The determination of organic substance in aqueous solution as total carbon by measurement of the carbon dioxide produced is the basis of several wet oxidation (1-4) and dry combustion (5, 6) methods, and provides a suitable subject for undergraduate laboratory experiments. We wish to describe an experiment, a modification after West (6), based on the combustion of organic material over hot copper oxide in a helium atmosphere and the measurement of the resultant carbon dioxide by gas chromatography. The experiment is designed to introduce students to gas chromatography, an instrumental technique basic to many industrial analytical laboratories, while making the student more aware of water pollution and the role of analytical chemistry in its control. A diagram of the apparatus used in the experiment is shown in the figure. Samples are charged through a rubber septum to the combustion tube (10 X in. stainless steel), a, which is packed with wire-form cupric oxide and placed in a vertically mounted tube type furnace, b, capable of being heated to S50°C. To avoid the possible interference of water with the carbon dioxide peak, a copper tube (6 X '/4 in.), c, packed with anhydrous magnesium perchlorate is placed in the combustion train prior to the gas chromatography unit. The chromatographic column, d, is a 3-ft X '/An. copper tube packed with silica gel (30-100 mesh). The gas chromatograph, e, is equipped with a thermal conductivity detector. A helium flow rate of 30 ml/min and a column temperature of 60°C give satisfactory results. The experiment is carried out as follows: Solutions of soluble organic compounds of known concentration containing from 0.2 to 2-3% weight carbon are prepared, or if desired, samples cont&ining organic impurities in this concentration range may be obtained from polluted streams or similar sources. The system is calibrated over the concentration range of the samples to be analyzed by charging different volumes of carbon dioxide to the gas chromatograph. The carbon content for each volume charged is calculated as micrograms of carbon. Up to 450 pg of carbon may be conveniently delivered from a '/rcc syringe which may be filled with carbon dioxide by inserting the needle into a container of crushed dry ice and pumping to expel air. The carbon dioxide peak areas are measured by multiplying the peak height by the width at half height. A plot of peak area versus micrograms of carbon should be a straight line passing through the origin. A known volume of the sample, approximat& 8 pl, (40-100 pg of carbon) is charged to the combustion tube and the resulting carbon dioxide peak area measured. The micrograms of carbon present are read
directly from the calibration curve. After determining the density of the sample solution, its carbon content can be calculated and reported as percent weight carbon. The percent weight carbon recovered has been found to vary with the type of compound present, and a recovery factor for each compound may be calculated as the ratio of micrograms of carbon found to the micrograms of carbon present. Typical recovery factors will vary from 0.85 to 1.0. We have found an average recovery factor of 0.95 for compounds ranging from low molecular weight alcohols to chlorinated hydrocarbons, and West (6) reports similar results. Therefore, it is suggested that a recovery factor of 0.95 be used for solutions of unknown composition. The less volatile compounds react more slowly with copper oxide, resulting in a broader peak (6). More uniform combustion could be obtained by injection of the sample into a heated vaporizer block (approximately 250°C) through which the carrier gas flows and which is placed prior to the combustion tube. The entire experiment including calibration can he accomplished in one laboratory session and repetitive analyses of the same sample can be performed rapidly with high precision. Duplicate analyses of the same sample can be made to agree to within 0.01% weight carbon. Literature Cited (1) VAN SLYKE,D. D., FOLCA,J., AND PLAZIN, J., J . Bwl. Chem., 136, 509 (1940). (2) KIESELBACH, R., Anal. Chem., 26, 1312 (1954). (3) PICKHAHDT, W. P., OEMLER, A. N.,AND MITCHELL, J., JR., Anal. Chem., 27, 1784 (1955). (4) BEATTIE, J., BRICKER, C., AND GARVIN, D., Anal. Chem., 33, 1890 (1961). J., AND STFINOER, V. A,, (5) VANHALL,C. E., SAPRANKO, Anal. Chem., 35, 315 (1963). (6) WEST,D. L.,Anal. Chem., 36, 2194 (1964).
Diagram of combustion-gar'shrom(~tographyopparatur
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