Sodium Hypochlorite

NaClO

IUPAC name: Sodium oxychlorate (I)
Other names: whitener, bleach, Javel water, hypochlorous acid, sodium salt.
CAS number: 7681-52-9

Sodium hypochlorite is sold in aqueous solution at a concentration that normally does not exceed 25% because it melts at about 18 °C in the pure state and is particularly unstable. It can decompose, even violently above 35 °C either by heating or by friction. The market value of a sodium hypochlorite solution varies depending on its oxidising power, which, by convention, is expressed as the “active chlorine” concentration; knowing this value is crucial for all those who trade or utilise sodium hypochlorite and the other various chlorine-based oxidising agents (calcium hypochlorite, chlorinated lime, chlorites, etc.)
The practice of expressing the oxidising power by the "active chlorine" content derives from the fact that it was once thought that the oxidising action of the hypochlorite was directly linked to its chlorine content. Consequently, the amount of “active chlorine” was defined as the amount of gaseous chlorine that a hypochlorite solution can develop if treated with a strong acid. Experience later showed that in fact pure chlorine cannot be considered by itself as an oxidising agent: indeed, it becomes one only in aqueous (chlorine water) or alkaline (sodium hypochlorite) solutions because of the oxygen whose release it makes possible. It follows that chlorine is the essential element to provoke the oxidising action, but its effects are obviously due to the oxygen released.
In the light of the above, today it would be logical to express the oxidising power of a hypochlorite solution as a function of the oxygen that it can release. However, because of the already established custom, and taking into account that there is a correspondence between the conventional active chlorine content and the amount of oxygen that the product can release, in commercial practice it is preferred to express the oxidising power of sodium hypochlorite by its “active chlorine” content.
The “active chlorine” in a sodium hypochlorite solution can be expressed:

  • as weight % (w/w) = g of active chlorine / 100 g of solution;
  • as volume % = g of active chlorine / 100 cm3 of solution;
  • in chlorometric o = l of active chlorine / l of solution

Sodium hypochlorite solutions lose spontaneously their initial active chlorine content. This natural degradation is usually faster immediately after production and, depending on the storage conditions, slows down progressively with time until it stops altogether.
The natural degradation is easily contained within commercially acceptable limits if the appropriate procedures for storing the product are followed.
The sodium hypochlorite solutions are subject to a decrease in the active chlorine content also because of other external factors, independent of the natural degradation, among which:

  • The action of sunlight
  • Temperatures above 35 °C
  • The action of metals, metal salts, and alloys
  • Acidification of the solutions
Sodium hypochlorite displays a higher stability in an alkaline environment; for this reason, a few grams of free caustic soda are always dissolved in the industrial products to ensure that there is always the amount required to guarantee a good stabilisation, even after prolonged storage.
It follows from these brief considerations that sodium hypochlorite is a product that requires careful handling and that its good quality depends, besides the purity of the raw materials employed, on the precautions used both during its production and during its transportation and storage.

Manufacture

Sodium hypochlorite is made by the exothermic absorption of chlorine into a caustic soda solution according to the reaction:

Cl2 + 2NaOH → NaClO + NaCl + H2O

This reaction also yields an equimolar amount of sodium chloride.

Principal uses

Because of its strong oxidising action, its low cost, its ease of application both in the industrial and domestic sectors, sodium hypochlorite is utilised in large quantities in a great many applications; the utilised concentrations vary depending on the case, within well defined limits.

Textile industry

Natural and synthetic textile fibres: hypochlorite can be used by itself or in combined treatments with other oxidising agents, such as sodium chlorite and hydrogen peroxide (this naturally depending on the material to be treated.)

Paper industry

Bleaching agent for wood pulp and cellulose.

Water treatment

Sterilisation, algae control treatment, etc.

Domestic applications and laundries

Preparation of laundry liquid products.

Chemical industry

Utilisation of sodium hypochlorite as intermediate oxidising agent in various organic syntheses.

Other applications

  • Bactericide
  • Sporicide
  • Fungicide
  • Virucide