used interchangeably in this article.
Chemical reactions at the liquid junction between the KCl reference electrolyte and the measured solution must be avoided at all costs. This reaction can lead to junction contamination, increased electrical resistance, and pH measurement errors. The reference electrolyte contains silver chloride (AgCl) which is prone to chemical reactions, especially with sulfide compounds and proteins found in biopharma processes. This is often referred to as poisoning of the pH sensor.
pH and Sulfides
Great care must be taken when measuring pH in solutions containing sulfides, as the diaphragm may be contaminated with silver sulfide precipitation. The precipitation occurs from chemical attack of the AgCl element by sulfides. Silver sulfide contamination can easily be identified by a blackened color of the diaphragm. As a result, the response time of an electrode assembly increases substantially, the liquid junction resistance increases radically, and calibration may become difficult or even impossible due to changes in the potential voltage.
pH and Proteins
Proteins are also well known to react with Ag+ ions that may be present in the reference electrolyte. The reaction forms a solid particulate that can clog the porous liquid junction causing loss of electrolyte flow and possible loss of measurement. Often these proteins can be removed through a proper cleaning process, however, it is better to prevent the initial precipitation through the reference design. Hamilton has developed special reference systems to separate the Ag-AgCl portions of the reference from the process liquid yet still maintain an electrical path to complete the pH voltage circuit.
The Everef-F Reference System
In order to counteract the effects of silver sulfide contamination and protein precipitation at the diaphragm Hamilton have invented the Everef-F reference system. The goal of the system is to isolate the Ag-AgCl element from the KCl reference electrolyte. This system consists of a silver chloride reservoir from which the silver reference wire leads to the sensor’s electrical connector. The reservoir is separated from the reference electrolyte by a diffusion barrier consisting of densely packed cotton wool in a glass tube. The barrier prevents the loss of silver chloride into the reference electrolyte induced by temperature variations. In front of the diffusion barrier is a Ag+ scavenger. The scavenger uses the principles of ion exchange resins to capture the Ag+ ions before they can escape into the reference electrolyte. The Everef-F reference system enhances the stability of the reference potential and extends the life of the combination electrode considerably. It is commonly used in Hamilton pre-pressurized pH sensors.
The Everef-L Reference System
In applications where a chemical reaction at the liquid junction is unavoidable, then lengthening the diffusion path through the reference electrolyte provides a solution. The Everef-L labyrinth liquid junction system from Hamilton addresses the long diffusion path idea. The Ag-AgCl element is extended the full length of the pH sensor. The element is enclosed in its own glass sheath with only a small opening near the top of the sensor where the reference electrolyte enters the sheath. The Everef-L design creates an extremely long diffusion path. Any ingress of sulfides into the electrolyte must travel this long distance before they can react with the AgCl and impact the potential voltage generated by the reference electrode. The Everef-L design is highly successful in industrial applications and commonly used with Hamilton’s polymer pH sensors.