Carbon-Isotope Fractionation During Wet Oxidation of Oxalic Acid

The usual procedure for preparation of CO (sub 2) from the oxalic-acid standard--wet oxidation by means of potassium permanganate in acid solution--suffers from difficulty in determining end point of the reaction. The method of preparation at the University of Arizona laboratory was reviewed to learn the factors responsible for fractionation. It was clear that fractionation progresses steadily throughout the wet oxidation of oxalic acid and that color change and cessation of bubbling are not adequate criteria for determining the end point. Freeze-thaw operations are layer processes occurring at cold-trap walls and may preserve serial fractionation through several cold-trap operations. It is necessary to monitor continually the delta C-13 values of the standard gas. Dry combustion affords an effective alternative to wet oxidation, but it also has problems.