The determination of total organic carbon in soils, fertilizers, sewage sludge, sediments, and humic extracts is widely performed by chemical oxidation methods with K2Cr2O7. The Yeomans-Bremner (YB) method is currently the one that stands out the most. The drawback of these methods is the large amount of concentrated H2SO4 used, which generates a large amount of hazardous waste. This work proposes using KMnO4 as an alternative to K2Cr2O7 for a lower consumption of H2SO4. The method uses the back titration of Fe2+ added to consume both the MnO2 produced and the excess KMnO4 that was not consumed in the OM oxidation. A non-trivial and yet not explored stoichiometry was applied for this purpose, providing a success not yet achieved in using permanganate to determine TOC by titration. The ideal condition for the oxidation of OC was determined by the analysis of a potassium hydrogen phthalate standard and involved the use of 0.125 mol L-1 H2SO4 and temperature of 70 °C, obtaining a significant advantage over the YB method (concentrated H2SO4 and 170 °C). The proposed method was applied to the analysis of soil samples, producing conversion factors for soil organic carbon that varied between 0.652 and 1.12.
Keywords:
Yeomans-Bremner; soil analysis; permanganate; organic carbon; hydrogen phthalate.