Soil Science Society of America Journal Soil Sci. Soc. Am. J. 76:1100–1106 doi:10.2136/sssaj 2011.0278 Received 5 Aug. 2011. *Corresponding author (hirmas@ku.edu). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Determination of Calcite and Dolomite Content in Soils and Paleosols by Continuous Coulometric Titration Soil Mineralogy T he ability to accurately determine the fraction of calcite and dolomite composing the total carbonate in a sample is important in investigations of soil genesis, determination of mineral weathering and formation rates, reconstruction of paleoenvironments, and identiication of sediment source areas (Levine et al., 1989; Long et al., 1997; Capo et al., 2000; Muhs et al., 2001; Bustillo and Alonso-Zarza, 2007). his ability has aided studies in discriminating pedo- genic pathways of soil inorganic carbon (SIC) accumulation from those where soil carbonates have been inherited from lithogenic sources or precipitated as a result of groundwater processes and is, thus, important for accurately inventorying true SIC sequestration (Bellanca and Neri, 1993; Wang and Anderson, 2000). In addi- tion, investigations that use stable isotopes of paleosol carbonates—such as paleo- climatic and paleohydrologic reconstructions—depend on an accurate determina- tion of the calcite and dolomite content in a sample. Daniel R. Hirmas* Dep. of Geography Univ. of Kansas Lawrence, KS 66045-7613 Brian F. Platt Stephen T. Hasiotis Dep. of Geology Univ. of Kansas Lawrence, KS 66045-7594 Quantitative determination of calcite/dolomite ratios in soil carbonate fractions is important for understanding the pedogenic history of a soil or paleosol. This is particularly relevant to stable carbonate-carbon and -oxy- gen isotope analyses where the presence of dolomite in a carbonate fraction can lead to erroneous results and inaccurate interpretations of the paleoenvi- ronment. The goal of this work was to develop and test an accurate method using coulometric titration combined with differential kinetic principles for determining calcite and dolomite fractions in a sample. An automated titra- tor was connected to a temperature-controlled carbonate reactor where 2 M HClO 4 was introduced to the sample and mixed by bubbling CO 2 –free air at a constant rate of 100 mL min –1 . Samples were ground to <53 μm to control for differences in particle size and reactive surface area. The concentration of C released from the reaction was monitored and recorded every 6 s for the length of the reaction. Data were it with both a pseudo-irst order kinet- ic model and a Weibull model for comparison. The latter outperformed the pseudo-irst order kinetic model and the Weibull parameter, λ, it to titration data from unknown samples was compared to λ values it to data from mix- tures of known fractions of pure calcite and dolomite to quantitatively obtain the content of the two minerals. Dolomite fractions obtained by this method qualitatively compared well with values obtained by X-ray diffraction. Our method allows a precise and accurate measurement for both total carbonate and the ratio of calcite to dolomite in a sample. Abbreviations: cal, calcite; C T , total dissolved inorganic carbon; DMSO, dimethyl sulfoxide; dol, dolomite; DTA, differential thermal analysis; ETA, ethanolamine; MS residuals , mean square of the residuals; PTFE, polytetraluoroethylene; RMSE, root mean square error; SIC, soil inorganic carbon; XRD, X-ray diffraction.