This International Standard specifies a method for the determination of the amount of free acid in unplasticized cellulose acetate. 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. The latter is usually a very small proportion of the total. This method is not suitable for cellulose acetate containing any additives which may affect the test.