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Preparation of Boric Acid. Boric acid can be prepared by the following methods.
(a) From borax. A hot and concentrated solution containing borax is boiled with hydrochloric or sulphuric acid. The solution upon concentration and cooling gives crystals of boric acid.
(b) From Colemanite. Sulphur dioxide gas is passed through the hot concentrated solution of mineral colemanite made in water. The solution upon concentration followed by cooling gives crystals of boric acid. Calcium bisulphite remains in solution as it is highly soluble in water.
(c) From boron compounds by hydrolysis. Certain boron compounds upon boiling withwater (upon hydrolysis) give boric acid.
BCl3 + 3H2O→ H3BO3 + 3HCl
BN + 3H2O → H3BO3 + NH3