Practical Equilibrium Saturation Studies

The numerator of the right side is the product of measured total concentrations of calcium and carbonate in the water—the ion concentration product (ICP). If n = 1 then the system is in equilibrium and should be stable. If O > 1, the waters are supersaturated, and the laws of thermodynamics would predict that the mineral should precipitate removing ions from solution until n returned to one. If O < 1, the waters are undersaturated and the solid CaCOa should dissolve until the solution concentrations increase to the point where 0=1. In practice it has been observed that CaCOa precipitation from supersaturated waters is rare probably because of the presence of the high concentrations of magnesium in seawater blocks nucleation sites on the surface of the mineral (e.g., Morse and Arvidson, 2002). Supersaturated conditions thus tend to persist. Dissolution of CaCOa, however, does occur when O < 1 and the rate is readily measurable in laboratory experiments and inferred from pore-water studies of marine sediments. Since calcium concentrations are nearly conservative in the ocean, varying by only a few percent, it is the apparent solubility product, and the carbonate ion concentration that largely determine the saturation state of the carbonate minerals. 

All the presented dependences are practically of the same kind they consist of an inclined section, whose position is exclusively defined by the value of the dissociation constant of the oxide, and a plateau beginning with the minimum (bending to the abscissa axis). This minimum occurs owing to partial dissolution of a portion of the oxide, immediately resulting in the saturation of the solution with respect to the oxide added. For zinc oxide there is an inflection at the first section of the SAM plot because, after small additions of ZnO, the equilibrium molality of oxide ions in the melt is comparable with that of the traces of oxygen-containing admixtures in the studied melt. 

Despite its industrial importance, adsorption from the liquid phase has been studied much less extensively than adsorption from the vapor phase. There is no difference in principle between adsorption from liquid and vapor phases since, thermodynamically, the adsorbed phase concentration in equilibrium with a liquid must be precisely the same as that which is in equilibrium with the saturated vapor. The differences arise in practice because in adsorption from the liquid phase one is almost invariably concerned with high adsorbed phase concentrations close to the saturation limit. The simple model isotherms, developed primarily to describe adsorption from the vapor phase, are at their best at low sorbate concentrations and become highly unreliable as saturation is approached. Such models are therefore of only very limited applicability for the correlation of liquid phase adsorption data. 

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