Borax [Na₂B₄O₇.10H₂O]

Borax is the sodium salt of tetraboric acid, formulated Na₂B₄O₇.10H₂O.

Preparation. Borax is obtained from colemanite ore by boiling its solution with sodium carbonate: 2Ca₂B₆O₁₁ + 3Na₂CO₃ + H₂O →(Δ)→ 3Na₂B₄O₇ + 3CaCO₃ + Ca(OH)₂.

Although normally written Na₂B₄O₇.10H₂O, borax actually contains the tetranuclear unit [B₄O₅(OH)₄]²⁻, and this is properly called the prismatic form. Borax also exists in two other known forms: jeweller's (octahedral) borax, Na₂B₄O₇.5H₂O, and borax glass, Na₂B₄O₇ (water-free).

Properties. Borax is basic in nature; dissolved in hot water it hydrolyses to give boric acid and sodium hydroxide, which is why its hot aqueous solution is alkaline: Na₂B₄O₇ + 7H₂O → 4H₃BO₃ + 2NaOH. On heating, it first loses its water of hydration and then breaks down further into a clear, transparent bead: Na₂B₄O₇.10H₂O →(Δ, -10H₂O)→ Na₂B₄O₇ → 2NaBO₂ + B₂O₃. Borax reacts with acids to precipitate the sparingly soluble boric acid, e.g. Na₂B₄O₇ + 2HCl + 5H₂O → 4H₃BO₃ + 2NaCl and Na₂B₄O₇ + H₂SO₄ + 5H₂O → 4H₃BO₃ + Na₂SO₄. Treated with ammonium chloride, it gives boron nitride: Na₂B₄O₇ + 2NH₄Cl → 2NaCl + 2BN + B₂O₃ + 4H₂O. …