Sodium hypochlorite

Sodium, Na   Oxygen, O   Chlorine, Cl
Names
IUPAC name Sodium hypochlorite
Other names Antiformin Bleach Chloride of soda Sodium chloroxide In dilution: Carrel-Dakin solution Modified Dakin's solution Surgical chlorinated soda solution
Identifiers
CAS Number	7681-52-9 (anhydrous) Y 10022-70-5 (pentahydrate) Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:32146 Y
ChemSpider	22756 Y
DrugBank	DBSALT001517
ECHA InfoCard	100.028.790
EC Number	231-668-3
KEGG	D01711 Y
PubChem CID	23665760
RTECS number	NH3486300
UNII	DY38VHM5OD Y
UN number	1791
CompTox Dashboard (EPA)	DTXSID8021276
InChI InChI=1S/ClO.Na/c1-2;/q-1;+1 Y Key: SUKJFIGYRHOWBL-UHFFFAOYSA-N Y InChI=1/ClO.Na/c1-2;/q-1;+1 Key: SUKJFIGYRHOWBL-UHFFFAOYAD
SMILES .Cl
Properties
Chemical formula	NaOCl
Molar mass	74.442 g/mol
Appearance	White crystalline solid (anhydrous); Colorless or slightly yellow[1] or greenish-yellow crystalline solid (pentahydrate)
Odor	Chlorine-like and sweetish (pentahydrate)[1]
Density	1.11 g/cm3
Melting point	18 °C (64 °F; 291 K) (pentahydrate)
Boiling point	101 °C (214 °F; 374 K) (decomposes) (pentahydrate)
Solubility in water	29.3 g/(100 mL) (0 °C)[2]
Acidity (pKa)	7.5185
Basicity (pKb)	6.4815
Thermochemistry
Std enthalpy of formation (ΔfH⦵298)	−347.1 kJ/mol
Pharmacology
ATC code	D08AX07 (WHO)
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	Oxidizer, corrosive[3]
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H302, H314, H410
Precautionary statements	P260, P264, P273, P280, P301+P330+P331, P303+P361+P353, P304+P340, P305+P351+P338, P310, P321, P363, P391, P405, P501
NFPA 704 (fire diamond)	2 0 1 OX
Safety data sheet (SDS)	ICSC 1119 (solution, >10% active chlorine) ICSC 0482 (solution, <10% active chlorine)
Related compounds
Other anions	Sodium chloride Sodium chlorite Sodium chlorate Sodium perchlorate
Other cations	Lithium hypochlorite Potassium hypochlorite Calcium hypochlorite Barium hypochlorite Silver hypochlorite
Related compounds	Hypochlorite Chlorine monoxide Hypochlorous acid Methyl hypochlorite
Except where otherwise noted, data are given for materials in their standard state (at 25 °C , 100 kPa). N verify (what is YN ?) Infobox references

Sodium hypochlorite is an alkaline inorganic chemical compound with the formula NaOCl (also written as NaClO). It is commonly known in a dilute aqueous solution as bleach or chlorine bleach.^[4] It is the sodium salt of hypochlorous acid, consisting of sodium cations (Na⁺) and hypochlorite anions (⁻OCl, also written as OCl⁻ and ClO⁻).

The anhydrous compound is unstable and may decompose explosively.^[5][6] It can be crystallized as a pentahydrate NaOCl·5H₂O, a pale greenish-yellow solid which is not explosive and is stable if kept refrigerated.^[7][8][9]

Sodium hypochlorite is most often encountered as a pale greenish-yellow dilute solution referred to as chlorine bleach, which is a household chemical widely used (since the 18th century) as a disinfectant and bleaching agent. In solution, the compound is unstable and easily decomposes, liberating chlorine, which is the active principle of such products. Sodium hypochlorite is still the most important chlorine-based bleach.^[10][11]

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 found in limescale-removing products, will release chlorine gas. Chlorine gas was utilized as a chemical weapon in World War I.^[12][13][14] A common misconception is that mixing bleach with ammonia also releases chlorine, but in reality they react to produce chloramines such as nitrogen trichloride. With excess ammonia and sodium hydroxide, hydrazine may be generated.

Chemistry

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.^[5] The decomposition is accelerated by carbon dioxide at Earth's atmospheric levels - around 4 parts per ten thousand.^[6][15] It is a white solid with the orthorhombic crystal structure.^[16]

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.^[6][7] The formula is sometimes given in its hydrous crystalline form as 2NaOCl·10H₂O.^[17] The Cl–O bond length in the pentahydrate is 1.686 Å.^[9] The transparent, light greenish-yellow, orthorhombic^[18][19] 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.^[8][20]

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 of 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.^[21]

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,^[22] and 1.21 g/mL at 14%, 20 °C.^[23] Stoichiometric solutions are fairly alkaline, with pH 11 or higher^[8] since hypochlorous acid is a weak acid:

OCl⁻ + H₂O ⇌ HOCl + OH⁻

The following species and equilibria are present in NaOCl/NaCl solutions:^[24]

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⁻.^[10] 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 + 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:

⁻OCl + H⁺ ⇌ HOCl

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.^[8] 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.^{[citation needed]}

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

Decomposition to chlorate or oxygen

In solution, under certain conditions, the hypochlorite anion may also disproportionate (autoxidize) to chloride and chlorate:^[25]

3 ClO⁻ + H⁺ → HClO₃ + 2 Cl⁻

In particular, this reaction occurs in sodium hypochlorite solutions at high temperatures, forming sodium chlorate and sodium chloride:^[25][26]

3 NaOCl(aq) → 2 NaCl(aq) + NaClO₃(aq)

This reaction is exploited in the industrial production of sodium chlorate.

An alternative decomposition of hypochlorite produces oxygen instead:

2 OCl⁻ → 2 Cl⁻ + O₂

In hot sodium hypochlorite solutions, this reaction competes with chlorate formation, yielding sodium chloride and oxygen gas:^[25]

2 NaOCl(aq) → 2 NaCl(aq) + O₂(g)

These two decomposition reactions of NaClO solutions are maximized at pH around 6. The chlorate-producing reaction predominates at pH above 6, while the oxygen one becomes significant below that. For example, at 80 °C, with NaOCl and NaCl concentrations of 80 mM, and pH 6–6.5, the chlorate is produced with ~95% efficiency. The oxygen pathway predominates at pH 10.^[25] This decomposition is affected by light^[26] and metal ion catalysts such as copper, nickel, cobalt,^[25] and iridium.^[27] Catalysts like sodium dichromate Na₂Cr₂O₇ and sodium molybdate Na₂MoO₄ may be added industrially to reduce the oxygen pathway, but a report claims that only the latter is effective.^[25]

Titration

Titration of hypochlorite solutions is often done by adding a measured sample to an excess amount of acidified solution of potassium iodide (KI) and then titrating the liberated iodine (I₂) with a standard solution of sodium thiosulfate or phenylarsine oxide, using starch as indicator, until the blue color disappears.^[19]

According to one US patent, the stability of sodium hypochlorite content of solids or solutions can be determined by monitoring the infrared absorption due to the O–Cl bond. The characteristic wavelength is given as 140.25 μm for water solutions, 140.05 μm for the solid dihydrate NaOCl·2H₂O, and 139.08 μm for the anhydrous mixed salt Na₂(OCl)(OH).^[21]

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.^[28]

In the presence of a phase-transfer catalyst, alcohols are oxidized to the corresponding carbonyl compound (aldehyde or ketone).^[29][8] Sodium hypochlorite can also oxidize organic sulfides to sulfoxides or sulfones, disulfides or thiols to sulfonyl halides, imines to oxaziridines.^[8] It can also de-aromatize phenols.^[8]

Oxidation of metals and complexes

Heterogeneous reactions of sodium hypochlorite and metals such as zinc proceed slowly to give the metal oxide or hydroxide:^{[citation needed]}

NaOCl + Zn → ZnO + NaCl

Homogeneous reactions with metal coordination complexes proceed somewhat faster. This has been exploited in the Jacobsen epoxidation.^{[citation needed]}

Other reactions

If not properly stored in airtight containers, sodium hypochlorite reacts with carbon dioxide to form sodium carbonate:

2 NaOCl + CO₂ + H₂O → Na₂CO₃ + 2 HOCl

Sodium hypochlorite reacts with most nitrogen compounds to form volatile monochloramine, dichloramines, and nitrogen trichloride:

NH₃ + NaOCl → NH₂Cl + NaOH

NH₂Cl + NaOCl → NHCl₂ + NaOH

NHCl₂ + NaOCl → NCl₃ + NaOH

Neutralization

Sodium thiosulfate is an effective chlorine neutralizer. Rinsing with a 5 mg/L solution, followed by washing with soap and water, will remove chlorine odor from the hands.^[30]

Production

Chlorination of soda

Potassium hypochlorite was first produced in 1789 by Claude Louis Berthollet in his laboratory on the Quai de Javel in Paris, France, by passing chlorine gas through a solution of potash lye. The resulting liquid, known as "Eau de Javel" ("Javel water"), was a weak solution of potassium hypochlorite. Antoine Labarraque replaced potash lye by the cheaper soda lye, thus obtaining sodium hypochlorite (Eau de Labarraque).^[31][32]

Cl₂(g) + 2 NaOH(aq) → NaCl(aq) + NaClO(aq) + H₂O

Hence, chlorine is simultaneously reduced and oxidized; this process is known as disproportionation.^{[citation needed]}

The process is also used to prepare the pentahydrate NaOCl·5H₂O for industrial and laboratory use. In a typical process, chlorine gas is added to a 45–48% NaOH solution. Some of the sodium chloride precipitates and is removed by filtration, and the pentahydrate is then obtained by cooling the filtrate to 12 °C .^[8]

From calcium hypochlorite

Another method involved the reaction of sodium carbonate ("washing soda") with chlorinated lime ("bleaching powder"), a mixture of calcium hypochlorite Ca(OCl)₂, calcium chloride CaCl₂, and calcium hydroxide Ca(OH)₂:

Na₂CO₃(aq) + Ca(OCl)₂(aq) → CaCO₃(s) + 2 NaOCl(aq)

Na₂CO₃(aq) + CaCl₂(aq) → CaCO₃(s) + 2 NaCl(aq)

Na₂CO₃(aq) + Ca(OH)₂(s) → CaCO₃(s) + 2 NaOH(aq)

This method was commonly used to produce hypochlorite solutions for use as a hospital antiseptic that was sold after World War I under the names "Eusol", an abbreviation for Edinburgh University Solution Of (chlorinated) Lime – a reference to the university's pathology department, where it was developed.^[33]

Electrolysis of brine

Near the end of the nineteenth century, E. S. Smith patented the chloralkali process: a method of producing sodium hypochlorite involving the electrolysis of brine to produce sodium hydroxide and chlorine gas, which then mixed to form sodium hypochlorite.^[34][32][35] The key reactions are:

2 Cl⁻ → Cl₂ + 2 e⁻ (at the anode)

2 H₂O + 2 e⁻ → H₂ + 2 ⁻OH (at the cathode)

Both electric power and brine solution were in cheap supply at the time, and various enterprising marketers took advantage of the situation to satisfy the market's demand for sodium hypochlorite. Bottled solutions of sodium hypochlorite were sold under numerous trade names.^{[citation needed]}

Today, an improved version of this method, known as the Hooker process (named after Hooker Chemicals, acquired by Occidental Petroleum), is the only large-scale industrial method of sodium hypochlorite production. In the process, sodium hypochlorite (NaClO) and sodium chloride (NaCl) are formed when chlorine is passed into cold dilute sodium hydroxide solution. The chlorine is prepared industrially by electrolysis with minimal separation between the anode and the cathode. The solution must be kept below 40 °C (by cooling coils) to prevent the undesired formation of sodium chlorate.^{[citation needed]}

Commercial solutions always contain significant amounts of sodium chloride (common salt) as the main by-product, as seen in the equation above.

From hypochlorous acid and soda

A 1966 patent describes the production of solid stable dihydrate NaOCl·2H₂O by reacting a chloride-free solution of hypochlorous acid HClO (such as prepared from chlorine monoxide ClO and water), with a concentrated solution of sodium hydroxide. In a typical preparation, 255 mL of a solution with 118 g/L HClO is slowly added with stirring to a solution of 40 g of NaOH in water 0 °C. Some sodium chloride precipitates and is removed by filtration. The solution is vacuum evaporated at 40–50 °C and 1–2 mmHg until the dihydrate crystallizes out. The crystals are vacuum-dried to produce a free-flowing crystalline powder.^[21]

The same principle was used in a 1993 patent to produce concentrated slurries of the pentahydrate NaClO·5H₂O. Typically, a 35% solution (by weight) of HClO is combined with sodium hydroxide at about or below 25 °C. The resulting slurry contains about 35% NaClO, and are relatively stable due to the low concentration of chloride.^[36]

Packaging and sale

Household bleach sold for use in laundering clothes is a 3–8% solution of sodium hypochlorite at the time of manufacture. Strength varies from one formulation to another and gradually decreases with long storage. Sodium hydroxide is usually added in small amounts to household bleach to slow down the decomposition of NaClO.^[10]

Domestic use patio blackspot remover products are ~10% solutions of sodium hypochlorite.

A 10–25% solution of sodium hypochlorite is, according to Univar's safety sheet, supplied with synonyms or trade names bleach, Hypo, Everchlor, Chloros, Hispec, Bridos, Bleacol, or Vo-redox 9110.^[37]

A 12% solution is widely used in waterworks for the chlorination of water, and a 15% solution is more commonly^[38] used for disinfection of waste water in treatment plants. Sodium hypochlorite can also be used for point-of-use disinfection of drinking water,^[39] taking 0.2–2 mg of sodium hypochlorite per liter of water.^[40]

Dilute solutions (50 ppm to 1.5%) are found in disinfecting sprays and wipes used on hard surfaces.^[41][42]

Uses

Bleaching

Household bleach is, in general, a solution containing 3–8% sodium hypochlorite, by weight, and 0.01–0.05% sodium hydroxide; the sodium hydroxide is used to slow the decomposition of sodium hypochlorite into sodium chloride and sodium chlorate.^[43]

Zdroj:https://en.wikipedia.org?pojem=Sodium_hypochlorite
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