The Science of the Total Environment, 81/82 (1989) 489-499 489 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands EVIDENCE OF CONFORMATIONAL CHANGES IN FULVIC ACIDS FROM DIALYSIS M.T.S.D.VASCONCELOS, A.P.L.M.G.SANTOS and A.A.S.C.MACHADO CIQ(LrP),Chemistry Department, Faculty of Science,P4000 Porto (Portugal) SUMMARY A dialysis study was conducted to evaluate the influence of the solution composition on the permeability of a membrane to fulvic adds (FA). It has been found how the permeability depends on the concentrations of the electrolytes in solution and pH, and that dialysis is not suitable for complexation studies on solutions with high ionic strength. Additional support for the present view of the dependence of the aqueous conformation of FA molecules on their solution environment has been found. INTRODUCTION The conformational state of molecules of humic substances is important because it conditions the extent of their interaction with other molecules in natural systems. In consequence, the study of the aggregation state and conformation in solution of molecules of humic materials, specially of fulvic adds (FA), has aroused much attention in recent years. Particular interest has been focussed on the influence of the solution characteristics (FA concentration, ionic strength, pH, presence of compexing cations,etc.) on the FA molecular shape and aggregation state, which have been studied by a variety of experimental tech- niques (refs.l-7). However, the results of these studies not always have been consistent and the use of different values for fixed experimental parameters make comparisons difficult or without meaning (ref.1). In the course of a comparative study (ref. 8) of three analytical methods for determina- tion of complexation parameters of copper(H) ion with FA extracted from soil (ion-selective electrodes, fluorescence and dialysis), we have confirmed that dialysis (ref. 9) is the most suitable technique for studies of metal-FA interactions in natural waters but we have found that it is not adequate for studies at higher concentrations of FA and of background electrolyte. Previous works on the use of dialysis for studying FA solutions have been 0048-9697/89/$03.50 © 1989 Elsevier Science Publishers B.V.