Magnesium bicarbonate and carbonate interactions in aqueous solutions: An infrared spectroscopic and quantum chemical study

Abstract

The interaction of magnesium with bicarbonate and carbonate ions in aqueous solutions was studied using infrared spectroscopy and quantum chemical calculations. Using the infrared vibrational bands for and at 1200–1450 cm−1 (δC-OH, vS and v3) together with their molar absorptivity (ε), the concentrations of the and ions and the corresponding Mg ion pairs have been determined. In the absence of Mg2+, measured spectra were accurately reproduced assuming that only and were present in solution. Upon addition of Mg2+ at fixed pH, infrared spectra were observed to shift indicating presence of the and ion pairs. From measurements, the second ionization constant of carbonic acid and the and ion pair formation constants have been obtained, these being logK2 = −10.34 ± 0.04, = 1.12 ± 0.11 and = 2.98 ± 0.06, respectively. To support our experimental infrared measurements and to gain further insight into the molecular nature of the ion pair formation, density functional theory (DFT) calculations with VPT2 anharmonic correction were conducted. The most stable geometries predicted for the and ion pairs were a bi-dentate [MgHCO3]+(H2O)n and a monodentate [MgHCO3]+(OH)(H2O)n complexes, respectively. The predicted frequencies for , and were found to shift toward those experimentally measured with an increasing H2O solvation number where possible band shifts were predicted for relative to , this being dependent on the exact structure and hydration of the bulk ion pair. Experimentally, the ion pair formations were found to have insignificant effects on the δC-OH, vS and v3 vibrational frequencies. The speciation of dissolved inorganic carbon may be significantly influenced by ion pair formation, particularly in alkaline solutions where they may be the predominant species.