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Measurement of Calcite Ion Activity Products in Soils
Author(s) -
Reddy K. J.,
Lindsay W. L.,
Workman S. M.,
Drever J. I.
Publication year - 1990
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1990.03615995005400010010x
Subject(s) - calcite , chemistry , soil water , titration , supersaturation , distilled water , carbonate , environmental chemistry , calcium carbonate , mineralogy , inorganic chemistry , geology , chromatography , soil science , organic chemistry
Soils often show supersaturation with respect to calcite. This study was undertaken to test alternative methods of measuring Ca 2+ and CO 2‐ 3 activities to assess their effects on measured ion‐activity products (IAP) of calcite in soils. Soils were reacted with atmospheric CO 2 (g) and distilled H 2 O 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 CO 2 gas in a NaOH + BaCl 2 solution (CO 2 gas release method). The IAP values for calcite were calculated in two ways. First, Ca 2+ 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 (Ca 2+ )(CO 2‐ 3 ) (equilibrium constant) of −8.48 for calcite. These results suggest supersaturation with respect to calcite. Secondly, when Ca 2+ activity was measured with a Ca 2+ ‐specific ion electrode and CO 2‐ 3 activity was estimated by the CO 2 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 Ca 2+ and contributed to titratably alkalinity over that of carbonate and bicarbonate alone.