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boric acid,any one of the three chemical compounds, orthoboric (or boracic) acid, metaboric acid, and tetraboric (or pyroboric) acid; the term often refers simply to orthoboric acid. The acids may be thought of as hydrates of boric oxide, B2O3. Orthoboric acid, H3BO3 or B2O3·3H2O, is colorless, weakly acidic, and forms triclinic crystals. It is fairly soluble in boiling water (about 27% by weight) but less so in cold water (about 6% by weight at room temperature). When orthoboric acid is heated above 170°C; it dehydrates, forming metaboric acid, HBO2 or B2O3·H2O. Metaboric acid is a white, cubic crystalline solid and is only slightly soluble in water. It melts at about 236°C;, and when heated above about 300°C; further dehydrates, forming tetraboric acid, H4B4O7 or B2O3·H2O. Tetraboric acid is either a vitreous solid or a white powder and is water soluble. When tetraboric or metaboric acid is dissolved it reverts largely to orthoboric acid. The major uses of the boric acids are in forming other boron compounds and in borate salts, e.g., boraxborax
or sodium tetraborate decahydrate
, chemical compound, Na2B4O7·10H2O; sp. gr. 1.73; slightly soluble in cold water; very soluble in hot water; insoluble in acids.
..... Click the link for more information. . A dilute water solution of boric acid is commonly used as a mild antiseptic and eyewash. Boric acid is also used in leather manufacture, electroplating, and cosmetics. Boric acid can be crystallized from an acidified borax solution. It occurs as the mineral sassolite in the Tuscan region of Italy, where it is also recovered from hot springs and vapors. In the United States boric acid is recovered from brines from Searles Lake in California.
(also orthoboric acid), H3BO3, a weak inorganic tribasic acid; colorless crystals in the form of flakes. Density, 1.48 g/cm3. Moderately soluble in cold water and more soluble in hot water, which is used to purify it (solubility per 100 g H2O: 2.66 g at 0° C; 39.7 g at 100° C). Boric acid is soluble in alcohol and other organic substances. When heated it loses water, turning first into metaboric acid, HBO2, and then into boric anhydride, B2O3. The salts of boric acid—borates—are derived mainly from various polyboric acids with the general formula nB2O3 · mH2O. When boric acid reacts with alcohols in the presence of concentrated H2SO4 (for bonding water), esters—for example, boric methyl ester—are formed:
3CH3OH + H3BO3 = B(OCH3)3 + 3H2O
When ignited, the esters of boric acid burn with a green flame, which is a qualitative reaction for boron. Boric acid is found in nature in hot springs as a dissolved form and as vapor. It evolves in the form of the mineral sassolite from hot springs and excrustations in volcanic craters. The industrial significance of natural boric acid is relatively small; it is usually obtained during the processing of borates.
Large quantities of boric acid are used to produce enamel wares. In laboratory practice boric acid is used in the preparation of buffer systems. In medicine it is used as an antiseptic. Aqueous solutions of boric acid serve as mouth and throat rinses and eyewashes. Ointments, pastes, and powders with boric acid and boric alcohol (boric acid dissolved in alcohol) are administered for some skin diseases (boric alcohol is also prescribed as ear drops). Boric acid is an ingredient in contraceptives.
V. L. VASILEVSKII