POTENTIOMETRIC ACID-BASE TITRATION OF A COLLOIDAL SOLUTION

An improved method for evaluating acid-base titration data obtained fr om the potentiometric titration of solutions containing highly charged colloidal particles, such as proteins or ionic micelles, is presented . The use of the method is demonstrated in the determination of the ch arge number, as a function of pH, of a colloid, in this case the prote in lysozyme titrated in differing sodium chloride concentrations, The method involves an improved calculation of the electrostatic potential and the concentration profiles of ions, e.g., the hydrogen ion, surro unding the charged colloidal particles. It uses a numerical program th at solves the Poisson-Boltzmann (PB) equation in a cell model, Knowled ge of the concentration profile of hydrogen ions enables a correct cal culation of the amounts of hydrogen ions that have protonated the prot ein, thus the charge number at differing pH, The PB calculation provid es an estimation of the solution concentration of hydrogen ions at the surface of the protein, It is shown that this surface concentration c orrelates well with the charge number on the protein over the wide pH range of 1.5-12, thus also at highly acidic pH, where the colloid is h ighly charged, This has been difficult to achieve before.