Assay Calculation by Titration

The formula for assay calculation by titration is based on the principle of chemical equivalence, which states that the number of moles of reactant in a reaction is equal to the number of moles of another reactant.

This technique used to work out the concentration of an unknown solution based on its chemical reaction with a solution of known concentration.


The titration process usually involves adding the known quantity of a solution (the titrant) to a unknown solution (the analyte) until the reaction is complete. To calculate the concentration of the analyte, you measure the volume of titrant used.


Assay by Titrations = [Titrate value of (sample - blank) x M x F x 100 x 100] / [Ws x (100- LOD)]


  • M  = Molarity of Volumetric Solution,
  • Ws = Weight of solution,
  • F  = Factor for drug substance,
  • LOD = Loss on Drying.


Assay Calculation Formula by Titration Method



  • A = reading (mL),
  • VSfactor = factor of VS (volumetric solution),
  • W = weight (mg),
  • LC = label claim,
  • f = factor of the titrant
  • Eq. wt = equivalent weight (molecular weight/ Valency).


Equivalent weight is the mass of the substance in grams that reacts with one mole of the titrant.

% Assay (on anhydrous basis) = (Assay × 100) ÷ (100-LOD/WC)


  • LOD = Loss on drying
  • WC = Water content


For a back-titration with a blank, use the following formula:

  • VBlank:  Volume of the equivalence point from the blank determination in mL
  • VEP:  Volume at the equivalence point in mL
  • cTitrant:  Nominal titrant concentration in mol/L
  • fTitrant:  Titer factor of the titrant (unitless)
  • r:  Stoichiometric ratio (unitless)
  • MA:  Molecular weight of analyte A in g/mol
  • mSample:  Weight of sample in mg
  • 100:  Conversion factor, to obtain the result in %

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