Coprecipitation reaction constants

In the coprecipitation of a phase associating two (or several) elements, if one of them is contained in an anion and the second in a cation, the precipitate will have a fixed or at least very inflexible composition. If both are cations (or both anions) the characteristics of the reactions with a common anion (or cation) of the solution, the solubility constants, and the super-saturation values will all be different, and the properties of the precipitate will change with time. Consequently, coprecipitation does not in general give homogeneous precipitates. Methods are available to produce homogeneous precipitates (see item c) below). 

Constant pH coprecipitation takes place by feeding acidic and basic solutions simultaneously into the precipitation tank. Usually, acidic solution is fed at a constant rate and the feed rate of basic solution is varied to maintain constant pH, typically at the acid-base neutralization point. The add feed rate is predetermined based on consideration of residence time, batch time, or aging time. The copredpitation reaction is typically controlled isothermally at 40-70°C with a constant stir... 

A reproducible coprecipitation or adsorption reaction of a constant amount of the element of interest can be used in substoichiometry. An interesting example is the substoichiometric radioactivation analysis for oxygen, based on the reproducible isolation of fluorine or fluorosilicate with a substoichiometric amount of hydrated tin dioxide It has been applied to the determination of oxygen in silicon crystal. 

Cu-Cr-Q] This compound had never been prepared previously by the coprecipitation method and the alternative salt-oxide method was successfully used to synthesized it. The reaction proceeds at a nearly constant pH, contrary to what is observed in the former cases.The pH drops from 6.8 rfter the first addition of CrCls, and then remains constant at ca. 4.5. The disappearance of CuO on diffractograms (Figure 7-14) occms at a Cu/Cr ratio near 2, and the phase is characterized by the chemical formula Cu2Cr(0H)6Cl.nH20. 

Sample A was prepared in the same way except that zirconia was substituted by alumina. Pure zirconia was prepared analogously by precipitation of zirconyl nitrate. Sample C was made by coprecipitation instead of sequential precipitation. Sample H was prepared by the method of deposition precipitation using urea. A suitable amount of amorphous zirconia was suspended in deionised water. After the addition of copper(II)nitrat and urea the temperatme was brought to 363 K under constant stirring. The reaction was accompanied by a rise of the pH to a final value of 8. The final product was treated in the same way as the sequentially precipitated catalysts. 

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