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WWRC 90-06
Measurement of Calcite Ion Activity Products in Soils

Abstract

Soils often show supersaturation with respect to calcite. This study was undertaken to test alternative methods of measuring Ca2+ and CO32- activities to assess their effects on measured ion-activity products (IAP) of calcite in soils. Soils were reacted with atmospheric C02(g) and distilled H20 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 HCI to pH 4.0 (direct-titration method). The second method consisted of acidifying soil extracts to pH 4.0 and trapping the evolved CO; 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 CO32- 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° (Ca2+CO32-) (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 CO32- 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 titratable alkalinity over that of carbonate and bicarbonate alone.

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