Boric acid

• BPBoric Acid
• JPBoric Acid
• Ph. Eur.Boric Acid
• USP–NFBoric Acid

Acidum boricum; boracic acid; orthoboric acid; trihydroxyborane; E284

Orthoboric acid · [10043-35-3]

UNII R57ZHV85D4 · PubChem CID 7628

H3BO3 · 61.83

Antimicrobial preservative, pH adjuster, buffering agent

Typical concentrations: — · Dosage forms: Sterile (parenteral/ophthalmic)

Used mainly as a buffering agent, with sodium borate, in ophthalmic preparations and contact-lens solutions, and to adjust tonicity; it has weak antimicrobial activity and may contribute to preservation. It is also used in some eye washes, otic and topical preparations. It should not be used in products for internal use, on broken skin or in infants.

Colourless, odourless, lustrous crystals or a white crystalline powder or granules that feel slightly unctuous.

pKa

≈ 9.2 (a weak Lewis acid; acidity is increased by polyols such as glycerol and mannitol)

Behaviour on heating

loses water above about 100 °C, forming metaboric acid and eventually boron trioxide

Solubility

soluble in water (about 1 in 20 at room temperature) and freely soluble in boiling water and in glycerol; soluble in ethanol

pH

weakly acidic in aqueous solution (about pH 3.8–4.8 for a 3.3% solution)

Stable and slightly hygroscopic. Store in an airtight container.

Incompatible with alkali carbonates and hydroxides, and forms complexes with polyhydric alcohols (glycerol, mannitol, sorbitol), which increases its acidity. It can form complexes with polyvinyl alcohol and some other polyols, causing gelation, and it is incompatible with tannins.

Prepared by treating borax (sodium tetraborate) or other borate minerals with sulfuric or hydrochloric acid, followed by crystallisation.

Boric acid is toxic if swallowed or absorbed in quantity, for example from large areas of damaged skin or in infants, causing gastrointestinal, skin, CNS and renal effects. It is classified in the EU as a reproductive toxicant (Category 1B), and EU product information requires warnings and limits for medicines containing boron compounds, particularly for children under 2 years. At the concentrations used in eye drops it is generally well tolerated.

Avoid ingestion and inhalation of the dust; use gloves, eye protection and a dust mask. Because of its reproductive-toxicity classification, exposure of workers should be minimised.

Included in the FDA Inactive Ingredient Database (ophthalmic, otic and topical preparations) and in non-parenteral medicines licensed in Europe. In the EU it is permitted as a food additive (E284) only for sturgeon roe (caviar).

Sodium borate (borax); sodium perborate.

Borate buffers are useful in the pH range of about 7–9 and are well suited to ophthalmic products, but they should not be combined with polyvinyl alcohol or with high levels of polyols without checking for complexation or gelation.