Applicable to products in which not all impurities are known or determined. The methods specified in ISO 3237 cannot be applied. The principle consists in dissolution of a test portion, removal of insoluble matter, iron, aluminium and calcium from the solution and evaporation to dryness in the presence of sulphuric acid. Heating and weighing of the total amount. Calculation of the sodium sulphate content by subtraction of the amount arising from any sodium chloride and sodium carbonate. Has also been approved by the IUPAC.