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

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 CO3(2-)(aq) and HCO3(-)(aq) species, which possess pseudo D3h 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 CO3(2-). 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 degrees C. The pK2 value of carbonic acid at 23 degrees C has been established as 10.35 by Raman spectroscopy, a value that compares favourably with published thermodynamic values.