Bleaching Action of Sulphur Dioxide: A Temporary Bleaching Agent
Sulphur dioxide (SO2) acts as a temporary bleaching agent through a reduction process, and is used especially for materials like wool, silk, and certain foods. The presence of moisture (H2O) is essential for the reaction to occur.
Mechanism of Bleaching
1. The Classical/Simplified Mechanism (The Nascent Hydrogen Model)
This mechanism is widely taught in introductory chemistry to easily explain SO2's reducing nature.
Formation of Nascent Hydrogen:
The bleaching action of sulphur dioxide is based on its reducing properties. When SO2 comes in contact with moisture (water), it undergoes the following reaction :
SO2 + 2H2O → H2SO4 + 2[H]
This reaction produces nascent hydrogen [H], which is the actual bleaching agent. The nascent hydrogen acts as a strong reducing agent that removes color from substances.
Reduction Process:
The nascent hydrogen reduces the colored compounds (chromophores) to colorless compounds through the following general reaction :
Colored matter + [H] → Colorless matter
Thus, the colored substances are reduced to colorless forms.
2. The Modern/Accurate Mechanism (The Sulphurous Acid Model)
The scientifically rigorous explanation focuses on the formation of sulphurous acid, which itself is the primary reducing agent.
In water, SO2 immediately forms an equilibrium with sulphurous acid (H2SO3). During bleaching, the sulphurous acid reduces the colored matter and is simultaneously oxidized to the more stable sulphuric acid (H2SO4).
Formation of Active Agent:
SO2 + H2O ⇌ H2SO3 (Sulphurous Acid)
Reduction (Bleaching):
The colored compound is reduced by the H2SO3, which gets oxidized (its sulfur changes from +4 to +6 oxidation state).
Colored Matter + H2SO3 → Colorless Matter (Reduced) + H2SO4
Explanation of Reduction:
The reduction of the colored matter is a result of the S+4 ion in H2SO3 acting as a reducing agent (a substance that is readily oxidized). The bleaching is temporary because the colorless compound is chemically unstable and can react with atmospheric oxygen to revert to its original colored structure.
Key Characteristics
1. Temporary Nature of Bleaching
The bleaching effect of SO2 is temporary and reversible. When the bleached material is exposed to atmospheric oxygen over time, the colorless reduced product gets re-oxidized and regains its original color.
Colorless matter + [O] → Original colored matter
2. Moisture Requirement
Dry sulphur dioxide does not have any bleaching action. The presence of moisture is essential for the bleaching process to occur. This is because water is required to form the nascent hydrogen that performs the actual bleaching.
3. Mild Bleaching Agent
SO2 is considered a mild bleaching agent compared to other bleaching agents like chlorine. This makes it suitable for delicate materials that might be damaged by stronger bleaching agents.