Study of the spontaneous crystallization of Na₂SO₄·7H₂O and Na₂SeO₄·7.5H₂O

The effects of temperature, cooling rate, and solution concentration on the spontaneous crystallization of Na₂SO₄·7H₂O and Na₂SeO₄·7.5H₂O from their supersaturated solutions was studied. It was found that the crystallization of Na₂SO₄·7H₂O almost always occurs prior to that of Na₂SO₄·10H₂O (except at high cooling rates), while the crystallization of Na₂SeO₄·7.5H₂O is rather hindered. Raman spectroscopic studies on υ₁ vibrations of SO₄²⁻/SeO₄²⁻ ions in their respective samples revealed that the totally symmetric υ₁ vibrations are shifted to the higher frequencies in the order: υ₁ (Na₂SO₄ solution) < υ₁ (Na₂SO₄·7H₂O) < υ₁ (Na₂SO₄·10H₂O) < υ₁ (Na₂SO₄) and υ₁ (Na₂SeO₄ solution) < υ₁ (Na₂SeO₄·7.5H₂O) < υ₁ (Na₂SeO₄·10H₂O) < υ₁ (Na₂SeO₄), respectively, which is an evidence of the order in which the deformation of the SO₄²⁻, respectively, SeO₄²⁻ ions, increases. This fact gives grounds to assume the existence of a closer proximity between the structures of the corresponding solutions and those of the metastable Na₂SO₄·7H₂O and Na₂SeO₄·7.5H₂O, than with their decahydrates. In this way, the preferred crystallization of Na₂SO₄·7H₂O, respectively, Na₂SeO₄·7.5H₂O, in comparison with that of Na₂SO₄·10H₂O, respectively, Na₂SeO₄·10H₂O, can be explained. The hindered crystallization of Na₂SeO₄·7.5H₂O in comparision with that of Na₂SO₄·7H₂O may be attributed to the lower symmetry of its crystal structure (monoclinic space group C2/c) in comparison with that of Na₂SO₄·7H₂O (tetragonal space group P4₁2₁2). Graphical abstract: .