Pseudo‐polymorphic Behavior of Precipitated Calcium Oxalate

Abstract

The precipitation of calcium oxalate pseudo-polymorphs was studied in a Y-mixer in series with a reaction tube. Experiments were performed with the aim to identify the decisive variables influencing the phase composition of the precipitate. Initial supersaturation, stoichiometric ratio of the lattice ions, ionic strength and pH were varied. The resulting phase distribution was determined with the help of DSC measurements. Mixing conditions were chosen such that mixing of the reactants had no influence on the experiments. Increasing supersaturation, ionic strength and pH each favor formation of the metastable calcium oxalate dihydrate over the thermodynamic stable monohydrate, although the influence of the two latter parameters is comparatively weak. If the ratio from calcium to oxalate activities was increased in the experiments a steep change in the phase composition from approximately 50 mass percent of the monohydrate to pure monohydrate was observed. Modeling of the experimental results is successful using a combination of classical nucleation theory with an equation that relates the adsorption of potential determining ions to the interfacial tension. The interfacial tension for the uncharged particles was obtained by regression from the experimental determined dependence of the phase distribution on the supersaturation.

Inorganic Precipitated Silica Gel. Part 1: Gelation Kinetics and Gel Properties

“…The reactants, diluted waterglass (sodium silicate solution, Na 2 O·3.3SiO 2 , q = 1.365 g/cm 3 ) and diluted sulfuric acid, were rapidly mixed in a Y-mixing nozzle [7]. Local concentration peaks, which may lead to partial instantaneous coagulation, could thus be avoided.…”

Consolidation of Inorganic Precipitated Silica Gel

“…The silica gels were generated by rapidly mixing alkaline water glass (sodium silicate solution Na 2 O·3.3SiO 2 ) and sulfuric acid in a Y-mixing nozzle [10][11][12][13]. Acidic and alkaline gels can be formed depending on the mixing ratio.…”

Investigation of Precipitation of Calcium Carbonate at High Supersaturations

“…For some experiments the suspension was injected directly from the mixing nozzle into liquid nitrogen at -196°C (for details see Haselhuhn and Kind [5]). The frozen suspension was then freeze-dried at -22°C for three days.…”