The previous Anatomy of a pH Electrode post we discussed why a reference system is needed when determining pH with a glass sensor as well as the different types of reference systems. This week we will be focusing on the reference electrolyte.
The structure of a typical pH electrode can be seen in figure 1 below.
Figure 1: Structure of a typical pH electrode
Properties of a Good Reference Electrolyte
The reference electrolyte has a connection to the sample through the junction, as it serves to close the electrical circuit in the electrode.
A good reference electrolyte must have certain qualities. The reference electrolyte must have good electrical conductivity and be chemically neutral. Since some electrolyte will typically leak into the sample during measurement, it is also important the electrolyte not react with the measurement solution.
The ions of an electrolyte solution must also be equally mobile. If ions in the electrolyte solution diffuse at different rates, an electrical potential (i.e. diffusion potential) can form due to the division between the positive and negative charge. This unwanted potential can be quite troublesome when measuring pH. With some ions (e.g. K+ and Cl-), the differences in diffusion speed are small, resulting in a much smaller diffusion potential.
Potassium chloride (KCl) possesses all of these qualities. As a result, KCl is the most commonly used electrolyte solution.
Form of Liquid Electrolyte
Electrodes can have a gel, polymer, or liquid electrolyte. All YSI field electrodes and some YSI lab electrodes (e.g. TruLine 25 and TruLine 26) feature a gel electrolyte, while most YSI lab electrodes feature a liquid electrolyte reference that can be refilled. Polymer electrolyte is used in specialized YSI electrodes with a flat tip (TruLine 27) and spear tip (TruLine 21).
Electrodes with liquid electrolyte can typically be refilled, thus resulting in a longer electrode life. Therefore, unlike an electrode with gel electrolyte, the electrolyte can easily be drained and replaced if the electrolyte becomes contaminated.
If using a YSI lab electrode with refillable reference, it is important to remember the refilling opening must always be open during calibration and measurement! Also, ensure the fill level of the electrolyte is at least 2 cm above the level of the calibration and/or measurement solution.
Figure 2: Slider (left) and stopper (right) refill openings for YSI lab electrodes
Response time is typically faster with electrodes featuring liquid electrolyte. Also, electrodes with liquid electrolyte have fewer limitations in their range of use, as gel and polymer electrolytes have a lower resistance to temperature and temperature changes. Unlike electrodes with liquid electrolyte, the incredibly small outflow rate of gel and polymer electrolyte in strongly acidic, basic, and low-ionic strength solutions can result in measurement errors due to the formation of diffusion potentials.
All YSI refillable lab electrodes utilize 3 M KCl for the reference electrolyte. However, since the electrolyte is connected to the sample via the junction, some applications require specialized electrolyte solutions.
Figure 3: YSI refillable lab electrodes (Science pHT-G and TruLine 17)
The YSI IoLine electrode design allows for the use of other electrolytes that meet the demands of the sampling application. For example, if electrolyte is needed that is completely chloride free (i.e. not KCl), another bridge electrolyte, such as 0.6 M potassium sulfate (K2SO4), can be used with the IoLine.
Gel and Polymer Electrolyte
Gel electrolyte still consists of KCl, but a gelling agent is added in order to prevent the electrolyte from readily leaking into the sample via the reference junction during measurement. Since there is virtually no loss of the electrolyte, these electrodes are easier to maintain, as there is no need to refill them with electrolyte. Since they cannot be refilled, electrodes with a gel electrolyte have a shorter lifetime than those with a liquid electrolyte.
Figure 4: YSI TruLine pH 25 electrode with gel electrolyte and plastic body
Polymer electrolyte is solid and can directly contact the sample during measurement. Due to the absence of electrolyte outflow, the mobility of all ions is greatly limited. This results in no precipitation of silver at the junction and makes the diffusion of foreign ions into the electrode virtually impossible.
The specialized YSI TruLine 21 and 27 pH electrodes feature the REFERID system, a unique form of polymer electrolyte. These electrodes are low maintenance and are characterized by high resistance to pressure and pressure changes. The REFERID system also features a visible KCl reserve.
Stay tuned...next pHriday we will be finishing our overview of glass pH sensor design by discussing reference junctions.
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 4 of 4
pH Meter Calibration Problems? Check Out These 12 Tips!