Sodium phosphate, monobasic

• Ph. Eur.Sodium Dihydrogen Phosphate Dihydrate
• USP–NFMonobasic Sodium Phosphate

Acid sodium phosphate; monosodium orthophosphate; monosodium phosphate; natrii dihydrogenophosphas dihydricus; primary sodium phosphate; sodium biphosphate; sodium dihydrogen phosphate; E339(i)

Sodium dihydrogen phosphate (anhydrous or as the monohydrate or dihydrate) · [7558-80-7]

UNII KH7I04HPUU · PubChem CID 23672064

NaH2PO4 (anhydrous); NaH2PO4·H2O; NaH2PO4·2H2O · 119.98 (anhydrous); 137.99 (monohydrate); 156.01 (dihydrate)

pH adjuster, buffering agent

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

Monobasic sodium phosphate is used as a buffering agent and acidifier, most often together with dibasic sodium phosphate, to prepare phosphate buffers for parenteral, ophthalmic, oral and topical preparations and protein formulations. It is also used as a sequestering agent, in effervescent and laxative products, and as a urinary acidifier. Typical buffer concentrations are low, of the order of 0.1–1% w/v.

The anhydrous form is a white, crystalline powder or granules. The monohydrate and dihydrate occur as colourless crystals or a white crystalline powder. It is odourless, slightly deliquescent and has an acidic, saline taste.

pH

about 4.1–4.5 (5% w/v aqueous solution)

pKa values (phosphoric acid)

about 2.1, 7.2 and 12.3 at 25 °C

Solubility

very soluble in water; practically insoluble in ethanol

Thermal behaviour

loses water of crystallisation on heating and, at higher temperatures, is converted to sodium acid pyrophosphate and then metaphosphate

Chemically stable, although slightly deliquescent. Aqueous solutions are stable and can be autoclaved. Store in an airtight container in a cool, dry place.

Incompatible with alkaline materials and carbonates; its acidic solutions cause carbonates to effervesce. Like other phosphates, it forms insoluble precipitates with calcium, magnesium, aluminium and other polyvalent metal ions.

Prepared by treating phosphoric acid with sodium carbonate or sodium hydroxide in a 1:1 molar ratio, followed by crystallisation of the required hydrate or drying to the anhydrous form.

Generally regarded as non-toxic at buffer levels. Large oral doses act as a saline laxative, and excessive phosphate intake can cause hyperphosphataemia and hypocalcaemia, particularly in patients with renal impairment.

Observe normal precautions for the quantity handled. Eye protection and gloves are recommended.

GRAS listed. Accepted as a food additive in Europe (E339). Included in the FDA Inactive Ingredient Database (injections, inhalation, nasal, ophthalmic, oral, otic, topical and vaginal preparations) and in parenteral and non-parenteral medicines licensed in Europe.

Sodium phosphate, dibasic; potassium phosphate, monobasic; phosphoric acid.

Phosphate buffers have their greatest capacity near pH 7.2, the second pKa of phosphoric acid; the ratio of monobasic to dibasic salt sets the pH and ionic strength also affects it. Specify the hydrate form used and correct quantities for water content.