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Determination of the Forms of Carbon in Geological Materials

DETERMINATION OF THE FORMS OF CARBON IN GEOLOGIC MATERIALS

Larry L. Jackson
U.S. Geological survey, Bx 25046,
MS 973, DFC, Denver, CO 80225, U.S.A.

Steven R. ROOF
Department of Geology and Geography, Morrill Science Center,
University of Massachusetts, Amherst, MA 01003, U.S.A.

Total carbon was determined in 30 geological reference materials by combustion/IR and combustion/coulometry methods. The combustion/coulometric method does not require empirical calibration and was found to be more precise than the combustion/IR method despite the use of smaller sample sizes. Carbonate carbon was determined by coulometric titration of acid-evolved CO2. Non-carbonate carbon was calculated as the difference between the total and carbonate contents.

Total, organic, and carbonate carbon are determined in geologic materials using numerous different methods. In general, total carbon is determined by combusting a sample and measuring the evolved carbon dioxide using infrared (IR) spectrometry (1 – 6), a thermal conductivity detector (7), coulometric titration (8), or gravimetrically (9 – 10). Organic carbon is determined as total carbon in a sample after removal of all carbonates by acid leaching (6 – 11), by direct oxidation (12), or indirectly by difference between the total and carbonate carbon. Carbonate carbon is commonly determined by measuring the acid-evolved carbon dioxide gravimetrically (13), volumetrically (14), or by coulometric titration (15 – 16).

Despite the importance of carbon in sedimentary and ore-forming processes relatively little work has been published on the characterization of carbon in geologic reference materials or on the comparison of methods of analysis. Terashima has determined total carbon in a large number of standards using combustion and infrared spectrometry (1 – 2, 5). Engleman et al. (15) and Chan (16) have determined carbonate carbon by coulometric titration of acid-evolved CO2. In general, non-carbonate carbon content has been reported infrequently (6, 11). As Terashima (5) points out too little data is available for most geological reference materials to develop reliable consensus values.

We selected a suite of 30 geologic reference materials spanning a range of carbon content and forms of carbon in order to compare analysis methods. We determined total carbon in this suite of reference materials by combustion with IR detection of evolved CO2 and by combustion with coulometric titration of the evolved CO2. We also determined carbonate carbon by coulometric titration of acid-evolved CO2. Non-carbonate carbon was calculated by difference.

METHODS

The entire contents of previously un-opened bottles of reference material as received from suppliers were mixed. The material was dried at 105°C for about 24 hrs. prior to analysis. All results are reported on a dry-weight basis.

Total carbon was determined by using a Leco1 model CR-12 instrument by combusting the sample at 1370°C in an oxygen atmosphere and detecting the evolved carbon dioxide with infrared spectrometry (3). The reference materials were divided into subsets based on carbon content. Different geologic reference materials were used to calibrate the non-linear infrared detector (Table 1) for each subset.