Redox Reactions: Principles, Volumetry, and Applications

Redox Reactions

Oxidation: A process in which an atom, ion, or molecule loses electrons.
Reduction: A process in which an atom, ion, or molecule gains electrons.

An oxidizing agent is a substance that gains the electrons lost during oxidation. A reducing agent is a substance that donates electrons, which are then gained during reduction. In short, a reducer is an electron donor, and an oxidant is an electron acceptor. Every redox reaction must involve at least one oxidizer and one reducer.

Balancing Redox Equations

Historically, oxidation was defined as the gain of oxygen. For example, in the reaction CO + 1/2O₂ → CO₂, carbon monoxide acts as the reducer, and the process is oxidation. Today, we follow the electron-transfer concept, and reactions are balanced using the ion-electron method.

General Principles of Redox Volumetry

Redox volumetry follows the same principles as other volumetric methods. The titrant is either an oxidizer or a reducer, and the analyte is oxidized or reduced accordingly. The general conditions for these reactions are:

• The reaction must be quantitative.
• The reaction must be rapid; if slow, it must be catalyzed.
• The reaction must be stoichiometrically defined.
• There must be a detectable change in a physical property at the equivalence point, typically identified by an indicator.

Basis of Redox Volumetry

The foundation of redox volumetry lies in the electrochemical cells formed and the property of potential varying with concentration. The titration curve, which plots potential versus the volume of titrant, is analogous to pH titration curves.

The equivalent weight is the mass of a substance that gives, receives, or is chemically equivalent to one mole of electrons transferred in the reaction. The valence is defined as the number of electrons transferred per mole of substance.

Common Oxidizing Agents

• HClO₄: Commonly used when hot; it is a powerful oxidant. An advantage is that cooling stops the oxidation process.
• NaBiO₃: Used to oxidize manganese to permanganate.
• H₂O₂: Excess can be eliminated by boiling.
• O₃: Does not introduce foreign ions into the solution, though its use is rare.
• HNO₃: Primarily used to dissolve samples.

Common Reducing Agents

• Metals: Excellent reducers because they lose electrons easily. Zinc (Zn) and Aluminum (Al) are commonly used, often in a mercury amalgam column.
• SnCl₂: Used to reduce Fe³⁺ to Fe²⁺.
• Ferrous Salts: Strong reducers; an example is Mohr's salt (FeSO₄ · (NH₄)₂SO₄ · 6H₂O).

Depending on the reagent used in the titration, these processes are classified as permanganometry (KMnO₄), dichromatometry (K₂Cr₂O₇), or iodimetry (I₂).