Calcium carbonate solubility-product constant

If S moles of CaCC>3 dissolve in a liter of water, then S moles each of calcium ion and carbonate ion form. With these ion concentrations equal to S, the solubility of CaCC>3 is calculated as 9.3 x 10 5 M. The higher solubility of magnesium carbonate in water, 6.3 x 10 3 M, results from the larger solubility product constant. Nevertheless, both of these carbonate salts are rather insoluble, and the excess carbonate anions provided by the sodium carbonate effectively precipitate the calcium and magnesium ions from solution. 

Note that the solubility product constant expression is true only when there is undissolved, solid material still present in the system. In other words, we are talking about (in this case) solid calcium carbonate in equilibrium with a saturated solution of calcium carbonate. 

Calcium hardness may either be carbonate or noncarbonate. The solubility product constant of CaC03 is K p = [Ca ][C03 ] = 5(10 ) at 25°C. A low value of the K p means that the snbstance has a low solnbility a valne of 5(10 ) is very low. Because of this very low solubility, calcium hardness is removed throngh precipitation of CaCOs. Becanse there are two types of calcinm hardness, there corresponds two general methods of removing it. When calcinm is associated with the bicarbonate ion, the hardness metal ion can be easily removed by providing the hydroxide radical. The of the bicarbonate becomes neutralized by the OH provided forming water and the ion necessary to precipitate calcinm carbonate. The softening reaction is as follows ... 

Calculate the solubility product constant for calcium carbonate, given that it has a solubility of 5.3 X 10 g/L of water. 

The solubility product constant, K p, for calcium carbonate at room temperature is approximately 3.0 X 10 . Calculate the solubility of CaCOj in grams per liter under these conditions. 

Seashells and limestone are primarily calcium carbonate. Calcium carbonate is fairly insoluble, as you would expect from its solubility product constant (3.8 X 10 ). But it dissolves readily in acidic solution. Water that has filtered through decomposing vegetation contains carbonic acid, as well as other acids. When such an acidic solution comes into contact with limestone, it carves out caverns. The water is now a solution of calcium hydrogen carbonate. 

Dissolution of Sparingly Soluble Salts. Obtain precipitates of calcium carbonate and calcium oxalate in test tubes by reacting the relevant salts. Decant the solutions and pour an acetic acid solution onto the moist precipitates. What happens Repeat the experiment, but use hydrochloric acid instead of the acetic acid. Write the equations of the chemical reactions in the molecular and net ionic forms. Explain the results obtained on the basis of the dissociation constants of the acids and the solubility product. 

Calcium carbonate scaling is perhaps the most common type of problem, with the possible exception of microbial fouling, that RO membranes experience. Fortunately, it is fairly easy to detect and handle. Basically, if the ion product (IP) of calcium carbonate in the RO reject is greater than the solubility constant (Ksp) under reject conditions, then calcium carbonate scale will form. If IP < Ksp/ scaling in unlikely. The ion product at any degree of saturation is defined as ... 

Alternatively, antisealants can be used to control calcium carbonate scale at LSI values as high as 2.0-2.5, depending on the specific antisealants. Calcium also forms scales with fluoride, sulfate, and phosphate. The LSI will not help predict these scales analysis of water quality, using the ion product and solubility constants, is required to determine the potential for scaling with calcium fluoride or calcium phosphate. Antisealants currently available can address calcium fluoride and calcium sulfate scale they do not address calcium phosphate scale (although newer antisealants will be available in the near future to address this scale). 

Barium and strontium form sulfate scales that are not readily soluble. In fact, barium is the least soluble of all the alkaline-earth sulfates. It can act as a catalyst for strontium and calcium sulfates scale.4 Analyses of the ion product with the solubility constants for barium and strontium sulfates is necessary to determine the potential for scaling with these species. If the ion product (IP) for barium sulfate exceeds the solubility constant, scale will form. Note that in the case of strontium sulfate, if IP > 0.8Ksp, scaling is likely. However, the induction period (the time it takes for scale to form) is longer for these sulfate-based scales than it is for calcium carbonate scale. 

Calcium phosphate has become a common problem with the increase in treatment of municipal waste-water for reuse. Surface waters can also contain phosphate. Calcium phosphate compounds can contain hydroxyl, chloride, fluoride, aluminum, and/ or iron. Several calcium phosphate compounds have low solubility, as shown in Table 7.2. Solubility for calcium carbonate and barium sulfate are also shown by comparison. The potential for scaling RO membranes with the calcium phosphate compounds listed in Table 7.2 is high and will occur when the ion product exceeds the solubility constant. This can occur at orthophosphate concentrations as low as 0.5 ppm. Sodium softening or antisealants together with low pH help to control phosphate-based scaling. 

General Considerations. In order to facilitate the discussion of methods for calculating the saturation state of seawater with respect to calcium carbonate, initial consideration will be given to pure calcium carbonate phases. The method most frequently used expressing the saturation state of a solution with respect to solid phase is as the ratio (Q) of the ion activity (a) product to the thermodynamic solubility constant (K). For the calcium carbonate phase calcite, the expression for the saturation state is defined as (e.g., 13) ... 

In high-salinity waters such as seawater, both ion-pairing and activity-coefficient effects (see Chap. 4) increase the concentrations of species limited by the solubility of minerals. For example, in pure water saturated with respect to calcite, the molal solubility product ZmCa x ZmCOf" = 10 whereas in seawater this product equals 10 If the concentration of carbonate is constant, this corresponds to a 250-fold increase in the concentration of dissolved calcium in seawater relative to that in pure water. 

Carbonate compounds are relatively insoluble, and Table 2.1 lists the solubility constants for a number of geologically important carbonates. Since these minerals are relatively insoluble, carbonates are precipitated at relatively low carbonate and counterion concentrations. As an example, a solution containing 10-4 M Ca2+ will be precipitated by a concentration of CO2 in excess of 10-4 32 M. This occurs because the product of the two ionic concentrations exceeds the solubility constant for calcium carbonate see Equation (2.1) ... 

For the calculation of pHg it is possible to derive the following equation from the equation for the second dissociation constant of carbonic acid K2 and the solubility product of calcium carbonate, K ... 

A principal cause of sinkholes is the dissolution of limestone, which is calcium carbonate, by groimdwater. Although CaC03 has a relatively small solubihty-product constant, it is quite soluble in the presence of acid. 

First acidity constant of carbonic acid (Eq. 2.43) Second acidity constant of carbonic acid (Eq. 2.44) Solubility product of calcium carbonate (Table 1.1)... 

Because of difficulties in precisely calculating the total ion activity coefficient (y) of calcium and carbonate ions in seawater, and the effects of temperature and pressure on the activity coefficients, a semi-empirical approach has been generally adopted by chemical oceanographers for calculating saturation states. This approach utilizes the apparent (stoichiometric) solubility constant (K ), which is the equilibrium ion molal (m) product. Values of K are directly determined in seawater (as ionic medium) at various temperatures, pressures and salinities. In this approach ... 

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