Boric Acid [H₃BO₃ or B(OH)₃]

Boric acid, H₃BO₃ or B(OH)₃, can be extracted from either borax or colemanite.

Preparation. From borax: Na₂B₄O₇ + H₂SO₄ + 5H₂O → 4H₃BO₃ + Na₂SO₄. From colemanite: Ca₂B₆O₁₁ + 11H₂O + 4SO₂ → 2Ca(HSO₃)₂ + 6H₃BO₃.

Properties. Boric acid forms colourless, transparent crystals. It is a very weak monobasic acid, and unusually, it does not behave as a Brønsted acid at all -- instead of donating a proton itself, it accepts a hydroxyl ion from a surrounding water molecule, releasing a proton in the process: B(OH)₃ + 2H₂O ⇌ H₃O⁺ + [B(OH)₄]⁻. It reacts with sodium hydroxide to give sodium metaborate or, with excess acid, sodium tetraborate: H₃BO₃ + NaOH → NaBO₂ + 2H₂O and 4H₃BO₃ + 2NaOH → Na₂B₄O₇ + 7H₂O.

Action of heat. Boric acid dehydrates in three stages on progressively stronger heating: at 373 K to metaboric acid (4H₃BO₃ →(373 K)→ 4HBO₂ + 4H₂O), at 413 K on to tetraboric acid (4HBO₂ →(413 K)→ H₂B₄O₇ + H₂O), and at red heat all the way to boric anhydride, a glassy mass (H₂B₄O₇ →(red hot)→ 2B₂O₃ + H₂O).

Action of ammonia. Fused with urea in an ammonia atmosphere at 800-1200 K, boric acid gives boron nitride: B(OH)₃ + NH₃ →(Δ)→ BN + 3H₂O.

Ethyl borate test. Heated with ethyl alcohol in the presence of conc. sulphuric acid, boric acid (or any borate salt) forms the ester triethyl borate, whose vapour burns with a distinctive green-edged flame -- the standard test used to identify a borate: H₃BO₃ + 3C₂H₅OH →(conc. H₂SO₄)→ B(OC₂H₅)₃ + 3H₂O. (This trialkyl borate, on further reaction with sodium hydride in tetrahydrofuran, gives the coordination compound Na[BH(OR)₃], a powerful reducing agent.)

Formation of boron trifluoride. Boric acid reacts with calcium fluoride in the presence of conc. sulphuric acid to give BF₃: 3CaF₂ + 3H₂SO₄ + 2B(OH)₃ → 3CaSO₄ + 2BF₃ + 6H₂O. Heated with soda ash, boric acid regenerates borax: Na₂CO₃ + 4B(OH)₃ → Na₂B₄O₇ + CO₂ + 6H₂O. …