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Potentiometry measures cell potential at zero current flow to determine analyte concentration, using the Nernst equation: E = E° − (RT/nF)ln(Q). The glass pH electrode is an ion-selective electrode (ISE) whose membrane potential varies with H⁺ activity; analogous membranes enable ISEs for F⁻, NO₃⁻, Ca²⁺, and other ions. Potentiometric titrations (pH, pIon, or pE vs volume) locate equivalence points precisely from inflection points, avoiding indicator ambiguity. Reference electrodes (SHE, Ag/AgCl, saturated calomel) provide a stable potential against which the indicator electrode is measured.
Calibrate a pH electrode using three buffers, measure unknown samples, then repeat a strong acid–strong base titration potentiometrically and graphically locate the equivalence point by the first or second derivative method. Comparing to the indicator endpoint quantifies the titration error.
Potentiometry is a form of electroanalytical chemistry that extracts concentration information from voltage, not from current. The key insight is the Nernst equation: at equilibrium (zero current), the potential of an electrochemical cell depends logarithmically on the activity of the ions in solution. By measuring that potential with a high-impedance voltmeter — so virtually no current flows — you can read out the analyte activity without disturbing the system.
The glass pH electrode is the most familiar ion-selective electrode. The electrode contains a thin glass membrane whose inner surface is in contact with a known reference solution, and whose outer surface is exposed to the sample. H⁺ ions exchange with sodium ions in the glass lattice, generating a membrane potential proportional to the difference in H⁺ activity across the glass. This potential, when measured against a stable reference electrode, gives pH directly. The elegance is that the membrane itself is the sensor — it is selective because only certain ions interact favorably with the glass lattice.
The same principle extends to other ions. Fluoride ISEs use a lanthanum fluoride crystal membrane; nitrate ISEs use a liquid membrane with a lipophilic ion exchanger. No membrane is perfectly selective: every ISE responds to some degree to interfering ions, described quantitatively by the Nikolsky–Eisenman equation. Understanding selectivity coefficients is critical when measuring dilute analytes in complex matrices.
A key misconception to address: the glass electrode measures H⁺ activity, not concentration. In pure water, activity ≈ concentration, so the distinction rarely matters in introductory work. But in high-ionic-strength solutions like blood or seawater, activity coefficients deviate substantially from 1, and ignoring this introduces systematic error. Calibrating in buffers that match the sample's ionic strength is standard practice in rigorous work.
Potentiometric titrations extend the technique to equivalence point location. Instead of watching a color change from an indicator, you plot cell potential versus volume of titrant added. The equivalence point appears as an inflection point — sharpest at the steepest part of the sigmoidal curve. Taking the first or second derivative of the potential-vs-volume plot localizes the equivalence point precisely, eliminating the subjectivity of indicator endpoint observations.