Group 13 - Boron Group (B, Al, Ga, In, Tl): Extraction, Properties

Group 13 - Boron Group (B, Al, Ga, In, Tl)

Boron does not occur in the free state in nature. In the combined state, it occurs mainly in the form of the salts of boric acid.

Ores of Boron

i)       Boric acid   H₃BO₃

ii)      Borax         Na₂B₄O₇.10H₂O

Extraction

On a large scale, boron is extracted from its minerals, borax Na₂B₄O₇ or colemanite Ca₂B₆O₁₁. The latter is first converted to borax by boiling with a solution of sodium carbonate in the requisite proportion.

2Ca₂B₆O₁₁ + 3Na₂CO₃ + H₂O -- >  3Na₂B₄O₇ + 3CaCO₃ + Ca(OH)₂ The insoluble calcium carbonate settles down and borax is crystallized

from the mother liquor. Boron is isolated from borax in the following two steps.

a) Preparation of boron trioxide:- Borax is treated with hot concentrated hydrochloric acid, when the sparingly soluble boric acid slowly separates out.

Na2B4O7 + 2HCl  -- >  2NaCl + H2B4O7 H2B4O7 + 5H2O  -- >  4H3BO3

Boric acid is strongly heated when borontrioxide is obtained 2H3BO3  -- >  B2O3 + 3H2O

b) Reduction of borontrioxide:- A mixture of borontrioxide with sodium, potassium or magnesium pieces is heated in a crucible to bright redness. The residual boron is broken up and boiled with concentrated HCl to dissolve out magnesium oxide and excess of boric acid when a dark brown powder of amorphous boron is obtained as a residue: It is washed with water and dried.

B2O3 + 3Mg  -- >  2B + 3MgO.

Pure boron is obtained in the crystalline form by passing a mixture of boron tribromide vapours and hydrogen over electrically heated filament of tungsten at 1470K. It may also be prepared by submitting a mixture of borontrichloride vapour and hydrogen to the action of a high tension electric arc, when boron is obtained on cooling as a hard black amorphous mass.

Physical properties

Boron exists in two allotropic forms amorphous and crystalline boron. Boron is a non-metallic element and is a non-conductor of electricity.

Chemical properties

1.     Action of air:- It is unaffected by air at ordinary temperature but when heated in air to about 975K, it burns forming boron trioxide and a little boron nitride, BN

4B + 3O2   -- >  2B2O3

2B + N2      -- > 2BN

2.     With acids: - Amorphous boron dissolves in hot concentrated sulphuric and in nitric acid to form boric acid.

B + 3HNO3   -- >  H3BO3 + 3NO2

2B + 3H2SO4  -- >  2H3BO3 + 3SO2.

3.     With caustic alkali:- It dissolves in fused caustic alkali and forms boric acid.

4.     As a reducing agent:- Boron is a powerful reducing agent and can even replace carbon from carbon dioxide and silicon from silica.

 3CO2 + 4B  -- >  2B2O3 + 3C 3SiO2 + 4B  -- >  2B2O3 + 3Si

5.     With metals:- It combines with metals (except Cu, Ag and Au) at high temperature in the electric furnace to form borides.

6.     With non-metals:- Boron combines with nitrogen, chlorine, bromine and carbon at higher temperature forming boron nitride, BN, boron trichloride, BCl3, boron tribromide, BBr3 and boron carbide, B4C respectively. Boron carbide is probably the hardest substance known.

Compounds of Boron

Borax (or) Sodium tetraborate, Na₂B₄O₇ - Tincal, a crude form of borax, contains 55% of it and is found in the land dried up lakes of Tibet.

Borax can be prepared

From colemanite:- It is boiled with concentrated solution of sodium carbonate.

Ca₂B₆O₁₁ + 2Na₂CO₃   -- >  2CaCO₃ + Na₂B₄O₇ + 2NaBO₂.

On filtration and concentration, crystals of borax separate. A current of CO2 is passed through the mother liquor to convert the metaborate into borax.

4NaBO₂ + CO₂  -- >  Na₂CO₃ + Na₂B₄O₇

The residual sodium carbonate is used again for the treatment of a fresh quantity of colemanite.

From Tincal - Naturally occurring crude borax (Tincal) is dissolved in water, filtered, concentrated and crystallized when pure borax is obtained.

Properties

 When borax is heated above its melting point until all the water of crystallization is expelled, it forms a colourless glassy substance known as borax glass. It then decomposes to give sodium meta borate and boron (III) oxide.

Na2B4O7.10H2O -- equ-- > ®Na2B4O7 + 10H2O

Na₂B₄O_{7-- - eq}-- > 2NaBO₂ + B₂O₃

When this mixture is fused with metallic oxide it forms characteristic coloured beads. With the help of the colour, the metal ions can be identified. For example

CuO + B₂O₃  -- >  Cu(BO₂)₂.

Uses: Borax is used

1.     to identify the metallic radicals in the qualitative analysis

2.     as a flux in welding metals

3.     in the manufacture of glass, soap and porcelin

4.     as cleaning and dyeing agent in tanneries

5.     as a food preservative.

Borax bead test

A pinch of borax is heated in a platinum loop, it melts to give a colourless glassy bead. It is then dipped in a coloured metallic salt solution and again heated. Characteristic coloured beads are formed. From the colour of the beads, the basic radicals are identified. Due to the formation of metallic metaborate, the characteristic colours are formed.

Example: Copper salts give blue beads

In an oxidising flame

CuSO₄ + B₂O₃  -- >  Cu(BO₂)₂ + SO₃

In a reducing flame

2Cu(BO₂)₂ + C  -- >  2CuBO₂ + B₂O₃ + CO 2CuBO₂ + C  -- >  2Cu + B₂O₃ + CO

Borax bead test is used to identify the coloured salts.