A comparison of four methods for determining carbonate contents

摘要

A common method for determining the carbonate content of solid samples is the gasometric determination according to Scheibler. However, it seems doubtful if the reported accuracy of 1 percent and reproducibility of this method could be achieved. Bymeasuring different standards and samples we studied the accuracy and precision, particularly with regard to different evaluation procedures given in the literature. Measurement of CaCO_3-standards and evaluation in accordance with German standard DIN 18 129 resulted in a median overestimation of the carbonate content of 5 percent, and in some cases as high as 20 percent. Calibration at the beginning of a series of measurements reduced this. Making allowance for vaporised hydrochloric acid did not improve precision or accuracy. The standard deviation of the sample results was generally in the range of 3 to 5 percent, independent of the evaluation method. A comparison of the Scheibler method with three geochemical methods used for the determination ofthe carbonate content, namely coulometric measurement, complexometric titration and indirect ICP-analysis of calcium and magnesium after the full disintegration of the sample, showed that, with the exception of ICP analysis, these procedures produce comparable results for the carbonate content. On the basis of our results we recommend calibrating the Scheibler apparatus at the beginning of each test series or once daily with a pure CaCO_3 standard to minimise errors which are specific to the experimental set-up. Especially in the case of low carbonate content, we recommended using an additional method to determine the carbonate content in addition to the Scheibler-method. The coulometric measurement and the complexometric titration were found to be suitable and accurate methods.