Hypochlorite (ClO–)
Table of Contents
The hypochlorite ion is a chlorine-based oxyanion with the chemical formula ClO–, belonging to a homologous series of chlorine oxyanions that includes chlorite (ClO2–), chlorate (ClO3–), and perchlorate (ClO4–). In practice, hypochlorite is most often encountered as reactive sodium or calcium salts dissolved in water.
The hypochlorite ion is vital due to its strong oxidizing and disinfecting properties. A common example is calcium hypochlorite, Ca(OCl)2, which is used to disinfect drinking water, swimming pools, and wastewater. Its solid form is preferred in remote areas because it is easier to transport concentrated and stable salts. These include rural villages or emergency relief camps, where electricity and refrigeration are unavailable. When dissolved in water, it releases hypochlorite ions that kill harmful microorganisms.
Structure and Bonding
The hypochlorite ion (ClO–) consists of a single chlorine atom covalently bonded to one oxygen atom. Its Lewis structure shows a polar Cl–O bond. The negative charge is largely localized on the more electronegative oxygen atom. This uneven distribution of charge contributes significantly to the chemical reactivity of the ion. [1]
In hypochlorite, chlorine is in the +1 oxidation state, which is the lowest among the common chlorine oxyanions. Because chlorine is bonded to only one oxygen atom, the ion is less oxidized but more chemically reactive than chlorite, chlorate, or perchlorate.
This structural feature explains its practical behavior in bleaching and disinfection: the weak Cl–O bond allows hypochlorite to release highly reactive chlorine species in water that rapidly attack microorganisms, stains, and odor-producing compounds.
Physical Properties of Hypochlorite Compounds [6,7]
| Property | Description |
|---|---|
| Molar mass of ClO⁻ ion | 51.45 g·mol–1 |
| Physical state and appearance * | Sodium hypochlorite is typically a pale yellow-green liquid, while calcium hypochlorite is a white to grayish solid powder. |
| Solubility in water | Both sodium hypochlorite and calcium hypochlorite are readily soluble in water. |
| Stability | Hypochlorite compounds are chemically unstable and decompose, especially in the presence of heat, light, or metal ions. |
| Moisture absorption | Calcium hypochlorite absorbs moisture from air; sodium hypochlorite is generally stored and used in aqueous solution. |
* The hypochlorite ion does not exist as an isolated solid. Therefore, the physical properties are given in terms of its commercially relevant salts, sodium hypochlorite (NaOCl) and calcium hypochlorite [Ca(OCl)2].
Preparation
1. Reaction of Chlorine with Cold Dilute Alkali
Hypochlorites are commonly prepared by passing chlorine gas through a cold, dilute solution of an alkali such as sodium hydroxide or potassium hydroxide. Under these conditions, chlorine undergoes a disproportionation reaction in which part of the chlorine is reduced to chloride ions while the remainder is oxidized to hypochlorite ions: [2,3]
Cl2 + 2 NaOH → NaCl + NaOCl + H2O
2 Cl2 + 2 Ca(OH)2 → CaCl2 + Ca(OCl)2 + 2 H2O
2. Electrolysis of Aqueous Sodium Chloride
Sodium hypochlorite can also be produced by the electrolysis of aqueous sodium chloride (brine). During electrolysis, chloride ions are oxidized at the anode to form chlorine gas, while water is reduced at the cathode to produce hydroxide ions:
At anode: 2 Cl– → Cl2 + 2 e–
At cathode: 2 H2O + 2 e− → H2 + 2 OH–
The chlorine formed at the anode then reacts with the sodium hydroxide produced in solution, yielding sodium hypochlorite:
Cl2 + 2 NaOH → NaCl + NaOCl + H2O
This process is the basis of electrochlorination used in modern water-treatment systems.
Chemical Reactions
1. Oxidizing Nature
The hypochlorite ion is a strong oxidizing agent. In aqueous solution, it readily gains electrons and oxidizes a wide range of substances. For example, it oxidizes iodide ions to iodine: [4,5]
ClO– + 2 I– + 2 H+ → Cl– + I2 + H2O
2. Bleaching Action
The bleaching action of hypochlorite begins when the hypochlorite ion reacts with water to form hypochlorous acid, a much more reactive chlorine species:
ClO– + H2O ⇌ HOCl + OH–
Hypochlorous acid is unstable and readily decomposes to produce highly reactive oxygen species:
HOCl → HCl + [O]
Hypochlorite is used as a bleaching agent because it removes colored stains. The reactive oxygen breaks the double bonds in chromophores, the parts of molecules that absorb visible light, thereby destroying their color. This oxidative process underlies its widespread use in laundry, paper bleaching, and surface cleaning.
Health and Safety Considerations
The same strong oxidizing power that makes hypochlorite an effective bleaching and disinfecting agent also accounts for its hazards. Concentrated solutions can irritate the skin, eyes, and lungs. Mixing hypochlorite with acids or ammonia can release toxic chlorine-containing gases. For this reason, hypochlorite must be stored in cool, well-ventilated areas and kept away from reactive chemicals.
Taken together, these properties demonstrate why hypochlorite remains indispensable in sanitation and public health. It combines exceptional oxidizing strength with water solubility, making it highly effective when used correctly.
