Preparation and uses of borax

Borax chemical formula and structure

Borax, commonly known as sodium tetraborate decahydrate is an important compound of born and broadly uses in our daily life. Two BO3 and two BO4 units form the closed anionic structure of Na2B4O7, 10H2O. Thus the chemical formula of borax

Na2B4O7, 10H2O
or Na2[B4O5(OH)4], 8H2O

Structure of borax

Borax structure, preparation and properties
Structure of Borax

It has two tetrahedral BO4 and two planner BO3 units. Each BO4 shares two oxygen with two BO3 and one oxygen with the second BO4. The fourth oxygen forms the chemical bond to the boron of BO4 unit and hydrogen.

The born in planner BO3 unit is SP2 hybridized but the tetrahedral BO4 unit is SP3 hybridized with an additional electron from sodium. These hybrid orbital overlap with suitable orbitals of oxygen.

Preparation of borax from colemanite

It made by boiling the mineral colemanite with Na2CO3 solution. The produced CaCO3 filtered out and concentration filtrate deposits crystals of borax.

2CaO,3B2O3+2Na2CO3=Na2B4O7+2CaCO3+2NaBO2

Another liquor containing NaBO2 again converted to borax by passing CO2.

NaBO2 + CO2 = Na2B4O7 + Na2CO3

Borax also obtained by heating the boric acid solution with NaCO3 and specific heat. It can be crystalline by cooling the solution.

4H3BO3 + Na2CO3 → Na2B4O7 + 6H2O + CO2↑

Physical and chemical properties

Physical properties of borax

  1. It is a white crystalline solid with soluble in water mainly hot water.
  2. At normal temperature, it contains ten molecule crystal water but at 62oC it contains five molecule crystal water.

Chemical properties of borax

An aqueous solution reacts alkaline due to hydrolysis to NaOH and H3BO3. Since boric acid is a weak acid and tritiated against a strong acid using methyl orange (pH scale 3.1 to 4.4)

Na2B4O7 +2HCl + 5H2O = 2NaCl + 4H3BO3

Borax bead test reaction

On heating borax first swells due to losing of hydrated water. Anhydrous borax on further heating produced metaborate and boron trioxide.

Na2B4O7, 10H2O → 2NaBO2 + B2O3 + 10H2O

But the fusion with metal oxides forms the bead of metal borates having characteristic glossy color. Thus this bead test used for the quantitative inorganic analysis metals, known as the borax bead test.

  1. When fusion with metal salts heated in oxidizing flame forms ic-metaborate with a characteristic glossy color.
  2. But when fusion with metal salts heated in reducing flame forms us-metaborate with a characteristic glossy color.
Metal compound Color of the bead
Oxidizing flame Reducing flame
Copper Green, blue Dull red
Iron Yellow Dark green
Cobalt Dark blue Dark blue
Nickel Brown colorless
Chromium Green Green

Reaction with copper sulfate

CuSO_{4},5H_{2}O\xrightarrow{Heat}CuSO_{4}+H_{2}O

2CuSO4 → 2CuO + 2SO2 + O2

Fusion this metal oxide with the borax bead in an oxidizing flame forms blue cupric metaborate but in reducing flame forms dull red cuprous metaborate.

CuO + B2O3 → Cu(BO2)2
2Cu(BO2)2 + C → Cu2(BO2) + CO + B2O3

Uses of borax in daily life

  1. Na2B4O7 is extensively used in the manufacture of optical and hard glasses.
  2. It used as an antiseptic in medicine and manufacturing of enamel.
  3. Due to the low solubility of calcium and magnesium borates, it used as a water softener.
  4. Borax uses in giving glaze to pottery, stiffening candle-wicks and in laundering.