Borates

Borates are salts or esters of boric acids . The salts are characterized in that they contain the borate ion BO ₃ ³⁻ or a condensed form of it (e.g. B ₄ O ₅ (OH) ₄ ²⁻ , tetraborate) as an anion in their ion lattice .

Borates form an independent mineral class .

Natural occurrence

Borax crystal

The breakdown of minerals containing boron , mostly containing water of crystallization, is called borate breakdown . The following minerals are important borates used in industry and technology:

• Boracit (magnesium chloroborate, anhydrous)
• Borax ( sodium tetraborate )
• Priceit (Pandermit) ( calcium borate )
• Colemanite (calcium borate)
• Sassolin (boric acid)
• Kernite (sodium tetraborate)
• Ulexit (calcium sodium borate)

presentation

A simple representation of borates is the reaction of orthoboric acid with caustic soda (sodium hydroxide solution):

${\displaystyle \mathrm {H_{3}BO_{3}+3\ NaOH\longrightarrow Na_{3}BO_{3}+3\ H_{2}O} }$

properties

trigonal-planar structure of BO ₃ ³⁻

Stick model of tetraborate

The isolated borate ion BO ₃ ³⁻ is trigonal-planar according to the VSEPR model (see figure).

There are a variety of structural variations in the borates; for example the dimer B ₂ O ₅ ⁴⁻ or the already mentioned tetraborate B ₄ O ₅ (OH) ₄ ²⁻ .

In addition, there are various metaborates, which differs from the metaboric derived and general formula (BO ₂ ) _n ⁿ have. You can form both ring-shaped and polymeric chain structures.

By reacting boric acid with polyhydric alcohols (for example glycerine or mannitol ), cyclic esters ( chelate complexes ) can be obtained.

Borates in which an oxygen atom has been replaced by a dioxygen group are called perborates or peroxoborates.

toxicology

Genotoxic or carcinogenic effects of boron have not yet been proven. Borates are excreted by the organism and do not accumulate. There is no absorption through the skin. In the case of sodium tetraborates (especially borax ) (as in the case of boric acid ), reproductive toxicity is assumed. This was observed when higher doses were administered to mice.

proof

Green-burning trimethyl borate

For the rapid qualitative detection of borates, the substance to be examined is placed in a test tube or porcelain dish, methanol and a few drops of concentrated sulfuric acid are added, resulting in trimethyl borate . The rising vapors of the methanol, which also contain the boric acid ester, are flammable and burn with a green flame.

${\displaystyle \mathrm {B(OH)_{3}+3\ CH_{3}OH\ {\overrightarrow {\longleftarrow }}\ B(OCH_{3})_{3}+3\ H_{2}O} }$

The presence of bivalent copper ions can interfere with the detection.

The curcumin method is suitable for the quantitative detection of borates in analytical chemistry. Curcumin and boric acid form the red complex rosocyanine , the concentration of which is determined colorimetrically . The curcumin method is therefore also suitable for the quantitative determination of boron after oxidative digestion of a sample of the boron-containing material.

use

Different forms of borates are used in wood preservatives . Borates are also used as buffer substances (see: buffer solution ). Perborates are used in detergents and bleaches.

literature

• Erwin Riedel, Christoph Janiak: Inorganic Chemistry. 7th edition. de Gruyter, Berlin 2007, ISBN 978-3-11-018903-2 , p. 582 ff.
• Arnold Willmes: Paperback Chemical Substances. 3. Edition. Verlag Harri Deutsch, Frankfurt am Main 2007, ISBN 978-3-8171-1787-1 , p. 213 ff.

Web links

Wikibooks: Practical course in inorganic chemistry / borate  - learning and teaching materials

Individual evidence

1. ↑ HERA - Human and Environmental Risk Assessment on ingredients of Household Cleaning Products, Substance: Boric Acid (CAS No 10043-35-3) - Edition 1.0 - 2005
2. ↑ BMG Engineering AG: Study of January 22, 2013 on the handling of boron in the assessment of contaminated sites on behalf of the Canton of Aargau, Switzerland; accessed in September 2016