The free acidity determined by this method includes acidity extractable by water and acidity due to acidic groups directly attached to the cellulose acetate, e.g. carboxyl groups. This method is not suitable for cellulose acetate containing any additives which may affect the test. The principle consists in treating a test portion of cellulose acetate with water and titrating the resultant solution with sodium hydroxide solution. The free acidity is calculated as the percentage, by mass, of free acetic acid in the cellulose acetate. This second edition cancels and replaces the first edition ISO 1061:1975, of which it constitutes a minor technical revision.