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Measurement of Calcite Ion Activity Products in Soils

Wyoming Water Research Center, P.O. Box 3067, Univ. of Wyoming, Laramie, WY 82071

W. L. Lindsay^* and S. M. Workman

Dep. of Agronomy, Colorado State Univ., Fort Collins, CO 80523

J. I. Drever

Dep. of Geology and Geophysics, Univ. of Wyoming, Laramie, WY 82071

* Corresponding author.

ABSTRACT

Soils often show supersaturation with respect to calcite. This study was undertaken to test alternative methods of measuring Ca²⁺ and CO^2-₃ activities to assess their effects on measured ion-activity products (IAP) of calcite in soils. Soils were reacted with atmospheric CO₂(g) and distilled H₂O for 1, 3, and 7 d. The soil extracts were analyzed for pH, electrical conductivity (EC), cations, anions, and dissolved organic carbon (DOC). Carbonate activities in soil extracts were obtained by two methods. The first consisted of titrating the soil extracts with standard HCl to pH 4.0 (direct-titration method). The second method consisted of acidifying soil extracts to pH 4.0 and trapping the evolved CO₂ gas in a NaOH + BaCl₂ solution (CO₂ gas release method). The IAP values for calcite were calculated in two ways. First, Ca²⁺ activity was obtained by speciating total soluble Ca while CO^2-₃ activity was obtained from the direct titration of soil extracts. This method gave mean log IAP values of calcite ranging from –7.90 to –7.78 compared with a log K^o (Ca²⁺)(CO^2-₃) (equilibrium constant) of –8.48 for calcite. These results suggest supersaturation with respect to calcite. Secondly, when Ca²⁺ activity was measured with a Ca²⁺-specific ion electrode and CO^2-₃ activity was estimated by the CO₂ gas release method, the mean log IAP values of calcite ranged from –8.45 to –8.35, which is very near equilibrium with calcite. The apparent supersaturation in the first case was probably caused by DOC, which both complexed Ca²⁺ and contributed to titratably alkalinity over that of carbonate and bicarbonate alone.

Received for publication December 10, 1988.