Does anybody have experience how to measure the point of zero charge of buffering materials (e.g., Mn oxides, zeolites, etc. in our case)? The common pH drift method is giving us very weird results, as the materials are strongly buffering the pH to their natural pH. E.g., birnessite, whose pHzpc should be according to the literature between 2 and 4, results in pHzcp of almost 10 when measured by the pH drift method, the similar case with zeolites....Also potentiometric titrations are giving us these highly alkaline values close to the natural pH of the materials....

Don´t you have a recommendation for any other method suitable for this type of materials?

Thank you very much.