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 antiseptic 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 (Üreretardant additive in plastics), and
boron phosphate (heterogeneous acid
catalyst in the petrochemicals industry).
borax-bead test A simple laboratory 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 platinum wire. Certain metals can be
identiÜed by the colour of the bead
produced in the oxidizing and reducing parts of a Bunsen Ûame. For example, iron gives a bead that is red
when hot and yellow when cold in
the oxidizing Ûame and a green bead
in the reducing Ûame.
borazon See boron nitride.
Bordeaux mixture A mixture of
copper(II) sulphate and calcium hydroxide in water, used as a fungicide.
boric acid Any 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 occurs 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 structure, which accounts for the easy
cleavage of the crystals. H 3 BO 3 molecules 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 accepting hydroxide ions:
B(OH) 3 + H 2 O ˆ B(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 manufacture of glass (borosilicate glass),
glazes and enamels, leather, paper,
adhesives, and explosives. It is widely
used (particularly in the USA) in detergents, and because of the ability of
fused boric acid to dissolve other
metal oxides it is used as a Ûux in
brazing and welding. Because of its
mild antiseptic properties it is used
in the pharmaceutical industry and
as a food preservative.
boric anhydride See boron(iii)
oxide.
boric oxide See boron(iii) oxide.
boride A 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 metallike conductivity. They can be made
by direct combination of the elements at high temperatures (over
2000°C) or, more usually, by hightemperature reduction of a mixture
79
boride
b
www.AzShimi.ir www.AzShimi.com
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 antiseptic 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 (Üreretardant additive in plastics), and
boron phosphate (heterogeneous acid
catalyst in the petrochemicals industry).
borax-bead test A simple laboratory 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 platinum wire. Certain metals can be
identiÜed by the colour of the bead
produced in the oxidizing and reducing parts of a Bunsen Ûame. For example, iron gives a bead that is red
when hot and yellow when cold in
the oxidizing Ûame and a green bead
in the reducing Ûame.
borazon See boron nitride.
Bordeaux mixture A mixture of
copper(II) sulphate and calcium hydroxide in water, used as a fungicide.
boric acid Any 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 occurs 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 structure, which accounts for the easy
cleavage of the crystals. H 3 BO 3 molecules 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 accepting hydroxide ions:
B(OH) 3 + H 2 O ˆ B(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 manufacture of glass (borosilicate glass),
glazes and enamels, leather, paper,
adhesives, and explosives. It is widely
used (particularly in the USA) in detergents, and because of the ability of
fused boric acid to dissolve other
metal oxides it is used as a Ûux in
brazing and welding. Because of its
mild antiseptic properties it is used
in the pharmaceutical industry and
as a food preservative.
boric anhydride See boron(iii)
oxide.
boric oxide See boron(iii) oxide.
boride A 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 metallike conductivity. They can be made
by direct combination of the elements at high temperatures (over
2000°C) or, more usually, by hightemperature reduction of a mixture
79
boride
b
www.AzShimi.ir www.AzShimi.com