Applicable to products containing at least 90 % (m/m) of tetrasodium pyrophosphate and in the absence of more condensed forms of phosphates. The reaction based on conversion of tetrasodium pyrophosphate to disodium dihydrogenpyrophosphate by addition of hydrochloric acid. Introduction of zinc sulfate in solution for precipitation of zinc pyrophosphate and formation of sulphuric acid. Potentiometric titration of the sulphuric acid formed with standard volumetric sodium hydroxide solution. Has also been approved by the IUPAC.