Calcium hypochlorite also referred to as Calcium oxychloride is an inorganic compound with formula Ca(ClO)₂. It is a white solid, although commercial samples appear yellow. Despite being quite stable at room temperatures, calcium hypochlorite slowly decomposes in moist environments, giving it a characteristic strong smell of chlorine.
This compound is relatively stable as a solid and solution and has greater available chlorine than sodium hypochlorite. “Pure” samples have 99.2% active chlorine. Given common industrial purity, an active chlorine content of 65-70% is typical. It is the main active ingredient of commercial products called bleaching powder, used for water treatment and as a bleaching agent.
Structure
Calcium hypochlorite has a crystal structure that consists of calcium cations (Ca2+) and hypochlorite anions (ClO^-) arranged in a specific pattern. The compound forms crystals in the orthorhombic crystal system, meaning that its unit cell has three axes of different lengths at right angles to each other.
The calcium cation (Ca2+) has a 2+ charge and is surrounded by six chloride ions (Cl^-) and six oxygen atoms from hypochlorite ions in an octahedral arrangement. Each chloride ion is tetrahedrally coordinated to four calcium ions, while each oxygen atom from the hypochlorite ion is octahedrally coordinated to three calcium ions and three chlorine ions. This arrangement forms a three-dimensional network structure, where the calcium ions are held in place by electrostatic forces of attraction to the chloride and hypochlorite ions.
In terms of its chemical formula, each calcium ion is associated with two hypochlorite ions (Ca(ClO)2), giving rise to the overall composition of calcium hypochlorite.
Physical Properties of Calcium Hypochlorite
- Physical state: Found as a white solid.
- Form: Available in powder or granular form.
- Odor: May have a strong chlorine-like odor.
- Solubility: Soluble in water, forming a solution that releases chlorine.
- Density: Density varies depending on the specific formulation, typically ranging from about 2.35 to 2.71 g/cm^3.
- Melting point: Melts at approximately 100°C (212°F).
- Stability: Can decompose upon exposure to heat, light, or organic materials.
- Hygroscopicity: Absorbs moisture from the air, affecting its stability and potency over time.
- pH: Typically alkaline in aqueous solution, contributing to its effectiveness as a disinfectant.
Chemical Properties of Calcium Hypochlorite
- Oxidizing agent: Calcium hypochlorite is a strong oxidizing agent, capable of oxidizing a wide range of substances, including organic matter, metals, and other chemicals.
- Disinfectant: It releases hypochlorous acid (HOCl) and hypochlorite ions (OCl^-) when dissolved in water, which are potent disinfectants, effectively killing bacteria, viruses, and other microorganisms.
- Bleaching agent: Due to its ability to release chlorine, calcium hypochlorite is used as a bleaching agent for textiles, paper, and other materials.
- Decomposition: It can decompose upon exposure to heat, light, or organic materials, releasing chlorine gas and oxygen.
- pH adjustment: In water, calcium hypochlorite can increase the pH due to the release of hydroxide ions (OH^-) from the reaction of hypochlorite ions with water.
- Reaction with acids: Reacts vigorously with acids, releasing chlorine gas and calcium salts.
- Compatibility with other chemicals: Must be carefully handled and stored to avoid reactions with incompatible substances, such as acids, ammonia, and reducing agents, which can lead to hazardous conditions or even explosions.
- Storage stability: Stability can be affected by factors such as temperature, humidity, and exposure to sunlight, leading to degradation over time.
Preparation of Calcium Hypochlorite
Calcium hypochlorite can be prepared through different methods, each involving the reaction of calcium compounds with chlorine gas or hypochlorous acid.
Chlorination of Slaked Lime (Calcium Hydroxide)
Reaction: Chlorine gas is bubbled through a suspension or solution of calcium hydroxide. Ca(OH)2+2Cl2→Ca(OCl)2+2HClCa(OH)2+2Cl2→Ca(OCl)2+2HCl
Procedure
- Calcium hydroxide (slaked lime) is prepared by adding water to calcium oxide (quicklime).
- Chlorine gas is then introduced into the calcium hydroxide suspension or solution.
- The chlorine gas reacts with calcium hydroxide to produce calcium hypochlorite and hydrochloric acid.
Isolation and Purification
- The resulting calcium hypochlorite may be obtained as a solid precipitate or as a concentrated solution, depending on the reaction conditions.
- Solid calcium hypochlorite can be filtered, washed, and dried to obtain the pure compound.
Direct Combination of Calcium Oxide with Chlorine Gas
Reaction: Calcium oxide reacts directly with chlorine gas. CaO+2Cl2→Ca(OCl)2CaO+2Cl2→Ca(OCl)2
Procedure
- Calcium oxide (quicklime) is heated and then exposed to chlorine gas.
- The chlorine gas reacts with calcium oxide to form calcium hypochlorite directly.
Isolation and Purification
- The resulting calcium hypochlorite may be obtained as a solid product and purified through methods such as filtration and recrystallization.
Reactions with Calcium Chloride and Sodium Hypochlorite
Reaction: Calcium chloride and sodium hypochlorite react to produce calcium hypochlorite. CaCl2+2NaOCl→Ca(OCl)2+2NaClCaCl2+2NaOCl→Ca(OCl)2+2NaCl
Procedure
- Calcium chloride and sodium hypochlorite are mixed together in a suitable solvent.
- The two compounds react to form calcium hypochlorite and sodium chloride.
Isolation and Purification
- The calcium hypochlorite formed may be isolated and purified through filtration and drying processes.
Electrolysis of a Calcium Chloride Solution
- Electrolysis Reaction: When a calcium chloride solution is electrolyzed, calcium hypochlorite can be formed along with other products. 2CaCl2+2H2O→Ca(OCl)2+Ca(OH)2+2HCl2CaCl2+2H2O→Ca(OCl)2+Ca(OH)2+2HCl
Procedure
- A solution of calcium chloride is subjected to electrolysis using inert electrodes.
- Chlorine gas is evolved at the anode, while calcium hypochlorite is formed along with calcium hydroxide and hydrogen gas at the cathode.
Isolation and Purification
- The calcium hypochlorite produced in this manner may require separation and purification steps to obtain the desired product.
Uses of Calcium Hypochlorite
Water Treatment
- Disinfection of drinking water in municipal water treatment plants.
- Purification of swimming pool water to kill bacteria, algae, and other microorganisms.
- Treatment of wastewater to control microbial growth.
Sanitation and Disinfection
- Disinfection of surfaces, equipment, and facilities in healthcare settings, food processing plants, and public spaces.
- Sterilization of medical instruments and equipment.
- Sanitization of household items, such as cutting boards and kitchen surfaces.
Bleaching Agent
- Bleaching of textiles, paper, and pulp in the textile and paper industries.
- Whitening of flour and other food products.
- Removal of stains from surfaces and fabrics.
Algae and Weed Control
- Control of algae and weed growth in lakes, ponds, and water bodies.
- Treatment of irrigation canals and water reservoirs to prevent algae buildup.
Mold and Mildew Remediation:
- Elimination of mold and mildew from surfaces in buildings, basements, and bathrooms.
Odor Control
- Neutralization of odors caused by organic matter decomposition in wastewater treatment plants, garbage dumps, and composting facilities.
Disaster Response
- Use in emergency response situations, such as disinfection of water supplies during natural disasters or outbreaks of waterborne diseases.
Industrial Applications
- Use in the production of other chemicals, such as chlorinated solvents and chlorinated paraffins.
- Application in pulp and paper bleaching processes.
Agricultural Uses
- Disinfection of animal drinking water and livestock facilities.
- Treatment of irrigation water to prevent microbial contamination in agricultural settings.
Laundry and Cleaning
- Addition to laundry detergents for whitening and brightening effects.
- Cleaning and disinfection of outdoor surfaces, such as decks, patios, and sidewalks.