Chlorite

For the mineral type, see chlorite group. For the neutral chemical compound, see chlorine dioxide.

The chlorite ion

The chlorite ion is ClO₂⁻. A chlorite (compound) is a compound that contains this group, with chlorine in oxidation state +3. Chlorites are also known as salts of chlorous acid.

Oxidation states

Chlorine can assume oxidation states of −1, +1, +3, +5, or +7 within the corresponding anions Cl⁻, ClO⁻, ClO₂⁻, ClO₃⁻, or ClO₄⁻, known commonly and respectively as chloride, hypochlorite, chlorite, chlorate, and perchlorate. An additional oxidation state of +4 is seen in the neutral compound chlorine dioxide ClO₂, which has the same structure as chlorite ClO₂⁻ and the cation chloryl (ClO₂⁺).

oxidation state	−1	+1	+3	+5	+7
anion named	chloride	hypochlorite	chlorite	chlorate	perchlorate
formula	Cl-	ClO−	ClO2−	ClO3−	ClO4−
structure

Some chlorite compounds

• sodium chlorite, NaClO₂
• magnesium chlorite, Mg(ClO₂)₂

Manufacture

The free acid, chlorous acid, HClO₂, is only stable at low concentrations. Since it cannot be concentrated, it is not a commercial product. However, the corresponding sodium salt, sodium chlorite, NaClO₂ is stable and inexpensive enough to be commercially available. The corresponding salts of heavy metals (Ag⁺, Hg⁺, Tl⁺, Pb²⁺, and also Cu²⁺ and NH₄⁺) decompose explosively with heat or shock.

Sodium chlorite is derived indirectly from sodium chlorate, NaClO₃. First, the explosively unstable gas chlorine dioxide, ClO₂ is produced by reducing sodium chlorate in a strong acid solution with a suitable reducing agent (for example, sodium chloride, sulfur dioxide, or hydrochloric acid).

H₂SO₄(aq) + NaClO₃(s) → NaHSO₄(aq) + HClO₃(aq)
3HClO₃(aq) → 2ClO₂(g) + HClO₄(aq) + H₂O(l)

(Other routes for the preparation of chlorine dioxide are available depending on the initial salt).

The chlorine dioxide is then absorbed into an alkaline solution and reduced with hydrogen peroxide, H₂O₂ yielding sodium chlorite (NaClO₂). (Sodium, Na⁺, spectator ions are not shown in the following equations).

2ClO₂(g) + 2OH^- → ClO₂^-(aq) + ClO₃^-(aq) + H₂O(l)
ClO₃^-(aq) + H₂O₂(l) → ClO₂^-(aq) + H₂O(l) + O₂(g)

Usage

The main application of sodium chlorite is the generation of chlorine dioxide for bleaching and stripping of textiles, pulp, and paper. It is also used for disinfection in a few municipal water treatment plants after conversion to chlorine dioxide. An advantage in this application, as compared to the more commonly used chlorine, is that trihalomethanes are not produced from organic contaminants. In the European Union however, the use of Sodium chlorite has been phased out by 24-10-2009 for use as certain types of biocides: Product-type(PT) 2: Private area and public health area disinfectants and other biocidal products, PT 3: Veterinary hygiene biocidal products, PT 4: Food and feed area disinfectants and PT 5: Drinking water disinfectants.^[1] Since September 2006, its application as molluscicide (PT 16) is forbidden as well. Sodium chlorite, NaClO₂ also finds application as a component of contact lens cleaning solution under the trade name purite.

Safety

Health related information: see sodium chlorite. Sodium chlorite, like many oxidizers, should be protected from inadvertent contamination by organic materials to avoid the formation of an explosive mixture.

References

1. ^ Commission Decision of 14 October 2008 concerning the non-inclusion of certain substances in Annex I, IA or IB to Directive 98/8/EC of the European Parliament and of the Council concerning the placing of biocidal products on the market (notified under document number C(2008) 5894)(Text with EEA relevance)(2008/809/EC) [1]

• Chemistry of the Elements, N.N. Greenwood and A. Earnshaw, Pergamon Press, 1984.
• Kirk-Othmer Concise Encyclopedia of Chemistry, Martin Grayson, Editor, John Wiley & Sons, Inc., 1985