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6.17 RESIDUAL TITRATIONS

      Some assays require the addition of a measured volume of a volumetric solution, in excess of the amount actually needed to react with the substance being assayed, the excess of this solution then being titrated with a second volumetric solution. This constitutes a residual titration and is known also as a “back titration”. The quantity of the substance being titrated may be calculated from the difference between the volume of the volumetric solution originally added and that consumed by the titrant in the back titration, due allowance being made for the respective molarity factors of the two solutions, and the equivalence factor for the substance given in the individual monograph.