The Liquid Junction (Diaphragm)
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.
The diaphragm or liquid junction as it is frequently called, is a very important and critical part of the reference electrode. It provides an electrolytic interface between the Ag-AgCl conducting system and the measured liquid solution. In most cases the diaphragm consists of a porous ceramic disc fused into the glass wall at the lower end of a reference electrode.
There are many different diaphragm designs differing in construction and shape currently available. Each type has its advantages and limitations.
It is normally the measurement application which determines the use of a specific diaphragm design.
Porous Ceramic (Coatramic) Diaphragm
The porous ceramic diaphragm is probably the most frequently used today. It possesses high chemical resistance and it is easy to manufacture. This liquid junction design provides a reproducible electrolyte flow but because of its small surface area it can be vulnerable to contamination. Coatramic and HP-Coatramic are the Hamilton trade names that refer to this style of diaphragm.
Platinum Fiber Diaphragm
Platinum fiber diaphragms consist of very fine platinum wires which are spun loosely together and fused into the glass. Platinum is desirable due to its corrosion resistance. This type of liquid junction has good resistance to contamination however the electrolyte flow is less reproducible than ceramic junctions.
Single Pore™ Diaphragm
The Single Pore diaphragm is strictly speaking not a porous liquid junction at all. It is a very small glass aperture which allows a larger leakage rate than similar ceramic or platinum style junctions. A constant and very reproducible electrolyte flow is assured. Due to the hole size, clogging or contamination is barely possible. It gives the most accurate and repeatable results. In combination with a polymer electrolyte the Single Pore junction design is best suited for industrial pH sensors where its lack of contamination and maintenance offers many advantages.
The German Federal Physical Technical Institute (PTB) decided during a traceability test in 1997 that the Single Pore pH sensor is the most accurate laboratory electrode. (“Traceability of pH measurement” by Petra Spitzer; ISBN 3-89429-877-4 or ISSN 0947-7063)
Annular Ceramic Diaphragm
The annular ceramic diaphragm is formed by a porous ceramic layer between two glass tubes that make up the shaft of the pH sensor. The directional flow rate of the measured liquid medium is not critical due to the annular shape of this junction. Due to the large surface the electrolyte flow is not as controlled, hence it is mainly applied in gel-type electrodes.
Ground Sleeve Diaphragm
Ground sleeve diaphragms are ideally suited for applications in suspensions and emulsions, as these diaphragms can be cleaned easily by simply moving or twisting the outer glass sleeve. Another successful application of this liquid junction design is the pH measurement in low ionic solutions such as pure water. The electrolyte flowrate depends on the roughness of the ground glass surface of the sleeve and the tightness of the sleeve fit. This diaphragm is not suitable for applications where the pH sensor is subjected to vibration as this might loosen the outer diaphragm sleeve.
The selection of the right liquid junction design for a measuring application is of utmost importance but not always easy. Very often only the experimental “trial and error” method will lead to a successful application of a certain diaphragm type. For detailed information one has to consult the technical data sheets of the sensor manufacturer.
A diaphragm provides a deliberate leak path of the electrolyte solution into the measured liquid medium whilst preventing unrestricted mixing of both solutions within the reference electrode. Penetration of the measured solution into the reference electrolyte, and thus poisoning of the reference conducting system occurs frequently during pH measurements, especially when the measured solution is pressurised.
pH sensor designs such as Hamilton’s EasyFerm and ChemoTrode products allow the reference electrolyte to be pressurised in order to counteract the penetration of the measured solution through the diaphragm. As a rule of thumb, a pressure of 1 bar (14.5 PSIG) above the pressure of the measured solution will normally suffice. As a result a small amount of electrolyte solution will penetrate into the measured solution which is generally of no significance to the process. This electrolyte flow decreases the resistance of the reference electrode to between 0.1 kΩ and 2 kΩ as well as improves the reproducibility of the measurement and prevents the diaphragm from clogging up.
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