Published online 2 June 2005
Published in Soil Sci Soc Am J 69:1066-1068 (2005)
DOI: 10.2136/sssaj2004.0010
© 2005 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
Soil Chemistry Note
A SIMPLE METHOD FOR MEASURING THE CARBONATE CONTENT OF SOILS
Balázs Horvátha,*,
Oliver Opara-Nadib and
Friedrich Beesea
a Inst. of Soil Sciences and Plant Nutrition, Georg-August Univ. of Göttingen, 2 Büsgenweg, Göttingen, 37077 F.R. Germany
b Faculty of Agriculture & Veterinary Medicine, Abia State Univ., P.M.B. 2000 Uturu, Nigeria
* Corresponding author (bhorvath{at}emk.nyme.hu)
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ABSTRACT
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The traditional measurement of the carbonate content of soils with the Scheibler apparatus is slow and labor intensive. Our objective was to develop a relatively simple and accurate method. The pressure calcimeter principle was used; the pressure change caused by the reaction between HCl and a weighed soil sample was measured with a digital plunge-in manometer through a silicone-rubber septum placed on a screw-capped tube. The method was calibrated with pure CaCO3. The correlation between the measured and weighed amounts was excellent (r2 = 0.999). The carbonate content of a natural soil sample was measured 10 times; the coefficient of variation was 1.9%. Ten ISE (International Soil-Analytical Exchange) samples were analyzed with this method which contained a carbonate content of 3 to 170 g kg–1. Satisfactory results were obtained, indicating the suitability of the proposed method for practical purposes.
Abbreviations: ISE, International Soil-Analytical Exchange • MAD, median of the absolute deviation
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INTRODUCTION
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THE CARBONATE CONTENT of soils is an important parameter determining the chemistry of a soil. The measurement of this parameter belongs to the standard investigation program of soil analysis. A number of methods have been described in the literature (Loeppert and Suarez, 1996), and most of them are based on the reaction between carbonates and strong acids, which results in carbonate dissolution and CO2 development. There are two simple ways to measure the amount of the released gas: (a) measure the volume of the gas under isobarometric conditions such as the use of a Scheibler apparatus, and (b) measure the pressure of the gas under isochore conditions. This so-called pressure calcimeter principle is used by several methods described in the literature (e.g., Williams, 1948; Martin and Reeve, 1954; Woodward, 1961). These procedures are quite old and use mechanical techniques. Wagner et al. (1998) developed a computerized volumetric inorganic carbon analysis system, which combines a pressure transducer with a personal computer, data acquisition board, and appropriately developed software. This method is on the one hand rapid (20 samples h–1) and precise (very good correlation with other standard methods), but on the other hand it is quite complex and expensive. We wanted to develop a more simple method, which is cost-efficient even by a moderate quantity of samples (up to about 100). Thus, our system requires equipment that is present in most soil laboratories. In addition to simplicity, rapidity, and accuracy were also important goals.
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Materials and Methods
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A screw-capped glass test tube (10–30 mL volume) was used as a reaction vessel. To achieve better sensitivity, the size of the reaction vessel was chosen according to the expected carbonate content of the sample. The test tube was closed with a plastic screw cap with a silicone-rubber septum (Fig. 1a)
. The pressure was measured with a digital plunge-in manometer with a measurement range of 0 to approximately 150 kPa above the barometric pressure. The needle in the manometer is placed in such a manner that during repeated use, the septum is most likely pierced in a different place (Fig. 1b). In this way, the septum remains airtight even after several measurements. The manometer we used measures pressure difference, although an absolute air pressure measuring instrument can also be used. The septum-equipped screw cap and the digital manometer is normally used for tensiometer measurements, a common piece of equipment in laboratories where the soil water regime is examined.
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Fig. 1. (a) A diagram of the carbonate content measuring apparatus. 1, Test tube; 2, septum (enlarged: laminated structure; a silicon layer between two rubber layers); 3, screw cap (enlarged top view: hole in the middle); 4, plastic container, with 3.5 M HCl; 5, sample. (b) The carbonate content measuring apparatus after mixing the sample and the HCl. 1, Mixture of sample and the acid; 2, developing CO2; 3, pressure sensor; 4, digital manometer (888 represents the displayed pressure).
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Different amounts of reagent-grade (dried at 105°C) CaCO3 were weighed in the reaction vessel. A plastic tube containing 1.0 mL of 3.5 M HCl was carefully inserted into the reaction vessel. Care should be taken that the acid does not contact the sample. The test tube was closed airtight and the silicon septum was pierced with a needle to release the pressure increase that occurred during securing of the cap. The reaction vessel was rotated and thoroughly shaken manually to mix carbonate with the acid. The pressure was measured after approximately 5 min of reaction time, when the reaction between the acid and CaCO3 had ceased, and the increased temperature resulting from the reaction had cooled to the ambient temperature. The barometric pressure and the temperature of the ambient air were recorded. By knowing the precise volumes of the tube, the HCl, and the plastic container, the reaction volumes were calculated. Using these values, and Ideal Gas Law, and Henry's Law, it was possible to determine the pressure increase caused by different amounts of carbonate. The following form of Henry's Law was used: nCO2 = KH x PCO2 x VHCl, where nCO2 is the released amount of CO2 (in mol), KH is Henry's constant (0.003524 mol MPa–1 L–1), PCO2 is the partial pressure of CO2 (in MPa), and VHCl is the volume of the acid (in L).
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Results and Discussion
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The comparison between the calculated and the weighed amount of CaCO3 is shown in Fig. 2
. High r2 value (0.9992) was obtained for the fitted line (y = 0.9986x, for n = 12). The calculated values were slightly smaller than the correct values due to an unaccounted small volume of air in the pressure sensor, which should have been added to the reaction volume. When this small volume (25 µL) was considered, the slope of the fitted lines was equal to 1.0000.
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Fig. 2. Comparison of the measured (msd.) and added values by different amounts of CaCO3. The equation of the fitted line is y = 0.9986x, r2 = 0.9992, and n = 12.
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A similar procedure as described above was employed to measure the carbonate content of soil samples. Table 1 shows the amount of the sample to be used per cubic centimeter of reaction volume. Depending on the expected carbonate content, the configuration of the equipment and the sample size should be selected to achieve good sensitivity and to stay in the measurement range of the manometer. The configurations which we have used are listed in Table 2. For low-carbonate content (<20 g kg–1) samples, we have added 3.0 mL instead of 1.0 mL of HCl. This has two advantages: (i) the larger soil sample (2.0 g instead of 0.5 g) is completely immersed by 3.0 mL of acid, and (ii) the reaction volume is reduced by the increased volume of the acid, providing better sensitivity. The calculated values of pressure increase for different contents of carbonate are shown in Fig. 3
. The volume of the soil sample should be considered in the calculation. We assumed the specific gravity of the soil to be 2.7 g cm–3. This was especially important for large samples of low carbonate content (<20 g kg–1).
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Table 1. Recommended weight of soil sample for each cm3 of reaction volume depending on the expected carbonate content.
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Fig. 3. Calibration lines for different soil carbonate contents. Sample size is 0.50 g. For the explanation of enlarged sample and reduced sample, see Table 2.
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Because of the small amount of soil used, it is very important to sieve and homogenize the sample carefully. FeCl2 or FeSO4 can reduce the oxidation of the organic material (Loeppert and Suarez, 1996), which should be added to samples of high organic content. In samples where dolomite [CaMg(CO3)2] is expected, the reaction time should be much longer, up to 2 h (Evangelou et al., 1984). In such cases, the reaction can be accelerated by placing the reaction vessels in a hot water bath (Woodward, 1961). In that case, the calculations should include the effects of the higher temperature on the pressure increase. We observed that the use of water bath complicated the measurements. When a water bath is used, it is important to pay attention to the septum that it does not get wet, otherwise water may enter the manometer needle and give erroneous results. It is recommended that replicate measurements be made which will help to trace and avoid errors, such as those resulting from any leak in the septum. It is also possible to inject the HCl with a syringe through the septum. In that case, the pressure increase caused partly by reduction in the test tube volume through the added HCl should be considered. Care should be taken that after injecting the acid to the sample that no air enters the syringe through the needle. This may not always be possible because of a vigorous reaction. Thus, the common way of acid addition is recommended.
To test our method, we examined natural carbonate of soil samples of the ISE. To asses our results, we compared our measurements with the results published in ISE Reports (International Soil-Analytical Exchange, 2004a, 2004b, and 2004c). We calculated median values after discarding statistical outliers. The sample 970/2003 was measured 10 times. The median of these values was 86.5 g kg–1 carbonate content. The median of the absolute deviation (MAD) was 1.3 g kg–1. The median of the results of about 130 laboratories was 86.7 g kg–1, and the MAD was 4.3 g kg–1. In addition, we examined nine other ISE soil samples, one time each (Table 3). The difference between our results and those of the ISE report was 6% or less. There was, however, one exception: sample 930/2004, where the measured value was 0.7 and the reported value is 3.0 g kg–1. The ISE reported a relatively large MAD value for this sample, the individual laboratories measured values ranged from 0.0 to 70 g kg–1. In conclusion, it is suggested that the proposed method is easy to handle, rapid, accurate, and the necessary equipment is easy to obtain. This method can be recommended for routine soil analysis.
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Table 3. Carbonate content of the ISE (International Soil-Analytical Exchange) soil samples measured with the proposed method.
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ACKNOWLEDGMENTS
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The first and second authors express their sincere gratitude to the German Academic Exchange Service (DAAD) for the financial grant for a visit to the Institute for Soil Science and Forest Nutrition, Georg-August-University of Göttingen.
Received for publication January 7, 2004.
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REFERENCES
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- Evangelou, V.P., L.D. Whiting, and K.K. Tanji. 1984. An automated manometric method for quantitative determination of calcite and dolomite. Soil Sci. Soc. Am. J. 48:1236–1239.[Abstract/Free Full Text]
- International Soil-Analytical Exchange. 2004a. Annual report 2003 [Online]. Available at www.wepal.nl [verified 21 Jan. 2005]. Wageningen Evaluating Programs for Analytical Laboratories, Wageningen University Environmental Sciences, Wageningen, the Netherlands.
- International Soil-Analytical Exchange. 2004b. Quarterly report 2004.1 [Online]. Available at www.wepal.nl [verified 21 Jan. 2005]. Wageningen Evaluating Programs for Analytical Laboratories, Wageningen University Environmental Sciences, Wageningen, the Netherlands.
- International Soil-Analytical Exchange. 2004c. Quarterly report 2004.2 [Online]. Available at www.wepal.nl [verified 21 Jan. 2005]. Wageningen Evaluating Programs for Analytical Laboratories, Wageningen University Environmental Sciences, Wageningen, the Netherlands.
- Loeppert, R.H., and D.L. Suarez. 1996. Carbonate and gypsum. p. 437–475. In D.L. Sparks et al. (ed.) Methods of soil analysis: Part 3—Chemical methods. SSSA Book Ser. No. 5. SSSA and ASA, Madison, WI.
- Martin, A.E., and R. Reeve. 1954. A rapid manometric method for determining soil carbonate. Soil Sci. 79:187–197.
- Wagner, S.C., D.J. Hanson, A. Olness, and W.B. Voorhess. 1998. A volumetric inorganic carbon analysis system. Soil Sci. Soc. Am. J. 62:690–693.[Abstract/Free Full Text]
- Williams, D.E. 1948. A rapid manometer method for the determination of carbonate in soils. Soil. Sci. Soc. Am. Proc. 13:127–129.
- Woodward, L. 1961. A manometric method for the rapid determination of lime in soils. Soil Sci. Soc. Am. Proc. 25:248–250.
TOP
ABSTRACT
INTRODUCTION
