True coprecipitation

Consider a solution with two metals A and B in solution with a predpitation agent X. With true coprecipitation, only one equilibria is 

Although these mixed metal alkoxide complexes are a means of obtaining atomic mixing of the various metals, the stoichiometric ratio of metals may or may not be that desired for the ceramic powder. A different stoichiometric ratio, n, for the same two metals is not likely to be precipitated out just because the initial solution contains a different stoichiometric ratio. Also for many electronic ceramic compositions it is desirable to have many other metals incorporated at the ppm level into the ceramic powder as sintering aids, grain growth inhibitors, and crystal phase stabilizers. Adding these otiier metals is very difficult 

The other precipitation possibility is the simultaneous precipitation of two insoluble species controlled by two separate precipitation reactions as follows  

FIGURE 6.37 Segregation of AX b) (black squares) and BX s) (open circles) during simultaneous precipitation and coaggregation. 

This type of behavior can be seen in the coprecipitation of BaYgCug (oxalate) produced by quickly mixiiig equal volumes of two solutions one 0.4 M in oxalic acid and the other 4.8 x 10 M in BafNOglg, 

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Sol id Sol utions. The aqueous concentrations of trace elements in natural waters are frequently much lower than would be expected on the basis of equilibrium solubility calculations or of supply to the water from various sources. It is often assumed that adsorption of the element on mineral surfaces is the cause for the depleted aqueous concentration of the trace element (97). However, Sposito (Chapter 11) shows that the methods commonly used to distinguish between solubility or adsorption controls are conceptually flawed. One of the important problems illustrated in Chapter 11 is the evaluation of the state of saturation of natural waters with respect to solid phases. Generally, the conclusion that a trace element is undersaturated is based on a comparison of ion activity products with known pure solid phases that contain the trace element. If a solid phase is pure, then its activity is equal to one by thermodynamic convention. However, when a trace cation is coprecipitated with another cation, the activity of the solid phase end member containing the trace cation in the coprecipitate wil 1 be less than one. If the aqueous phase is at equil ibrium with the coprecipitate, then the ion activity product wi 1 1 be 1 ess than the sol ubi 1 ity constant of the pure sol id phase containing the trace element. This condition could then lead to the conclusion that a natural water was undersaturated with respect to the pure solid phase and that the aqueous concentration of the trace cation was controlled by adsorption on mineral surfaces. While this might be true, Sposito points out that the ion activity product comparison with the solubility product does not provide any conclusive evidence as to whether an adsorption or coprecipitation process controls the aqueous concentration. 

The behavior of solid phase activity coefficients is a more complex issue. We can view this behavior in two ways. The first is where only a trace mole fraction of component i ("trace component") is coprecipitated in a relatively pure component j ("carrier component"). The activity of the carrier component is well approximated as being equal to unity. When this is true, the activity coefficient of the trace component in the solid is given by the following relation ... 

The preparation of synthetic silica-alumina catalysts is a relatively simple one, involving the coprecipitation or cogelation of the two hydrous oxides from mixed solutions of sodium silicate and aluminum sulfate. Depending on how the solutions are mixed and on the pH and concentration of the resulting mixture, the combined hydrous oxides will be formed as a coprecipitate, which separates from a greater part of the aqueous phase, or as a true hydrogel, which embraces the entire solution volume. 

Since vanadium oxide had been used as an effective catalyst for the dehydrogenation of hydrocarbons, it was expected from purely thermodynamic considerations that conditions could be found for the reverse reaction of hydrogenation to take place. Experiments carried out in our laboratory with coprecipitated vanadia-alumina catalyst showed this to be true. 

The soluble complexes of heptaenes commercially available are formulated with detergent-like substances, e.g. sodium desoxycholate or sodium lauryl sulphate, and do not form true solutions but consist of colloidal dispersions in an aqueous medium. The in vitro solubility of drugs with low aqueous solubility may be enhanced by coprecipitation with poly(vinyl pyrrolidone) (PVP) [376, 377]. The preparation of nystatin—PVP complexes has been described [378-380]. The solubility of PVP coprecipitates was eight to ten times greater than the parent compound [381] but the coprecipitates showed decreased antibiotic stability. 

Among the many types of blends discussed, an obvious deficiency is the absence of polymer/polymer eutectics from the polymer literature. Here we imagine two polymers that form a true solution in the melt, and coprecipitate in crystalline form to make a type of crystalline/crystalline polyblend. 

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