What is Sodium Hypochlorite?

Sodium hypochlorite is a liquid chlorine, and is the active ingredient in many laundry bleach products and household cleaning products. It also has several large-scale applications. For example, it’s used in agriculture to protect crops from pathogens, and it’s also used in the synthesis of various pharmaceutical products. Perhaps the biggest use of this chemical is in bleaching textiles.

As well as all this, swimming pools are regularly treated with sodium hypochlorite to prevent the growth of potentially pathogenic microorganisms in the water. This chemical is also used as part of the water treatment process of tap water, and is added to wastewater to deodorise it.

Is Sodium Hypochlorite the Same As Chlorine?

Sodium hypochlorite is a liquid chlorine, containing around 10-12% of available chlorine. It also has similar oxidising properties. When dissolved in water, sodium and hypochlorite ions dissociate to form a weak base solution. The hypochlorite ions react as an oxidising agent, similar to elemental chlorine. However, sodium hypochlorite and chlorine are not the same thing. Chlorine is an element, while sodium hypochlorite is a chemical compound that contains chlorine.

Chlorine is a greenish-yellow gas that is highly reactive and corrosive. It belongs to the halogen group, which includes four other elements: fluorine (F), bromine (Br), iodine (I), and astatine (At). These are chemically similar elements whose group name literally means salt-producing. This is because they produce different types of salts when they react with metals.

Sodium hypochlorite can either exist as an aqueous solution or in solid crystalline pentahydrate form (NaOCl·5H2O). That said, the anhydrous form of this substance is uncommon because it’s highly unstable and can potentially explode. Even as an aqueous solution, the compound is still relatively unstable. It either liberates chlorine or converts the ions into sodium chloride and sodium chlorate, as shown in the equation below:

Sodium hypochlorite equation

What is the Chemical Formula for Sodium Hypochlorite?

The chemical formula for sodium hypochlorite can either be written as NaOCl or NaClO.

In aqueous solution, it splits into a sodium cation (Na+) and a hypochlorite anion (OCl− or ClO−). Technically, it’s considered as a salt of hypochlorous acid (HOCl or HClO), although it acts as a weak base in aqueous solution.

As an aqueous solution, especially in a sealed container, this substance is in dynamic equilibrium, continually decomposing and reintegrating. However, the stability of the chemical equilibrium weakens over time. This is why laundry bleaches that have NaClO as the active ingredient have expiration dates. Temperature is a main factor that affects its stability.

The following chemical species and equilibria can be found in an aqueous solution of sodium hypochlorite:

Chemical species and equilibria found in an aqueous solution of sodium hypochlorite

The first time that this chemical was synthesised was in 1789. Claude Louis Berthollet synthesised the substance by forcing chlorine gas to bubble through a solution of sodium potash lye (KOH). Another more cost-effective method was developed by Antoine Labarraque by replacing the expensive potash lye with soda lye (NaOH). The balanced equation is shown below:

Equation showing reaction between chlorine gas and soda lye

These methods are known as the chlorination of soda. But these are no longer the main ways of mass-producing this substance. Another method that was developed was the reaction between sodium carbonate and chlorinated lime, as shown in the chemical equations below:

Chemical equations showing the reaction between sodium carbonate and chlorinated lime

Other products are also produced with mixtures of calcium hypochlorite, calcium chloride, and calcium hydroxide. This method was used to produce antiseptics for hospitals after World War I. The product was sold as Eusols, which is in reference to the institution that developed it: Edinburgh University Solution of (chlorinated) Lime.

This antiseptic solution was commonly used to treat wounds before antibiotics were discovered and mass-produced.

Sodium Hypochlorite properties and structure

What is the pH Level of Sodium Hypochlorite?

This substance is sold commercially as an aqueous solution of around 5% concentration. In laundry bleach solutions with 5% concentrations, the pH value of sodium hypochlorite is around 11. Meanwhile, for solutions with higher concentrations (between 10-15%), the pH value is about 13.

Although the pH value of a solution is directly proportional to its acidity and basicity, the real measure of strength of an acid or base is the dissociation constant, or extent by which the ions split. Weaker acids or bases do not completely dissociate into ions. That said, the pKb value for this substance is 6.4815.

What is the Freezing Point of Sodium Hypochlorite?

The freezing point of sodium hypochlorite varies depending on several factors, such as its state of matter, concentration, and atmospheric pressure. Generally, the freezing point refers to the temperature at which a substance becomes solid from liquid state, which is just below the melting point.

Therefore, if we extrapolate the melting point of the solid crystalline pentahydrate form of sodium hypochlorite, which is 18°C, we can safely say that the freezing point of this substance is just below this temperature under ideal conditions.

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