Chemical formula BH3O3
Molar mass 61.83 g·mol−1
Appearance White crystalline solid
Density 1.435 g/cm3
Melting point 170.9 °C (339.6 °F; 444.0 K)
Boiling point 300 °C (572 °F; 573 K)
Solubility in water 2.52 g/100 mL (0 °C)
4.72 g/100 mL (20 °C)
5.7 g/100 mL (25 °C)
19.10 g/100 mL (80 °C)
27.53 g/100 mL (100 °C)
Solubility in other solvents Soluble in lower alcohols
moderately soluble in pyridine
very slightly soluble in acetone
log P -0.29
Acidity (pKa) 9.24, 12.4, 13.3
Conjugate base Borate
Magnetic susceptibility (χ) -34.1·10−6
However, some of its behaviour towards some chemical reactions suggest it to be tribasic acid in the Brønsted sense as well. Boric acid is often used as an antiseptic, insecticide, flame retardant, neutron absorber, or precursor to other chemical compounds.
It has the chemical formula H3BO3 (sometimes written B(OH)3), and exists in the form of colorless crystals or a white powder that dissolves in water. When occurring as a mineral, it is called sassolite
Boric acid is soluble in boiling water. When heated above 170 °C, it dehydrates, forming metaboric acid (HBO2):
H3BO3 → HBO2 + H
Metaboric acid is a white, cubic crystalline solid and is only slightly soluble in water. Metaboric acid melts at about 236 °C, and when heated above about 300 °C further dehydrates, forming tetraboric acid, also called pyroboric acid (H2B4O7):
4 HBO2 → H2B4O7 + H
The term boric acid may sometimes refer to any of these compounds. Further heating (to about 330 °C) leads to boron trioxide.
H2B4O7 → 2 B2O3 + H
There are conflicting interpretations for the origin of the acidity of aqueous boric acid solutions. Raman spectroscopy of strongly alkaline solutions has shown the presence of B(OH)−
4 ion, leading some to conclude that the acidity is exclusively due to the abstraction of OH− from water:
B(OH)3 + H
2O ⇌ B(OH)−
4 + H+ (K = 7.3×10−10; pK = 9.14)
or more properly expressed in the aqueous solution:
B(OH)3 + 2 H
2O ⇌ B(OH)−
4 + H
This may be characterized as Lewis acidity of boron toward OH−, rather than as Brønsted acidity.
Polyborate anions are formed at pH 7–10 if the boron concentration is higher than about 0.025 mol/L. The best known of these is the 'tetraborate' ion, found in the mineral borax:
4 [B(OH)4]− + 2 H+ ⇌ [B4O5(OH)4]2− + 7 H
Boric acid makes an important contribution to the absorption of low frequency sound in seawater.
The primary industrial use of boric acid is in the manufacture of monofilament fiberglass usually referred to as textile fiberglass. Textile fiberglass is used to reinforce plastics in applications that range from boats, to industrial piping to computer circuit boards.
In the jewelry industry, boric acid is often used in combination with denatured alcohol to reduce surface oxidation and firescale from forming on metals during annealing and soldering operations.
Boric acid is used in the production of the glass in LCD flat panel displays.
In electroplating, boric acid is used as part of some proprietary formulas. One such known formula calls for about a 1 to 10 ratio of H3BO3 to NiSO4, a very small portion of sodium lauryl sulfate and a small portion of H2SO4.
Boric acid, mixed with borax (sodium tetraborate decahydrate) at the weight ratio of 4:5, is highly soluble in water, though they are not so soluble separately. The solution is used for fire retarding agent of wood by impregnation.
It is also used in the manufacturing of ramming mass, a fine silica-containing powder used for producing induction furnace linings and ceramics.
Boric acid is one of the most commonly used substances that can counteract the harmful effects of reactive hydrofluoric acid (HF) after an accidental contact with the skin. It works by forcing the free F− anions into complex salts. This process defeats the extreme toxicity of hydrofluoric acid, particularly its ability to sequester ionic calcium from blood serum which can lead to cardiac arrest and bone decomposition; such an event can occur from just minor skin contact with HF.
Boric acid is added to borax for use as welding flux by blacksmiths.
Boric acid, in combination with polyvinyl alcohol (PVA) or silicone oil, is used to manufacture Silly Putty.
Boric acid is also present in the list of chemical additives used for hydraulic fracturing (fracking) in the Marcellus Shale in Pennsylvania. Indeed, it is often used in conjunction with guar gum as cross-linking and gelling agent for controlling the viscosity and the rheology of the fracking fluid injected at high pressure in the well. Indeed, it is important to control the fluid viscosity for keeping in suspension on long transport distances the grains of the propping agents aimed at maintaining the cracks in the shales sufficiently open to facilitate the gas extraction after the hydraulic pressure is relieved. The rheological properties of borate cross-linked guar gum hydrogel mainly depend on the pH value