Potentiometric investigation of the effect of the pH on the ionic transfer of some amino acids at the interface between two immiscible electrolyte solutions

The effect of the pH on the ionic transfer of glycine and β-alanine at the interface between two immiscible electrolyte solutions (ITIES) was investigated by a simple potentiometric method. Upon addition of small amounts of solution containing the investigated amino acids, a variation of the potential drop across the interface was recorded, which was found to be pH-dependent. This behavior was explained in terms of a preferential orientation of the amino acid molecules at the ITIES, induced by the different lipoficility of the functional groups. The results enabled the measurement of this voltage variation to be used as the basis for a simple and rapid method for determining the isoelectric point of the investigated compounds. The agreement between the pHi values thus estimated and those reported in the literature suggests the possibility of using the method for the interpretation of processes occurring at the level of biological membranes.