Phosphoric acid

• BPPhosphoric Acid
• JPPhosphoric Acid
• Ph. Eur.Phosphoric Acid, Concentrated
• USP–NFPhosphoric Acid

Acidum phosphoricum concentratum; orthophosphoric acid; E338

Orthophosphoric acid · [7664-38-2]

UNII E4GA8884NN · PubChem CID 1004

H3PO4 · 98.00

pH adjuster, buffering agent

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

Used as an acidifying agent and for pH adjustment in oral, topical and parenteral formulations, normally in dilute form (for example diluted phosphoric acid, about 10% w/v) and in small, quantity-sufficient amounts. It is also used with its salts to prepare phosphate buffers, and in foods and soft drinks as an acidulant.

The concentrated acid is a clear, colourless, odourless, syrupy liquid that is strongly corrosive. It may crystallise on standing at low temperatures.

Concentration

concentrated grades about 85% w/w H3PO4 (pharmacopoeial limits roughly 84–90%)

pKa

pKa1 ≈ 2.1; pKa2 ≈ 7.2; pKa3 ≈ 12.3 (25 °C)

Density

about 1.7 g/cm³ (85% acid)

Solubility

miscible with water and ethanol, with evolution of heat

Chemically stable. The concentrated acid is hygroscopic and should be stored in an airtight container made of a material resistant to the acid, such as glass or suitable plastic.

Reacts vigorously and exothermically with alkalis. It attacks many metals, releasing flammable hydrogen gas, and is incompatible with strong oxidising agents and many organic materials.

High-purity acid is made by the thermal process, in which elemental phosphorus is burned to phosphorus pentoxide and then hydrated. Technical grades are made by the wet process, by treating phosphate rock with sulfuric acid, and then purified.

Dilute phosphoric acid in the amounts used as an excipient is not considered toxic and is widely consumed in foods and drinks. The concentrated acid is corrosive and causes severe burns to the skin, eyes and mucous membranes. EFSA has set a group ADI for phosphates of 40 mg/kg body weight per day, expressed as phosphorus.

The concentrated acid is corrosive. Wear acid-resistant gloves, eye and face protection and protective clothing. When diluting, always add the acid slowly to water, never the reverse.

GRAS listed. Accepted as a food additive in Europe (E338). Included in the FDA Inactive Ingredient Database (injections; oral, topical and other preparations) and in parenteral and non-parenteral medicines licensed in Europe.

Diluted phosphoric acid; potassium phosphates; sodium phosphates.

Because it has three dissociation steps, phosphoric acid and its sodium or potassium salts can buffer around pH 2, 7 and 12; the pH 6–8 phosphate buffers are the most widely used in pharmaceutical formulations. Phosphate can precipitate with calcium and some other polyvalent cations, which matters in parenteral nutrition admixtures.