The principle consists in a decomposition of most of the formic acid in a test portion by sulphuric acid an of the remainder by chromic acid. Steam distillation of acetic acid and/or other volatile acids and titration of the distillate with standard volumetric sodium hydroxide solution in the presence of phenolphthalein as indicator. The method is applicable to products containing less than 0,5 % (m/m) of other volatile acids, expressed as acetic acid. The ISO Recommendation R 1913-1971 was review and has been changed to ISO 731/7. ISO 731/7 replaces ISO/R 1913-1971, to which it is technically identical.