Composition seeded growth, constant

Figure 5. Constant composition growth of HAP seed at low supersaturation (Tea = 0.30 mU, Tp = 0.18 mM, [KCl] = 2 mM, 37°C, pH 8.50). Plots of titrant added as a function of time. Experiments in the presence of magnesium and...

In a typical CC growth kinetics experiment, carefully prepared and well-characterized seed crystals are introduced to a thermo-stated crystallization cell containing solution at a preset supersaturation. The reactant solutions are prepared with appropriate stoichiometry matching that of the precipitating phase. To maintain the composition of the supersaturated solution at a constant value, both reactants (titrant solutions) are added at the rate sufficient to replace reactant ions that precipitated, and to bring the supersaturation of the resultant total volume to the value of the initial supersaturated solution. 

This equation is first order in [Ca ], [P04 ] and s. The average rate constant kf was determined to be 173 11 mof m sec The apparent Arrhenius activation energy was determined over a temperature range of 10 to 40°C to be 186 15 kJ mol Inskeep and Silvertooth (1988) speculated that the rate-limiting step is a surface process, specifically that it is the surface diffusion of Ca and PO4 ions and dehydration commensurate with binding at surface sites of incorporation. Using a constant-composition method and seeded solutions with Sha = 3.05-5.58 Koutsopoulos (2001) found hydroxylapatite formed without a precursor phase. The spiral growth model was found to best fit their kinetic data. 

Walker et al. (2007) developed 6 QSARs to predict the toxicity of 17 cations to sunflower seeds (F.l. Helianthus annuus Sunspot) from the metal s physical properties and natural occurrence levels (Table 5.4). The QSARs predicted EC50 values based on the cation concentration producing a 50% inhibition of radicle growth one day after emergence. The QSAR developed with density of the elements (p), enthalpy of formation of metal sulfides (AHJ, and the stability constants of metal ions with sulfate (log Ki [sulphate]) produced the highest adjusted (Table 5.20). For natural occurrence levels, the QSAR developed with metal concentrations in soil (log Msoi,), the median elemental composition of soils (mg X/kg soil), and the calculated mean of the elemental content in land plants (Land Plants) produced the highest adjusted... 

The nucleation/growth process, most commonly encountered, consists of two separate steps. The first one is the nucleation, corresponding to the formation of seeds, also called nuclei, whose chemical composition is different from that of the initial glass, thus forming a new phase. The composition of this new phase remains constant over the whole nucleation/growth process (Figure 9.2). 

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