Last pHriday we discussed the wide range of possible hydrogen ion activities that can be observed. Due to this wide range, hydrogen ion activity is typically represented on a logarithmic scale, the pH scale. For the next several pHridays we will be focusing on the most common method of measuring pH, the glass pH electrode.
Methods of pH Measurement
Visual, photometric, and potentiometric methods can be used to measure the hydrogen ion activity of a solution. Visual and photometric methods rely on color changes of specific organic pigments in order to determine pH. Visual methods are completed with visual indicators such as pH test strips, while photometric determination involves shining a light through the sample and measuring the absorbance.
The application of visual or photometric determination of pH is limited. Measurements will be unreliable if the solution to be measured is cloudy or has an inherent color. Some measurement solutions also contain chemical bonds which destroy the color indicators through oxidation or reduction and produce incorrect results.
Potentiometric methods determine pH by using the electrical potential of pH-sensitive electrodes as a measurement signal. The disadvantages of visual and photometric methods are not present with potentiometric methods. Potentiometric determination of pH can be used in almost any application, as potentiometric sensors are very sensitive and selective.
A distinction is made between hydrogen, metal (e.g. antimony), and glass electrodes, with the glass electrode being the most commonly used pH sensor.
The glass pH sensor is an example of an ion selective electrode (ISE). This measuring system basically consists of the ISE reacting on a special ion type, in this case the hydrogen ion, and a reference electrode that are jointly immersed in the sample to be measured.
The hydrogen ISE provides an electrochemical potential that is influenced by the hydrogen ion activity of the solution. The reference electrode, however, is intended to build up an electrochemical potential that does not depend on the composition of the sample. The difference between these potentials, the voltage (mV) displayed on a pH meter, determines the pH value based on the Nernst equation.
Glass pH Electrode Structure and Design
A typical pH electrode consists of several unique structures, each of which can be seen in Figure 1. The principle components are the electrode body, pH-sensitive glass membrane, reference electrode (i.e. reference system), reference electrolyte, and the reference junction.
Figure 1: Structure of a typical pH electrode
The term ‘glass electrode’ is not indicative of the material used to construct the electrode body, as electrodes can have either a plastic or glass electrode body. Rather, ‘glass electrode’ is used to describe the membrane material (i.e. glass membrane).
All YSI field electrodes are constructed from plastic, as plastic body electrodes are more rugged and less likely to crack than glass. YSI offers both glass and plastic body electrodes for the lab, with glass electrodes typically possessing a greater range of operating temperature.
YSI field pH probes are not always entirely plastic. The pH sensing module on ProDSS and EXO pH sensors are plastic, but the rest of the electrode body is constructed from titanium in order to ensure durability in the field. A ProDSS pH/ORP sensor can be seen in Figure 2.
Figure 2: YSI ProDSS pH/ORP sensor
With the glass electrode, a glass membrane is fused on as a pH sensor. This membrane is filled with a buffer solution of known pH (typically pH = 7). This electrode design creates an environment with constant binding of H+ ions on the inside of the glass membrane, while the outside of the glass membrane is exposed to the sample where a variable amount of H+ ions exist. The difference in H+ ions creates a potential that is read versus the stable potential of the reference electrode.
In order to ensure optimal moistening of the glass membrane, the membrane shape can vary. Sphere and cone membrane shapes can be used for most applications, but unique applications may require a specialized membrane, such as the spear tipped TruLine 21 for penetrating semi-solid media and the TruLine 27 with a flat membrane for surface measurements (Figure 3). Table 1 provides an overview of the different types of pH membranes. (View the pH Probe Comparison Guide).
Figure 3: YSI TruLine 21 (spear tip membrane) and YSI TruLine 27 (flat membrane)
Table 1: Glass Membrane Shapes
Additional Blog Posts of Interest:
Anatomy of a pH Electrode | Glass pH Probes, Part 2 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!