Borax

Borax contains the tetranuclear unit i.e. [B₄O₅(OH)₄]^2– . The correct formula is Na₂[B₄O₅(OH)₄].8H₂O.

Preparation of Borax

1) From Tincal

Naturally occurring borax is called tincal or suhaga. Tincal obtained from dried up lakes contains 50% borax. It is boiled with water and filtered to remove insoluble impurities of clay ,sand. The filtrate is concentrated when crystals of borax separate out.

2) From Colemanite

The mineral colemanite is finely powdered and is boiled with sodium carbonate solution.

Ca₂B₆O₁₁ + 2 Na₂CO₃ ——–> Na₂B₄O₇ + 2 NaBO₂ + 2 CaCO₃ ↓

The precipitate of calcium carbonate thus formed is removed by filtration. The filtrate is concentrated and cooled when crystals of borax separate out. Sodium metaborate present in the mother liquor can be converted into borax by passing a current of carbon dioxide through it.

3) From Boric Acid

Borax can also be prepared in small amounts by neutralising boric acid with sodium carbonate.

On cooling crystal of borax i.e. Na₂B₄O₇.10H₂O separate out.

 

Properties of Borax

1) It is a white crystalline solid, less soluble in cold water but more soluble in hot water.

2) The aqueous solution of borax is alkaline due to hydrolysis. Borax is therefore, used as a water softener and cleaning agent.

Na₂B₄O₇ + 2 H₂O ———-> 2 NaOH + H₂B₄O₇

H₂B₄O₇ + 5 H₂O ———-> 4 H₃BO₃

Action of heat- borax Bead Test

Borax  loses its water of crystallisation and swells up to form a puffy mass. On further heating , it melts into a clear liquid which solidifies to a transparent glass like bead which consists of sodium metaborate (NaBO₂) and boric anhydride(B₂O₃)

Na₂B₄O₇.10H₂O ————–> Na₂B₄O₇ + 10 H₂O

Na₂B₄O₇ ————-> 2 NaBO₂ + B₂O₃

The glassy bead is commonly known as borax bead and is employed in qualitative analysis for the detection of certain coloured basic radicals such as Ni²⁺ , Co²⁺, Cr³⁺, Cu²⁺, Mn²⁺ . Whenever a coloured salt containing these cations is heated with borax bead on a platinum wire , the salt decomposes to form the corresponding metal oxide which then combines with B₂O₃ present in the glassy bead to form coloured metaborates. This test is called borax bead test.

CoSO₄ ———> CoO + SO₃

CoO + B₂O₃ ——> Co(BO₂)₂

NiO + B₂O₃ ———-> Ni(BO₂)₂

Cr₂O₃ + B₂O₃ ———-> 2 Cr(BO₂)₃

MnO + B₂O₃ ———-> Mn(BO₂)₂

CuO + B₂O₃ ———-> Cu(BO₂)₂

Action of Sodium Hydroxide

On adding a calculated quantity of sodium hydroxide to borax , sodium metaborate is formed.

Na₂B₄O₇ + 2 NaOH ———–> Na₂BO₂ + H₂O

Action of Sulphuric Acid

On adding a calculated quantity of concentrated sulphuric acid to a hot concentrated solution of borax, boric acid is produced.

Na₂B₄O₇ + H₂SO₄ ———> Na₂SO₄ + H₂B₄O₇

H₂B₄O₇ + 5 H₂O ——-> 4 H₃BO₃

Action of Ethyl Alcohol and Sulphuric Acid

On heating borax with ethyl alcohol and concentrated sulphuric acid , vapours of triethylborates are produced. When ignited these vapours burn with a green edged flame.

Na₂B₄O₇ + H₂SO₄ +5 H₂O ———-> Na₂SO₄ + 4 H₂BO₃

H₃BO₃ + 3 C₂H₅OH ——-> B(OC₂H₅)₃ + 3 H₂O

Uses of Borax

Borax is used in:

1) in the manufacture of enamels and glazes for earthen ware. The glazed surface is resistant to heat ,stains and scratches.

2) as a flux in soldering

3) in the preparation of medicinal soaps due to its antiseptic properties.

4) in the manufacture of heat resistant Borosilicate glass.

5) to make peroxoborate- an important cleansing and bleaching agent present in washing powder.

6) as a stiffening agent for candle wicks.

7) to produce a good finish in laboratory.

8) in the laboratory for borax bead test.

9) in softening of water.