Potentiometry is a branch of analytical chemistry that measures the electric potential (voltage) of an electrochemical cell, consisting of two electrodes placed in a solution when there is zero current flow. This potential difference arises from redox reactions—chemical reactions that involve the transfer of electrons—occurring at the surfaces of the electrodes. At each electrode, an equilibrium is established between the ions in the solution and the electrode material. This equilibrium creates a stable potential, known as the electrode potential, which depends on the type and concentration of ions in the solution. ^[1–4]

By measuring the potential difference between the two electrodes, potentiometry provides valuable information about a solution’s composition. It helps determine properties such as pH, the concentration of specific ions, and how these properties change during a chemical reaction.

To better understand how potentiometry works in practice, let us explore the key components of a typical potentiometric system.

Basic Principles

The above diagram shows a typical potentiometric cell, where a zinc anode (left) undergoes oxidation and a silver cathode (right) undergoes reduction. The salt bridge (KCl solution) connects the two half-cells, allowing the flow of ions (K⁺ and Cl⁻) to maintain electrical neutrality in each half-cell while preventing the mixing of different solutions. It also ensures the cell can reach equilibrium without significant disruption. ^[1,2,4,5]

The potentiometer measures the potential difference (voltage) between the electrodes, which depends on the ion concentrations in the solutions. The system is designed for no net electron flow, ensuring the potential difference reflects the true equilibrium potential of the cell. ^[1–5] 

Chemical Reactions ^[1]

Anode

Zinc atoms are reduced, lose electrons, and become zinc ions.

Reaction:

Zn (s) → Zn²⁺ (aq) + 2e⁻

This releases two electrons per zinc atom. The Zn anode dissolves over time as it loses mass.

Cathode

Silver ions in solution gain electrons to form solid silver.

Reaction:

Ag⁺ (aq) + e⁻ → Ag (s)

Electrons from the Ag electrode reduce the silver ions at the cathode. As solid silver deposits onto the silver cathode, its mass increases.

Overall Cell Reaction

By combining the half-reactions and balancing electrons, the overall redox process is:

Zn (s) + 2Ag⁺ (aq) → 2Ag (s) + Zn²⁺ (aq)

Equation

The relationship between the measured potential and ion concentration is described by the Nernst equation: ^[1,2,4,5]

E = E⁰ – (RT/nF) ln Q

Where:

– E: Measured potential

– E⁰: Standard cell potential

– R: Gas constant

– T: Temperature in Kelvin

– n: Number of electrons transferred

– F: Faraday constant

– Q: Reaction quotient (ratio of product and reactant concentrations)

By applying the Nernst equation to the measured potential, we can calculate the concentration of the unknown ion in the solution.

Applications ^[1]

1. pH Determination: A key use of potentiometry is to measure pH, which indicates whether a solution is acidic or basic. This is done using pH electrodes (commonly called glass electrodes). The Nernst equation is applied to convert the measured electrode potential into a pH value.
2. Measurement of Ion Concentration: Potentiometry plays an important role in determining the concentration of specific ions in a solution. This is achieved using ion-selective electrodes (ISEs), which are designed to respond to particular ions like sodium, potassium, calcium, or chloride.
3. Complexometric Titration: In complexometric titrations, potentiometry helps measure the concentration of metal ions. A titrant is added that forms a stable complex with the metal ion in the solution, and the endpoint is indicated by a sudden change in electrode potential.
4. Redox Titration: Potentiometric methods are also used in redox titrations to find the concentration of reducing or oxidizing agents. A redox indicator electrode (Pt, Au, Pd) detects the changes in potential. This technique is commonly used in determining substances like vitamin C and in analyzing redox-active compounds in solution.
5. Trace-Level Analysis: Potentiometry is highly effective for detecting trace amounts of substances due to its sensitivity and selectivity. Environmental testing, food safety assessments, and pharmaceutical quality control often utilize potentiometry to monitor contaminants at very low concentrations.