pH Measurement By Glass Electrode
A glass electrode is a specialized type of ion-selective electrode (ISE) used widely across analytical chemistry for determining the pH of a solution. It offers exceptional performance because its surface potential is completely unaffected by strong oxidizing or reducing agents in the target medium, and it is highly resistant to chemical poisoning factors.
The standard electrochemical half-cell shorthand notation of a classic glass electrode is written as follows:
The fundamental principle governing a glass electrode relies on phase boundary potentials: when a thin glass membrane separates two solutions of differing hydrogen ion activities, an electrical potential difference develops across the interior and exterior surfaces of the glass. This boundary potential varies linearly with the difference in pH between the two solutions.
Structurally, the electrode consists of a thin-walled bulb blown from a specialized glass* mixture possessing an exceptionally low melting point paired with high internal electrical conductivity.
The internal core is filled with a stable reference solution of $0.1\text{ N}$ hydrochloric acid ($\text{HCl}$) that is either saturated with quinhydrone or maintains equilibrium with a built-in Silver/Silver Chloride ($\text{Ag/AgCl}$) reference element. An internal connection wire passes out of the protective glass support sleeve. To compute values, this assembled bulb is immersed inside a sample beaker alongside a standard saturated calomel reference electrode ($\text{SCE}$) to establish a complete measuring circuit.
Due to the extremely high electrical resistance ($10^7$ to $10^9\ \Omega$) of even an ultra-thin glass membrane, ordinary terminal potentiometers cannot record the voltage profile accurately without dropping out. Consequently, specialized modern electronic valve electrometers or high-input impedance digital pH meters must be utilized to prevent measurement distortions.
The total boundary potential ($E_\text{G}$) of the active glass electrode follows the mathematical Nernst relationship:
EG = E°G − 0.0591 log₁₀[H⁺] at 25°C
Substituting the standard logarithmic identity of $\text{pH} = -\log_{10}[\text{H}^+]$, the expression simplifies directly to:
The standard potential parameter ($E^\circ_\text{G}$) is determined experimentally by calibrating the active digital meter instrument against standardized reference buffer solutions of known pH. Because the reference value of the auxiliary calomel half-cell is completely fixed, the true sample pH value can be instantly resolved from the net electromotive force ($\text{emf}$) generated across the complete cell matrix.
Advantages of Glass Electrode for pH Measurement
The common advantages of the application of glass electrodes in pH measurement are-
- The ion selective glass electrode gives a fairly accurate measurement of pH within an error of ± 0.05.
- The equilibrium of the electrode is quickly attained. It cannot be influenced by any gas, oxidizing, or reducing agents.
- The range of pH which can be measured is 2 to 9. However, new glasses have been developed with which the pH range can be extended up to 13 or 14.
- Glass electrodes are used for a long time for pH measurement if carefully handled and stored in distilled water.
Limitations of Glass Electrode for pH Measurement
Limitations of glass electrodes in pH measurement are-
- The glass membrane though it is very thin, it offers high resistance. Therefore ordinary potentiometers cannot be used; hence it is necessary to use electronic potentiometers.
- This electrode cannot be used to determine the pH above 12.
Related Topic:
Quinhydrone Electrode for pH Measurement Explained