Sodium hypochlorite is a chemical compound with the formula NaOCl or NaClO, comprising a sodium cation (Na+
) and a hypochlorite anion (OCl−
or ClO−
). It may also be viewed as the sodium salt of hypochlorous acid. The anhydrous compound is unstable and may decompose explosively.[3][4] It can be crystallized as a pentahydrate NaOCl·5H
2O, a pale greenish-yellow solid which is not explosive and is stable if kept refrigerated.[5][6]
Sodium hypochlorite is most often encountered as a pale greenish-yellow dilute solution commonly known as liquid bleach or simply bleach, a household chemical widely used (since the 18th century) as a disinfectant or a bleaching agent. The compound in solution is unstable and easily decomposes, liberating chlorine, which is the active principle of such products. Indeed, sodium hypochlorite is the oldest and still most important chlorine-based bleach.[7][8]
How is sodium hypochlorite applied in swimming pools?
Its corrosive properties, common availability, and reaction products make it a significant safety risk. In particular, mixing liquid bleach with other cleaning products, such as acids or ammonia, may produce toxic fumes
Sodium hypochlorite is applied in swimming pools for water disinfection and oxidation. It has the advantage that microorganisms cannot build up any resistance to it. Sodium hypochlorite is effective against Legionella bacteria and bio film, in which Legionella bacteria can multiply.
Hypochlorous acid is produced by the reaction of sodium hydroxide with chlorine gas. In water, the so-called 'active chlorine' is formed.
There are various ways to use sodium hypochlorite. For on-site salt electrolysis, a solution of salt (NaCl) in water is applied. Sodium (Na+) and chloride (Cl-) ions are produced.
4NaCl- → 4Na+ + 4Cl-
By leading the salty solution over an electrolysis cell, the following reactions take place at the electrodes:
2Cl- → Cl2 + 2e- 2H2O + 2e- → H2 + 20H-
2H20 → O2 + 4H++ 4e-
Subsequently, chlorine and hydroxide react to form hypochlorite:
OH- + Cl2 → HOCl + Cl-
The advantage of the salt electrolysis system is that no transport or storage of sodium hypochlorite is required. When sodium hypochlorite is stored for a long time, it becomes inactive. Another advantage of the on site process is that chlorine lowers the pH and no other acid is required to lower pH. The hydrogen gas that is produced is explosive and as a result ventilation is required for expolsion prevention. This system is slow and a buffer of extra hypochlorous acid needs to be used. The maintenance and purchase of the electrolysis system is much more expensive than sodium hypochlorite.
When sodium hypochlorite is used, acetic or sulphuric acid are added to the water. An overdose can produce poisonous gasses. If the dosage is too low, the pH becomes to high and can irritate the eyes.
Because sodium hypochlorite is used both to oxidize pollutions (urine, sweat, cosmetics) and to remove pathogenic microorganisms, the required concentration of sodium hypochlorite depends on the concentrations of these pollutions. Especially the amount of organic pollution determines the required concentration. If the water is filtered before sodium hypochlorite is applied, less sodium hypochlorite is needed.
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