THE DETERMINATION OF THE MOLECULAR-MASS OF HUMIC SUBSTANCES FROM NATURAL-WATERS BY ANALYTICAL ULTRACENTRIFUGATION

UV scanning analytical ultracentrifugation has been applied to the determination of the weight-average molecular masses (M(w)BAR) for a range of fulvic and humic acids extracted from natural waters and soils. It is demonstrated that the technique of approach-to-equilibrium (Archibald method) gives a relatively rapid means of determining M(w)BAR which for low molecular mass fulvic acids is in good agreement with the conventional sedimentation equilibrium method. For higher molecular mass humic substances it is found that the sedimentation equilibrium method underestimates M(w)BAR. due to the loss of material from the concentration gradient and the approach-to-equilibrium method is the only means of estimating the molecular mass for the whole sample. For very high molecular mass humic substances (M(w)BAR) of 30 000 or more) a double extrapolation procedure is described for obtaining the true weight-average molecular mass at zero time and zero force field. The application of the approach-to-equilibrium method to samples taken directly from natural water without purification and to ion binding of fulvic acids is described.