The principle consists in a quantitative oxidation of the formic acid in a test portion with excess mercury(II) oxide. Potentiometric titration of the residual acids with standard volumetric sodium hydroxide solution. The method is applicable to products containing 0,5 to 6,0 % (m/m) of other acids, expressed as acetic acid. ISO/R 731 has been subdivised into seven parts. ISO 731/3 replaces clause 4 of ISO Recommendation R 731-1968.