145 SOLUBILITY OF CALCIUM CARBONATE 10.4 Substituting into equation 10.58, dropp

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145 SOLUBILITY OF CALCIUM CARBONATE 10.4 Substituting into equation 10.58, dropping the predicted Ca concentration in the Mississippi (H+) and combining the IOH terms as River only to 22.0 mg/L and therefore does not before, we obtain: resolve the discrepancy. The water in the Mississippi River, like all 14.600) 2 X 10 natural solutions, contains a mixture of ions and (2 x 106.385)[OH-P DOH-2 2+ molecules derived from different sources. Its Ca (10.69) content originates from the solution of calcium 102 634[OH-] carbonate calcite and aragonite) calcium sul Note that this equation is stated in terms of the fate (anhydrite and gypsum), and calcium phos activities of OH which express the concentra phate (apatite) and from the transformation of tions of the other ions. By solving equation 10.69 plagioclase to kaolinite or other clay minerals 5.644 In addition, 2+ may be adsorbed on clay min Ca as before, we obtain OH 10 mol/L. This value yields for the other ions erals and on the surfaces of other small mineral particles in suspension in the water. Finally, cal LHCO1] 9.42 x 10-4 mol/L cium occurs in many complex ions and mole COA 1.07 x 10-5 mol/L cules whose presence permits the calcium concentration of natural waters to exceed the [Ca 4.17 x 10-4 mol/L concentration of Ca2 based on the solubility of CH 10-83se or pH 8.36 its principal minerals. A partial list of the com plex ions of Ca includes CaHCO CaCO The ionic strength, based on these results CaC CaC CaNO Ca(NO3) CaOH is I 1.3 x 10 which does not differ enough from Ca(OH) CaPO CaH PO CaHPO the previous estimate to justify a repetition of the CaP,O CaHP,O7, CaOHP,O CaSO 2+ calculation. Therefore, the concentration of Ca (Lindsay, 1979). The point is that natural waters in the solution is: are very complex and their compositions are not xplainable by the solubility of a few common 4.17 X 10 (Ca? 4.86 x 10-4 mol/L minerals. We therefore abandon the effort to 0.857 explain the concentration of calcium or any other or 19.5 mg/L. The solubility (S) of calcite in water element in the Mississippi River. Instead, we use in equilibrium with CO2 of the atmosphere at the discrepancy between the Ca 2+ concentration 25 OC is of a saturated calcite solution and the calcium n the water of the Mississippi concentration 4.86 X 10 100.0787 4.86 x 10-2 g/L River to consider how the solubility of calcite is In addition, note that the pH of this solution is affected by variations in the partial pressure CO 8.36, thus confirming our prediction based on the pH, and the temperature. the effect of hydrolysis of the carbonate ion of The reaction of calcite with carbonic acid calcite. between pH values of 6.35 and 10.3 can be repre The chemical analysis in Table 10.2 indicate sented by the equation: that the water in the Mississippi River has a Ca CaCO3 H,CO Cat 2HCO (10.70) 2+ concentration of 34 ppm, which is nearly 75 where carbonic acid is also in equilibrium with larger than the value we obtained above. The dif aqueous CO2, which in turn is in equilibrium ference is partly due to the higher ionic strength of water in the Mississippi River (I 34.4 x 10 3) with CO2 gas: and to the correspondingly lower activity coeffi H CO CO2 (aq) H2O (10.71) cients (y 1 0.759) 0.936 However, Y-2 CO2 (aq) CO2 (g) 10.72) allowance for the higher ionic strength increases

Explanation / Answer

[CO2] = 4.0 X 10-4 mol/L

Molecular weight of CaCO3 (Calcite), M = 100.0787 g/mol

Solubility of calcite in water = [CO2] X M = 4.0 X 10-4 mol/L X 100.0787 g/mol = 4.00 X 10-4 g/L

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