Last pHriday we discussed visual, photometric, and potentiometric methods of determining pH. Potentiometric determination of pH is very common, and the most widely used electrode is the glass pH electrode. This week we will continue to discuss the glass electrode by describing the reference system.
The structure of a typical glass pH electrode can be seen in figure 1 below.
Figure 1: Structure of a typical glass pH electrode
Why Is a Reference Needed?
pH measurement with a glass pH electrode relies on the measurement of a voltage. In order to measure the voltage, two points with different electrical potential values are required. The reference electrode is designed to maintain a constant electrical potential that is independent of the sample composition and temperature. In contrast, the hydrogen ion selective electrode (ISE) with glass membrane provides an electrical potential that is dependent upon the activity of hydrogen (H+) ions in the sample solution. Therefore, both the reference electrode and the hydrogen ISE are needed when determining pH.
Types of Reference Systems
The reference electrode/system and the hydrogen ISE (i.e. the electrode with the glass membrane) can be separate electrodes (figure 2) or they can be combined into a single electrode for convenience (figure 1 and figure 3). The combination-style electrode is very common and all YSI electrodes are of the combination type.
Regardless of the electrode design or the type of reference system used, the reference electrode is immersed in reference electrolyte (typically KCl). The reference electrolyte will be discussed in more detail next pHriday.
Figure 2: Schematic structure of a hydrogen ISE, reference, and voltmeter
Figure 3: Combination pH electrode with KCl gel reference electrolyte
The most common type of reference electrode used today is the silver/silver chloride (Ag/AgCl) system. Since silver is non-toxic to humans, Ag/AgCl electrodes can also be used in medicine and food technology where the poisonous mercury and thallium systems are prohibited. Disposal is also less critical with Ag/AgCl than with thallium and mercury. Ag/AgCl has a wide range of application with respect to temperature (up to 140°C) and is therefore also suitable for sterilizable electrodes. Most YSI electrodes feature a Ag/AgCl reference system.
The mercury chloride (Hg/Hg2Cl2 ) system has been in use for the longest time. This reference system displays the most stable potential in the presence of KCl. However, today it is only used in exceptional cases due to the toxicity of mercury. Also, the reliable temperature range when using mercury chloride is relatively narrow. At temperatures above 60°C the Hg2Cl2 begins to degrade.
The iodine/iodide system, a relatively new reference system with a fast response time, has recently been developed. Compared to conventional electrodes with Ag/AgCl reference systems, electrodes with iodine/iodide reference systems have the advantage of much lower temperature sensitivity, as the temperature coefficient of this reference system is almost zero.
The iodine/iodide system is also metal ion free, a feature that is especially useful when measuring in Tris buffer and protein solutions. Reference systems with metal ions (e.g. Ag/AgCl) will interact with these solutions, ultimately resulting in the reference junction becoming clogged.
The advanced YSI IoLine series of pH electrodes features an iodine/iodide reference system. The unique color of the IoLine reference can be seen in figure 4.
Figure 4: YSI IoLine pH electrode
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 3 of 4
Anatomy of a pH Electrode | Glass pH Probes, Part 4 of 4
pH Meter Calibration Problems? Check Out These 12 Tips!