A simple and accurate determination of the point of zero charge of ceramic membranes

A simple and accurate determination of the point of zero charge of a ceramic 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 ceramic membranes. In fact, we present and test a new experimental set-up that allows the performance of pH measurements on a solution continuously circulating 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 electrokinetic methods. The shift in the point of zero charge towards a higher and a lower pH, in presence of N2SO4 and CaCl2 solutions, respectively, confirms 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 different value of the pzc, thus showing the importance in performing measurements directly on membranes themselves. Another interesting aspect in the method presented here is that it allows to assess directly to the absolute value of the surface charge density of the membrane. Results obtained are in good agreement with the data reported in the literature on mineral oxides.