Calcite solubility product

Figure 2 Rate of carbonate dissolution from deep-sea sediment versus (1 — O). SoUd Une from Hales and Emerson (1996), dotted line from Keir (1980), dashed line from Atlantic Ocean, and dotted and dashed line from Pacific Ocean sediment results of Morse (1978). Note that Hales and Emerson (1996) used a different calcite solubility product.

Calcite solubility product constants range over 0.3 log units. The major tabulated differences are in the free energies of calcite and of the calcium ion. The most reliable measurement and evaluation of calcite solubility is that of Plummer and Busenberg (31). They found log Ksp = -8.48(+ 0.02) at 298.15 K which agrees excellently with the CODATA value of -8.47. The main source of error can be traced to a 2 kJ/mol difference between the CODATA and USGS values for the enthalpy of formation of calcite from the elements. The recent CODATA revisions of the calcium ion and calcite values take into account many different properties including the Plummer and Busenberg solubility value (31). Hence, they are the most reliable values for this system. 

Usually, however, the distribution coefficients determined experimentally are not equal to the ratios of the solubility product because the ratio of the activity coefficients of the constituents in the solid phase cannot be assumed to be equal to 1. Actually observed D values show that activity coefficients in the solid phase may differ markedly from 1. Let us consider, for example, the coprecipitation of MnC03 in calcite. Assuming that the ratio of the activity coefficients in the aqueous solution is close to unity, the equilibrium distribution may be formulated as (cf. Eq. A.6.11)... 

Let us equilibrate some pure calcite crystals with an aqueous phase at T = 25 °C and P = bar. Calcite partially dissolves to reach the solubility product (eq. 8.77). 

Each compartment of the cell contains a mixture of solid PbC03 ( sp = 7.4 X 10 l4) and either calcite or aragonite, both of which have Ksp 5 X 10-9. Each solution was buffered to pH 7.00 with an inert buffer, and the cell was completely isolated from atmospheric C02. The measured cell voltage was — 1.8 mV. Find the ratio of solubility products, Kif> (for calcite)/ (for aragonite)... 

Buffering of pH during the early heavier precipitation of calcite and sepiolite is clear and is reflected in a near constancy of HCCV and CO32. However, after Ca2+ and Mg2+ are substantially reduced, the pH again rises with further concentration. Because the Sierra waters are so low in sulfate, gypsum does not precipitate abstraction of Ca2+ as calcite never permits the solubility product of gypsum to be exceeded. 

Figure 1. Saturation of lake water with respect to calcite. Double line (C for calcite, A for aragonite) represents the solubility product as a function of temperature...

Increase in water depth will shift the equilibrium and between 3.000 and 4.000 m the ocean starts to become unsaturated with respect to aragonite and calcite due to pressure effects122-124. In contrast, the majority of river and lake waters are unsaturated at all depths, because precipitation of CaC03 will readily take place the moment saturation is exceeded. The solubility products are92 ... 

During the past decade there have been four major studies of calcite solubility in dilute solutions, three of which also included aragonite (Jacobsen and Langmuir, 1974 Berner, 1976 Plummer and Busenberg, 1982 Sass et al., 1983). These studies were aimed at more accurate and precise determinations of the thermodynamic solubility product for these minerals. Earlier work on this topic was well summarized by Jacobsen and Langmuir (1974). 

Jacobsen and Langmuir (1974) determined a value for pKSp (25°C) for calcite of 8.42 0.01, whereas Berner s (1976) value was 8.45 0.01. Berner also determined the pKSp for aragonite at 25°C to be 8.28 0.03. An aspect of particular interest of both studies was that to obtain internal consistency for the carbonic acid system or constant values for the solubility products over the range of conditions studied, it was necessary to neglect ion pair formation. The potentially important ion pairs that could have formed in the experimental solutions are CaHCC>3+ and CaC03°. The former is by far the most important species, and a vast body of previous literature supported its existence (see Plummer and Busenberg, 1982, for summary). 

Sass E Morse J.W. and Millero F.J. (1983) Dependence of values of calcite and aragonite thermodynamic solubility products on ionic models. Amer. J. Sci. 283, 218-229. 

Prepare a graph of log Kf0 against the mole fraction of dolomite for magnesian calcite. Use 0 < x < 0.2 in the graph and find the value of x for which the solid phase has its smallest solubility product constant. Show that ln[(Mg2 )/(Ca2 )] = 2 (d In K s0 /dx)T P and determine whether a value of x exists such that (Mg2 ) = (Ca2 ). (Hint Combine Eqs. 3.27 and 3.35 aq applied to magnesian calcite in Eq. 3.36. Note that the slope of a log K s0 plot against x vanishes when the log solubility product constant has a minimum value.)... 

Using the solubility product constants for calcite and strontianite and assuming a calcium activity of 1.6 mmol/L, a distribution coefficient of 0.8 for strontium and 0.98 for calcite, and a ratio of 50 1 (=0.02) in the solid-solution mineral, the following equation gives the activity of strontium ... 

In the presence of both calcite and dolomite, dolomite dissolves while calcite precipitates (Table 42, Fig. 50), because the solubility product of CaC03 is exceeded. This process is called incongment dissolution (see chapter 1.1.4.1.3). The maximum of dissolution/ precipitation shifts to lower temperatures (6°-7°C). 

Calculation of Apparent Calcium Carbonate Solubility Products The precise determination of the solubility of calcite... 

The equation developed by Ingle (32) for determining the effect of pressure on the apparent solubility product of calcite in seawater is ... 

Here, K2 is the dissociation constant for bicarbonate, Kc is the solubility product constant for calcite, ki is the rate constant for reaction (11), and the subscript (s) refers to concentrations in the surface adsorption layer. Chou et al. (1989) suggested a... 

The apparent solubility products of calcite and aragonite have been determined repeatedly... 

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