Borax (Na2B4O7)

Borax, Na2B4O7, is one of the most important compounds of boron. The crystalline solid is usually written as Na2B4O7.10H2O, though its structural formula is more accurately given as Na2[B4O5(OH)4].8H2O. It occurs naturally as the mineral tincal (about 90% borax) in certain inland lakes of India, Tibet and California, and is also prepared industrially from the mineral colemanite (Ca2B6O11) by boiling it with a sodium carbonate solution, giving borax alongside sodium metaborate and calcium carbonate as by-products: Ca2B6O11 + 2Na2CO3, heated, → Na2B4O7 + 2NaBO2 + 2CaCO3. Borax is a white crystalline solid that dissolves in water to give an alkaline solution because it hydrolyses, releasing orthoboric acid and sodium hydroxide: Na2B4O7 + 7H2O → 2NaOH + 4H3BO3; this hydrolysis reaction also serves as a test for, and a means of removing, borate ion in qualitative analysis. On heating, borax first loses its water of crystallisation and swells, then, on further heating, melts to a clear liquid that solidifies on cooling into a glassy 'borax bead' made of sodium metaborate and boric anhydride: Na2B4O7.10H2O, heated, loses 10H2O to give Na2B4O7, which on further heating gives 2NaBO2 + B2O3 (the borax bead). This borax bead is put to direct analytical use in the borax bead test: heating borax with a coloured transition-metal compound (for example CoO) on a platinum wire loop in a Bunsen flame produces a characteristically coloured bead — CoO gives a blue Co(BO2)2 bead — which helps identify the metal ion present. Finally, heating borax with ethyl alcohol and concentrated sulfuric acid liberates volatile vapours of triethyl borate, which burn with a distinctive green-edged flame: Na2B4O7 + H2SO4 + 5H2O → Na2SO4 + 4H3BO3, a …