The principle consists in heating a test portion with an excess of potassium dichromate solution in the presence of sulphuric acid. Iodometric titration of the residual potassium dichromate. The method is applicable to products having dichromate inexes equal to or greater than 0,04 ml. It has also been approved by the International Union of Pure and Applied Chemistry (IUPAC). ISO 753/1 to ISO 753/11 cancel and replace ISO Recommendation R 753-1968.