Sodium Hypochlorite

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.[2][3] It can be crystallized as a pentahydrate NaOCl·5H2O, a pale greenish-yellow solid which is not explosive and is stable if kept refrigerated.

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.

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.

Stability of the solid

Anhydrous sodium hypochlorite can be prepared but, like many hypochlorites, it is highly unstable and decomposes explosively on heating or friction. The decomposition is accelerated by carbon dioxide at atmospheric levels.[3][8] It is a white solid with the orthorhombic crystal structure.

Sodium hypochlorite can also be obtained as a crystalline pentahydrate NaOCl·5H2O, which is not explosive and is much more stable than the anhydrous compound. The formula is sometimes given as 2NaOCl·10H2O.[citation needed] The transparent light greenish yellow orthorombic[10][11] crystals contain 44% NaOCl by weight and melt at 25–27 °C. The compound decomposes rapidly at room temperature, so it must be kept under refrigeration. At lower temperatures, however, it is quite stable: reportedly only 1% decomposition after 360 days at 7 °C.

A 1966 US patent claims that stable solid sodium hypochlorite dihydrate NaOCl·2H2O can be obtained by carefully excluding chloride ions (Cl−), which are present in the output of common manufacturing processes and are said to catalyze the decomposition of hypochlorite into chlorate (ClO−3) and chloride. In one test, the dihydrate was claimed to show only 6% decomposition after 13.5 months storage at −25 °C. The patent also claims that the dihydrate can be reduced to the anhydrous form by vacuum drying at about 50 °C, yielding a solid that showed no decomposition after 64 hours at −25 °C.

Equilibria and stability of solutions

At typical ambient temperatures, sodium hypochlorite is more stable in dilute solutions that contain solvated Na+ and OCl−ions. The density of the solution is 1.093 g/mL at 5% concentration,[14] and 1.21 g/mL at 14%, 20 °C.[15] Stoichometric solutions are fairly alkaline, with pH 11 or more[5] since hypochlorous acid is a weak acid:

OCl−+ H2O ⇌ HOCl + OH−

The following species and equilibria are present in solutions of NaOCl:

HOCl (aq) ⇌ H++ OCl−

HOCl (aq) + Cl−+ H+⇌ Cl2 (aq) + H2O
Cl2 (aq) + Cl−⇌ Cl−3
Cl2 (aq) ⇌ Cl2 (g)
The second equilibrium equation above will be shifted to the right if the chlorine Cl2 is allowed to escape as gas. The ratios of Cl2, HOCl, and OCl−in solution are also pH dependent. At pH below 2, the majority of the chlorine in the solution is in the form of dissolved elemental Cl2. At pH greater than 7.4, the majority is in the form of hypochlorite ClO−. The equilibrium can be shifted by adding acids (such as hydrochloric acid) or bases (such as sodium hydroxide) to the solution:
ClO−(aq) + 2 HCl (aq) → Cl2 (g) + H2O (aq) + Cl−(aq)
The second equilibrium equation above will be shifted to the right if the chlorine Cl
2 is allowed to escape as gas. The ratios of Cl
2, HOCl, and OCl− in solution are also pH dependent. At pH below 2, the majority of the chlorine in the solution is in the form of dissolved elemental Cl
2. At pH greater than 7.4, the majority is in the form of hypochlorite ClO−
. The equilibrium can be shifted by adding acids (such as hydrochloric acid) or bases (such as sodium hydroxide) to the solution:
ClO−(aq) + 2 HCl (aq) → Cl2 (g) + H2O (aq) + Cl−(aq)
Cl2 (g) + 2 OH−→ ClO−(aq) + Cl−(aq) + H2O (aq)

At a pH of about 4, such as obtained by the addition of strong acids like hydrochloric acid, the amount of undissociated (nonionized) HOCl is highest. The reaction can be written as:

ClO−+ H+⇌ HClO

Sodium hypochlorite solutions combined with acid evolve chlorine gas, particularly strongly at pH < 2, by the reactions:

HOCl (aq) + Cl−+ H+⇌ Cl2 (aq) + H2O
Cl2 (aq) ⇌ Cl2 (g)

At pH > 8, the chlorine is practically all in the form of hypochlorite anions (OCl−). The solutions are fairly stable at pH 11–12. Even so, one report claims that a conventional 13.6% NaOCl reagent solution lost 17% of its strength after being stored for 360 days at 7 °C.[5] For this reason, in some applications one may use more stable chlorine-releasing compounds, such as calcium hypochlorite Ca(ClO)2 or trichloroisocyanuric acid (CNClO)3.

Anhydrous sodium hypochlorite is soluble in methanol, and solutions are stable.[citation needed]

Oxidation of organic compounds

Oxidation of starch by sodium hypochlorite, that adds carbonyl and carboxyl groups, is relevant to the production of modified starch products.[20]

In the presence of a phase-transfer catalyst, alcohols are oxidized to the corresponding carbonyl compound (aldehyde or ketone).[21][5] Sodium hypochlorite can also oxidize organic sulfides to sulfoxides or sulfones, disulfides or thiols to sulfonyl chlorides or bromides, imines to oxaziridines.[5] It can also de-aromatize phenols.[5]

Oxidation of metals and complexes

Heterogeneous reactions of sodium hypochlorite and metals such as zinc proceed slowly to give the metal oxide or hydroxide:

NaOCl + Zn → ZnO + NaCl

Homogeneous reactions with metal coordination complexes proceed somewhat faster. This has been exploited in the Jacobsen epoxidation.

Sodium hypochlorite has a long history. Around 1785 the Frenchman Berthollet developed liquid bleaching agents based on sodium hypochlorite. The Javel company introduced this product and called it ‘liqueur de Javel’. At first, it was used to bleach cotton. Because of its specific characteristics it soon became a popular compound. Hypochlorite can remove stains from clothes at room temperature. In France, sodium hypochlorite is still known as ‘eau de Javel’.

What are the characteristics of sodium hypochlorite?

Sodium hypochlorite is a clear, slightly yellowish solution with a characteristic odor.
Sodium hypochlorite has a relative density of is 1,1 (5,5% watery solution).
As a bleaching agent for domestic use it usually contains 5% sodium hypochlorite (with a pH of around 11, it is irritating). If it is more concentrated, it contains a concentration 10-15% sodium hypochlorite (with a pH of around 13, it burns and is corrosive).?

Sodium hypochlorite is a clear, slightly yellowish solution with a characteristic odor.
Sodium hypochlorite has a relative density of is 1,1 (5,5% watery solution).
As a bleaching agent for domestic use it usually contains 5% sodium hypochlorite (with a pH of around 11, it is irritating). If it is more concentrated, it contains a concentration 10-15% sodium hypochlorite (with a pH of around 13, it burns and is corrosive).
Sodium hypochlorite is unstable. Chlorine evaporates at a rate of 0,75 gram active chlorine per day from the solution. Then heated sodium hypochlorite disintegrates. This also happens when sodium hypochlorite comes in contact with acids, sunlight, certain metals and poisonous and corrosive gasses, including chlorine gas. Sodium hypochlorite is a strong oxidator and reacts with flammable compounds and reductors. Sodium hypochlorite solution is a weak base that is inflammable.
These characteristics must be kept in mind during transport, storage and use of sodium hypochlorite.

What happens to the pH value when sodium hypochlorite is added to water?

Due to the presence of caustic soda in sodium hypo chlorite, the pH of the water is increased. When sodium hypo chlorite dissolves in water, two substances form, which play a role in for oxidation and disinfection. These are hypochlorous acid (HOCl) and the less active hypochlorite ion (OCl-). The pH of the water determines how much hypochlorous acid is formed. While sodium hypochlorite is used, hydrochloric acid (HCl) is used to lower the pH. Sulfuric acid (H2SO4) can be used as an alternative for acetic acid. Less harmful gasses are produced when sulfuric acid is used. Sulfuric acid is a strong acid that strongly reacts with bases and that is very corrosion.