Glass Electrode Principles and pH Measurement
Understanding the Glass Electrode
The glass electrode is an ion-selective membrane indicator electrode used for pH measurement. In potentiometry, its potential is measured against a reference electrode with a constant and known potential. Therefore, changes in the measured cell potential can be related to changes in the activity of H+ in the analyte solution.
Components of a Glass Electrode
The glass electrode can be used together with a separate reference electrode, or both systems can be incorporated into a single combination electrode. The glass electrode consists of:
- A thin pH-sensitive glass membrane
- An internal solution with constant H+ activity
- An internal Ag/AgCl electrode
The external side of the glass membrane is in contact with the analyte solution of unknown pH, whereas the internal side is in contact with a solution of known and constant composition.
Ion-Exchange Mechanism and Hydrated Gel Layers
The glass membrane is based on a silicate structure containing cations such as Na+ or Li+. When the glass is in contact with water, hydrated gel layers are formed on both surfaces of the membrane. Hydrogen ions from the solution can interact with these hydrated layers by ion exchange, for example:
H+solution + Na+glass ⇌ Na+solution + H+glass
Therefore, the surface of the glass membrane is sensitive to the activity of hydrogen ions. The composition of the glass determines its sensitivity to H+ and other cations. Ion-exchange equilibria occur at both surfaces of the membrane.
Boundary Potential and the Nernst Equation
At the external surface, a potential E1 develops depending on the H+ activity of the analyte solution. At the internal surface, another potential E2 develops. Since the H+ activity of the internal solution is kept constant, E2 is essentially constant, whereas E1 changes when the pH of the sample changes. Electrical conductivity through the glass membrane is associated with the movement of ions such as H+ and Na+.
The difference between the potentials at the two surfaces is called the boundary potential (Eb):
Eb = E1 − E2
The boundary potential depends on the difference in hydrogen-ion activity on both sides of the glass membrane. Since the internal hydrogen-ion activity is constant, the measured potential depends mainly on the H+ activity of the analyte solution. According to the Nernst relationship, at 25°C:
Eb = L − 0.0592 pH
Where L contains the constant terms of the electrode system. Therefore, a change of one pH unit produces a potential change of approximately 59.2 mV per pH unit at 25°C.
Thus: change in H+ → change in membrane potential → pH measurement.
Calculating Total Indicator Electrode Potential
The total potential of the glass indicator electrode (Eind) contains three main contributions: the boundary potential, the potential of the internal Ag/AgCl reference electrode, and a small asymmetry potential:
Eind = Eb + EAg/AgCl + Easym
The asymmetry potential is a small potential caused by differences between the two surfaces of a real glass membrane and may change slowly with time.
pH Measurement and Calibration
Finally, the glass-electrode potential is measured relative to an external reference electrode:
Ecell ≈ Eglass − Ereference
Because the reference-electrode potential is constant, changes in Ecell are mainly caused by changes in the glass-electrode potential and therefore by changes in the pH of the sample. After calibration with buffer solutions of known pH, the measured potential can be converted directly into pH.