DETERMINATION OF PHENOLIC-ACIDS AND TANNINS IN SOIL-WATER EXTRACTS USING THE TECHNICON AUTOANALYZER-II SYSTEM

A manual colorimetric procedure for determination of phenolic compounds in food samples was modified for automated analysis of soil water extracts using the Technicon Autoanalyzer II (AA II). The Folin Denis color reagent consists of a mixture of molybdic and tungstic complexes which are reduced to oxides by phenolic acids. The oxides form blue salts under alkali conditions. This reaction was assumed to be specific to the hydroxy and methoxy bonds of phenolic acids. EDTA was added to complex calcium and magnesium in the sample extracts, which otherwise would have interfered with the colorimetric reaction. The method was useful for tracing the disappearance of 11 individual phenolic acids added to soil samples. Accurate measurement of the total concentration of mixed phenolic acids was not possible because of variation in the degree of color development between phenolic acids. The method was sensitive to tannic acid in soil water extracts.