Chemistry of Hypochlorous Acid

Chemistry 

Hypochlorous Acid 

IUPAC Name : Hypochlorous acid, chloric(I) acid, chloranol, hydroxidochlorine 
Other Names : Hydrogen hypochlorite, chlorine hydroxide, electrolyzed water, electrolyzed oxidizing water, electro-activated water 
CAS Number : 7790-92-3
Molar Mass : 52.46 g/mol
Molecular Formula : HOCl
Appearance : Colorless aqueous solution 
Solubility in water : Soluble 
Acidity : 7.53

Electrolysis 

In chemistry and manufacturing, electrolysis is a technique that uses a direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from naturally occurring sources. Electrolysis of sodium chloride (NaCl) and water (H2O) can be used to generate hypochlorous acid. Electrolysis technology was first explained by Michael Farraday when he developed the Laws of Electrolysis in the 1830s. Conducting electrical current across two electrodes in a salt brine solution may produce chlorine gas, sodium hypochlorite (bleach or NaOCl), hypochlorous acid, sodium hydroxide, hydrogen gas, ozone, and traces of other nascent oxidants. 

The key process of electrolysis is the interchange of atoms and ions by the removal or addition of electrons from the external circuit. An electrical potential is applied across a pair of electrodes immersed in the electrolyte. Each electrode attracts ions that are of the opposite charge. Positively charged ions (cations) move towards the electron-providing (negative) cathode. Negatively charged ions (anions) move towards the electron-extracting (positive) anode. In chemistry, the loss of electrons is called oxidation, while electron gain is called reduction. 

For example, the first step in making hypochlorous acid is the electrolysis of a salt water brine to produce hydrogen and chlorine, the products are gaseous. These gaseous products bubble from the electrolyte and are collected. 

2 NaCl(s) + 2 H20(l) → 2 NaOH(aq) + H2(g) + Cl2(g)

Membrane Cell Technology 

The ion exchange membrane is made from a polymer which only allows positive ions to pass through it. That means that the only the sodium ions from the sodium chloride solution can pass through the membrane, and not the chloride ions. The advantage of this is that the sodium hydroxide solution being formed in the right-hand compartment never gets contaminated with any sodium chloride solution. The sodium chloride solution being used has to be pure. If it contained any other metal ions, these would also pass through the membrane and so contaminate the sodium hydroxide solution. 

Hydrogen is produced at the cathode :
2H+(aq) + 2e- → H2(g)

Sodium hydroxide is produced at the cathode :
Na+(aq) + OH-(aq) → NaOH(aq)

Chlorine is produced at the anode :
2Cl-(aq) - 2e- → Cl2(g)

It is contaminated with some oxygen because of the reaction :
4OH-(aq) - 4e- → 2H2O(l) + O2(g)

Addition of chlorine to water gives both hydrochloric acid (HCl) and hypochlorous acid (HOCl) :
Cl2(g) + H2O ⇌ HOCl(aq) + HCl(aq)
Cl2(g) + 4 OH− ⇌ 2 ClO-(aq) + 2 H2O(l) + 2 e−
Cl2(g) + 2 e− ⇌ 2 Cl-(aq)

The pH dictates the free chlorine species present in aqueous solutions. At a pH of between 5-6, the chlorine species is nearly 100% hypochlorous acid (HOCl). As the pH drops below about 4, a growing share converts to Cl2 (chlorine gas). Above a pH of 6, it starts to convert to the hypochlorite ion (OCl-). 

Hypochlorous acid is a weak acid (pKa of about 7.5), meaning it dissociates slightly into hydrogen and hypochlorite ions as noted in equation: : HOCl ⇌ H+ + OCl-

Between about pH 6.5 and 8.5, this dissociation is partial and both HOCl and OCl- are present in significant amounts (about equal at pH 7.5). Below about pH 6.5, HOCl predominates; above about pH 8.5, OCl- predominates. 

The distribution of chlorine species between HOCl and OCl- is determined by pH, as discussed above. 

HOCl is the main form of free chlorine below about pH 7.5 (its pKa) and nearly the only form at about pH 4–6; above about pH 7.5, the hypochlorite ion (OCl-) predominates. 

This biggest challenge has been to create hypochlorous acid at a near neutral pH instead of chlorine gas or hypochlorite. Hypochlorous acid is a meta-stable molecule. It wants to revert back to salt water or convert to hypochlorite. 

Single Cell Technology 

One of the greatest advancements has been the development of single cell technology where a single stream of free chlorine is generated without a byproduct of sodium hydroxide (NaOH). This technology also allows greater control over the pH of the free chlorine generated. Since water pH is different depending on its source throughout the world, altering the pH of the brine allows for greater control and consistency in generating a free chlorine solution between pH 5 and 7 that is dominated by hypochlorous acid (HOCl). 

Anode reaction :
2Cl-(l) → Cl2(g) + 2e-

Cathode reaction :
2H2O(l) + 2e- → H2(g) + 2OH- (aq)

Free Chlorine Generation :
Cl2 (g) + H2O → HOCl + HCl
Cl2 (g) + 2OH-(aq) → OCl- (aq) + Cl-(aq) + H2O(l)