Sodium perborate

Sodium perborate is chemical compound whose chemical formula may be written NaH2BO4, Na2H4B2O8, or, more properly, [Na+]2[B2O4(OH)4]2−. Its name is sometimes abbreviated as PBS (not to be confused with phosphate-buffered saline).

Sodium perborate
Perborate unit in the "monohydrate"
Names
Other names
Sodium peroxoborate,[1] PBS-1 ("monohydrate"), PBS-4 ("tetrahydrate")
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.035.597
EC Number
  • 231-556-4
RTECS number
  • SC7350000
UNII
UN number 1479
  • InChI=1S/B2H4O8.2Na/c3-1(4)7-9-2(5,6)10-8-1;;/h3-6H;;/q-2;2*+1 checkY
    Key: JBUKJLNBQDQXLI-UHFFFAOYSA-N checkY
  • InChI=1/B2H4O8.2Na/c3-1(4)7-9-2(5,6)10-8-1;;/h3-6H;;/q-2;2*+1
    Key: JBUKJLNBQDQXLI-UHFFFAOYAG
  • [Na+].[Na+].O[B-]1(OO[B-](O)(O)OO1)O
Properties
NaBO3·nH2O
Molar mass 99.815 g/mol ("monohydrate");
153.86 g/mol ("tetrahydrate")
Appearance White powder
Odor Odorless
Melting point 63 °C (145 °F; 336 K) ("tetrahydrate")
Boiling point 130 to 150 °C (266 to 302 °F; 403 to 423 K) ("tetrahydrate", decomposes)
2.15 g/(100 mL) ("tetrahydrate", 18 °C)
Pharmacology
A01AB19 (WHO)
Hazards
NFPA 704 (fire diamond)
NFPA 704 four-colored diamond
1
1
0
Flash point Non-flammable
Safety data sheet (SDS) ICSC 1046
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)
Infobox references

The compound is commonly encountered in anhydrous form or as a hexahydrate (commonly called "monohydrate" or PBS-1 and "tetrahydrate" or PBS-4, after the early assumption that NaBO3 would be the anhydrous form).[2] They are both white, odorless, water-soluble solids.[3]

This salt is widely used in laundry detergents, as one of the peroxide-based bleaches.

Structure

Unlike sodium percarbonate and sodium perphosphate, the compound is not simply an adduct with hydrogen peroxide – known only since 1961.[4][1] Rather, it contains a perborate anion [(B(OH)2OO)2]2− consisting of a cyclic −B−O−O−B−O−O− core with two hydroxy groups attached to each boron atom. The ring adopts a chair conformation.[5]

Hydrates

The compound also crystallizes from water as the hexahydrate, Na2H4B2O8·6H2O, that is, Na2H16B2O14 or NaH8BO7.

The anhydrous compound is commonly but incorrectly called a "monohydrate" after the historical formulation NaBO3·H2O instead of the correct Na2H4B2O8. Likewise, the hexahydrate is usually called "tetrahydrate" and formulated as NaBO3·4H2O.[2] Both forms are white, odorless, water-soluble solids.[3] The "monohydrate" and the "tetrahydrate" are the commercially important forms.[3]

There does exist a true tetrahydrate Na2H4B2O8·4H2O, traditionally known as the "trihydrate", with 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, totalling seven.[1]

Chemistry

Sodium perborate undergoes hydrolysis in contact with water, producing hydrogen peroxide and borate.[3]

More precisely, in solution the cyclic anion hydrolizes into two anions [B(OH)3(OOH)]−, which then enter in equilibrium with boric acid B(OH)3, hydrogen peroxide H2O2, the hydroperoxyl anion −OOH, and the tetrahydroxyborate anion [B(OH)4]−:[2]

[(B(OH)2OO)2]2− + 2 H2O ⇌ 2 [B(OH)3(OOH)]−
[B(OH)3(OOH)]− ⇌ B(OH)3 + −OOH
B(OH)3 + −OOH + 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 dimeric anion crystallizes out, due to its relatively low solubility.[2]

The "monohydrate" form dissolves faster than the "tetrahydrate" and has higher heat stability; it is prepared by heating the "tetrahydrate".[1] The commercial "anhydrate", or Oxoborate, is prepared by further heating of "monohydrate" and actually consists of sodium borate and boron–oxygen radical.[1]

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:[3][6]

Na2B4O7 + 2 NaOH → 4 NaBO2 + H2O
2 NaBO2 + 2 H2O2 → Na2B2O4(OH)4

A surfactant may be added to control crystal size.[7][8]

It may also be produced in the electrolysis of an aqueous solution of a 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.[9]

Uses

Sodium perborate serves as a stable source of active oxygen in many detergents, laundry detergents, cleaning products, and laundry bleaches.[3] 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 (40–60 °C), one must mix it with a suitable activator, typically tetraacetylethylenediamine (TAED).

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.[10]

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.[11]

Safety

In the European Union, sodium perborate, like most borates, 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.[10]

See also

References
  1. Harald Jakob; et al. "Peroxy Compounds, Inorganic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a19_177.pub2.
  2. Alexander McKillop and William R Sanderson (1995): "Sodium perborate and sodium percarbonate: Cheap, safe and versatile oxidising agents for organic synthesis". Tetrahedron, volume 51, issue 22, pages 6145-6166. doi:10.1016/0040-4020(95)00304-Q
  3. B.J. Brotherton "Boron: Inorganic Chemistry" in Encyclopedia of Inorganic Chemistry (1994) Ed. R. Bruce King, John Wiley & Sons ISBN 0-471-93620-0
  4. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  5. Carrondo, M. A. A. F. de C. T.; Skapski, A. C. (1978). "Refinement of the X-ray crystal structure of the industrial bleaching agent disodium tetrahydroxo-di-μ-peroxo-diborate hexahydrate, Na2[B2(O2)2(OH)4]·6H2O". Acta Crystallogr B. 34: 3551. doi:10.1107/S0567740878011565.
  6. "Sodium Perborate REACH Consortium" (PDF). Archived from the original (PDF) on 2014-04-26. Retrieved 2012-06-07.
  7. J. Dugua and B.Simon (1978): "Crystallization of sodium perborate from aqueous solutions: I. Nucleation rates in pure solution and in presence of a surfactant". Journal of Crystal Growth, volume 44, issue 3, pages 265-279.doi:10.1016/0022-0248(78)90025-8
  8. J. Dugua and B.Simon (1978): "Crystallization of sodium perborate from aqueous solutions: II. Growth kinetics of different faces in pure solution and in the presence of a surfactant". Journal of Crystal Growth, volume 44, issue 3, pages 280-286.doi:10.1016/0022-0248(78)90026-X
  9. Paul, Mollard (12 June 1962). "US3038842A Process of making sodium perborate by electrolysis". Google Patents.
  10. Nature Inc. (2015): "Chemical used in beauty salon teeth whitening banned by EU". BDJ Team, volume 2, article 15075, 26 June 2015. doi:10.1038/bdjteam.2015.75
  11. McKillop, Alexander; Kabalka, George W.; Reddy, Marepally Srinivasa (2008-09-15), "Sodium Perborate", in John Wiley & Sons, Ltd (ed.), Encyclopedia of Reagents for Organic Synthesis, Chichester, UK: John Wiley & Sons, Ltd, pp. rs094.pub2, doi:10.1002/047084289x.rs094.pub2, ISBN 978-0-471-93623-7, retrieved 2021-01-18
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