The sodium hypochlorite (NaOCl) is a ternary and inorganic sodium salt. It is commercially available as aqueous solutions and is the active agent in these domestic and industrial products. These solutions are known as chlorine bleach, soda bleach, liquid bleach or, even more refined, Javel liquor.
In water, sodium hypochlorite exhibits the same oxidizing properties as chlorine gas, so a solution of this salt is equivalent to carrying said compound inside a plastic bottle. In fact, the available chlorine in these containers is an indicator of the concentration and bleaching power of the sodium hypochlorite solution.
This ternary salt in water may well be considered a simple aqueous solution of chlorine; however, there are also other forms available, such as solid in the case of calcium hypochlorite, and liquid chlorine. All three have the same oxidizing power and their uses depend on variables such as comfort, performance or time.
The chemical formula for sodium hypochlorite is NaClO. It is made up of a Na + cation and a ClO – anion . The Na + ions feel electrostatic attraction by the ClO – ions , the latter adopting a tetrahedral structure with an sp 3 hybridization .
The formula indicates the ratio of the ions in the anhydride state, which is equal to 1: 1. However, this applies only to anhydride.
In the case of hydrated sodium hypochlorite —one of the most stable forms for this salt— its chemical formula is NaClO · 5H 2 O.
Where is sodium hypochlorite found?
NaClO is a synthetic product and, due to its reactivity, it is only found in local regions where it is used, especially in wastewater, soils or even in drinking water emanations.
Chloride ions are abundant in the human body, which could participate in redox reactions that produce sodium hypochlorite within the body.
Throughout history NaClO has been prepared by various methods. The simplest consists of a solution of chlorine in water, or in solutions of Na 2 CO 3 that liberate CO 2 by the action of hypochlorous acid (HClO).
Other methods use sea brine as raw material through an electrolytic process. Under efficient mechanical stirring, the Cl 2 and NaOH generated by NaCl and water react and become NaClO:
Cl 2 (g) + 2NaOH (aq) => NaClO + NaCl + H 2 O + Q (heat)
Today the Hooker process manufactures this compound on large scales, which consists of an improved version of the previous method described.
Sodium Chlorite Uses / Applications
The most common applications of sodium hypochlorite are:
- Sodium hypochlorite is used as a bleaching agent in textiles, in detergents that incorporate it, and in the paper industry.
- Its use as a bactericidal and disinfectant agent is very wide, being used in the purification of water and in the treatment of wastewater.
- Its usefulness in disinfecting the equipment used in food preparation and in the processing of fruits and vegetables is known. Likewise, it is used with the same disinfectant function in the production of mushrooms, cattle, pigs and poultry.
- Sodium hypochlorite is used in the oil industry in the refining stage.
- At home, the bleaching capacity of sodium hypochlorite is used in washing white clothes, and the disinfectant action in cleaning bathrooms, floors, etc.
- Sodium hypochlorite is used in endodontic therapy, specifically in the treatment of the root canal of the tooth. In this treatment, Dakin’s solution (0.5% ClONE) is used, which preserves vital dental tissue by dissolving necrotic tissue.
Properties of sodium hypochlorite
Sodium hypochlorite solutions at room temperature are colorless and have a sweet chlorine odor. The physical properties vary depending on the concentrations of the salt dissolved in water. All have yellow colorations.
Through crystallization techniques, the solid NaClO · 5H 2 O is obtained from these solutions , the crystals of which are pale green.
This hydrated salt has an approximate molecular weight of 164 g / mol, a density of 1.11 g / mL, is very soluble in water and decomposes at 101 ºC. NaClO · 5H 2 O is also sensitive to the same anhydride reactions.
Why is salt pentahydrated? As NaClO crystallizes in its aqueous environment, the water molecules envelop the ions in an aqueous sphere.
Three of these molecules can be thought of as interacting with Cl’s unshared electron pairs: one forms hydrogen bond with O, and the latter is attracted to Na.
However, it is the studies focused on the crystalline structure of this solid that have the true answer to this question.
The anion ClO – participates in the following hydrolysis equilibrium:
HClO (aq) + H 2 O (l) <=> ClO – (aq) + H + (aq)
If the acidity of the solution increases, the equilibrium shifts to the left, producing HClO.
This acid is even more unstable than hypochlorite and therefore decomposition lowers the concentration of the active agent. If the pH is basic (greater than 11), it guarantees the existence of ClO – and the life of the product.
However, excessive alkalinity triggers other problems in your applications. For example, a very basic NaClO solution damages clothes rather than just bleaching them.
Likewise, in aqueous medium, HClO is also transformed into chlorine, which explains the yellow coloration of these solutions:
HClO (aq) <=> Cl 2 (g) + H 2 O (l)
The chlorine atom in sodium hypochlorite has an oxidation state of +1, requiring only two electrons to complete its valence octet.
On the other hand, its electronic configuration is [Ne] 3s 2 3p 5 , and it can also empty all the electrons from its more energetic “p” orbitals.
This results in hypochlorite undergoing disproportionate reactions in ions with oxidation states of +1 and +5:
3ClO – (aq) <=> 2Cl – (aq) + ClO 3 – (aq)
This reaction in aqueous solution is accelerated with increases in temperature and hypochlorite concentrations. Likewise, the reaction proceeds by a different mechanism catalyzed by light and metallic oxides of copper, nickel and cobalt:
2NaOCl (aq) => O 2 (g) + 2NaCl (aq)
The anhydrous NaClO disproportionates at a much faster rate, even exploding.
The chlorine atom can accept electrons from negative (nucleophilic) species. Anhydride is a powerful oxidizing agent, reducing itself to chloride anions (Cl – ).
In the case of NaClO · 5H 2 O, it is thought that the water molecules partially prevent the ClO – from undergoing nucleophilic attacks.
However, given the structural linearity of ClO – , these water molecules do not slow enough “attacks” on the Cl atom. It is for this reason that sodium hypochlorite is a strong oxidizing agent.