In an attempt to grow pure crystals of the iron(II) phosphate vivianite, Fe₃(PO₄)₂,8H₂O, from a solution of Mohr's salt, Fe(NH₄)₂(SO₄)₂,6H₂O, added to a solution of ammonium phosphate, hydroxylammonium chloride, NH₃OHCl, was added to the stock solution of Mohr’s salt to eliminate oxidation of iron(II) by oxygen from the air. However, the effect turned out to be the opposite of the expected: whereas hydroxylamine reduces iron(III) in bulk solution, it acted as a strong oxidant in the presence of growing iron phosphate crystals, causing the crystallization of the iron(III) phosphate strengite, FePO₄,2H₂O, as the only solid phase. Evidently the crystal surface catalyses oxidation of iron(II) by hydroxylamine, the actual precipitation process being
2 Fe²⁺ + 2 H₂PO₄^- + NH₃OH⁺ + 3 H₂O → 2 FePO₄,2H₂O + NH₄⁺ + 2 H⁺
As H⁺ is liberated, causing decrease in pH, the crystallization process may be monitored by recording pH versus time. The graphs of growth rate versus supersaturation as calculated from pH and initial solution composition were significantly more regular with this system than those for crystallization of vivianite or strengite from solutions without hydroxylamine. The usual composite kinetics of spiral growth and surface nucleation was found. At 25ºC the surface-nucleation part yielded the edge free energy λ = 25±10 pJ/m, which is about half the lowest value found for strengite crystallizing from an iron(III) solution without hydroxylamine and in the range of values previously found for vivianite. The scatter of values for λ presumably arises from contributions from different crystal forms to the overall growth rate. The low mean value points to strong adsorption of iron(II), which is subsequently oxidized at the crystal surface, forming strengite. The state of the system did not tend to thermodynamic equilibrium, but to a metastable state, presumably controlled by the iron(II) rich surface layer of the crystal. In addition to crystal growth, it was possible to measure nucleation kinetics by light scattering (turbidimetry). A point of transition from heterogeneous to homogeneous nucleation was found, and from the results for the homogeneous domain a rather precise value of crystal surface free energy γ = 55 mJ/m² was found. This is a relatively low value as well, indicating that the redox process plays a role already at the nucleation stage.

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1. Kinetics of crystal growth of vivianite, Fe₃(PO₄)₂,8H₂O, from solution at 25, 35 and 45ºC