Boric acid is also called “orthoboric acid”. Chemical formula H3BO3. Molecular weight 61.83. White triclinic scales with glossy crystals. The relative density is 1.43514, and the refractive index is 1.337, 1.461, and 1.462. Soluble in water, ethanol and glycerin. The aqueous solution is acidic and is a monovalent weak acid (Ka=6×10-10). Solubility in water increases with temperature. Heated to 169℃ (±1℃), dehydration will produce metaboric acid, and 300℃ will form boric anhydride. Neutralize with strong alkali to obtain metaborate. Tetraborate is obtained under weakly alkaline conditions.
The source of acidity does not itself donate protons. Since boron is an electron-deficient atom, it can add hydroxide ions of water molecules and release protons. Taking advantage of this electrically deficient property, polyhydroxy compounds (such as mannitol and glycerin, etc.) are added to form a stable complex to strengthen its acidity. It reacts with methanol or ethanol to generate volatile borate esters, which produce green flames when burned. This property can be used to identify borate radicals.
Boron oxide hydrate. There are mainly tetraboric acid H2B4O7, metaboric acid (HBO2)n and orthoboric acid H3BO3 (also called orthoboric acid). Among them, orthoboric acid has the most water content and is the most stable. Orthoboric acid, also referred to as boric acid, is a white shiny flaky crystal with a solubility of 2.6g at 0°C and 37g at 107°C, so it can be purified by recrystallization. Boric acid can be produced by applying sulfuric acid to the hot solution of borax:
Na2B4O7+H2SO4+5H2O=Na2SO4+4H3BO3
It can crystallize when cooled.
Northeast my country is rich in boron magnesium ore MgB2O5·H2O or 2MgO·B2O5·H2O. Boric acid can also be obtained by directly decomposing boronite with sulfuric acid: Mg2B2O5·H2O+2H2SO4=2MgSO4+2H3BO3
Boric acid
Physical and chemical properties
Colorless and transparent triclinic flake crystals, or white crystalline powder. Shiny. Has a slight bitter taste. It feels greasy in contact with skin. Odorless. Relative molecular mass 61.83. Relative density 1.435 (15℃). The melting point is 170±1°C (decomposition), and transforms into metaboric acid HBO2. At 300°C, 1 (1/2) molecule of water is lost to form boron anhydride B2O3. Soluble in water, ethanol and glycerol. Solubility in water: 2.77 at 0℃, 3.65 at 10℃, 4.87 at 20℃, 5.74 at 25℃, 6.77 at 30℃, 8.90 at 40℃, 11.39 at 50℃, 14.89 at 60℃, 23.54 at 80℃, 38.00 at 100℃. The weakly acidic aqueous solution is a typical Lewis acid, which can promote solubility in water in the presence of hydrochloric acid, citric acid or tartaric acid. Boric acid can react with methanol to form an ester (trimethyl borate), which is volatile. Burns beautifully green.
3CH3OH+H3BO3=(CH3O)3B+3H2O
However, when reacting with polyol, an ester-type coordination ion with two hydroxyl groups is generated at the cis position, which is highly acidic. In titration, boric acid can be quantitatively detected. Rat oral LD505140mg/kg.
Boric acid
Preparation
① In industry, boron magnesium ore is used as raw material and decomposed with sulfuric acid to obtain crude product [1]. Then the crude product [1] is repeatedly recrystallized to obtain refined product [1]. Nitrophenylboric acid Mg2B2O5·H2O+2H2SO4→2[1]+2Mg di-n-butyl carbonate SO4
② Use borax as raw material, dissolve it in hot water, filter it while hot, remove insoluble matter, add 25 % hydrochloric acid, concentrate, cool, filter, wash the crystals with cold water, then wash with ethanol until there is no chloride, and then recrystallize, it can be obtained [1].
Na2B4O7·10H2O+2HCl→4[1]+2NaCl+5H2O
Boric acid
use
Boric acid reacts with methanol or ethanol to form volatile trimethyl borate or triethyl borate: H3BO3+3C2H5OH=B(OC2H5)3+3H2O
According to this reaction principle, boric acid or borate can be made from acid-soluble separated from minerals. When trimethyl borate (or triethyl borate) burns, it generates a green flame. This characteristic can be used to qualitatively identify the presence of boric acid or its salts.
Boric acid aqueous solution has weak acidity and bactericidal ability. It is used as a disinfectant and preservative in the pharmaceutical and food industries, papermaking and tanning industries. Boric acid is also used in the manufacture of enamel and glaze, and is also an important ingredient in the manufacture of glass fiber. One of the raw materials.
Boric acid is a weak acid. It has a weak antibacterial effect and is less irritating. It can also be used for the antisepsis of skin, mucous membranes and wounds. Boric acid powder or ointment is used medically to treat eczema and diaper rash. Talcum powder is used daily, and cockroach-killing pills also contain boric acid.
Boric acid
Antiseptics
Boric acid is an antiseptic and has a weak bacteriostatic effect on bacteria and fungi, but has no bactericidal effect. This product exerts its antibacterial effect by releasing hydrogen ions. Little irritation, no tissue damage.
[Function and use] This product can release hydrogen ions and has antibacterial effect. It is a weak preservative and has no obvious irritation. It can be formulated with neutral substances into powder, ointment, cream, etc. Can clean and lubricate the skin; soften scabs.
[Usage and dosage] 3% boric acid solution is used for wet compresses and cleaning; 5% to 10% boric acid ointment is used for scabs and scaly skin diseases; dusting powder is used for mild dermatitis mainly erythema and papules.
【Preparation Specifications】External preparation: 3% boric acid solution. 5% to 10% boric acid ointment. Face powder (10% boric acid, 20% zinc oxide, 70% talc).
[Precautions] 1. Long-term external use of large doses of this product can cause absorption poisoning, manifested as nausea, vomiting, abdominal pain, diarrhea, headache, visual impairment, hair loss and renal damage. 2. Not for use by infants and young children.
