Improvements to two routine methods for calcium carbonate determination in soils

Abstract

In the course of routine analytical work, wide discrepancies were noted between results from alternative, established procedures for measuring calcium carbonate (CaCO₃) in soils. In one procedure (Method I), the CaCO₃ content is calculated from the weight of CO₂ lost after treating a sample with excess hydrochloric acid. Results of an investigation using this procedure in our laboratory tended to be inaccurate and poorly reproducible. The method was therefore modified by using as the reaction vessel a plastic vial with pin‐holes in its lid, instead of a glass Erlenmeyer flask with a stopper removed at intervals, to let CO₂ escape. Further, the weighed soil sample was placed in a disposable cup inside the vial of acid, instead of being weighed and transferred. These modifications greatly improved accuracy and reproducibility of results obtained by Method I. In another procedure, the CaCO₃ content was calculated from the pH of a suspension of the soil in dilute acetic acid (Method II). This method tends to give results appreciably greater than zero for acidic soils containing no free lime. This undesirable tendency was reduced after Method II was modified by calibrating it against soils spiked with known amounts of CaCO₃, instead of against CaCO₃ alone. As a result of the modifications, agreement between results for soils analyzed by both methods was greatly improved. Method I is considered more suitable for soils with appreciable free lime, or for liming materials, and Method II for soils with low CaCO₃ content (.5% or less).