What exactly is sodium hypochlorite?
No, it’s not common table salt (that’s sodium chloride), but it is a compound of sodium and chlorine (this time with additional oxygen) that you may be more familiar with than perhaps you first think.
Sodium hypochlorite NaOCl, also known as sodium chlorate(I), is a crystalline, pale-yellow solid that looks a lot like regular salt, but that has a very different role. Like table salt it is a highly soluble compound in water, and in most applications it comes in the form of a clear, pale-yellow, aqueous solution. Still doesn’t sound familiar? Well, you probably know it simply as ‘bleach’.
Bleach in the home
Most domestic bleach is no more than an aqueous solution of somewhere between 3 and 10% sodium hypochlorite by mass. Its common uses around the home are usually restricted to laundry – where it acts as a whitener to brighten dulled, white clothing, and as a stain remover – and as a universal disinfectant and germ-killer, mostly in kitchen, bathroom, and toilet facilities. As such, sodium hypochlorite is probably a very familiar chemical to most people, even if they don’t recognize it by its formal chemical name.
Sodium hypochlorite in industrial and commercial settings
Some of the most common industrial uses of NaOCl reflect the domestic applications almost exactly. As a sanitizer and a whitener, one finds it in water treatment and the paper industry.
Swimming pools and other water treatment
Once again, there’s familiar application found in water treatment, i.e., the ‘chlorination’ of swimming pools where the NaOCl is used to kill potentially harmful water-borne bacteria. The use of chlorine gas to directly disinfect any type of water supply is tricky. While the gas itself can be an effective disinfectant on one hand, it is extremely toxic to humans, and because it’s a gas it is difficult to transport and use. NaOCl is a much better option as a source of the bacteria killing chlorine since it can be stored and used as a solid or in solution. In solution it is sometimes referred to as ‘liquid chlorine’ which is technically incorrect but draws attention to the fact that we are not dealing with a gas. In order to understand how NaOCl works in this situation, one needs to appreciate a little of the chemistry involved.
It’s all a balance
The sodium hypochlorite reacts with water to set up an equilibrium according to the equation below.
NaOCl + H2O ⇌ HOCl + Na+ + OH–
The hypochlorous acid (HOCl) that is produced in the reaction above, also exists in equilibrium in water according to this equation.
HOCl + H2O ⇌ OCl– + H3O+
In each case, the ‘equilibrium’ means that all of the products and reactants are present in the mixture at any given time, with the balance of either being controlled by external factors such as temperature or pH.
Together, the HOCl and the OCl– (the hypochlorite ion) that are formed in water from the sodium hypochlorite are often referred to as Free Available Chlorine or FAC, and these act as the chief sanitizers in pools.
HOCl is a strong oxidizing agent and will kill bacteria very efficiently. The OCl– ion formed in the second reaction is much less effective as a disinfectant, but the second equilibrium can be forced backwards to produce more HOCl by carefully monitoring and controlling the pH (acidity) of the pool water.
It’s complex balance. A very acidic pool with a low pH would be great at driving the second reaction backwards, and thus producing lots of bacteria-killing HOCl, but it makes no sense to swim in water that’s highly acidic! A pH in the 7-8 range usually creates the best compromise between acidity and a safe water environment.
The combination of the equilibrium above means that swimming pool water is usually a complicated mixture of chemicals at any given time, and this is exacerbated by the fact that each HOCl and OCl– are both broken down by sunlight to produce Cl– and O2 in the first case, and HCl and O2 in the second. (As an aside, because of the reaction with sunlight, outdoor pools tend to lose their FAC more quickly than indoor pools, and need to be monitored and treated more frequently than those inside). NaOCl can also be used as a de-odorizer for water, and for removing mold from pools.
In the textile, paper, and pulp industries
Much like bleach in the home, sodium hypochlorite can be used as a whitening agent in the textile, paper and pulp industries. The purpose is usually to simply remove any natural occurring color to either achieve the desired ‘whiteness’, or to create a completely neutral color that can then be colored, treated, or dyed more easily to the desired hue. Other chemicals such as hydrogen peroxide are also used for whitening, and the specific application will determine the agent ultimately employed, but environmental concern about any chlorine-containing compounds is always a factor.
As a widely used disinfectant
In many commercial and industrial situations where disinfection is vital, sodium hypochlorite is often a good choice. In agriculture, equipment that comes into contact with animals and animal accommodations is often disinfected with NaOCl. As we have seen above, sodium hypochlorite is also an excellent chemical for fighting water-borne nuisances. It can be used to clean farming irrigation systems, and to control the growth of algae in bodies of water such as ponds. Sodium hypochlorite’s use in such situations must be regulated carefully, as it does have the potential to be harmful to both the animals and the environment if not used correctly.
In food processing
The germ-killing nature of sodium hypochlorite is utilized in the processing of food. In any situation where surfaces come into contact with food, or even those where the food itself needs to be sanitized, a solution of NaOCl can help.
The results of a 2021 study indicated that chlorine-based disinfectants such as NaOCl could be used to great effect in the poultry processing industry. It was found that sodium hypochlorite was especially effective as a sanitizer on surfaces where raw chicken may have been, being an excellent disinfectant against salmonella, and thus greatly enhancing food safety.
As with the concerns over the potential harmful nature of the chemicals on the environment, when used in food processing the NaOCl must be carefully regulated.
Byun, Kye Hwan & Han, Sang & Yoon, Jang-won & Park, Si Hong & Ha, Sang-Do. (2020). Efficacy of chlorine-based disinfectants (sodium hypochlorite and chlorine dioxide) on Salmonella Enteritidis planktonic cells, biofilms on food contact surfaces and chicken skin. Food Control. 123. 107838. 10.1016/j.foodcont.2020.107838.< Back