Experimental determination of partial specific volumes of humic substances in aqueous solutions

Partial specific volume is an important molecular parameter in the measurement of molecular masses by analytical ultracentrifugation and X-ray diffraction studies. Humic substances (aquatic fulvic and humic acids and peat humic acid) were isolated from a natural source (Whitray Fell, UK). The partial specific volumes (v̄2) have been studied by two independent methods, an analytical ultracentrifugation technique based on simultaneous sedimentation equilibrium measurements in water and deuterium oxide and by high precision digital densimetry. Measurements were made in a range of conditions from 0.01 M to 0.5 M salt concentration and in the humic substance concentration range 0.01 to 2 mg ml−1. Partial specific volumes in the range 0.43 to 0.63 cm3 g−1 were found depending on the samples and conditions. No evidence was found for a dependence of (v̄2) on humic concentration within the ranges studied. Both methods of measurements gave results in reasonable accord. The ultracentrifugation method was more precise for the aquatic fulvic acid than for the more polydisperse humic acids. For the fulvic sample the reproducibility of the ultracentrifugation method was ± 4.31%, comparable to the average reproducibility of the densimetry method which was ± 4.07% over a range of 12 different systems.