Sodium hypochlorite, commonly known in a dilute solution as (chlorine) bleach, is an inorganic chemical compound with the formula NaOCl (or NaClO), consisting of a sodium cation () and a hypochlorite anion (or ). It may also be viewed as the sodium salt of hypochlorous acid. The anhydrous compound is unstable and may decompose explosively. It can be crystallized as a pentahydrate ·5, 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 referred to as liquid bleach, which is a household chemical widely used (since the 18th century) as a disinfectant or a bleaching agent. In solution, the compound is unstable and easily decomposes, liberating chlorine, which is the active principle of such products. 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 found in limescale-removing products, will produce chlorine gas, which was used as a chemical weapon in World War I. 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. The decomposition is accelerated by carbon dioxide at atmospheric levels. It is a white solid with the orthorhombic crystal structure.
Sodium hypochlorite can also be obtained as a crystalline pentahydrate ·5, which is not explosive and is much more stable than the anhydrous compound. The formula is sometimes given in its hydrous crystalline form as 2·10. The Cl–O bond length in the pentahydrate is 1.686 Å. The transparent, light greenish-yellow, orthorhombic 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 ·2 can be obtained by carefully excluding chloride ions (), which are present in the output of common manufacturing processes and are said to catalyze the decomposition of hypochlorite into chlorate () 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.
Equilibria and stability of solutions
At typical ambient temperatures, sodium hypochlorite is more stable in dilute solutions that contain solvated and ions. The density of the solution is 1.093 g/mL at 5% concentration, and 1.21 g/mL at 14%, 20 °C. Stoichiometric solutions are fairly alkaline, with pH 11 or higher since hypochlorous acid is a weak acid:
+ HOCl +
The following species and equilibria are present in solutions:
(aq) ⇌ +
(aq) + + ⇌ (aq) +
(aq) + ⇌
(aq) ⇌ (g)
The second equilibrium equation above will be shifted to the right if the chlorine is allowed to escape as gas. The ratios of , HOCl, and 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 . At pH greater than 7.4, the majority is in the form of hypochlorite . The equilibrium can be shifted by adding acids (such as hydrochloric acid) or bases (such as sodium hydroxide) to the solution:
(aq) + 2 HCl (aq) → (g) + (aq) + (aq) (g) + 2 → (aq) + (aq) + (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:
+ ⇌ HClO
Sodium hypochlorite solutions combined with acid evolve chlorine gas, particularly strongly at pH < 2, by the reactions:
(aq) + + ⇌ (aq) +
(aq) ⇌ (g)