boric acid

boric acid

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℃ 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℃, and when heated above about 300℃ 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., borax. 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.

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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Boric Acid

 

(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

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

boric acid

[¦bȯr·ik ′as·əd]

(inorganic chemistry)

H₃BO₃ An acid derived from boric oxide in the form of white, triclinic crystals, melting at 185°C, soluble in water. Also known as boracic acid; orthoboric acid.

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

boric acid

1. a white soluble weakly acid crystalline solid used in the manufacture of heat-resistant glass and porcelain enamels, as a fireproofing material, and as a mild antiseptic. Formula: H₃BO₃

2. any other acid containing boron

Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005