A Simple Method to Determine the Reactivityof Calcium Carbonate in SoilsShuela M. Sheikh-Abdullah1, Daniel R. Hirmas2, and Aaron N. Koop2

1Dept. of Soil and Water Sci., College of Agric. Sci., Univ. of Sulaimani, Sulaimani, KRG, Iraq; 2Dept. of Geography and Atmospheric Sci., Univ. of Kansas, Lawrence, KS

IRAQ

IRAN

JORDAN

SYRIA

TURKEY

KUWAIT

SulaimaniProvince

SAUDI ARABIA

Khalakan

DukanSurdash

Bazian

Khewata

BakhybakhtyareGoizha

Chwarta

Bakrajo

Qaradagh Qaragol

ArbatKanipanka

Penjwen

Barzinjah

Saidsadiq

KhurmalHalabja

50 km

N

Sampling location

We thank Eric Zautner for his help with the coulometry work in this study.

Future work: Oxalate should be reacted with the active carbonate during coulometric titration to continuously monitor the CO2 evolved during the precipitation of calcium oxalate to further simplify and expedite the procedure. Because the reaction of oxalate and CaCO3 proceeds on the order of hours, the shape of the evolved CO2 vs. time curve for the whole reaction should be determined in order to predict total CO2 evolved using only measurements near the start of the reaction.

Dissolved ammonium oxalate will react with ACC in a soil sample to precipitate calcium oxalate via the reaction:

Conventional Approach: Oxalate used up in the reaction is determined as the difference between the initial and final concentrations of oxalate in the extracting solution. Potassium permanganate is used to determine the final con-centration in the solution through the following reaction:

The mass fraction of ACC (fACC) in the sample is determined as:

where Ve = volume of the (NH4)2C2O4 extractant solution used in the procedure, Ce = extractant molar concentration, Vt = volume of KMnO4 used to titrate an aliquot volume (Va) of the extractant after reaction with ACC, Ct = titrant molar concentration, Ms = sample mass, 5/2 = stoichiometric molar ratio of C2O4

2 -:Mn2+ in the previous reaction, and 100 = formula weight of CaCO3.Proposed Method: The mass fraction of ACC is calculated as the difference between the mass fractions of CaCO3 determined from an acidification and coulometric titration procedure in samples reacted and unreacted with (NH4)2C2O4.

The reactive quantity of calcium carbonate—that is, active CaCO3 or ACC—in soils influences adsorption and pre-cipitation reactions, volatization of ammonia, micronutrient availability, and water retention, and is important for pedogenic interpretations. Several procedures have been proposed in the literature for estimating ACC including methods that characterize particle-size distribution of the carbonate fraction, rely on acid dissolution kinetics, or in-volve the surface complexation of Ca. Perhaps the most common of the surface complexation procedures accounts for the difference in concentrations of an oxalate solution measured by titration with potassium permanganate before and after reaction with a sample. However, KMnO4 takes approximately 24 hours to prepare, requires standardiza-tion, and has a relatively short shelf life making the procedure cumbersome for routine analysis. Also, one of the major sources of error is the presence of exchangeable Ca which reacts with oxalate in the extractant. Objective: We sought to develop a simple and reliable method for determining ACC in soils by modifying the oxalate procedure. This modification should remove both the need for KMnO4 and the interference from exchangeable Ca.

The proposed method performed as well as the conventional approach with ACC determined from the difference be-tween the carbonate fractions of samples before and after reaction with ammonium oxalate showing a high correla-tion with the ACC determined from titration of the remaining oxalate using potassium permanganate. The method was considerably faster than the conventional approach and was unaffected by the assumed presence of exchangeable Ca which can significantly affect final oxalate concentrations especially in samples with modest amounts of carbon-ate. Thus, the proposed method is particularly well suited for samples with relatively low amounts of ACC or CCE.

Samples within the upper 30 cm of the land surface were taken from a mou- ntainous province in the Kurdish Region of northern Iraq. Sites ranged in el-

evation from 497 to 1103 m asl. At least three of the sites (Dukan, Goizha, and Bakhyabakhtyare) were formed on limestone parent

materials and one of the sites (Khalakan) was formed on dolomite. Particle-size distributions ranged from 27 to 38% sand, 30 to 56% silt, and 25 to 59% clay. Cation

exchange capacity data were only available on 4 of the samples with clay contents >43% and

ranged from 32.7 to 37.6 cmolc kg-1 with exchangeable Ca comprising 69.7 to 81.2% of those exchange

sites.

The active fraction of the total calcium carbonate equivalent (CCE) in the samples remained relatively constant across the wide range of CCEs (0.4 to 46% CCE). One notable deviation from this trend was the Barzinjah sample with a non-sensical value of nearly 3 times the total CCE in the sample. One plausible explanation for this high value is the likely presence of high ex-changeable Ca; this sample contained approximately 59% clay. With respect to the active fractions of total CCE, the largest differences be-tween the two procedures occurred at the lowest CCE contents. This is due to the increasing sensitivity of the low range of total CCE due to the effect of small deviations in ACC on the fraction of total CCE. In this range (i.e., <20% CCE), significant errors in the conventional approach can occur due to the presence of exchangeable

Ca. The deviation in ACC between the two procedures also suggests the pres-ence of exchangeable Ca in the samples: ACC values determined by the con-ventional approach had a higher slope (0.421) than those determined by the proposed method (0.373) suggesting the new method is insensitive to the presence of exchangeable Ca. Values from the two procedures were highly correlated and exhibited a large coefficient of determination (0.960).

0 10 20 30 40 500.0

1.0

2.0

3.0

Total CCE (%)

Conventional ApproachProposed Method

0 10 20 30 40 500.0

1.0

2.0

3.0

Act

ive

frac

tion

of to

tal C

CE

High exchangeable Ca?

0 10 20 30 40 500.00

50.

050

0.50

0

Total CCE (%)

log(y) = −0.023x − 0.569r2 = 0.271

0 10 20 30 4 00.00

50.

050

0.50

0

Activ

e fr

actio

n to

tal C

CE]

log[Δ of

0 10 20 30 40 50

05

1015

20

Total CCE (%)

ACC

(%) y = 0.421x + 0.554

r2 = 0.828y = 0.373x + 0.192r2 = 0.815

0 10 20 30 40 50

05

1015

20 Conventional ApproachProposed Method

Khalakan site(dolomite)

0 5 10 15 20 25

05

1015

ACC (conventional approach, %)

y = 0.875x − 0.207r2 = 0.960

0 5 10 15 20 25

05

1015

ACC

(pro

pose

d m

etho

d, %

)

95% confidence bands

Mix at 160 cpm in a 30°C waterbath for 3 h

Add 25 mL of0.1 M (NH4)2C2O4

Weigh 1 g soil sample

Grind to <840 μm

Centrifuge at 3kG for 20 min

Supernatant Pellet

Sample 10 mL aliquot Dry at 60°C

Determine remaining [C2O4

2 -] by titration with 0.02 M KMnO4 at 85°C

Acidify with 5 mL of 3 M H2SO4

Calculate ACC from the difference between initial

and remaining [C2O42 -]

Determine gravimetric

moisture

Determine remaining CaCO3 via acidification

and C coulometry

Calculate ACC as the difference between CaCO3 before and after

treatment with (NH4)2C2O4

Separately deter-mine total CaCO3

on original sample

Conventional Approach

Proposed Method

BACKGROUND

SAMPLES

THEORY

PROCEDURE

RESULTS

CONCLUSIONS

ACKNOWLEDGMENTS

CaC2O4(s) + 2NH3 + H2O + CO2CaCO3 + 2NH4+ + C2O4

2 -

2Mn2+ + 10CO2 + 8H2O2MnO4- + 5C2O4

2 - + 16H+

100Ve[CeVa-VtCt(5/2)]VaMs

fACC =