This is derived from alginic acid, a colloidal substance obtained from the sea weeds Laminaria digitata and Laminaria cloustoni. Alginic acid is a polyuronide built up from d-mannuronic acid units. Its carboxyl groups react with metallic ions to form alginates. Since the parent acid is unstable, the water-soluble sodium salt is used as the source of other alginates. If ionized calcium salt is added to sodium alginate solution, instantaneous precipitation of calcium alginate occurs, which is a sensitive reaction that can be used for preparing foams, fabrics and other physical forms. These can be sterilized by autoclaving or dry heat.

Calcium alginate dressings have a marked hemostatic effect. By including a small proportion of sodium alginate the hemostatic effect can be increased and it can be used to arrest the external bleeding, e.g., from surgical incisions, tooth sockets and sites from which grafts have been taken. A transparent protective film can be made in situ over a burn, wound or incision by applying a solution of sodium alginate and then spraying with calcium chloride solution. This film is impervious to water but permeable to water vapor.