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Simple, Simultaneous Gravimetric Determination of Calcite and Dolomite in Calcareous Soils

Juan B. Rodriguez
Soil Science Society of America journal 2017 v.81 no.6 pp. 1615-1620
calcareous soils, calcite, carbonates, dolomite, equipment, ferrous chloride, glass, rubber
Core Ideas The carbonate meter (CM) method was modified to measure calcite and dolomite. The modified carbonate meter (MCM) method efficiently separated calcite and dolomite. The MCM method can be done in any soil testing lab. The MCM technique not only measures calcite and dolomite but also total inorganic C The MCM categorized calcareous soils as calcitic, dolomitic, and calcitic‐dolomitic. Literature pertaining to the determination of calcite and dolomite is not modern and generally describes slow methods that require expensive specialized apparatus. The objective of this paper is to describe a new method that requires no specialized equipment. The separation of calcite and dolomite was done by placing 0.2000 ± 0.0200 g of calcareous soil, 6‐mL glass vial with 4.0 mL of fresh 4 mol L–¹ HCl–3% FeCl₂ 4H₂O in a 50‐mL Erlenmeyer flask. The Erlenmeyer flask was capped with the tubed rubber stopper and the carbonate meter's (CM) initial weight was recorded to the accuracy of four decimal places. A second weight was recorded after 30 s of reaction, a third weight was recorded after 30 min of reaction, and the last weight was taken after 22 to 24 h. The linear regressions and correlation coefficient results for calcite and dolomite in calcite–dolomite–soil mixtures with the modified carbonate meter (MCM) were excellent. The precision for calcite was <5% in 92.6% of samples and that for dolomite was <10% in 98.8% of samples. Recovery for calcite was 100 ± 10% in 86.4% of samples and recovery for dolomite was 100 ± 10% in 97.5% of samples. The MCM technique efficiently separated calcite and dolomite in calcareous soils. This procedure is recommended for routine analysis of calcite and dolomite concentrations up to 450 g kg–¹. The MCM is easy to follow with good precision and recovery.