Estimation of the carbon valence from its average formal oxidation state in the soil organic matter

The wet dichromate methods for the quantification of soil organic carbon on the basis of the reductant ability of soil organic matter have been widely used and they are still nowadays. The results obtained with these methods depart from the reference dry combustion with CO2 measurement and the discrepancies are interpreted in terms of incomplete oxidation under the wet dichromate conditions. However, in the calculations accompanying the dichromate methods a carbon valence (V-C) equal to four is systematically used, even though the actual V-C could be different. To assess the importance that an actual V-C different from four could have in the mismatch between the wet dichromate methods and the reference dry combustion, the relationship between V-C and the average formal oxidation state of carbon (z(C)) in soil organic matter should be known. On the basis of the definition of the equivalent weight and the average formal oxidation state, it is shown that a simple linear expression of the form V-C = 4 - z(C) relates both concepts in carbon-hydrogen-oxygen-nitrogen-phosphorus-sulphur compounds, which are representative of soil organic matter. On the basis of this equation the actual V-C in soil organic matter can be assessed, thus helping in the interpretation of soil organic carbon determinations with the wet dichromate methods and, therefore, in the comparison between the dichromate methods and the reference dry combustion. Highlights Although questioned, a carbon valence of four is always used in the wet dichromate methods for SOC. How the valence of carbon depends on its average formal oxidation had not been previously shown. Both concepts are related through a simple linear equation: V-C = 4 - z(C). The mismatch between the dichromate oxidation and the dry combustion will be better interpreted.