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SCHNITZER M.
Soil Science: February 1974
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ABSTRACTSoil scientists methylate humic substances in order to measure the concentration of acidic functional groups, to assess the role of specific functional groups in metal-organic interactions, to enhance the definition of IR spectra, and to make humic materials more soluble in organic solvents so that more efficient methods of fractionation and analysis can be used. At the present time reagents widely used for this purpose are diazomethane in ether and dimethylsulfate over anhydrous potassium carbonate in acetone. While reports in the literature have noted that diazomethane often adds N to humic substances during methylation, I have observed that dimethylsulfate (over potassium carbonate in acetone) adds large amounts of C to certain humic acids. Thus, I set out to examine reactions between these reagents and humic substances of widely differing origins and pedological histories with the objective of uncovering the most favorable conditions for the methylation of humic substances.Diazomethane was found to be the most suitable reagent for the methylation of HA's extracted from soils that tended towards a neutral pH. Relatively little C and N were added and the efficiency of methylation was high. On the other hand, methylation with dimethylsulfate appeared to be the most useful procedure for HA's extracted from acid soils and for fungal “HA's.” Little C and no N were added under these conditions. The most promising procedure for the methylation of FA and other water-soluble humic substances was that involving CH3I-Ag2O. The results presented herein show that there exists at this time no universal procedure for the methylation of humic materials regardless of origin and method of fractionation. This is probably not too surprising if one considers the chemical complexity of humic substances.

Soil scientists methylate humic substances in order to measure the concentration of acidic functional groups, to assess the role of specific functional groups in metal-organic interactions, to enhance the definition of IR spectra, and to make humic materials more soluble in organic solvents so that more efficient methods of fractionation and analysis can be used. At the present time reagents widely used for this purpose are diazomethane in ether and dimethylsulfate over anhydrous potassium carbonate in acetone. While reports in the literature have noted that diazomethane often adds N to humic substances during methylation, I have observed that dimethylsulfate (over potassium carbonate in acetone) adds large amounts of C to certain humic acids. Thus, I set out to examine reactions between these reagents and humic substances of widely differing origins and pedological histories with the objective of uncovering the most favorable conditions for the methylation of humic substances.

Diazomethane was found to be the most suitable reagent for the methylation of HA's extracted from soils that tended towards a neutral pH. Relatively little C and N were added and the efficiency of methylation was high. On the other hand, methylation with dimethylsulfate appeared to be the most useful procedure for HA's extracted from acid soils and for fungal “HA's.” Little C and no N were added under these conditions. The most promising procedure for the methylation of FA and other water-soluble humic substances was that involving CH3I-Ag2O. The results presented herein show that there exists at this time no universal procedure for the methylation of humic materials regardless of origin and method of fractionation. This is probably not too surprising if one considers the chemical complexity of humic substances.

© Williams & Wilkins 1974. All Rights Reserved.