Sodium perborate is chemical compound whose chemical formula may be written,, or, more properly, ·. Its name is sometimes abbreviated as PBS. The compound is commonly encountered in anhydrous form or as a hexahydrate. They are both white, odorless, water-soluble solids. This salt is widely used in laundry detergents, as one of the peroxide-based bleaches.
The compound also crystallizes from water as the hexahydrate, ·6, that is, or. The anhydrous compound is commonly but incorrectly called a "monohydrate" after the original formulation · instead of the correct. Likewise, the hexahydrate is usually called "tetrahydrate" and formulated as ·4. Both forms are white, odorless, water-soluble solids. The "monohydrate" and the "tetrahydrate" are the commercially important forms.
Chemistry
Sodium perborate undergoes hydrolysis in contact with water, producing hydrogen peroxide and borate. More precisely, in solution the cyclic anion hydrolizes into two anions, which then enter in equilibrium with boric acid, hydrogen peroxide, the hydroperoxyl anion, and the tetrahydroxyborate anion : + 2 H2O <=> 2 - - <=> B3 + HOO- B3 + HOO- + H2O <=> - + H2O2 As the concentration of the solution increases, other peroxoborate species become significant. With excess, the anions,, and eventually appear. At high borate concentrations, the sodium perborate with dimeric anion crystallizes out, due to its relatively low solubility. The monohydrate form dissolves better than the tetrahydrate and has higher heat stability; it is prepared by heating the tetrahydrate.
Preparation
Sodium perborate is manufactured by reaction of borax and sodium hydroxide to give sodium metaborate, which is then reacted with hydrogen peroxide to give hydrated sodium perborate: Na2B4O7 + 2NaOH -> 4 NaBO2 + H2O NaBO2 + 2H2O2 -> Na2B2O44 A surfactant may be added to control crystal size.
Uses
Sodium perborate serves as a stable source of active oxygen in many detergents, laundry detergents, cleaning products, and laundry bleaches. It is a less aggressive bleach than sodium hypochlorite and other chlorine-based bleaches, causing less degradation to dyes and textiles. Borates also have some non-oxidative bleaching properties. Sodium perborate releases oxygen rapidly at temperatures over 60 °C. To make it active at lower temperatures, it has to be mixed with a suitable activator, typically tetraacetylethylenediamine. Sodium perborate is also present in some tooth bleaching formulas for non vital root treated teeth. The compound is inserted in the root canal and left in place for an extended period of time to allow it to diffuse into the tooth and bleach stains from the inside out. However, this use has been banned in the European Union. The compound has antiseptic properties and can act as a disinfectant. It is also used as a "disappearing" preservative in some brands of eye drops. Sodium perborate is also used as an oxidizing reagent in organic synthesis. For example, it converts thioethers into sulfoxides and sulfones.
Safety
In the European Union, sodium perborate, like most borates, was classified as "carcinogenic, mutagenic, or toxic for reproduction", category 1B of Regulation 790/2009, as a result of being included in Part 3 of Annex VI of the regulation 1272/2008 on Classification, Labelling and Packaging of substances and mixtures. As a result, their use has been automatically banned in cosmetic products in the EU, in any concentration, starting 1 December 2010. That extends to the use of perborates for tooth whitening.
