Influence of Organic Materials on the Determination of the Spectfic Surface Areas of Soils

Summary The surface areas of a number of soils have been determined before and after removal of organic matter by treatment with hydrogen peroxide. The external area as determined by low‐temperature nitrogen adsorption increased after peroxidation. The total area derived from the nitrogen area and cetyl pyridinium bromide (CPB) adsorption generally decreased. Part of the clay‐organic complex was separated from some soils, and the external and total surface areas of this material before and after peroxidation determined similarly. Both the external and total areas increased after peroxidation. These results indicate that soils contain organic materials which are not part of the clay‐organic complex and which adsorb large amounts of CPB on surfaces not accessible to nitrogen molecules. A sample of peat showed a correspondingly high CPB adsorption and low nitrogen area. The large differences that occur in the nitrogen and CPB areas after peroxidation show the importance of removal of organic matter prior to the determination of the surface areas of soil clays. The surface areas were only slightly greater when carbon was completely removed by 30 per cent peroxide than when it was partly removed by 6 per cent peroxide. Organic compounds associated with the clay in the clay‐organic complex are apparently able to prevent access of nitrogen molecules to many of the clay surfaces, and also probably exclude CPB from some of these surfaces.