ity of molten borates to dissolve
metal oxides. In solution it partially
hydrolyses to boric acid and can thus
act as a buffer. For this reason it is
used as a laundry pre-soak. It is used
medicinally as a mild alkaline anti-
septic and astringent for the skin and
mucous membranes.
Disodium tetraborate is the source
of many industrially important boron
compounds, such as barium borate
(fungicidal paints), zinc borate (Üre-
retardant additive in plastics), and
boron phosphate (heterogeneous acid
catalyst in the petrochemicals indus-
try).
borax-bead testA simple labora-
tory test for certain metal ions in
salts. A small amount of the salt is
mixed with borax and a molten bead
formed on the end of a piece of plat-
inum wire. Certain metals can be
identiÜed by the colour of the bead
produced in the oxidizing and reduc-
ing parts of a BunsenÛame. For ex-
ample, iron gives a bead that is red
when hot and yellow when cold in
the oxidizingÛame and a green bead
in the reducingÛame.
borazonSee boron nitride.
Bordeaux mixture A mixture of
copper(II) sulphate and calcium hy-
droxide in water, used as a fungicide.
boric acidAny of a number of
acids containing boron and oxygen.
Used without qualiÜcation the term
applies to the compound H 3 BO 3
(which is also called orthoboric acid
or, technically, trioxoboric(III) acid).
This is a white or colourless solid
that is soluble in water and ethanol;
triclinic; r.d. 1.435; m.p. 169°C. It oc-
curs naturally in the condensate
from volcanic steam vents (sufÜoni).
Commercially, it is made by treating
borate minerals (e.g. kernite,
Na 2 B 4 O 7 .4H 2 O) with sulphuric acid
followed by recrystallization.
In the solid there is considerable
hydrogen bonding between H 3 BO 3
molecules resulting in a layer struc-
ture, which accounts for the easy
cleavage of the crystals. H 3 BO 3 mol-
ecules also exist in dilute solutions
but in more concentrated solutions
polymeric acids and ions are formed
(e.g. H 4 B 2 O 7 ; pyroboric acid or
tetrahydroxomonoxodiboric(III) acid).
The compound is a very weak acid
but also acts as a Lewis *acid in ac-
cepting hydroxide ions:
B(OH) 3 + H 2 O ˆB(OH) 4 – + H+
If solid boric acid is heated it loses
water and transforms to another acid
at 300°C. This is given the formula
HBO 2 but is in fact a polymer
(HBO 2 )n. It is called metaboric acid or,
technically, polydioxoboric(III)acid.
Boric acid is used in the manufac-
ture of glass (borosilicate glass),
glazes and enamels, leather, paper,
adhesives, and explosives. It is widely
used (particularly in the USA) in de-
tergents, and because of the ability of
fused boric acid to dissolve other
metal oxides it is used as aÛux in
brazing and welding. Because of its
mild antiseptic properties it is used
in the pharmaceutical industry and
as a food preservative.boric anhydrideSee boron(iii)
oxide.boric oxideSee boron(iii) oxide.borideA compound of boron with
a metal. Most metals form at least
one boride of the type MB, MB 2 , MB 4 ,
MB 6 , or MB 12. The compounds have a
variety of structures; in particular,
the hexaborides contain clusters of
B 6 atoms. The borides are all hard
high-melting materials with metal-
like conductivity. They can be made
by direct combination of the el-
ements at high temperatures (over
2000 °C) or, more usually, by high-
temperature reduction of a mixture79 boride
b