What is the Structure of Borax ?

Structure of Borax

In borax, two boron atoms are in triangular geometry and two boron atoms are in tetrahedral geometry. The ion is [B₄O₅(OH)₄]²⁻ and the remaining eight water molecules are associated with the two sodium ions. Hence, the borax contains tetranuclear units [B₄O₅(OH)₄]²⁻ and therefore is formulated as Na₂[B₄O₅(OH)₄].8H₂O.Check out Chemistry Formulas and NCERT Solutions for class 11 Chemistry prepared by Physics Wallah. 

Detail structure of Borax

Structure and Use of Borax

Borax occurs naturally as tincal (contains about 55% borax) in certain inland lakes of India, Tibet and California (U.S.A.).Borax can also be prepared from certain other minerals such as boracite, colemanite and boranatrocalcite. The minerals are powdered and boiled with sodium carbonate solution.

Ca₂B₆O₁₁ + 2Na₂CO₃ → 2CaCO₃  + Na₂B4O₇ + 2NaBO₂

Borax is crystallised from the filtrate. Sodium metaborate, present in the mother liquor, is converted into borax by passing carbon dioxide through it

4NaBO₂  + CO₂  →  Na₂B4O₇  +  Na₂CO₃

sod. metaborate

Borax is also obtained by the action of Na₂CO₃ on orthoboric acid.

Two important hydrates of borax are known. These are,

• Pentahydrate, Na₂B₄O₇.5H₂O.
• Decahydrate or monoclinic borax, Na₂B₄O₇.10H₂O.

The pentahydrate is obtained when the solution is crystallised at above 60°C, while the monoclinic variety is obtained when crystallisation is carried out below 60°C. Both the varieties on heating form anhydrous borax.

Properties of Borax:

Borax is a colourless, crystalline solid sparingly soluble in cold water but dissolves readily in hot water.

Borax is used,

1. For manufacturing enamels, glazes and optical glass.

2. As a flux for soldering and welding.

3. As an analytical reagent, e.g. in borax bead test.

4. In borax bead test

5. In purifying gold