Carbonate solubility equilibria

Consider just a few cases of aqueous equilibria. The magnificent formations i n limestone caves and the vast expanses of oceanic coral reefs result from subtle shifts in carbonate solubility equilibria. Carbonates also influence soil pH and prevent acidification of lakes by acid rain. Equilibria involving carbon dioxide and phosphates help organisms maintain cellular pH within narrow limits. Equilibria involving clays in soils control the availability of ionic nutrients for plants. The principles of ionic equilibrium also govern how water is softened, how substances are purified by precipitation of unwanted ions, and even how the weak acids in wine and vinegar influence the delicate taste of a fine French sauce. In this chapter, we explore three aqueous ionic equilibrium systems acid-base buffers, slightly soluble salts, and complex ions. 

A convenient method of interpreting water analysis for the purpose of determining the calcium carbonate solubility equilibrium conditions is embodied in the Langelier equation. The Langelier equation can be used to... 

The concentrations of free carbonate and bicarbonate ions determined at solubility equilibrium as a function of pH. Decrements of the concentration near pH = 10 suggest the formation of the Pu(0H)2C03 precipitate and hence lowering solubilities of Pu02 (cf. Figure 2). 

Over the eons, the flow and evaporation of water inside a cavern creates a stunning array of rock sculptures. Stalagmites grow upward from the floor, sometimes joining stalactites to form massive columns. Limestone dams create beautiful pools of water. Limestone draperies fall like curtains from water flowing around overhanging rock. Delicate mineral flowers sprout from the walls. All these features result from the aqueous solubility equilibrium of calcium carbonate. 

The limestone deposits that decorate Carlsbad and other caverns are the result of the solubility equilibrium of calcium carbonate in groundwater, as described in Chapter 16 ... 

Calcium carbonate is insoluble in pure water but dissolves in weakly acidic water. The role of this solubility phenomenon in the geochemistry of caverns is described in Box. We can understand this dependence on pH by examining the acid-base properties of the species involved in the solubility equilibrium. 

C18-0074. For the following salts, write a balanced equation showing the solubility equilibrium and write the solubility product expression for each (a) lead(II) chloride (b) magnesium carbonate (c) nickel(II) hydroxide and (d) silver acetate. 

Equilibrium with precipitation. The previous example calculated carbonate speciation admitting unrestricted solubility of all species. Actually, it is easily verified that the calculated calcium and carbonate concentrations exceed calcium carbonate solubility as measured by the solubility product... 

Carbon dioxide is a symmetric linear molecule with zero dipole moment. Hence its interaction with a dipolar molecule like H20 is weak. However, it is moderately soluble in water and in many other organic solvents. The solubility equilibrium can be described by Eq. (1) ... 

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. 

For these alloys we determined the equilibrium carbon solubility and calculated carbon supersaturation with respect to diamond. We also grew diamond using the alloys and established the average yield of the diamond transformation. The data are listed in Table 1. 

SGTE data bank, except for nickelocene, and for CjHj and C Hg. Results show that in a large domain of operating conditions, including those investigated in the present work, the Ni(C) solid solution is the only stable condensed phase. Carbon solubility in Ni is very low (below 0.1 at.%). CH is the only stable gas under these conditions. In view of the misfit between calculated and experimental results (the latter showing that CH is not the major gas and that carbon content in the film is higher) it is concluded that equilibrium was not attained in the experiments. 

For solubility equilibrium to predict the aqueous concentration of a trace element, thermodynamic equilibrium is required and the solid phases must also be identified. In the case of oxyhydroxides and carbonates, it is reasonable to assume a close approach to equilibrium because the characteristic reaction times of dissolution of these minerals are in the range of a few days to a few hundred years (Bruno et al., 2002). For silicate phases the assumption of thermodynamic equilibrium is more problematic due to the low reaction rates compared to the residence time of waters in hydrosystems. Examples of codes and database used by modelers to calculate the speciation and the solubility of a number of trace elements can be found in Bruno et al. (2002). 

As is to be expected from relative solubility, the first minerals to form from concentrating waters are the alkaline-earth carbonates. The equilibrium precipitation of calcite also illustrates... 

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... 

Dissolution of carbonates can only occur if the solution is undersaturated with regard to the solid carbonate. The solubility equilibrium of carbonates and especially of calcite has been discussed extensively in Chapters 4 and 7. 

The three Salem samples show increased leaching rates as a function of increased acidity, but not the rate values predicted by simple chemical stoichiometry. A pH decrease from 5.6 to 4.0 is a 39.8x increase in acidity, while a change from 4.0 to 3.0 is a 10x acidity increase. The observed changes were factors of 2.88x and 1.58x, respectively. These discrepancies can be attributed to the complex equilibrium interactions involved in the solubilities of the metal carbonates. These two solubility equilibria are further complicated by the two acid equilibria for the carbonic acid/bicarbon-ate/carbonate system in addition to the equilibrium solubility of congas in water. The solution of these simultaneous equilibria processes to determine the relationship between carbonate solubility and acid concentration is a non-trivial one (sixth degree in concentration). This solubility problem has been approached from several different viewpoints (31-35), the most convenient being a graphical solution of the solubility as a function of initial solution and final solution pH. From this method, it can be theoreti-... 

Figure 3. Plot of log total calcium vs ionic strength. Solid line, calculated solubility of calcium carbonate in equilibrium with atmospheric CO2. Calculated solubility of calcium carbonate in the absence of atmospheric CO2 for Circle, deionized water triangle, acid solution at pH = 4.0 star, acid rain at pH = 4.0.

Figure 19.11 Test for the presence of a carbonate. When a mineral that contains carbonate ion is treated with strong acid, the added HsO shifts the equilibrium position of the carbonate solubility. More carbonate dissolves, and the carbonic acid that is formed breaks down to water and gaseous CO2.

First we find the molar solubility and then convert moles to grams. The solubility equilibrium for silver carbonate is ... 

The equilibrium equations that normally have to be considered in the EKR modeling of a soil contaminated by heavy metals can be classified into one of the following categories complex formation reactions, precipitation of the metal hydroxides or of other species, ion exchange reactions, surface complexation reactions, etc. Anyway, the autoionization of water always has to be considered and the precipitation of carbonates, together with the carbonate-bicarbonate equilibrium, should normally also be considered. However, the above equations have only considered the species in aqueous phase, so if a species precipitates, a new master species has to be included in this equilibrium system, whose concentration would be the amount of the precipitated species per unit volume of water. This additional degree of freedom is constrained by the solubility product constant of the precipitate (KO, because the new solid phase is in equilibrium with the aqueous phase. If there exists Np precipitated species, the pure-phase equilibria can be represented with the following equation ... 

A further constraint of pH buffering and of the availability of TIC, Ca, Fe and Mn is caused by the solubility equilibrium with rock forming solid carbonates, as calcite, rhodochrosite, and siderite. The solubility products Lcakite. and Ls.dente... 

Solid carbonate precipitation/dissolution are included as kinetic processes to the overall DOC biodegradation reactive model depending on the actual saturation. Further TIC (total inorganic Carbon) and the Calcite solubility equilibrium (dissolved Calcium and solid Calcite) are considered as main pH buffering factors. The actual... 

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