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.