Effect of Al(III) impurity on the crystallization of ammonium dihydrogen phosphate (ADP) from aqueous solutions by cooling method

• Metastable zone width (MSZW) of aqueous ADP solutions containing Al(III) ions is studied. • Morphology of self-nucleated ADP crystals after MSZW measurements is examined. • Crystal structure and segregation coefficient of Al(III) impurity in the crystals are studied. • Analysis of MSZW data suggests the occurrence of crystallization by heterogeneous nucleation. • Chemical complexes determining crystal morphology and segregation coefficient are discussed. The experimental results of a study of the effect of concentration of Al(III) ions on the metastable zone width (MSZW) of aqueous ammonium dihydrogen phosphate [NH 4 H 2 PO 4 ; ADP] solutions saturated at 45 o C by cooling them at different rates and the growth morphology of the crystals from these solutions are described and discussed. Crystal structure and segregation coefficients of impurity in the crystals were determined by X-ray diffraction analysis and microwave plasma atomic absorption spectrometry, respectively. It was observed that: (1) the value of MSZW of the solutions increases both with increase in cooling rate R L as well as impurity concentration c i , (2) impurity leads to a change in the lattice parameters of the crystal samples but their tetragonal crystal system I 4 ¯ 2d space group remain unchanged, (3) the morphology and size of ADP crystals changes with an increase in the concentration c i of the Al(III) ions and the cooling rate R L , and (4) segregation coefficient of Al(III) impurity in the crystals decreases with an increase in impurity concentration c i in the solution. It was found that: (1) simple solute-supply-based model for MSZW based on the classical nucleation theory satisfactorily describes the MSZW data and enables to establish trends of the dependence of the preexponential factor A and the effective solution–solution interfacial energy γ eff on Al(III) concentration in the solution, (2) the observed morphology of ADP crystals is related to the available supersaturation in the solution created by the applied cooling rate R L , and the supersaturation σ available for the crystal growth is more at high cooling rate R L of the solutions and the value of σ increases with impurity content c i in the solution, and (3) bare Al(III) ions are trapped in the ADP lattice at interstitial sites.