Alkaline-earth-metal(II) complexes with hydroxide and fluoride in molten NH4NO3· 1.5H2O at 50 °C

Abstract

The formation of alkaline-earth-metal(II) hydroxide and fluoride complexes in molten NH4NO3· 1.5H2O has been investigated by potentiometric, NMR and Raman spectroscopic methods at 50 °C. Complexes with the formal composition MgOH+, Mg2OH3+ and CaOH+ were detected in the hydroxide systems. The formal stability constants are: β11[MgOH+]= 3.2 ± 0.1 kg mol–1, β21[Mg2OH3+]= 1.6 ± 0.5 kg2 mol–2, β11[CaOH+]= 0.8 ± 0.1 kg mol–1 obtained from potentiometric measurements. In the fluoride systems the following complexes were detected: MgF+, MgF2, Mg2F3+, CaF+, CaF2, SrF+, Sr2F3+ and BaF+ with formal stability constants: β11[MgF+]= 3.3 ± 0.5 × 102 kg mol–1, β12[MgF2]= 1.2 ± 0.2 × 106 kg2 mol–2, β21[Mg2F3+]= 2.5 ± 0.2 × 102 kg2 mol–2, β11[CaF+]= 1.0 ± 0.2 × 102 kg mol–1, β12[CaF2]= 2.9 ± 0.6 × 105 kg2 mol–2, β11[SrF+]= 1.5 ± 0.2 kg mol–1, β21[Sr2F3+]= 0.9 ± 0.2 kg2 mol–2 and β11[BaF+]= 0.7 ± 0.1 kg mol–1 . 19F NMR experiments reveal only small changes in the 19F chemical shift upon addition of Mg2+ or Ca2+, a fact which indicates a predominantly ionic M—F bonding character in the complexes formed. 14N NMR and Raman spectroscopy experiments using M(NO3)2· 3H2O–NH4NO3· 1.5H2O melts (M = Mg, Ca, Sr, Ba) indicate that nitrate ions are weakly coordinated to the metal ions and suggest the following order of increasing metal ion–nitrate interaction: Ba2+ < Sr2+ < Ca2+ < Mg2+. Difference Raman spectra of melts with magnesium or calcium ions, with and without fluoride present, show that nitrate ions, rather than water molecules, are most likely released upon metal–fluoride complex formation according to: M(H2O)x(NO3)y2–y+nF–⇌ MFn(H2O)x(NO3)y–z2 –n–y+z+zNO3–.