Boric acid can refer collectively to three different compounds. Orthoboric acid [B(OH)3] consists of a central boron (B) atom attached to three hydroxide (OH) ions. The term "boric acid" typically refers to boric acid unless otherwise specified. The other two forms of boric acid are metaboric acid (HBO2) and tetraboric acid (H2B4O7).
An acid can be added to an aqueous solution of borax (Na2B4O7) to form a salt and boric acid. Borax normally occurs as its decahydrate (Na2B4O7.10H2O), so the acid breaks the boron-oxygen bond and replaces it with a boron-hydroxide bond. The additional hydroxide ions are supplied by the water molecules, resulting in an aqueous solution of a salt and boric acid. The solution is cooled after the reaction is complete, which causes the boric acid to drop out of the solution in the form of solid white flakes. The boric acid can then be separated from the aqueous salt solution by mechanical means. This general reaction can be shown as aqueous Na2B4O7.10H2O + Acid -> Aqueous salt + Water + Boric acid.
The specific equation for hydrochloric acid can be shown as Na2B4O7.10H2O + 2HCl -> 2NaCl(aq) + 5H2O + 4B(OH)3. Here is a similar reaction using sulfuric acid (H2SO4): Na2B4O7.10H2O + H2SO4 -> Na2SO4(aq) + 5H2O + 4B(OH)3.
Boric acid can also be obtained by dissolving various boron compounds in water. These compounds are extremely sensitive to water and will eventually hydrolyze into boric acid. For example, boron nitride and water will form boric acid and ammonia, as shown in the following equation: BN + 3H2O -> B(OH)3 + NH3. Similar reactions occur with boron compounds, such as boron sulfide (B2S3), as follows: B2S3 + 3H2O -> 2B(OH)3 + 3H2S. Here is a similar reaction involving boron chloride (BCl3): BCl33 + 3H2O -> B(OH)3 + 3HCL.
Orthoboric acid can be heated to 102 degrees Celsius to yield metaboric acid as follows: B(OH)3 -> HB02 + H2O. Furthermore, metaboric acid can be heated to 162 degrees Celsius to yield tetraboric acid as follows: 4HBO2 -> H2B407 + H2O.