A simple and accurate determination of the point of zero charge of ceramicmembranes

A simple and accurate determination of the point of zero charge of a cerami c membrane is reported. It is based on pH variation measurements on adding an amphoteric oxide in a solution of a given pH. Up to now this method was effective for studying powder dispersions. In this work it is extended to c eramic membranes. In fact, we present and test a new experimental set-up th at allows the performance of pH measurements on a solution continuously cir culating through the membrane. It is verified that the point of zero charge , determined in presence of an indifferent electrolyte, is the same as the isoelectric point (or point of zero charge) determined with standard electr okinetic methods. The shift in the point of zero charge towards a higher an d a lower pH, in presence of N2SO4 and CaCl2 solutions, respectively, confi rms the specific adsorption of sulphate and calcium ions and validates the method used. We have compared this new method with the known salt addition method, carried out on the crushed membrane. This last one leads to a diffe rent value of the pzc, thus showing the importance in performing measuremen ts directly on membranes themselves. Another interesting aspect in the meth od presented here is that it allows to assess directly to the absolute valu e of the surface charge density of the membrane. Results obtained are in go od agreement with the data reported in the literature on mineral oxides.