Sodium alginate

• BPSodium Alginate
• Ph. Eur.Sodium Alginate
• USP–NFSodium Alginate

Algin; alginic acid, sodium salt; natrii alginas; sodium polymannuronate; Kelcosol; Keltone; Manucol; Protanal; E401

Sodium salt of alginic acid, a linear copolymer of (1→4)-linked β-D-mannuronic acid and α-L-guluronic acid residues · [9005-38-3]

(C6H7NaO6)n · ≈ 10 000–600 000 (grade-dependent); repeat unit 198.11

Gelling agent, viscosity modifier, Binder, Disintegrant

Typical concentrations: 1–3% binder · 2.5–10% disintegrant · Dosage forms: Liquid, Solid, Semi-solid

Used as a suspending and thickening agent (1–5%), as an emulsion stabiliser (1–3%), as a tablet binder (1–3%) and disintegrant (2.5–10%), and in pastes and creams (5–10%). It is also used in hydrophilic matrices for modified release, in microencapsulation and bead formation by ionotropic gelation with calcium ions, and with sodium bicarbonate and calcium carbonate in raft-forming preparations for gastro-oesophageal reflux. Calcium alginate made from it is used in wound dressings.

An odourless, almost tasteless, white to pale yellowish-brown powder. It is extracted from brown seaweeds; the ratio and sequence of mannuronic (M) and guluronic (G) acid blocks, which depend on the seaweed source, determine the gel strength and viscosity of each grade.

Viscosity

about 20–400 mPa s for a 1% w/v aqueous solution at 20 °C, depending on grade

pH

about 7 (1% w/v aqueous solution)

Loss on drying

not more than 15% w/w

Solubility

slowly soluble in water, forming a viscous colloidal solution; practically insoluble in ethanol, ether, chloroform and aqueous acids below about pH 3

Gelation

forms gels with calcium and other divalent cations; precipitates as alginic acid at low pH

A hygroscopic material that is stable if stored in an airtight container in a cool, dry place. Aqueous solutions are most stable at pH 4–10; they lose viscosity on heating or autoclaving through depolymerisation and are susceptible to microbial spoilage, so a preservative is needed.

Incompatible with acridine derivatives, crystal violet, phenylmercuric acetate and nitrate, calcium salts, heavy metals and ethanol at concentrations above about 5%. Low concentrations of electrolytes raise viscosity, but high concentrations cause salting out.

Brown seaweeds (Phaeophyceae, for example Laminaria and Macrocystis species) are extracted with dilute alkali; the alginic acid is precipitated by acid or as calcium alginate, purified and converted to the sodium salt with sodium carbonate or hydroxide, then dried and milled.

Widely used in oral and topical products and foods, and generally regarded as non-toxic and non-irritant. It is not digested and passes through the gut largely unchanged. Inhalation of the dust may cause respiratory irritation.

Observe normal precautions for the quantity handled. Avoid generating dust; eye protection, gloves and a dust mask are recommended.

GRAS listed. Accepted as a food additive in Europe (E401). Included in the FDA Inactive Ingredient Database (oral capsules, suspensions and tablets) and in non-parenteral medicines licensed in Europe.

Alginic acid; calcium alginate; potassium alginate; propylene glycol alginate.

Disperse the powder slowly into vigorously stirred water, or pre-wet it with a non-solvent such as glycerol or ethanol, to avoid lumps. Grades rich in guluronic acid give strong, brittle calcium gels, whereas mannuronic-rich grades give softer, more elastic gels.