Degree of super saturation

For solution on the other hand no such difficulties occur. It has already been noted that the edges of the planes are already stripped under conditions of equilibrium a small decrease in the concentration of the solution simply extends the edges further towards the centre of the plane until it is stripped entirely. The mechanism of crystal growth thus presupposes a finite degree of super-saturation, this no doubt varies with the surface energies of the crystal faces and will probably be greatest for the surfaces for which <7,1 is least,... 

Since the pH directly controls the degree of super-saturation, at least if hydroxides arc precipitated, this should be one of the crucial factors in precipitation processes. As for many other parameters, the influence of pH is not a simple one and it has to be investigated experimentally for a specific system. Even in a relatively well-known system such as iron oxide [30] the effect which the precipitation pH has on the properties of the final product is not yet clear. There is no clear relationship between the precipitation pH and the textural and catalytic properties of the precipitate. It is relatively well known what phases are formed under specific conditions, due to the importance of iron... 

In this situation the degree of super saturation is highly dependent on the physicochemical properties of the gases (diffusion coefficient and solubility). Less soluble gases are more supersaturated than soluble ones at steady state because the water has lower relative concentrations of these gases to start with. For this reason, a small amount of air forced into the water by bubble processes has a much larger effect on the relative concentrations of Ne or N2 than it does on CO2. 

Calcium oxalate is a hard crystalline precipitate and is therefore similar to barium sulfate with respect to the process of precipitation. This means that the crystal size distribuhon of the precipitated form varies a lot witir precipitation parameters such as sample solution temperature, degree of super saturation, and hme allowed from precipitation to evaluation of turbidity. For a more thorough discussion of this phenomenon, please see Chapter 4, "Precipitahon in Limit Tests."... 

FIGURE 64.1 A schematic presentation of the metastable zone in primary and secondary nucleation. The width of the metastable zone, which is the concentration difference between saturated and operational concentration, shows the maximum degree of super-saturation where spontaneous nucleation starts. 

P the degree of super-saturation (measured by the difference between the concentration present at a given moment and the normal saturation concentration,... 

Degree of super-saturation, saturation index, saturation quotient... 

Nuclei of the new phase can only be originated after a certain degree of super-saturation or supercooling has been reached. The nuclei then grow further by asslmi lating more material until the supersaturation or supercooling is abolished. 

A reasonable assumption about the rate of mineral precipitation or dissolution might be that the rate is proportional to the degree of super- or under-saturation. Expressing this idea mathematically, we would say that the rate R is... 

The initial conditions have been shown to be very important as they determine how rapidly the calcium oxide dissolves [22.1]. When quicklime is mixed with water, the most highly reactive particles dissolve producing a very high level of super-saturation with respect to calcium hydroxide. This results in heavy primary nucleation (i.e., the formation of a very large number of calcium hydroxide nuclei). Less reactive particles dissolve more slowly and produce a lower degree of supersaturation, which largely results in crystal growth on the primary nuclei. 

Marine chemists are presently exploring the use of an extended suite of inert gas tracers to separate the effects of bubbles, gas exchange, temperature change and atmospheric pressure on the degree of gas super saturation. When more high quality data become available for more gases the importance of the different mechanisms will be clearer. 

Thus, the substitutional carbon is reduced and the same amount of SiC particles is generated in the melt. The formation rate of SiC particles and the destruction rate of substitutional carbon are equal and proportional to the super-saturation degree of substitutional carbon and the speed of the chemical reaction (4.3) ... 

Crystals are always of good quality and the size exceeds generally the values observed in alkaline-type synthesis. Addition of seeds decreases the crystallization time and allows tailoring of the size. The morphology is the same as for crystals obtained by the usual alkaline route. The length/width ratio decreases with x in the template and the substitution degree of Si (16). The substitution by trivalent elements leads generally to crystals with less flat faces which indicates that crystallization occured in a more super-saturated medium. 

In calorimetry, not only is the degree of solubility and degree of dissociation important, but the slow rate of dissolution of many electrolytes in organic solvents must also be considered. It is advisable to make an initial study of the nature of the system by first observing the dissolution rate of the solute and then determining the degree of dissociation by conductance measurements. Much time can be saved by these preliminary examinations. The slow rate of solution of many salts may also pose problems in solubility measurements. It is advisable, as in the case of aqueous solutions, that solubility studies be conducted from both super- and under-saturation to ensure that equilibration has been attained. 

---