Determination of organic and inorganic carbon in soils by potentiometry

Abstract

A method is described for the direct, rapid and precise determination of 0–6 mg of organic and/or inorganic carbon in soils. The equivalent amount of carbon dioxide, liberated by improved combustion techniques, is absorbed and precipitated in a solution containing 2.50 mmol of sodium hydroxide and 0.96 mmol of barium chloride. Next, the resultant barium carbonate is dissolved by the addition of 0.99 mmol of EDTA (disodium salt). The pH of the final solution is related to the amount of carbon dioxide present by means of a calibration graph. By use of a pH meter with an expanded range of 1 pH unit, sensitivity to 10 µg of carbon is achieved. Analyses of pure organic compounds, such as benzoic acid and hydroquinone, show recoveries accurate to within 0.1 mg of carbon. Results for calcareous soils are in close agreement with values for calcium oxide obtained by use of X-ray fluorescence spectrometry.

Interference from chloride is effectively eliminated by a preliminary evaporation step.