Inorganic Carbon Analysis by Modified Pressure-Calcimeter Method

Wagner et al. (1998) showed that inorganic C determined as the difference between total C by combustion Soil organic C (SOC) analyses using high temperature induction furnace and SOC where soils are acidified before comfurnace combustion methods have become increasing popular because of advances in instrumentation. Combustion methods, however, also bustion had a close correlation with their volumetric IC include C from CaCO3 and CaMg(CO3 )2 found in calcareous soils. system. However, at concentrations 5.0 g IC kg 1, the Separate analysis of the inorganic C (IC) must be done to correct C combustion method difference was less precise. This dedata from combustion methods. Our objective was to develop a effitermination also requires the sample be run twice through cient and precise IC method by modification of the pressure-calcimea dry combustion system which dramatically increases the ter method. We modified the method by using Wheaton serum bottles cost of the analysis. Acidification of soils prior to analysis (20-mL and 100-mL) sealed with butyl rubber stoppers and aluminum by dry combustion is also time-consuming and presents tear-off seals as the reaction vessel and a pressure transducer monidifficulties because destruction of organic matter can tored by a digital voltmeter. Our gravimetric IC determination of six occur (Nelson and Sommers, 1996). soils showed a strong correlation when regressed against IC from the We propose a modification of the reaction vessel which modified pressure-calcimeter method (slope of 0.99, r 2 0.998). The method detection limit (MDL) was 0.17 g IC kg 1 for the 20-mL facilitates effective analysis of batch runs of 80 to 120 serum bottles and the limit of quantification (LOQ) was 0.30 g IC samples per day. Our main objective was to develop a kg 1. The 100-mL serum bottle had a MDL of 0.42 with a LOQ of fast and routine method that could produce quantitative 2.4 g IC kg 1. When using a 100-mL Wheaton serum bottle as the analysis of total IC for both extremes of the analytical reaction vessel with a 0.50-g sample size, soils containing up to 120 g range. Five subobjectives were (i) to compare gravimetIC kg 1, which represent a 100 % CaCO3 equivalent, can be analyzed rically-determined IC with IC determined by our modiwithin the V output range of the pressure transducer. Soil organic C fied pressure-calcimeter method; (ii) to determine the determined by subtraction of IC from total C from combustion analyMDL and the LOQ; (iii) to evaluate the effect of reducsis correlated well with SOC determined by the Walkley-Black. ing the sample size on precision; (iv) to evaluate the time required to neutralize a range of carbonate-containing S carbonate analysis methods typically utilize acid soils; (v) to compare the SOC values obtained from dissolution with the consumption of H or the detercombustion analysis corrected with this method to those mination of Ca and Mg, or CO2 production (Loeppert SOC values obtained from a Walkley–Black analysis. and Suarez, 1996). Quantitative determination of carbonates can be accomplished using the gravimetric method MATERIALS AND METHODS based on the reaction of HCl with soil carbonates and the gravimetric loss of CO2 as described by the U.S. Apparatus Description Salinity Laboratory Staff (1954). This method however The modified calcimeter apparatus (Fig. 1) consists of a presis not practical for large sample runs and is not suitable sure transducer (Model 280E Serta, 0-105 kPa, output .03–5.03 for low IC contents. The measurement of CO2 is preferred VDC from Setra Systems, Inc., Boxborough, MA) connected because of its simplicity and because it is a direct measure to a power supply (24 V DC) with 14 gauge wire, and a digital of carbonate when precautions are used to eliminate orvoltmeter wired in line to monitor output from the transducer. ganic matter oxidation which would give a positive interAttached to the base of the pressure transducer is 20 cm of clear laboratory Tygon R-3603 tubing (9.5-mm i.d.) connected ference (Loeppert and Suarez, 1996). The analysis of CO2 to an 18 guage Luer-loc hypodermic needle with a particle by pressure-calcimeter method described by Loeppert filter (0.6 m for removing particles from carrier gas, Leco and Suarez (1996) is a direct, accurate and inexpensive #768-980; Leco Co., St. Joseph, MI) spliced in the middle of method, but the complexity of the pressure calcimeter the tubing to prevent any reflux from reaching the pressure apparatus makes large sample runs impractical. The voltransducer (Fig. 1). A 20-mL wheaton serum bottle (Wheaton umetric IC analysis system described by Wagner et al. Science Products, Millville, NJ) serves as the reaction vessel (1998) improved the pressure-calcimeter apparatus by for soils containing up to 18 g IC kg 1 and a 100-mL bottle utilizing a pressure transducer, which is monitored by a for soils above 18 g IC kg , in which a 2-mL (0.50 dram) data acquisition card connected to a personal computer. autosampler vial is inserted containing 2-mL of the acid reHowever, the reaction vessel used by Wagner et al. (1998) agent of 6 M HCl containing 3% by weight of FeCl2 · 4H2O (Fig. 2). The FeCl2 is used to eliminate the release of CO2 is susceptible to leaks, and therefore requires leak checks from organic matter (Loeppert and Suarez, 1996). The 2 mL while the reaction takes place in a constant temperaof acid reagent is able to neutralize up to 72 g IC kg . ture bath. 1 Mention of a commercial product does not impy either USDA– L.A. Sherrod, G. Dunn, Great Plains Systems Res. Unit, USDAARS or Colorado State University endorsement of that product or ARS, P.O. Box E, Fort Collins, CO 80522; G.A. Peterson, Dep. of similar products. Soil and Crop Science, Colorado State Univ., Fort Collins, CO 80523; R.L. Kolberg, USDA-ARS, P.O. Box 1109, Sidney, MT. Received 6 Abbreviations: IC, inorganic C; LOQ, limit of quantitation; MDL, July 2000. *Corresponding author ([email protected]). method detection limit; NAPT, North American Proficiency Testing program; RSD, relative standard deviation; SOC, soil organic C. Published in Soil Sci. Soc. Am. J. 66:299–305 (2002).