Calcium hypochlorite – Structure, Properties and its Uses

You must know the chemical ‘bleaching powder’ and its key usage. Bleaching powder is a general name, and the exact content of any bleaching powder depends on the industrial processes used to produce it.

But do you know what the chemical components of bleaching powder are? What is calcium hypochlorite? What is the calcium hypochlorite structure? What are its pros and cons?

To get an answer to all these questions, follow this article.

What Is Calcium Hypochlorite?

A chemical compound, it has a Ca(ClO)₂ chemical formula. It is commonly known as bleaching powder, hypochlorous acid calcium salt, lime chloride, and chlorinated lime. It is an electrostatic or ionic compound containing a Ca²⁺ (calcium ion) and two ClO⁻ (hypochlorite ions).

To answer, what is calcium hypochlorite? It is a white, concentrated chemical available on the market in powder, crystal, or tablet form. The key use of it is to disinfect and sanitize swimming pool water and drinking water. Compared with other chlorine sources for water treatment, it is easier and safer to use.

Though it is stable at room temperature, it decomposes slowly in moist conditions, giving it a characteristic ‘chlorine’ smell. It is an active ingredient in bleaching powder, chlorinated lime, etc.

History of Calcium Hypochlorite

In the late 18th century, Charles Tennant and Charles Macintosh developed an industrial process for manufacturing lime chloride, i.e., calcium hypochlorite. In 1799, it was patented. During World War I, doctors used it heavily to disinfect wounds and trenches.

Calcium Hypochlorite Structure

It is an inorganic complex having the chemical formula Ca(ClO)₂. It is a white, slightly soluble solid. It is one of the major compounds of hypochlorites.

Its structure consists of a hypochlorite anion and a calcium cation. The calcium cation (Ca²⁺) coordinates to six O-atoms in a distorted octahedral geometry. The hypochlorite anion (ClO⁻) is straight or linear in structure and is bonded to the calcium cation through oxygen atoms.

The calcium hypochlorite structure is given below:

Preparation of Calcium Hypochlorite

There are two ways to prepare calcium hypochlorite. These are

1. Calcium process:

Industrially, It is prepared by treating lime water with chlorine gas. As lime water contains calcium, this process is generally known as the calcium process. The chemical reaction takes place as follows

2Ca(OH)₂ + 2Cl₂ → Ca(OCl)₂ + CaCl₂ + 2H₂O

The reaction is managed in stages to give different compositions, each with various calcium chloride concentrations, hypochlorite, and unconverted lime.

2. Sodium process:

In this process, slaked lime is treated with chlorine gas and sodium hydroxide to give a mixture of calcium hypochlorite Ca(ClO)₂, calcium chloride CaCl₂, water, and sodium chloride NaCl. The reaction takes place as follows

2Ca(OH)₂ + 3Cl₂ + 2NaOH → Ca(OCl)₂ + CaCl₂ + 2H₂O + 2NaCl

Calcium Hypochlorite Properties

The properties are classified into two classes. These are given below:

1. Physical properties:

Among these properties, physical properties define its appearance, chemical formula, nature, mass, and other characteristics. Some of them are given below:

• The molecular formula of calcium hypochlorite is Ca(OCl)₂.
• It exists in white-grey granular material form.
• The molecular mass of calcium hypochlorite is 142.98 g/mol.
• At 20°C, its density is 2.35 g/cm³.
• The melting point of calcium hypochlorite is 100°C, and its boiling point is 175°C.
• It is highly soluble in alcohol.
• At room temperature, the solubility of this in water is 21 g/100 mL.
• It is a non-flammable substance but will accelerate the rate of burning of combustible materials.
• It has an odour of chlorine.
• This is generally available as pellets, white powder, or flat plates.
• The specific gravity of calcium hypochlorite in water is 1.21 g/cm³.

2. Chemical properties:

Among these properties, chemical properties define the nature of the compound toward different reagents. Some of them are given below:

• It readily decomposes in water.
• On heating, it releases chlorine and oxygen.
• This is not combustible but acts as an oxidiser with flammable material.
• It may react explosively with amines, ammonia, or organic sulphides.
• As it readily accepts H⁺ ions, It is a strong base.
• Dissolving calcium hypochlorite in water decomposes into hypochlorite ions and calcium ions. Furthermore, the hypochlorite ions accept H⁺ ions from water molecules and liberate OH⁻ ions.

ClO⁻ + H₂O → HClO + OH⁻

• It can readily accept electrons. Hence, it acts as a powerful oxidising agent.
• It reacts with hydrochloric acid and gives calcium chloride, chlorine, and water. The reaction follows as

4HCl + Ca(ClO)₂ → CaCl₂ + Cl₂ + H₂O

• It reacts with carbon dioxide (CO₂) to form calcium carbonate (CaCO₃) and release chlorine. 

2Ca(ClO)₂ + 2CO₂ → 2CaCO₃ + 2Cl₂ + O₂

Uses of Calcium Hypochlorite

The uses of these are to make drinking water safe and to disinfect large volumes of water. In swimming pools, it helps sanitising water bodies and kills germs. Some other uses of calcium hypochlorite are

• In organic chemistry, it is highly useful due to its oxidising agent behaviour.
• The uses of calcium hypochlorite are for bleaching of hemp, cotton, fibre, disinfection of surfaces, pulp and starch, and as a disinfectant for water treatment.
• It is used in some bleaching solutions too.
• It is also used as a solid fire extinguisher.
• Due to the presence of reactive chlorine, this is used to treat waste water and drinking water.
• It cleaves the keto acid and glycol bonds by obtaining disintegrated carboxylic acids or aldehydes.
• It is used in the preparation of chloroform during haloform reactions.
• It is also used in hard waters to prevent clogging up of filters or pipes.
• It is used in disinfecting and deodorising in beverage, food processing, and sugar industries.
• It helps in refinishing and extra sheen in the carpet industry.
• It (known as bleaching powder) is also used for linen and bleaching cotton.
• It works as a mosquito controller by eradicating mosquito larvae in drains, gutters, ponds, etc.

Is Calcium Hypochlorite Dangerous?

Although there are many uses for calcium hypochlorite, a question may still arise: is it dangerous? If you use it with care and precaution, it is not so dangerous. But using it without precautions may cause violent effects. Some of the side effects of using calcium hypochlorite are

• In contact with water or moisture, it releases chlorine gas. It may lead to some health issues.
• In the presence of an acid, it produces highly toxic chlorine gas fumes.
• The moderate use of calcium hypochlorite solution can cause skin irritation. Sometimes it can cause serious skin burns.
• It shows corrosive effects on moist tissues like eyes, upper respiratory tracts, etc.
• Inhaling it may cause cough, respiratory problems, and dyspnea.
• More concentrated solutions of calcium hypochlorite may cause significant esophageal and gastric burns.

Summary

To summarise, what is calcium hypochlorite is an important salt of hypochlorous acid and is prepared by treating lime slurry, Ca(OH)₂ with Cl₂. It is a stable compound and has the commercial name ‘perchloron.’ Sometimes, it is used in place of bleaching powder.

Because of its non-hygroscopic properties, it is superior to bleaching powder. It is available commercially but is most often generated in situ at the site of usage. It is a widely used inorganic compound.

This decomposes in contact with water and releases chlorine. It stays stable because calcium hypochlorite only decomposes in contact with water. Generally, bleaching powder is a mixture of many chlorine compounds which release nascent oxygen.

Frequently Asked Questions

1. What are the advantages and disadvantages of using calcium hypochlorite?

 A) The key advantages and disadvantages of using calcium hypochlorite are

Advantages:

• It is easy for transport and storage purposes.
• You can store it for a long time.
• Its simple and efficient use makes it preferable to other chlorine water treatment compounds.
• It is a cost-effective chemical and easily available.

Disadvantages:

• It might cause blockage.
• It needs proper storage. Leakage of even a small amount can cause violent results.
• It has low solubility.
• Exposure to fire may lead to a sharp increase in the fire’s intensity.
• The final chlorine concentration in water depends on the dissolution extent achieved.

2) Why should we store calcium hypochlorite in a dry place?

 A)   It must be stored in a dry, well-aerated area below 50ºC, isolated from acids, amines, ammonia, and other chlorinating or oxidising agents. As these compounds are highly flammable when exposed and it acts as an oxidising agent, it is stored in a dry place.

3) What is the function of calcium hypochlorite?

  A)  It is a bleaching agent. It can produce chlorine which acts as an oxidising agent. That is why calcium hypochlorite or bleaching powder is used as a cleansing agent and disinfectant.

4) What is the difference between calcium hypochlorite and bleaching powder?

 A) It is an inorganic compound with the formula Ca(OCl)₂. This compound is relatively stable and has greater available chlorine than the bleaching solution. It’s not highly soluble in water and is preferably used in soft to medium hard water.

Bleaching powder combines calcium chloride, calcium hypochlorite, and calcium hydroxide. It’s a mixture consisting chiefly of dibasic calcium hypochlorite Ca₃(OCl)₂(OH)₄, calcium hypochlorite Ca(OCl)₂, and dibasic calcium chloride Ca₃Cl₂(OH)₄. It is formed from slightly moist slaked lime.