Sodium hypochlorite is in a number of everyday household cleaning products, such as laundry detergent. It also has a variety of industrial applications, including large-scale water treatment and the synthesis of pharmaceutical products.
Although an aqueous solution of sodium hypochlorite is a weak base, it’s considered as a salt of hypochlorous acid. The hypochlorite ions are the ones that oxidise, or remove electrons from substances, including the protective membranes of bacteria and viruses. This is how this chemical acts as a disinfectant.
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What is Sodium Hypochlorite Found in?
Except for a few rare occurrences, sodium hypochlorite is too unstable to exist in nature. It has to be synthesised from other chemicals and manufactured on an industrial scale. As such, it’s most commonly found in cleaning products, bleach products, and disinfectants.
This substance exists both as a solid and as a liquid or aqueous solution. In solid form, it can either be an anhydrous (doesn’t contain water) or hydrous (contains water) compound. The hydrous form is light greenish-yellow in colour, and has an orthorhombic crystal structure, which is about 44% sodium hypochlorite by weight. Its melting point is between 25-27°C in one atmosphere.
The chemical formula for the anhydrous compound can be written as either NaOCl or NaClO. Meanwhile, the hydrous crystalline form has a chemical formula that can be written as either NaOCl·5H2O or 2NaOCl·10H2O.
Just like other hypochlorite compounds, anhydrous sodium hypochlorite is potentially explosive. It’s so unstable, in fact, that even a small amount of heat or friction could ignite it. Even in hydrous and aqueous form, sodium hypochlorite decomposes over time.
When left in the open air at room temperature, anhydrous sodium hypochlorite decomposes more rapidly because of exposure to carbon dioxide in the air. This is why it must be kept refrigerated. Indeed, lower temperatures make sodium hypochlorite much more stable, with the optimum temperature being 7°C. At this temperature, only about 1% of the compound decomposes in the course of a year.
Meanwhile, high temperatures aid in the decomposition of sodium hypochlorite. In solid, crystalline pentahydrate form, the chemical decomposes at 101°C, which is also its boiling point. In an aqueous solution, high temperatures cause it to decompose into sodium chloride and sodium chlorate:
3 NaOCl (aq) → 2 NaCl (aq) + NaClO3 (aq)
To some extent, an aqueous solution of sodium hypochlorite is in dynamic equilibrium. This is because although it continues to decompose overtime, its ions tend to continuously react and balance the equations. For example, the hypochlorite component of the compound is in dynamic equilibrium with water in aqueous solution, as shown in the balanced equation below:
OCl− + H2O ⇌ HOCl + OH−
As you can see, the reaction between the hypochlorite ions and a molecule of water forms hypochlorous acid and a hydroxide ion. This is a relatively weak acid. Therefore, overall, the solution maintains its alkalinity.
What is Sodium Hypochlorite Used For in Swimming Pools?
According to a study conducted by UK researchers, maintaining swimming pools based on UK guidelines can help in preventing the transmission of SARS-CoV-2 (COVID-19) between pool users, especially in public swimming pools.
The use of sodium hypochlorite in swimming pools also stops the growth of pathogenic bacteria. In the UK, this substance is used as a swimming pool disinfectant at a concentration range of 1.5-3.0 mg/l or parts per million. The pH level is maintained at either neutral or slightly basic level of 7.6.
Viruses like the coronavirus variants are surrounded by protective fatty and protein envelopes that have glycoprotein spikes. The spikes are the ones that attach to the receptors of target cells, like the lung epithelial cells. Once the spikes successfully bind with the receptors, the viral RNA is injected into the cell. This genetic material hijacks the infected cell to produce copies of the virus, which in turn infect other cells.
Although there is no evidence to suggest that COVID-19 can be transmitted via swimming pool water, surfaces such as railings and the hands of possibly infected individuals could be disinfected by coming into contact with the chlorinated swimming pool water.
Sodium hypochlorite solution in water disinfects by oxidising the protective outer layers of bacteria and viruses. The cell membrane of bacteria or the lipo-protein envelope of viruses is destroyed, spilling out the content. This process kills the microorganisms.
Will Sodium Hypochlorite Kill Grass?
The same oxidising reaction of an aqueous solution of sodium hypochlorite can kill the cells of multicellular organisms like grass. It’s the same principle that kills microorganisms by destroying the protective membranes or lipoprotein envelopes of bacteria, fungi, and viruses.
In fact, you can use a laundry bleach solution as herbicide to control the spread of weeds in your garden. This is because sodium hypochlorite is the active ingredient in many of these types of products. Aside from destroying the cells of weeds, the bleach solution also prevents weeds from growing back.
As we touched on earlier, once exposed to air, sodium hypochlorite decomposes into sodium chloride and sodium chlorate. These salts kill the beneficial bacteria in the soil and destroy the essential nutrients. In effect, the weeds are starved in the process. You may need to regularly apply bleach solution on areas where weeds grow until they’re completely exterminated. It may take several days or up to a week to kill the weeds.
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