MOLECULAR WEIGHT AND SHAPE OF HUMIC ACID FROM SEDIMENTATION AND DIFFUSION MEASUREMENTS ON FRACTIONATED EXTRACTS

Abstract

Summary Whole humic acid extracts are usually too polydisperse for reliable molecular‐weight measurement to be made in the ultracentrifuge by the sedimentation velocity technique. Consequently, the humic acid used in this study was fractionated with respect to molecular weight into fractions of low polydispersity by extensive use of gel‐permeation chromatography and other fractionation techniques. The sedimentation and diffusion coefficients of the fractions were determined and molecular‐weight values calculated. These values ranged approximately from 2 × 10 3 to 1.5 × 10 6 , the higher figure not necessarily representing the upper limit for these substances. On the basis of the frictional parameters calculated from the experimental data, it is proposed that the molecule adopts the solution conformation of a randomly coiled polymer in which branching may be significant, particularly at higher molecular weights.