The determination of orthophosphate is not possible if the content is lower than 0.1 % (m/m). Tetrametaphosphates and pentametaphosphates are not usually present in commercial products. Specifies the absorption of the phosphate anions on an anionic ion-exchange resin. Elution with potassium chloride solutions of increasing concentrations. Calculation of the P2 O5 content in the different eluate volumes by means of a conversion factor to the corresponding form. The annex gives a photometric method of analysis of eluates. The principle consists in formation of molybdatophosphate in a dilute sulphuric acid medium of eluate, reduction with ascorbic acid and photometric measurement of the reduced complex at a wavelength of about 650 nm. Typical ion-exchange columns are shown in figures. Cancels and replaces the first edition of ISO 3358/1976.