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

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 HCO3- and CO32- at 1200-1450 cm−1 (δC-OH, vS and v3) together with their molar absorptivity (ε), the concentrations of the HCO3- and CO32- ions and the corresponding Mg ion pairs have been determined. In the absence of Mg2+, measured spectra were accurately reproduced assuming that only HCO3- and CO32- were present in solution. Upon addition of Mg2+ at fixed pH, infrared spectra were observed to shift indicating presence of the MgHCO3+ and MgCO3(aq) ion pairs. From measurements, the second ionization constant of carbonic acid and the MgHCO3+ and MgCO3(aq) ion pair formation constants have been obtained, these being logK2 = −10.34 ± 0.04, logKMgHCO3+ = 1.12 ± 0.11 and logKMgCO3 = 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 MgHCO3+ and MgCO3(aq) ion pairs were a bi-dentate [MgHCO3]+(H2O)n and a monodentate [MgHCO3]+(OH)(H2O)n complexes, respectively. The predicted frequencies for HCO3-, CO32- and MgHCO3+ were found to shift toward those experimentally measured with an increasing H2O solvation number where possible band shifts were predicted for MgCO3(aq) relative to CO32-, this being dependent on the exact structure and hydration of the bulk MgCO3(aq) 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.