Borax

Borax, $\text{Na}_2\text{B}_4\text{O}_7\!\cdot\!10\text{H}_2\text{O}$, is by far the most important naturally occurring borate mineral and the starting point for almost all commercial boron chemistry, including the preparation of boric acid and boron oxide described in the previous section. Its formula is more informatively written as $\text{Na}_2[\text{B}_4\text{O}_5(\text{OH})_4]\!\cdot\!8\text{H}_2\text{O}$, which reveals the structure of the tetraborate anion itself: it contains four boron atoms, of which two are three-coordinate (planar, $sp^2$-hybridised $\text{BO}_3$ triangular units, exactly as in boron oxide) and two are four-coordinate (tetrahedral, $sp^3$-hybridised $\text{BO}_4$ units). These four boron centres are linked together through shared bridging oxygen atoms into a single closed ring-like unit, $[\text{B}_4\text{O}_5(\text{OH})_4]^{2-}$, with the two sodium ions and the water of crystallisation completing the salt. This mixed triangular/tetrahedral boron coordination — two of each — is characteristic of borax and distinguishes it from simpler, single-boron borate ions.

When an aqueous solution of borax is heated (or simply dissolved), it hydrolyses, because it is the salt of a strong base (NaOH) and a weak acid (boric acid). The tetraborate ion reacts with water to liberate boric acid while releasing hydroxide ion, so an aqueous borax solution is characteristically alkaline (basic), a property exploited in its traditional use as a mild cleaning agent and water softener, as well as a buffer in some laboratory and industrial applications. …