Sodium perborate are chemical compounds with chemical formula [Na+]2[B2O4(OH)4]2−(H2O)x. Commonly encountered salts are the anhydrous form (x = 0) and as a hexahydrate (x = 6). These two species are sometimes called, respectively, "monohydrate" or PBS-1 and "tetrahydrate" or PBS-4, after the historic assumption that NaBO3 would be the anhydrous form). Both the anhydrous and hexahydrate salts are white, odorless, water-soluble solids. Peroxyborates are widely used in laundry detergents, as one of the peroxide-based bleaches. Sodium perborate was first obtained in 1898, independently, by Sebastian Tanatar and by P. Melikoff and L. Pissadewsky; the researchers prepared sodium perborate by treating sodium borate with a solution of hydrogen peroxide and sodium hydroxide, but Tanatar also obtained sodium perborate by electrolysis of a solution of sodium borate.
Structure Unlike sodium percarbonate, the peroxyborates are not adducts of hydrogen peroxide. Rather, they contain a peroxyborate anion [(B(OH)2OO)2]2−, which consists of a cyclic −B−O−O−B−O−O− core with a pair of hydroxy groups bonded to each boron atom. The ring adopts a chair conformation. The hexahydrate has the formula Na2H4B2O8·6H2O, that is, Na2H16B2O14 or NaH8BO7. The anhydrous compound is commonly but incorrectly called a "monohydrate" with the historical but misleading formula NaBO3·H2O. Instead, a more descriptive formula is Na2[(HO)2B]2(O2)2. Likewise, the hexahydrate is usually called "tetrahydrate" and formulated as NaBO3·4H2O. Both forms are white, odorless, water-soluble solids. The "monohydrate" and the "tetrahydrate" are the commercially important forms. A true tetrahydrate Na2H4B2O8·4H2O, traditionally known as the "trihydrate", is also known but has no industrial significance. There is a CAS number for each of the three traditional "hydrates", the three "peroxyborate" versions of each (interpreted as a hydrogen peroxide adduct) and the poorly defined "anhydrate" NaBO3, a total of seven.
Chemistry Sodium perborate hydrolyzes (i.e. breaks down in contact with water), producing hydrogen peroxide and borate. This reaction proceeds through formation of a boron-hydroperoxide which enters an equilibrium with boric acid B(OH)3, hydrogen peroxide H2O2, the hydroperoxyl anion −OOH, and the tetrahydroxyborate anion [B(OH)4]−:
[B(OH)3(OOH)]− ⇌ B(OH)3 + HO−2 B(OH)3 + HO−2 + H2O ⇌ [B(OH)4]− + H2O2 As the concentration of the solution increases, other peroxoborate species become significant. With excess H2O2, the anions [B(OH)2(OOH)2]−, [B(OH)(OOH)3]−, and eventually [B(OOH)4]− appear. At high borate concentrations, the sodium perborate with a dimeric anion crystallizes out, due to its relatively low solubility. The "monohydrate" form dissolves faster than the "tetrahydrate" and has higher heat stability; it is prepared by heating the "tetrahydrate". The commercial "anhydrate", or Oxoborate, is prepared by further heating of the "monohydrate" and is believed to consist of sodium borate and an uncharacterized boron–oxygen radical.
Preparation Sodium perborate is manufactured by reaction of borax Na2B4O7 and sodium hydroxide NaOH to give sodium metaborate NaBO2, which is then reacted with hydrogen peroxide to give hydrated sodium perborate:
Na2B4O7 + 2 NaOH → 4 NaBO2 + H2O 2 NaBO2 + 2 H2O2 → Na2B2O4(OH)4 A surfactant may be added to control crystal size. It may also be produced in the electrolysis of an aqueous solution containing borax, sodium carbonate and sodium bicarbonate (potassium dichromate is added to improve yield along with sodium silicate). A copper pipe is used as a cathode and platinum for the anode, the current being 6 amperes at 7 to 8 volts, and the temperature 10 °C.
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. Sodium perborate releases oxygen rapidly at temperatures greater than 60 °C. Addition of the activator, typically tetraacetylethylenediamine (TAED), makes it active at lower temperatures (40–60 °C). 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.
Dental use Sodium perborate monohydrate is quickly hydrolyzed into hydrogen peroxide and borate on contact with water. A 1979 double-blind crossover study suggests that hydrogen peroxide, which is released during the use of this product, may prevent or retard colonization and multiplication of anaerobic bacteria, such as those that inhabit oral wounds. 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.
Organic synthesis Sodium perborate is also used as an oxidizing reagent in organic synthesis. For example, it converts thioethers into sulfoxides and sulfones. Another use of sodium perborate in organic synthesis is as a convenient alternative to H2O2, for example in the 2 step conversion of an aryl halide to a phenol (the first step is a Miyaura borylation).
Safety In the European Union, sodium perborate, like most other inorganic boron compounds, was classified as "carcinogenic, mutagenic, or toxic for reproduction" (CMR), category 1B of Regulation (EC) 790/2009, as a result of being included in Part 3 of Annex VI of the regulation 1272/2008 on Classification, Labelling and Packaging (CLP) 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.
Brands
Bocasan Bocasan was an oral wound cleanser manufactured in the United Kingdom by Knox Laboratories Ltd of London from 1960–1975, before being rebranded as an Oral-B product
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