Chlorite refers to two entirely different scientific concepts: a specific chemical anion (\(\text{ClO}_2^-\)) used extensively in industrial processes and water treatment, and a large group of common silicate minerals found in the Earth’s crust. This article clarifies the distinctions between the chemical compound and the mineral group.
The Chemical Compound
The chemical compound chlorite is defined by the chlorite ion (\(\text{ClO}_2^-\)). This is an oxyanion containing oxygen combined with chlorine, which is in the less common oxidation state of \(+3\).
The free acid of this ion, chlorous acid (\(\text{HClO}_2\)), is highly unstable and is not commercially available. Chlorite is almost exclusively encountered as a salt, with Sodium Chlorite (\(\text{NaClO}_2\)) being the only commercially significant form. This salt is typically a white to slightly yellowish crystalline solid that is highly soluble in water. Due to its chemical structure, the chlorite ion is a powerful oxidizing agent, fundamental to its utility in many industrial and disinfection applications.
Primary Commercial Applications
The primary commercial application of chemical chlorite stems from its role as a precursor for generating Chlorine Dioxide (\(\text{ClO}_2\)). This is a potent biocide and oxidizing agent that is too hazardous to transport, so it is often generated on-site from Sodium Chlorite. In water treatment, this process is widely used for the disinfection and purification of municipal drinking water supplies.
The resulting Chlorine Dioxide effectively kills bacteria, viruses, and other microorganisms. A significant advantage of this method over traditional chlorine is that it minimizes the formation of trihalomethanes, which are undesirable disinfection byproducts. The United States Environmental Protection Agency (EPA) has set a maximum contaminant level for the chlorite ion in drinking water at \(1.0\) milligram per liter (\(\text{mg/L}\)). Beyond water treatment, Sodium Chlorite is also extensively used as a bleaching agent in the paper pulp and textile industries.
Distinguishing from Related Ions
Chlorite (\(\text{ClO}_2^-\)) is one of a family of chlorine-oxygen anions, frequently confused with its relatives due to similar naming conventions. The chemical behavior of these ions changes drastically based on the number of oxygen atoms and the resulting oxidation state of the chlorine atom.
- The simplest is the Chloride ion (\(\text{Cl}^-\)), which has chlorine in the oxidation state of \(-1\) and contains no oxygen atoms. This is the common ion found in table salt, or sodium chloride (\(\text{NaCl}\)).
- Chlorite (\(\text{ClO}_2^-\)) has a chlorine oxidation state of \(+3\).
- Chlorate (\(\text{ClO}_3^-\)) contains three oxygen atoms and a chlorine oxidation state of \(+5\).
- Perchlorate (\(\text{ClO}_4^-\)) has four oxygen atoms and features chlorine in its highest possible oxidation state of \(+7\).
The increasing number of oxygen atoms changes the chemical reactivity and oxidizing strength of the ion. For example, the Perchlorate ion is chemically very stable and is primarily known for its use in rocket propellants and fireworks, a distinct application from the disinfection role of the chlorite ion.
The Silicate Mineral Group
The second, entirely unrelated meaning of chlorite refers to a common and widespread group of silicate minerals. These minerals are classified as phyllosilicates, meaning they have a sheet-like, layered structure similar to mica. The name for the mineral group comes from the Greek word chloros, meaning “green,” which is the characteristic color of most chlorite specimens.
The general chemical composition of chlorite minerals is complex, but it consistently involves iron (\(\text{Fe}\)), magnesium (\(\text{Mg}\)), aluminum (\(\text{Al}\)), and silicon (\(\text{Si}\)). Chlorite minerals are frequently found in low-grade metamorphic rocks, such as greenschist, where they form under conditions of low to medium temperature and pressure. These minerals are soft, with a Mohs hardness of \(2\) to \(3\), and they exhibit perfect cleavage, often appearing as flexible flakes or foliated masses.