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Concept
Boric acid
Summary
Boric acid, more specifically orthoboric acid, is a compound of boron, oxygen, and hydrogen with formula . It may also be called hydrogen orthoborate, trihydroxidoboron or boracic acid. It is usually encountered as colorless crystals or a white powder, that dissolves in water, and occurs in nature as the mineral sassolite. It is a weak acid that yields various borate anions and salts, and can react with alcohols to form borate esters.
Boric acid is often used as an antiseptic, insecticide, flame retardant, neutron absorber, or precursor to other boron compounds.
The term "boric acid" is also used generically for any oxoacid of boron, such as metaboric acid and tetraboric acid .
Orthoboric acid was first prepared by Wilhelm Homberg (1652–1715) from borax, by the action of mineral acids, and was given the name sal sedativum Hombergi ("sedative salt of Homberg"). However boric acid and borates have been used since the time of the ancient Greeks for cleaning, preserving food, and other activities.
The three oxygen atoms form a trigonal planar geometry around the boron. The B-O bond length is 136 pm and the O-H is 97 pm. The molecular point group is C3h.
Two crystalline forms of orthoboric acid are known: triclinic and hexagonal. The former is the most common; the second, which is a bit more stable thermodynamically, can be obtained with a special preparation method.
The triclinic form of boric acid consists of layers of molecules held together by hydrogen bonds with an O...O separation of 272 pm. The distance between two adjacent layers is 318 pm.
Boric acid may be prepared by reacting borax (sodium tetraborate decahydrate) with a mineral acid, such as hydrochloric acid:
10 + 2 HCl → 4 + 2 NaCl + 5
It is also formed as a by product of hydrolysis of boron trihalides and diborane:
6 → 2 + 6
3 → + 3 HX (X = Cl, Br, I)
When heated, orthoboric acid undergoes a three step dehydration. The reported transition temperatures vary substantially from source to source.
Official source
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