A colourless monoclinic solid, Na2B4O7.10H2O, soluble in water and very slightly soluble in ethanol; monoclinic; r.d. 1.73; loses 8H2O at 75°C; loses 10H2O at 320°C. The formula gives a misleading impression of the structure. The compound contains the ion [B4O5(OH)4]2− (see borate). Attempts to recrystallize this compound above 60.8°C yield the pentahydrate. The main sources are the borate minerals kernite (Na2B4O7.4H2O) and tincal (Na2B4O7.10H2O). The ores are purified by carefully controlled dissolution and recrystallization. On treatment with mineral acids borax gives boric acid.
Borax is a very important substance in the glass and ceramics industries as a raw material for making borosilicates. It is also important as a metallurgical flux because of the ability of molten borates to dissolve metal oxides. In solution it partially hydrolyses to boric acid and can thus act as a buffer. For this reason it is used as a laundry pre-soak. It is used medicinally as a mild alkaline antiseptic and astringent for the skin and mucous membranes.
Disodium tetraborate is the source of many industrially important boron compounds, such as barium borate (fungicidal paints), zinc borate (fire-retardant additive in plastics), and boron phosphate (heterogeneous acid catalyst in the petrochemicals industry).