Chlor-Alkali Process and Bleaching Powder Chemistry

Common Salt as a Raw Material for Chemicals

Common salt, chemically known as sodium chloride ($\text{NaCl}$), serves as a fundamental raw material in the chemical industry for synthesizing a wide range of essential commercial and industrial compounds. Everyday products used in households and chemical manufacturing processes rely directly or indirectly on sodium chloride as a starting material. Among the key chemical compounds manufactured using common salt are sodium hydroxide ($\text{NaOH}$), baking soda, washing soda, and bleaching powder ($\text{CaOCl}_2$).

The Chlor-Alkali Process and Sodium Hydroxide Production

Sodium hydroxide ($\text{NaOH}$) is industrially produced through the electrolytic decomposition of an aqueous solution of sodium chloride, commonly referred to as brine. Brine is prepared by dissolving sodium chloride in water to form a concentrated aqueous solution. When an electric current is passed through brine, the solution undergoes chemical decomposition, generating sodium hydroxide along with chlorine gas and hydrogen gas.

This industrial procedure is formally termed the chlor-alkali process. The name is derived directly from the two major products generated during electrolysis: "chlor" denotes chlorine gas, and "alkali" denotes sodium hydroxide, which is a strong soluble base or alkali. The overall balanced chemical equation representing the chlor-alkali process is:

$2\text{NaCl}(aq) + 2\text{H}_2\text{O}(l) \rightarrow 2\text{NaOH}(aq) + \text{Cl}_2(g) + \text{H}_2(g)$

Analysis of the Electrolytic Cell and Products

The electrolytic cell utilized in the chlor-alkali process consists of an anode positive electrode ($+\text{ve}$) and a cathode negative electrode ($-\text{ve}$) separated by a membrane. During electrolysis, ionic dissociation occurs within the brine solution, yielding sodium ions ($\text{Na}^+$), chloride ions ($\text{Cl}^-$), hydrogen ions ($\text{H}^+$), and hydroxide ions ($\text{OH}^-$). Chloride ions migrate to the positively charged anode, where chlorine gas ($\text{Cl}_2$) is evolved. Concurrently, hydrogen ions migrate to the negatively charged cathode, where hydrogen gas ($\text{H}_2$) is liberated. Sodium hydroxide solution ($\text{NaOH}$) accumulates near the cathode compartment and is continuously extracted as brine containing sodium hydroxide.

Each of the three co-products derived from the chlor-alkali process possesses distinct industrial applications:

Chlorine gas ($\text{Cl}_2$) emitted at the anode ($+\text{ve}$) is extensively utilized in municipal water treatment facilities, swimming pool disinfection, the manufacturing of polyvinyl chloride (PVC) plastics, the production of disinfectants, chlorofluorocarbons (CFCs), and various agricultural pesticides.

Hydrogen gas ($\text{H}_2$) liberated at the cathode ($-\text{ve}$) serves as a clean fuel source, a key reactant in the hydrogenation of vegetable oils to form margarine, and an essential feed material for synthesizing industrial ammonia used in agricultural fertilizers.

Sodium hydroxide ($\text{NaOH}$) produced near the cathode compartment is employed in de-greasing metal surfaces, manufacturing soaps and detergents, paper making industries, producing artificial fibers, refining petroleum products, formulating drain cleaners, and processing salt metals.

Manufacture and Chemistry of Bleaching Powder

Bleaching powder is chemically designated as calcium oxychloride and represented by the empirical formula $\text{CaOCl}_2$. The industrial synthesis of bleaching powder was developed in 1799 by Charles Tennant. The process utilizes the chlorine gas produced during the electrolysis of aqueous sodium chloride brine as a primary reactant.

Bleaching powder is manufactured by passing chlorine gas over dry slaked lime, which is chemically calcium hydroxide [$\text{Ca(OH)}_2$]. The chemical reaction between dry slaked lime and chlorine gas yields calcium oxychloride along with water as a byproduct. The chemical equation for this reaction is expressed as:

$\text{Ca(OH)}_2 + \text{Cl}_2 \rightarrow \text{CaOCl}_2 + \text{H}_2\text{O}$

Although bleaching powder is conventionally represented by the simple formula $\text{CaOCl}_2$, its actual chemical composition and structure in solid form are complex and consist of a mixture of calcium compounds.