Last pHriday we discussed the properties of a good reference electrolyte and the difference between liquid and gel electrolyte. This pHriday we will be finishing our discussion of pH electrode anatomy by focusing on the reference junction.
Figure 1: Structure of a typical pH electrode
pH Reference Junction, What’s Your Function?
The reference junction, also known as a diaphragm, creates electrical contact between the reference system and the solution. Much like the reference electrolyte, the reference junction must possess certain qualities.
Diffusion voltages at the junction are a common measurement error, so the junction plays a major role in the precision of measurements. To keep these disruptive potentials small, the junction must guarantee a relatively large and consistent outflow of reference electrolyte. However, the junction must only be slightly permeable to prevent electrolyte from escaping too quickly, which is especially important with electrodes utilizing liquid electrolyte. Different junction types have different outflow rates of electrolyte.
In addition to the permeability of the junction, its electrical resistance should be as low as possible and it must be chemically inert.
Types of Reference Junctions
There are several types of junctions, each with unique characteristics. Depending on the application, a decision must be made for YSI electrodes with liquid electrolytes as to whether a ceramic, ground-joint, or platinum junction best fits the measurement conditions. Table 1 provides an overview of the different junction types, including the annular gap (TruLine 27) and fiber (TruLine 25 and TruLine 26, MultiLab IDS 4110) junctions.
Table 1: Types of reference junctions
A ceramic junction uses the porosity of unglazed ceramic. Ceramic junctions have a KCl outflow rate of ~0.2 mL/day and a relatively high electrical resistance (1 kΩ). Diffusion potentials are easily created in measurement solutions with greater ionic strength, as the concentration gradient at the junction is very large. In lower ionic strength solutions, the resistance of the test material may be too high for exact measurements. Both effects are amplified by the low outflow rate, so ceramic junctions are less suitable in such cases. Due to the high risk of blockage of its pores, it is also not suitable for solutions containing suspended particles. Only in measurement solutions that contain oxidizing substances is the ceramic junction superior to the platinum junction.
The platinum junction consists of fine, twisted platinum filaments between which the electrolyte flows out along precisely defined channels. The platinum junction features a very constant outflow and does not easily become blocked. With an outflow rate of ~ 1 mL / day and electrical resistance of ~ 0.5 kΩ, it has advantages over ceramic junctions. The platinum junction is more sensitive to mechanical stress and is also less than optimal for strongly oxidizing or reducing solutions due to the occurrence of disruptive potentials. However, the platinum junction can be used almost universally.
Figure 2: TruLine pH 17 electrode with platinum junction
The ground-joint junction works with the thin gap of the unlubricated ground glass as an outflow opening for the electrolyte. The outflow rate is ~ 3 mL / day and greater. It features a very low electrical resistance (0.1 kΩ). The ground-joint junction is suitable for measurements in contaminated solutions, as it is easy to clean. Due to the high outflow rate, it is suitable for both high and low-ion solutions.
Figure 3: Science pHT-G electrode with ground-joint junction
Junctions constructed from plastic fibers can also be used for special applications. For example, combination electrodes with a plastic shaft often have junctions made from nylon fibers to avoid contamination of the connection hole. For process measurements in solutions that contain fluoride, electrolyte keys with PTFE junctions are used.
Figure 4: TruLine pH 25 electrode with plastic fiber junction
Additional Reference Junctions
Additional junctions can be used to prevent contamination of the reference system. With this design, the reference electrode is immersed in electrolyte solution within an additional chamber. This additional chamber acts as an extra barrier against contamination while additional junctions are used to ensure the reference system still has contact with the measurement solution. The reference system can still become contaminated by the measurement solution, but the solution must first diffuse through the additional junction(s).
The Silamid reference system by YSI is a special type of double junction electrode that utilizes a unique construction of the silver/silver chloride reference system. Most electrodes featuring a Ag/AgCl system are built with an Ag wire coated with AgCl. Silamid reference systems have a glass tube with the inner part coated with Ag, then filled with AgCl, and plugged with a polyester fiber. This reference system creates greater contact surface area between Ag and AgCl compared to the standard Ag/AgCl wire system, resulting in a reference system that is long lasting and very stable. The Science series of electrodes from YSI feature the Silamid reference system.
YSI offers a variety of platforms for the measurement of pH. Whether for the lab (MultiLab, TruLab), environmental sampling (Pro Plus, ProDSS, Pro10), or for long-term monitoring (EXO1, EXO2), YSI has what you need!
Additional Blog Posts of Interest:
Anatomy of a pH Electrode | Glass pH Probes, Part 1 of 4
Anatomy of a pH Electrode | Glass pH Probes, Part 2 of 4
Anatomy of a pH Electrode | Glass pH Probes, Part 3 of 4
pH Meter Calibration Problems? Check Out These 12 Tips!