How is orthoboric acid prepared?
Orthoboric acid is also called boric acid (H3BO3) and prepared as follows:
(i) From borax: It may be prepared by adding an excess of sulphuric acid or hydrochloric acid to a hot saturated solution of borax.
On cooling the reaction mixture, crystals of boric acid separate out.
(ii) From colemanite: The mineral is powdered, mixed with boiling water and sulphur dioxide is passed through the mixture.
On cooling, the reaction mixture, crystals of boric acid separate out.
(iii) From colemanite: The mineral is powdered, mixed with boiling water and sulphur dioxide is passed through the mixture. 
On crystallisation, boric acid separates out while calcium sulphite remains in solution.
Boric acid can be purified by recrystallization.
(iv) From boron halides: When boron halides are hydrolysed, orthoboric acid is formed. 
