Hypochlorous Acid – Structure, Properties, Uses Of HOCl

Hypochlorous acids are weak acids containing hydrogen, oxygen, and chlorine. Although a weak acid with pKa=7.5, hypochlorous acid is a strong oxidising agent. It is responsible for the killing action of pathogens by phagocytes. 

What is HOCl?

Hypochlorous acid is a weak acid. Its chemical formula is HOCl. 

Other names: It also has several other names, including:

• Chlorine hydroxide
• Hydrogen hypochlorite
• Hypochloric acid
• Hydroxidochlorine (the IUPAC name)
• Chloric (I) acid
• Chlorine hydroxide
• Chloranol

It is often referred to as an oxyacid of chlorine.

Discovery: A French chemist, Antoine Ballard, discovered hypochlorous acid in 1834. Its monovalent chlorine functions as an oxidising agent or reducing agent. It serves as a human metabolite. Hypochlorous acid is an unstable acid. It is the conjugate acid of hypochlorite and belongs to the reactive oxygen family.

Hypochlorous Acid Formula and Structure

Hypochlorous acid formula is HOCl or HClO. The Lewis structure of hypochlorous acid is as follows:

Lewis Structure of Hypochlorous Acid

Thus, from the above structure, you can conclude as follows:

• It has two covalent bond pairs. 
• It has five lone pairs of electrons.
• There are ten non-bonding electrons in the HOCl. 

The shape of HOCl is bent, as presented in the following diagram,

Structure of HOCl

The presence of the hydrogen atom in the compound suggests that it is an acid. It tends to get deprotonated, i.e., it loses its hydrogen ion.

Properties of HOCl- Hypochlorous Acid

The pKa value of hypochlorous acid is relatively high. It is 7.53. This negative log base ten of the acid dissociation constant suggests a lower acid dissociation. Hence, proving HOCl to be a weak acid. This acid dissociates partially into its constituent ions, and the inability is the reason for its weak nature. 

HOCl acid is a colourless liquid at room temperature. Its chlorine is mild and fades away quickly. It is a polar molecule. The difference in electronegativities of the atoms and the electron lone pairs creates an uneven charge distribution. This asymmetrical distribution gives rise to polarity. The taste of hypochlorous acid is not precisely known, and its pure form is unstable. Hypochlorous acid exists majorly as an aqueous solution in water.

HOCl acid is an oxoacid, i.e., an acid containing an oxygen atom. Here is a summary of the important properties of HOCl acid.

• Molar Mass or Molecular weight: 52.457 g/mol
• Monoisotopic mass: 51.972 g/mol
• No. of hydrogen bond acceptor: 1
• Hydrogen bond donor: 1
• Physical State: Liquid
• Colour: Colourless
• Solubility: Water Soluble
• Odour: Pungent, chlorine-like
• Acid Strength: Weak
• Acidity pKa: 7.53
• Corrosiveness: Corrosive
• Toxicity: Toxic
• Polarity: Polar
• Shape: Bent

Calculating the molecular weight of Hypochlorous Acid

To calculate the molar mass or molecular weight of hypochlorous acid, you must know the atomic mass of its constituent atoms:

Chlorine = 35.5

Hydrogen = 1

Oxygen = 16

So, the molecular weight of HOCl can be calculated as follows:

= Molecular weight of hydrogen + oxygen + chlorine

= 1 + 16 + 35.5 

= 52.5 g/mol

Thus, the molecular weight of HOCl is 52.5 g/mol.

Hypochlorous Acid Uses

As stated earlier, the main use of hypochlorous acid is as a disinfecting agent. However, its uses are not limited to killing germs. The acid finds various other applications in different industries. The following list summarises the key uses of HOCl acid:

• It is an ingredient in several consumer items like deodorants. 
• It can be found naturally in the white blood cells of mammals, including humans. 
• In marine sanitation devices wherein seawater is converted to HOCl.
• It is used in transforming alkenes into chlorohydrins.
• HOCl is also used in cosmetics like baby products.
• It is added to swimming pools.
• In the manufacturing of safe disinfectants
• It is popularly used for wound care purposes.
• For treating several infections in humans and pets 
• HOCl has proven to cleanse, calm, and soothe the skin. The hypochlorous acid spray is often used for treating redness and other damaging changes from exposure to harmful substances. 
• Sodium hypochlorite is used in root canals in dentistry. 

The Use of Hypochlorous Acid on Skin

HOCl exists in the human body. It is manufactured by white blood cells for defence against bacteria, infection, and general ickiness.

It attacks invading pathogens and serves as an antimicrobial acid. The human skin is tough and durable, acting as an outer protective barrier. This front-line defence often leaves your skin vulnerable to cuts, dust, scrapes, and more. HOCl offers your skin some extra backup by 

• Fighting bacteria that leads to clogged pores and acne.
• Speeding up repair damage and wound healing. 
• Combating inflammation and skin problems like eczema or psoriasis.

Thus, the hypochlorous acid spray is incorporated into a daily skin maintenance routine to eliminate harmful bacteria. However, those with sensitive skin should stay away from HOCl sprays with fragrance. 

Production of HOCl Acid

Hypochlorous acid is obtained when chlorine (Cl₂) partially dissociates in water (H₂O) to give a hypochlorite ion (ClO⁻). On protonation, it gets converted to HClO when oxygen bonds with hydrogen from the water molecules. The following equation demonstrates the formation of hypochlorous acid:

Cl₂ + H₂O ⇌ HClO + HCl

It must be noted that the formation of hypochlorous acid also gives rise to an aqueous solution of hydrogen chloride HCl. 

Chemical Reactions

Following are some important chemical reactions of HOCl acid. 

Dissociation 

Hypochlorous acid dissociates partially into hypochlorite anion ClO⁻ in aqueous solutions. The dissociation reaction is shown below:

HClO ⇌ ClO⁻ + H⁺

This hypochlorite anion forms various salts. A common hypochlorite- sodium hypochlorite (NaClO) is an ingredient in bleach.

Oxidation

As stated in the important properties, hypochlorous acid is a strong oxidising agent. It is a stronger oxidant than chlorine, under standard conditions.

2 HClO(aq) + 2  H+ + 2  e− ⇌ Cl2(g) + 2 H₂O

Reaction with HCl

It reacts with HCl or hydrochloric acid to give chlorine gas. The chemical reaction is as follows:

HClO + HCl → H₂O + Cl₂

Reaction with Ammonia

It reacts with ammonia to produce chloramines and water. The balanced  reaction is as follows:

NH₃ + HClO → NH₂Cl + H₂O

Reaction with Lipids

Hypochlorous acid reacts with lipids uniquely. It reacts only with unsaturated lipid bonds and not saturated bonds. The hypochlorite ion or ClO⁻does not participate in the reaction. 

The reaction involves hydrolysis and the addition of chlorine to a carbon atom and hydroxyl to the other carbon atom. The process leads to the formation of chlorohydrin. Chlorohydrin formation in RBC’s lipid bilayers increases their permeability. So, disruptions can occur if more chlorohydrin is formed.

Is Hypochlorous Acid Safe for Use?

The EPA (Environmental Protection Agency, US) considers hypochlorous acid non-hazardous. 

However, it is an oxidising agent and can be corrosive depending on its pH and concentration. Several clinical tests have been conducted to test hypochlorous acid water for eye and skin irritation. It was concluded that HOCl water is non-toxic and doesn’t cause irritation to the skin and eyes. 

A 2017 study confirmed that a saline hygiene solution, when preserved with pure hypochlorous acid, had reduced bacterial load without altering the bacterial species’ diversity on the eyelids. There was a 99% reduction in bacteria after 20 minutes of treatment.

Furthermore, hypochlorous acid is quite unstable, and the active compounds easily deteriorate back into salt water. Thus, it readily loses its disinfecting capability. This property makes HOCl difficult to transport. Also, its high cost makes it less popular for use as a disinfectant compared to alcohol and bleach. With technological advancement, the manufacturing costs of hypochlorous acid water have been significantly reduced for home and commercial use. However, its short shelf life is still a matter of concern. 

Conclusion

Hypochlorous acid is a highly useful mild acid. It contains chlorine, oxygen and hydrogen. It has a bent structure, and it is an unstable compound. Famous for its antibacterial properties, HOCl is safe for topical applications despite being an acid. It is an active wound care agent and its spray is also used for treating skin problems.

Frequently Asked Questions

Q1. What happens when an acid is added to the aqueous salt of HOCl?

A. When an is added to the aqueous salts of hypochlorous acid like sodium hypochlorite in bleach solutions, the reaction gets pushed to the left. It results in the formation of chlorine gas. The chlorine gas dissolves into simple water solutions like sodium hydroxide. Thus, solid hypochlorite bleaches are formed.

Q2. How does HOCl acid react with amino acids?

A. Hypochlorous acid and amino acids react readily because the latter possess side chains. The hypochlorous acid chlorine displaces hydrogen in amino acids, resulting in organic chloramine formation. However, chlorinated amino acids break down quickly in contrast to protein chloramines, which last longer.

Q3. Why is oxygen and not chlorine the central atom in HOCl?

A. The structure is usually constructed by determining the least electronegative atom. The least electronegative atom is then placed in the centre. In HOCl, Oxygen’s electronegativity is 3.44; chlorine’s is 3.16; Hydrogen’s electronegativity is 2.2. So, hydrogen is the least electronegative atom. However, it can’t be the central atom because it can only form a single bond. Chlorine comes next in terms of minimum electronegativity. Still, it cannot be the central atom as oxygen should be directly bonded to hydrogen, as chlorine in the centre would obstruct the structure. Thus, even with the highest electronegativity, oxygen is the central atom.