Inorganic Carbon Analysis by Modified Pressure-Calcimeter Method

Abstract

Soil organic C (SOC) analyses using high temperature induction furnace combustion methods have become increasing popular because of advances in instrumentation. Combustion methods, however, also include C from CaCO3 and CaMg(CO3)2 found in calcareous soils. Separate analysis of the inorganic C (IC) must be done to correct C data from combustion methods. Our objective was to develop a efficient and precise IC method by modification of the pressure-calcimeter method. We modified the method by using Wheaton serum bottles (20-mL and 100-mL) sealed with butyl rubber stoppers and aluminum tear-off seals as the reaction vessel and a pressure transducer monitored by a digital voltmeter. Our gravimetric IC determination of six soils showed a strong correlation when regressed against IC from the modified pressure-calcimeter method (slope of 0.99, r2 = 0.998). The method detection limit (MDL) was 0.17 g IC kg−1 for the 20-mL serum bottles and the limit of quantification (LOQ) was 0.30 g IC kg−1 The 100-mL serum bottle had a MDL of 0.42 with a LOQ of 2.4 g IC kg−1 When using a 100-mL Wheaton serum bottle as the reaction vessel with a 0.50-g sample size, soils containing up to 120 g IC kg−1, which represent a 100 % CaCO3 equivalent, can be analyzed within the V output range of the pressure transducer. Soil organic C determined by subtraction of IC from total C from combustion analysis correlated well with SOC determined by the Walkley-Black.