| Resum: |
The cation exchange capacity (CEC) is one of the most important properties of clays in terms of their performance in both natural and technical processes. For decades, common methods for determining exchangeable cations have failed when calcareous clays or soils were examined, because calcite is at least partly dissolved throughout the exchange experiment which in turn increases measureable Ca concentrations. As a result, exchangeable Ca and the sum of exchangeable cations by far exceed the amount of negative charges. In the past, the silver-thiourea method (AgTU) has been modified to overcome this problem (AgTU ), but is unsatisfactory as the method is laborious. In the present study three new methods based on two alternative metal complexes, cobalt(III) hexamine (CoHex) and copper(II) triethylenetetramine (Cu-trien), are proposed. The optimum solid/liquid ratios of these methods and the optimum complex concentration of Cu-trien are reported, dependingon the mineralogical composition of the samples. The key development is that the exchange solutions are saturated with respect to calcite prior to the experiment. Approximately 70-90% of the dissolution of calcite present as an admixture in a clay sample is suppressed in the subsequent cation exchange experiment, but not all. The Ca exchange is not suppressed and there is no evidence for any precipitation of this Ca . Three possibilities for how to handle this problem are discussed, one of which is to perform no further correction. The resultinge rror arises from the remainingcalc ite solubility of the different solutions after pre-treatment with calcite. This corresponds to errors of 0. 2-1. 3 (CoHex ) and 0. 7-8. 4 (Cu-trien ) meq/100 gC a for samples with small and large CEC values, respectively. As a consequence of the poor performance of Cu-trien for samples with large CEC, a more concentrated Cu-trien solution was developed which performed much better: 0. 1-0. 8 meq/100 g(Cu-trien ). For Cu-trien and CoHex at least, the errors are in the range of the non-systematic scattering for exchangeable Ca determination. Therefore, the methods suggested provide 'operationally correct' Ca values without additional effort. Moreover, owingto the high selectivity of the index cations applied in the present study, only one exchange step is required, providing a significant advantage over the AgTU method. |
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