Potentiometry is an electrochemical analytical technique that measures the potential difference between two electrodes in a solution to determine the concentration of specific ions. It is widely used in pH measurement, ion-selective analysis, and endpoint detection in titrations.

Fundamental Principles

1. An electrochemical cell consists of a reference electrode (constant potential) and an indicator electrode (potential varies with analyte concentration).
2. The measured cell potential follows the Nernst Equation: E = E° - (RT/nF) ln Q, where E° is the standard potential, R is the gas constant, T is temperature, n is the number of electrons, F is Faraday’s constant, and Q is the reaction quotient.
3. At 25°C, the equation simplifies to E = E° - (0.0592/n) log Q for each decade change in ion concentration.

Electrode Systems

1. Reference Electrodes: Silver/silver chloride (Ag/AgCl) or saturated calomel electrode (SCE) provide a stable, known potential independent of sample composition.
2. Glass pH Electrode: A hydrogen-ion-selective glass membrane generates a potential proportional to the pH of the solution, following E = constant + 0.0592 pH.
3. Ion-Selective Electrodes (ISEs): Membranes selective for specific ions such as fluoride (LaF3 crystal), potassium (valinomycin membrane), calcium, or nitrate.
4. Solid-State Electrodes: Use sparingly soluble salt membranes, such as the silver sulfide electrode for sulfide or silver ion detection.

Instrumentation

1. A pH meter or potentiometer measures the voltage between the electrode pair with high input impedance (10^12 Ω or higher) to prevent current draw.
2. Temperature compensation is essential as the Nernst slope is temperature-dependent.
3. Calibration with standard buffer solutions (pH 4, 7, 10) or standard ionic solutions is required before measurement.

Practical Measurement Techniques

1. Direct Potentiometry: The electrode is immersed in the sample and the potential is read directly after calibration.
2. Potentiometric Titration: The potential is monitored as a titrant is added; the endpoint corresponds to the largest change in potential per unit volume of titrant.
3. Standard Addition: A known amount of analyte is added to the sample, and the change in potential is used to calculate the original concentration.

Applications

1. Routine pH measurement in laboratories, environmental monitoring, and industrial process control.
2. Determination of fluoride in drinking water and toothpaste using the fluoride ISE.
3. Potentiometric titration of halides, heavy metals, and acids in complex matrices.
4. Clinical analysis of electrolytes (Na+, K+, Ca2+, Cl-) in blood serum and urine.