Optimizing the removal of carbon phases in soils and sediments for sequential chemical extractions by coulometry

We have developed a coulometric technique to optimize the removal of the ca rbonate and organic fractions for sequential chemical extractions of soils and sediments. The coulometric system facilitates optimizing these two frac tions by direct real-time measurement of carbon dioxide (CO2) evolved durin g the removal of these two fractions. Further analyses by ICP-MS and alpha- spectrometry aided in interpreting the results of coulometry experiments. T he effects of time, temperature, ionic strength and pH were investigated. T he sensitivity of the coulometric reaction vessel/detection system was suff icient even at very low total carbon content (<0.1 mol kg(-1)). The efficie ncy of the system is estimated to be 96% with a standard deviation of 8%. E xperiments were carried out using NIST Standard Reference Materials 4357 Oc ean Sediment (OS), 2704 Buffalo River Sediment (BRS), and pure calcium carb onate. Carbonate minerals were dissolved selectively using an ammonium acet ate-acetic acid buffer. Organic matter was then oxidized to CO2 using hydro gen peroxide (H2O2) in nitric acid. The carbonate fraction was completely d issolved within 120 min under all conditions examined (literature suggests up to 8 h). For the OS standard, the oxidation of organic matter self-perpe tuates between 45 and 50 degrees C, a factor of two less than commonly sugg ested, while organic carbon in the BRS standard required 80 degrees C for t he reaction to proceed to completion. For complete oxidation of organic mat ter, we find that at least three additions of H2O2 are required (popular me thods suggest one or two).