Boric acid may be prepared by reacting borax (sodium tetraborate decahydrate) with a mineral acid, such as hydrochloric acid:
Na2B4O7·10H2O + 2 HCl → 4 B(OH)3 [or H3BO3] + 2 NaCl + 5 H2O
It is also formed as a by product of hydrolysis of boron trihalides and diborane:[3]
B2H6 + 6 H2O → 2 B(OH)3 + 6 H2
BX3 + 3 H2O → B(OH)3 + 3 HX (X = Cl, Br, I)