Reliable pKw values from potentiometric titrations: the striking effect of the titration final added volume.

Errors affecting the estimation of the ionic product of water from the potentiometric titration of weak acids have been analysed. A Fortran programme has been developed in order to simulate experimental titration data sets. This programme allows one to isolate experimental error sources and analyse their effect upon the final fitted equilibrium constant. pKw dependence with titration final added volume and the pK value of the weak acid, predicted with the simulation programme are coincident with experimental data, suggesting that obtaining reliable pKw values entails using experimental data of titrations up to a sufficiently large hydroxyl concentration which depends on the value of pK.