Determination of carbon by using barium sulfate as an oxidizing flux

Thermal behavior of mixtures of amorphous carbon and sulfates (K2SO4, Na2SO4, CaSO4, and BaSO4) was investigated by differential thermal analysis (DTA) and thermogravimetric analysis (TGA). Samples for DTA and TGA were prepared by mixing amorphous carbon ( 1 % of the mixture) with one of these sulfates, and the curves were recorded for these samples under an atmosphere of helium. The DTA curves for the mixtures of the carbon and one of the sulfates are shown in Fig. 1 where endothermic peaks occur at 940, 980, 1000, and 980°C. These peaks correspond to those by the reaction of amorphous carbon with potassium, sodium, calcium, and barium sulfates, respectively. During these experiments, evolution of carbon dioxide, carbon monoxide, and a small amount of sulfur dioxide was observed at a temperature of the endothermic peak mentioned above. From the result of X-ray diffraction analysis of the mixture of carbon and barium sulfate, which had been heated at 1200°C, it was found that barium sulfide was formed. An applicability of this fact to determine carbon was investigated. Barium sulfate was selected as an oxidizing flux for the determination of carbon, because barium sulfate and its reaction product, barium sulfide, were stable at atemperature as high as 1300°C. Factors which affect the determimtion of carbon were studied. such as extraction time of carbon, extraction temperature, and a ratio of barium sulfate to carbon. Extraction curves of carbon from elemental- and carbide carbons with barium sulfate are shown in Figs. 3 and 4. The recommended procedure is as follows: mix 0.1 g of elemental carbon with 2 g of barium sulfate, transfer 0.1 to 0.2 g of the mixture to a porcelain combustion boat, ignite the mixture under an atmosphere of helium (150 ml/min) at a temperature of 1200°C for 15 min, and determine the carbon dioxide coulometrically. In the case of carbides, mix 1 g of uranium carbide with 1 g of barium sulfate and for titanium carbide with 4 g of barium sulfate. Then continue the procedure described above. Analytical results of carbon in amorphous carbon, charcoal, active carbon, graphite, uranium dicarbide, uranium monocarbide, titanium carbide, and chromium carbide are listed in Table II. Except for the results of graphite and chromium carbide, the results obtained by the proposed method are in good agreement with those obtained by the combustion method in a stream of oxygen.