BORAX

Salts of boric acids (H₃BO₃) are called borates. Borax is an important borate. Borax is a sodium salt of tetraboric acid (H₂B₄O₇). Ten molecules of water, as water of crystallization are present in borax compound. So, borax is sodium tetraborate decahydrate [Na₂B₄O₇.10H₂O].

Preparation of Borax

(1) Borax from boric acid (H₃BO₃) ---

(i) Borax is obtained, when boric acid (H₃BO₃) is boiled with sodium carbonate (Na₂CO₃) solution- 

4H₃BO₃ + Na₂CO₃ = Na₂B₄O₇ + CO₂ + 6H₂O

(ii) Borax is obtained, when boric acid (H₃BO₃) is boiled with excess sodium hydroxide (NaOH) solution---

4H₃BO₃ + 2NaOH + 3H₂O = Na₂B₄O₇.10H₂O

(2) Borax from colemanite (2CaO, 3B₂O₃) ---

Borax is obtained, when colemanite (2CaO, 3B₂O₃) is boiled with sodium carbonate (Na₂CO₃) solution ---

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

At first, precipitate calcium carbonate (CaCO₃) is filtered out and then concentrated the remaining filtrate. As a result, crystals of borax are deposited. NaBO₂ present in the mother liquor is converted to borax by passing CO₂.

4NaBO₂ + CO₂ = Na₂B₄O₇ + Na₂CO₃

Structure of Borax

The crystal structure study of borax has indicated that, two H₂O molecules participate in the formation of borax anion. So, borax is now represented as----

Na₂[B₄O₅(OH)₄].8H₂O

The anion of borax [B₄O₅(OH)₄]^2- is made up of two trigonal planer BO₃ units and two tetrahedral BO₄ units. The B atom of BO₃ unit is sp² hybridised and each B atom of BO₃ unit forms three B―O sigma(σ) bonds. The B atom of BO₄ unit is sp³ hybridised. one extra electron is present in each BO₄ units and each B atom of BO₄ unit forms four B―O sigma (σ) bonds. Four OH groups linked with four B atoms. A bridge bond is formed among two B atoms and one O atom. So, the structure of the borax anion [B₄O₅(OH)₄]^2- is as follows---

Properties of Borax

White crystalline solid borax exists in three different crystal structure.

(1) Monoclinic or prismatic borax---

The formula of monoclinic or prismatic borax is Na₂B₄O₇.10H₂O, it is the most common form of borax.

(2) Octahedral or jeweller’s borax---

The formula of octahedral or jeweller’s borax is Na₂B₄O₇.5H₂O. The octahedral form of borax (Na₂B₄O_7.5H₂O) is obtained, when monoclinic form of borax (Na₂B₄O₇.10H₂O) is heated above 60⁰C.

(3) Borax glass or anhydrous borax----

The formula of borax glass or anhydrous borax is Na₂B₄O₇. The anhydrous borax (Na₂B₄O₇) is obtained when monoclinic form of borax (Na₂B₄O₇.10H₂O) is heated above its melting point.

In water borax gets hydrolysed, and gives a mixture of strong base NaOH and weak acid H₃BO₃. Since boric acid (H₃BO₃) is weak so, the aqueous solution of borax is alkaline.

Na₂B₄O₇ + 7H₂O <=====> 2NaOH + 4H₃BO₃

On heating borax first swells losing water molecules. On further heating it gives NaBO₂ and B₂O₃. A glassy bead is formed.

Na₂B₄O₇.10H₂O = 2NaBO₂ + B₂O₃ + 10H₂O

Borax bead test-----

When this glassy bead (B₂O₃) is heated strongly with coloured metal salts, specific coloured beads are obtained. This offers a means of identifying certain metal ions like Cu, Co, Cr, Ni, Fe etc.

CuSO₄ = CuO + SO₃

CuO + B₂O₃ = Cu(BO₂)₂

This gives green colour in hot condition and blue colour in cold condition.

When this Cu(BO₂)₂ is again heated in reducing flame it becomes dull red and opaque.

2Cu(BO₂)₂ (Cu = +2) + C ----> 2CuBO₂ (Cu = +1) + B₂O₃ + CO

2CuBO₂ (Cu = +1) + C ----> 2Cu (Cu = 0) + B₂O₃ + CO

Reaction of borax with conc HCl or H₂SO₄ forms boric acid---

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

Na₂B₄O₇ + H₂SO₄ + 5H₂O = Na₂SO₄ + 4H₃BO₃

When borax is heated with ammonium chloride, boron nitride is formed---

Na₂ [B₄O₅(OH)₄] + 2NH₄Cl ----> 2NaCl + 2BN + B₂O₃ + 6H₂O

Uses of Borax

Borax is used in the manufactured of enamels, washing powders, various types of glasses such as optical and hard glasses. Borax is also used in making antiseptic.