Unit 24
Electroanalytical chemistry
Voltages, currents and conductances are all windows onto chemistry. Starting from reference electrodes and the Nernst slope, this unit explains how glass pH electrodes, ion-selective electrodes and the zirconia oxygen sensor in a car exhaust work, finds titration end points from potentials, explains conductivity through ionic mobilities, counts silver atoms and micrograms of water in coulombs, simulates potential steps and cyclic voltammograms from the diffusion equation, and finally measures dissolved oxygen by titration and with an oxygen electrode — and you calibrate a pH meter yourself.
- 1 Potentiometry: reference and indicator electrodes A voltmeter only measures differences. The same silver electrode reads differently against calomel and against silver–silver chloride; learn to convert between potential scales and to turn a reading into a concentration with the Nernst slope. 25 min
- 2 The glass pH electrode and ion-selective electrodes Why does a thin glass membrane respond to H⁺? Two-point calibration, slope efficiency, sodium and acid errors — and the LaF₃ electrode with TISAB that measures fluoride in drinking water. 30 min
- 3 Solid electrolytes: the oxygen sensor in a car exhaust Yttria-doped zirconia conducts O²⁻ through its oxide vacancies, and the difference in oxygen pressure across it gives a Nernst voltage. Compute the oxygen pressure of exhaust from combustion equilibria and see why it jumps by more than ten orders of magnitude at λ = 1. 30 min
- 4 Potentiometric titration: end points from potentials Coloured or cloudy samples defeat indicators, but an electrode works regardless. Find end points from the first and second derivatives of the E–V curve and from Gran plots, and see how an autotitrator doses cleverly. 25 min
- 5 Conductometry: how fast ions move Ultrapure water conducts only 0.055 µS/cm while seawater reaches 50 mS/cm. Ionic molar conductivities, Kohlrausch’s law and the proton relay of H⁺ explain why a conductometric titration curve is V-shaped. 25 min
- 6 Coulometry: counting electrons, counting water In 1975 the US National Bureau of Standards dissolved a silver bar at constant current for six hours and obtained the Faraday constant from its mass loss and the charge. The same Faraday’s law lets a coulometric Karl Fischer titrator count a few micrograms of water with electrogenerated iodine: 10.71 C per milligram, with a dead-stop end point at two platinum wires. 30 min
- 7 Voltammetry: diffusion written in current A glucose meter reads a current. After a potential step the current decays as 1/√t (Cottrell); in a cyclic voltammogram the peak current grows as √v and reversible peaks sit about 59/n mV apart — all computed from the diffusion equation. 30 min
- 8 Dissolved oxygen: the Winkler titration and the Clark electrode Warm water holds less oxygen, seawater and mountain lakes less still: compute the saturation values with the Benson–Krause equations. The Winkler method fixes oxygen as MnO(OH)₂ and counts it by iodometry; a Clark electrode’s current follows the oxygen partial pressure, and the meter needs the salinity to give mg/L. 30 min