EXTRACTION OF HUMIC ACIDS FROM A NATURAL MATRIX BY ALKALINE PYROPHOSPHATE - EVALUATION OF THE MOLECULAR-WEIGHT OF FRACTIONS OBTAINED BY ULTRAFILTRATION

Fractions of humic acids, resolved by ultrafiltration of extracts from a sample of peat treated with alkaline pyrophosphate solution, have b een submitted to high-performance size-exclusion chromatography, with the aim of determining the molecular weight distributions. Anomalous p eaks, located at retention volumes higher than those corresponding to the main signal, are present in the elution profiles relative to the l ightest fractions. These peaks are more intense when using a refractiv e index detector rather than a UV detector. Elemental analysis data fo r the lightest fractions suggested that the spurious peaks are due to the presence of inorganic material. The hypothesis that pyrophosphate interacts with humic acids in the course of the extraction has been co nfirmed by colorimetric determination of the phosphorus content in the different fractions. As a consequence, the extraction procedure has b een modified by using 0.1 mol/L NaOH as extractant. The average molecu lar weights of the various fractions, following NaOH extraction, resul t in substantial agreement with those obtained following pyrophosphate solution extraction (if the 'pyrophosphate peaks' present in the chro matographic profile are discarded in the calculations).