Calcite CaCO solubility

Some solids are only weakly soluble in water but dissolve readily in acidic solutions. Copper and nickel sulfides from ores, for example, can be brought into solution with strong acids, a fact that aids greatly in the separation and recovery of these valuable metals in their elemental forms. The effect of pH on solubility is shown dramatically in the damage done to buildings and monuments by acid precipitation (Fig. 16.8). Both marble and limestone are made up of small crystals of calcite (CaCOs), which dissolves to only a limited extent in natural rain (with a pH of about 5.6) but dissolves much more extensively as the rainwater becomes more acidic. The reaction... 

As an example of the effect of these interactions, consider CaCOs (calcite), whose solubility product, Kjp = 4.5 X 10 , gives a calculated solubility of 6.7 X 10 mol/L, and correcting for ionic interactions in the solution yields 7.3 X 10 mol/L. The reported solubility, however, is 1.4 X 10 mol/L, indicating that there must be additional fectors involved. 

The solubility of calcite (CaCOs) (Appendix, Plate 3) in H2O at atmospheric Pcoj is determined by the following reactions. 

During hardening, calcium hydroxide and carbon dioxide form the sparingly soluble compound calcium carbonate that has binder properties. The raw material Ca(OH)2 form naturally occurring calcite CaCOs by the following two steps... 

The activities of Mg++ and Ca++ obtained from the model of sea water proposed by Garrels and Thompson have recently been confirmed by use of specific Ca++ and Mg++ ion electrodes, and for Mg++ by solubility techniques and ultrasonic absorption studies of synthetic and natural sea water. The importance of ion activities to the chemistry of sea water is amply demonstrated by consideration of CaC03 (calcite) in sea water. The total molality of Ca++ in surface sea water is about 10 and that of COf is 3.7 x 1C-4 therefore the ion product is 3.7 x 10 . This value is nearly 600 times greater than the equilibrium ion activity product of CaCO of 4.6 x 10-g at 25°C and one atmosphere total pressure. However, the activities of the free 10ns Ca++ and COj = in surface sea water are about 2.3 x 10-3 and 7.4 x 10-S, respectively thus the ion activity product is 17 x 10 which is only 3,7 rimes greater than the equilibrium ion activity product of calcite. Thus, by considering activities of sea water constituents rather than concentrations, we are better able to evaluate chemical equilibria in sea water an obvious restatement of simple chemical theory but an often neglected concept in sea water chemistry. 

Besides organic growth modifiers. Mg and Sr play a primary role the incorporation of Mg + has been shown to modily the morphology, solubility, and polymorphic expression of CaCOs biominerals. Furthermore, Mg is the principal inhibitor of calcite growth in natural waters. However, the ftmdamental mechanistic interactions of Mg with the calcite surface remain controversial. In vitro experiments show that Mg contents of ACC depend on the Mg/Ca ratio solution, and that Mg stabilizes ACC. ... 

Several mineral phases are produced by phytoplankton and exported to depth in the ocean, associated with and analogous to the production and export of organic matter. First among these is CaCOs, comprised of two mineral phases calcite and the more soluble aragonite (Milliman, 1974 Mucci, 1983). The solubility of calcite depends also on the concentration of magnesium higher... 

As in most parts of today s deep ocean the concentrations of Ca and of CO are nearly constant with water depth, profiles of CaCOs content with depth reflect mainly the increase in the solubility of the mineral calcite with pressure (see Figure 2). This increase occurs because the volume occupied by the Ca and ions... 

Biogenous oozes are either calcareous or siliceous. Calcareous oozes are predominantly the calcitic tests of coccolithophores and/or foraminifera, or the aragonitic tests of pteropods. The solubility of CaCOs increases with decreasing temperature and increasing pressure, and thus with increasing depth in the oceans. Aragonite is 1.45 times more soluble than calcite (Morse and Mackenzie, 1990), so aragonitic oozes are confined to shallower depths than the calcitic oozes. The compensation depth for each mineral is defined as the depth at which the rates of... 

Example 4.12. Solubility of CaCO as a Function of pco2 How does the solubility of CaCOs (calcite) increase, for example, in a groundwater carrier with increase in CO2 pressure The system is characterized by Tableau 4.5. [Ca leq can be calculated from the equilibrium... 

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... 

Tableau 7.5. Solubility of CaCO (Calcite) in Pure Water...

This model has been applied successfully to irrigated arid soils after the refinements of activity coefficients, presence of ion pairs (especially CaCO ), slight differences in solubility products between calcite and aragonite, and the inhibitory effect of organic matter on carbonate precipitation are taken into account. The Pco2 is also a major chemical variable in soils. 

The predominant species in terms of concentration are soluble calcium up to pH 8 to 9 and calcite precipitate at a higher pH. From pH 9 to 12 calcite (or calcium carbonate) precipitates fully at all concentrations as the only solid, except for the 0.5 mM calcium case, when calcite and aragonite coexist. Both precipitates have the same formula, CaCOs, and the difference between the two is probably insignificant (Morel and Hering (1993)) for the membrane experiments. 

Figure 2.41 Calcite solubility vs. in CaCO -COj-HjO solution at 25 C and 1 atm. Dashed line is solubility calculated from pH, squares, from...

Let us see if we can use a combination of the lattiee energy and the hydration energy to understand why a componnd like CaCOs (calcite) is not very soluble in water. 

Dolomite can be thought of as a double salt of MgCOs and CaCOs. Double salts usually have a solubility similar to the solubility of the least soluble simple salt constituent. Because MgCOs is less soluble than CaCOs, dolomite is less soluble than calcite. 

Minerals containing the carbonate ion are plentiful. The principal carbonate minerals are caldte (CaCOs), magnesite (MgCOs), dolomite [MgCa(C03)2], and siderite (FeCOs). Calcite is the principal mineral in limestone rock, large deposits of which occur in many parts of the world. It is also the main constituent of marble, chalk, pearls, coral reefs, and the shells of marine animals such as clams and oysters. Although CaCOs has low solubility in pure water, it dissolves readily in acidic solutions with evolution of CO2. 

Poly(N-vinylpyrrohdone) (PVP) is a water-soluble and imcharged polymer. The presence of PVP has no influence on the polymorphs of CaCOs precipitation, but has a morphological effect on vaterite and calcite at high PVP concentration [101]. The precipitate obtained in the initial presence of P VA was calcite (run 2 of Table 3). The crystalline products obtained with N-vinylpyrrolidone without addition of the radical initiator were calcite with a trace amount of aragonite (run 1 of Table 3). These results indicate that both the polymer and monomer did not exert any influence on the nucleation and crystal growth of CaCOs. On the contrary, in-situ polymerization of the monomer during the precipitation of CaCOs was carried out by the doublejet method (Keiun et al, 2006, personal communication). After addition of the calcium reactants into the aqueous solution of the monomer was completed, an aqueous solution of KPS as a water-soluble radical initiator was added to the reaction mixture after incubation at 30 °C for several minutes (1, 3, or 20 min). All the products obtained by the in-situ polymerization were pre-... 

---