Speciation studies in aqueous HCO3−–CO32− solutions. A combined Raman spectroscopic and thermodynamic study

Abstract

Raman (and a few additional FT-IR) spectroscopic measurements of sodium and potassium carbonate and hydrogencarbonate in aqueous solution have been carried out over wide concentration ranges at room temperature and at elevated temperatures. The bands of the CO₃²⁻(aq) and HCO₃⁻(aq) species, which possess pseudo D_3h and C₁ symmetry respectively, have been assigned and discussed. Quantitative Raman measurements and thermodynamic calculations on KHCO₃ solutions show that the salt does not dissolve congruently in aqueous solutions but forms small amounts of CO₃²⁻. Quantitative Raman spectroscopic measurements have also been carried out on K₂CO₃ solutions and the hydrolysis of the carbonate ion has been determined as a function of concentration at room temperature and as a function of temperature up to 219 °C. The pK₂ value of carbonic acid at 23 °C has been established as 10.35 by Raman spectroscopy, a value that compares favourably with published thermodynamic values.