Borax is the sodium salt of tetraboric acid. It is also called Tincal. Its chemical name is sodium tetraborate decaHydrate. It is found in Dry lake area of tibet and california. The common name of borax is Suhaga.
Preparation of Borax:
- Borax is prepare by reaction of soda ash with hot solution of boric acid ( H3BO3 ).
4H3BO3 + Na2CO3 → Na2B4O7+6H2O+CO2
2) Bo-rax is prepare by reaction of finely powder Colemanite with boiling solution of Na2CO3 .
Ca2B6O11 + 2Na2CO3 → 2CaCO3 + Na2B4O7 + 2NaBO2
The CaCO3 Settles down as precipitate and clear solution of borax and NaBO2 is taken off. Its crystallization gives fine crystals of borax. We can get more borax by passing CO2 through mother liquor .
4NaBO2 + CO2 → Na2B4O7+ Na2CO3
3) When natural tincal is treated with water and its clear solution is evaporate, Then we get crystal of Borax.
Properties of Borax:
(1) Borax is White crystalline solid. It is less soluble in cold water and more soluble in hot water. At 10 ℃ its solubility is 3g in 100g of water and at 100 ℃ its solubility is 99.3g in 100g of water.
(2) The Saturated solution of borax on crystallization gives two types of crystals. Below 62 ℃ it gives monoclinic crystals of Na2B4O7.10H2O. Above 62 c it gives octahedral crystals of Na2B4O7.5H2O.
When borax is dissolve in water, then it shows hydrolysis. Due to hydrolyse NaOH and H3BO3 are produce. Now NaOH is Strong alkali and H3BO3 is a weak acid , So aqueous solution of borax is alkaline . It turns red Litmus to blue.
Na2B4O7+7H2O → 2NaOH + H3BO3
OR (Strong alkali) (Weak acid)
Na2B4O7 ⇌ 2Na+ +B4-2O7
B4-2O7 + 7H2O ⇌ 4H3BO3 + 2OH–
(4) Borax on heating with NH4Cl gives boron Nitride (BN).
Na2B4O7 + 2NH4Cl → Δ 2NaCl + 2BN+B2O3+4H2O
(5) Reaction with acid:
Borax reacts with acids to form boric acid.
Na2B4O7 + 2HCl + 5H2O → 2NaCl + 4H3BO3
Na2B4O7+ H2SO4+ 5H2O → Na2SO4 + 4H3BO3
(6) Borax Bead Test:
Borax bead test is use to identify the basic radical or cation of a coloured salt. Make a loop at the end of a platinum wire. Heat the loop and dip it into borax. Heat it again. The bo-rax melts and for,ms a transparent bead or glassy mass on the loop. Now dip this bead into coloured salt and re-heat it, First in oxidizing flame and then in reducing flame. A color bead is form on the loop. From this specific coloured bead. We identify the cation of the coloured salt.
Chemistry of borax bead test:
Bo-rax on heating forms sodium metaborate and B2O3 (Boric anhydride).
Na2B4O7 → 2NaBO3 (In the presence of heat)
The coloured salt decomposes into its oxide .
CuSO4.5H2O → CuO + SO3 + 5H2O
The metal Oxide reacts with B2O3 to form metal metaborate with Specific Colour.
CuO + B2O3 → Cu(BO2)2
Cu(BO2)2 is colour in reducing flame and it is blue color in oxidizing flame.
Uses Of Borax:
- Bo-rax is use in softening of hard Water.
- It is use in making washing powders.
- This is use to prepare borate glass. The Borate Glass is heat resistant.
- It is use in borax bead test of salt analysis.
- It is use as a flux in welding and soldering.
- It’s use in leather industry for tanning and dyeing.
- It is use in metallurgical operations.
- It’s use in preparation of soap, Paints, medicins, match, and textiles.
- It is use as an antiseptic.
- Borax is use as Preservative.
- It is use in cosmetic.
There are four important boric acids.
- Orthoboric acid H3PO3.
- Metaboric acid HBO2.
- Tetraboric acid H2B4O7.
- Pyroboric acid H6B4O9.
Out of these acids orthoboric acid is very stable. All other acids are stable in solid state but in aqueous solution they change into orthoboric acid.
HBO2 + H2O → H3BO3 (Boric acid)
H2B4O7 + 5H2O → 4H3BO3