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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