Equilibrium constant calcite, solubility

Table 15.1 Solubility as (S = 35) Stoichiometric Equilibrium Constants (-logKip) for a Function of Temperature and Pressure Where K p Calcite and Aragonite Has Units of (mol/kg) ...

Figure 15.8 Equilibrium constant for the solubility of calcite [reaction (15.25)] as a function of temperature obtained from Line 1 Measured solubility (fitting equation assumes ACp is linear in temperature). Line 2 Thermodynamic data assuming ACp equals zero. Line 3 Thermodynamic data assuming ACp is constant. Line 4 Thermodynamic data assuming ACp is constant with corrected CP(CO2-).

Hiickel theory [or the Giintelberg or Davies equation (Table 3.3)] may be used to convert the solubility equilibrium constant given at infinite dilution or at a specified / to an operational constant, valid for the ionic strength of interest. In seawater solubility equilibrium constants, experimentally determined in seawater, may be used. For example, the CaC03 calcite solubility in seawater of specified salinity may be defined by = [Ca " ] [CO f ], where [Caj ] and [C03f ] are the total concentrations of calcium and carbonate ions, for example,... 

The equilibrium constants used to characterize solubility equilibria are summarized in Table 7.2 and for the CaCOs (calcite) system in Table 4.3.) The various solubility expressions (6-11, Table 7.2) are interrelated and can all be expressed in terms of the conventional solubility product K q. A listing of the different formulations should indicate merely that the solubility can be characterized by different experimental variables. For example, we can fully define a solubility equilibrium with a solid carbonate by Pco2 [Me " ], and [H ] equation 10, Table 7.2 by pco2 [Me ], and [HCO ] (Equation 9, Table 7.2) or by [H ], [Me ], and [HCOb ] (Equation 7, Table 7.2). Parameters such as these are more accessible to direct analytical determination than... 

The reader will observe that equilibrium constants for the same reactions may differ depending on the source of the data. (Often, different stability constants for ion pairs are the reason for the difference.) For the solubility of calcite and aragonite in seawater, see Table A6.4. 

It is useful to construct a graph relating carbonate mineral solubilities to CO2 pressure. This can be done for calcite starting with equilibrium constant expression (6.2) above. If done rigorously, the derivation accounts for the effects of ion activity coefficients and the presence of CaHCOI and CaCOf ion pairs and of CaOH. Considering all complexation, the exact charge-balance equation for a pure water in which calcite is dissolving is... 

This expression is the familiar solubility product. This equilibrium constant does not contain a term for CaCOa (calcite). To the extent that the calcite is pure (does not contain dissolved impurities) and consists of particles large enough that surface effects are unimportant, this equilibrium does not depend on the "concentration" of calcite or particle size. Exceptions to this case are important in the formation of cloud droplets (where the particle size dependence is known as the Kelvin Effect) and in the solubility of finely divided solids. 

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