Antisolvent crystallization of ammonium dihydrogen phosphate (ADP) from aqueous solutions containing Al(III) and Fe(III) impurities by addition of ethanol

Abstract

Experimental results of a study of the effect of concentration of Al(III) and Fe(III) ions on the metastable zone width (MSZW) of aqueous ADP saturated solutions, as determined by the maximum antisolvent content Δxmax in them for spontaneous three-dimensional (3D) nucleation, by feeding ethanol antisolvent to them at different rates RA and the nucleation and growth of crystals from these solutions are described and discussed. Crystal structure and segregation coefficients of impurity in the crystals were also studied by X-ray diffraction analysis and microwave plasma atomic absorption spectrometry, respectively. It was observed that: (1) the value of Δxmax for spontaneous 3D nucleation increases with an increase in ethanol feeding rate RA as well as impurity concentration ci, (2) the morphology and size of the ADP crystals changes with an increase in impurity concentration ci and antisolvent feeding rate RA, and (3) the effective segregation coefficients keff of the impurities in ADP crystals at a constant RA of ethanol feeding decreases with an increase in impurity concentration ci in the solution. The experimental data of the dependence of Δxmax on RA for solutions containing different impurity concentration ci were analyzed according to the approach based on the classical theory for 3D nucleation to calculate values of the preexponential factor A and the effective interfacial energy γeff of the theory, those of the morphology of ADP crystals from consideration of adsorption of impurity particles on the surfaces of growing crystals, and those of the dependence of segregation coefficient keff of the impurities on their concentration ci in the solution using a model based on competitive adsorption of solute and impurity molecules. It is argued that the formation of 3D nuclei of ADP during MSZW measurements and growth of these ADP nuclei as large crystals from aqueous solutions containing impurities and segregation of impurities in the crystals may be explained from consideration of different rates of ligand exchange of the surface-adsorbed complexes formed by interaction between univalently-charged cation complex present in aqueous solution and the H2PO4− ions of kinks in steps.