Chemical Knowledge Share – Boric Acid

Thread Content

This post was last edited by sdwfsgyykai on 2019-10-2 at 11:58. The hydrates of boron oxides are called boric acid. Boric acids mainly include tetraboric acid H2B4O7, metaboric acid (HBO2)n, and boric acid H3BO3 (also known as protoboric acid). Among them, boric acid in its positive form has the highest water content and is the most stable. Orthoboric acid, also simply known as boric acid, is a white, shiny flaky crystal that is soluble in hot water. Boric acid can be obtained by treating a hot solution of borax, Na2B4O7, with sulfuric acid: Na2B4O7 + H2SO4 + 5H2O = Na2SO4 + 4H3BO3. Since boric acid is insoluble in cold water, it crystallizes out as the solution cools. The northeast of our country is rich in boromagnesite, Mg2B2O5·H2O or 2MgO·B2O3·H2O; boric acid can also be obtained by directly decomposing this boromagnesite with sulfuric acid: Mg2B2O5·H2O + 2H2SO4 = 2MgSO4 + 2H3BO3. When the boric acid solution is boiled, some of the boric acid evaporates along with water vapor. This explains why the water vapor emitted from the fissures in the strata of volcanic areas contains boric acid. The structural unit of boric acid crystals is the planar triangular \"BO3\" unit, which, in addition to being bonded to one boron atom via covalent bonds, is also bonded to two other hydrogen atoms through both covalent and hydrogen bonds, forming layered macromolecules. Due to the weak binding force of hydrogen bonds, upon heating, the macromolecules are broken down, resulting in an increase in solubility. Boric acid is a monoprotic acid with extremely weak acidity (K=5.8×10-10), similar to silicic acid (K1=2×10-10). When it ionizes in water, it does not lose protons; instead, it combines with the OH- ions present in water: B(OH)3 + 2H2O = H3O+ + B(OH)4-. This mode of ionization is related to the electron-deficient structure of boron compounds. The boron atom in boric acid can act as an acceptor of electron pairs and combine with another OH-, while the oxygen atom in OH- is a typical Lewis acid. When glycerol (glycerin, CH2OH·CHOH·CH2OH) or mannitol (CH2(OH)·(CHOH)4CH2OH) is added to boric acid, its acidity increases. This is because when boric acid combines with glycerol to form a complex anion, it pushes the H+ ions outside, facilitating ionization. Boric acid reacts with methanol or ethanol to produce volatile trimethyl borate or triethyl borate: B(OH)3 + 3C2H5OH = B(OC2H5)3 + 3H2O. Based on this reaction principle, boric acid or borates can be separated from acid-soluble minerals. When trimethyl borate (or triethyl borate) burns, it produces a green flame; this property can be used to qualitatively detect the presence of borates. Aqueous solutions of boric acid have weak acidity and antibacterial properties, which is why they are used as disinfectants and preservatives in the pharmaceutical, food, papermaking, and leather industries. Boric acid is also used in the manufacture of enamel and glazes, and it is one of the important raw materials for producing glass fibers.