Precipitation of CaCO

The presence of a time limitation suggests that there must be a kinetically controlled interference, possibly arising from a competing chemical reaction. In this case the interference is the possible precipitation of CaCOs. 

A big concern in swimming pools is prevention of etching and scaling (ie, precipitation of CaCO ) which can be controlled by maintenance of a proper degree of saturation with respect to calcium carbonate. The calcium carbonate dissolution-precipitation equiUbrium is represented by ... 

Sca.Ie nd Sta.in Controllers. Polyacrjiates (low molecular weight) and organic phosphonates, eg, (l-hydroxyethyhdene)diphosphonic acid, prevent or control precipitation of CaCO by acting as chelating agents (qv) or dispersants (qv) to prevent excessive formation of hard scale by promoting crystal distortion. 

Table 13.1 Comparison between the reactive precipitation of CaCOs in conventional equipment and in rotating packed bed reactor.

In addition to changing the pH of the water, the uptake and release of CO2 alter the buffer capacity of the water. The effect upon buffer capacity is the result of two factors (1) the dependence of buffer capacity on the hydrogen ion concentration, and (2) the dependence of buffer capacity on the total concentration of weak acid and conjugate base in solution (67, 68). The precipitation of CaCO in natural waters reduces the buffer capacity to a value lower than that predicted on the basis of pH change and respiratory or photosynthetic changes in COL content of the water. 

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 ... 

A shift in community composition may also be important to the biological pump if it is from a calcareous to a noncalcareous phytoplankton, as precipitation of CaCOs diminishes the ocean s ability to hold dissolved CO2. DIC (XCO2) is present in seawater as several species, dissolved CO2, carbonic acid, and the dissociated forms, bicarbonate ion, and carbonate ion ... 

At steady state the rate of bicarbonate removal by CaCOs precipitation in the ocean is given by two times the flux of calcium from rivers, based on the river flow rate and Ca + concentration in Table 2.3. (This is an upper limit for the removal of HCOif by precipitation of CaCOs because a small amount of the Ca + from rivers is precipitated as anhydrite in the Mackenzie and Carrels mass balance, Table 2.4.)... 

Boiling. Boiling will remove temporary hardness, but the resulting precipitation of CaCOs may be precisely what one is trying to avoid. 

The precipitation of CaCO, and Fe-oxides was considered to be a possible source... 

A further aspect of decompression that should be kept in mind is the degassing of CO and the resulting precipitation of CaCO, which might affect pore water concentrations of phosphate by adsorption or co-precipitation (Jahnke et al. 1982). This effect can even imply negative fluxes as discussed below (see Section 6.5.1). 

The Effect of Magnetic Fields on the Precipitation of CaCO 100 mg/L C02/Tapwater Solution Temperature Dependence tabulated approximate values are based on Fig. 2.24 [3]). 

First, the gases could be tested to see if they supported combustion. O2 would support combustion, CO2 would not. Second, if CO2 is bubbled through a solution of calcium hydroxide [Ca(OH)2], a white precipitate of CaCOs forms. No reaction occurs when O2 is bubbled through a calcium hydroxide solution. 

In a softening plant, hardness ions (e.g., Ca and Mg " ) are removed by precipitation of CaCOs and Mg(OH)2 at pH levels of nominally 10.3 and 11.0, respectively (Fig. 2). Lime is added to raise the pH to the necessary levels, and soda ash (Na2C03) is added if insufficient carbonate is present to precipitate the calcium hardness. Filtration and oxidants are also used as described above. 

The visual proof is the formation of a white precipitate of CaCOs. Why would a water solution of NaOH be unsuitable to qualitatively test for carbon dioxide ... 

Carbon dioxide also has sources other than the cycle represented by Eq. (17) and its reverse (Fig. 2). It is one of the major gases emitted during volcanic eruptions, and it is produced in large amounts by the precipitation of CaCOs from bicarbonate solutions ... 

Scale formation results from the precipitation of CaCOs andMg(OH)2. Precipitation of CaCOs is a particular problem in heated water systems such as steam power generation systems, industrial cooling systems, and home hot water systems because the equilibrium between HCOJ and CO3 increasingly favors as the temperamre in-... 

In similar experiments CO2 was shown to be present in the gases vented from the reactor by precipitation of CaCOs from a saturated Ca(OH)2 solution through which the gases were dispersed. 

Fig. 2 Experimental setup of a double-jet reactor for the precipitation of CaCOs...

Fig. 4 Schematic depiction for the precipitation of CaCOs and the transformations from ACC to crystalline phases...

A delay addition of sodium salt of PAA for precipitation of CaCOs was carried out by the double-jet method [97]. After addition of the aqueous solutions of 0.1 M CaCl2 and 0.1 M (NH4)2C03 into an aqueous solution, an aqueous solution of sodium salt of PAA (Mw = 1200) was added to the reaction mixture after incubation at 30 °C for several minutes. The feed ratio of a repeating unit of PAA to calcium ions was 0.62. Little crystalline CaCOs was collected after incubation at 30 °C imder N2 for 1 day when the sodium salt of PAA (Mw = 1200) was added to the reaction mixture after incubation at 30 °C for 1 min. When the sodium salt of PAA (Mw = 1200) was added to the reaction mixtiue after incubation for 20 min, a small amoimt of vaterite was obtained. 

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-... 

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