PAPER www.rsc.org/dalton | Dalton Transactions Speciation studies in aqueous HCO 3 − –CO 3 2− solutions. A combined Raman spectroscopic and thermodynamic study Wolfram W. Rudolph,* a Gert Irmer b and Erich K ¨ onigsberger c Received 29th August 2007, Accepted 9th November 2007 First published as an Advance Article on the web 10th December 2007 DOI: 10.1039/b713254a 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 CO 3 2− (aq) and HCO 3 − (aq) species, which possess pseudo D 3h and C 1 symmetry respectively, have been assigned and discussed. Quantitative Raman measurements and thermodynamic calculations on KHCO 3 solutions show that the salt does not dissolve congruently in aqueous solutions but forms small amounts of CO 3 2− . Quantitative Raman spectroscopic measurements have also been carried out on K 2 CO 3 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 pK 2 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. Introduction Equilibria involving hydrogencarbonate (HCO 3 − ) and carbonate (CO 3 2− ) species in aqueous solution have been studied extensively due to their important role in the earth and life sciences. 1 However, spectroscopic characterizations of these species in solution are sparse and only concentrated HCO 3 − and CO 3 2− solutions 2–6 as well as anhydrous and hydrate carbonate melts 5,6 have been studied due to methodological limitations at the time. Dilute solutions, which occur in natural waters and biological fluids, have been omitted from the above mentioned studies because the carbonate species are not very strong Raman scatterers and are therefore hard to measure with sufficiently high quality. In the present Raman study we restrict ourselves to dilute sodium and potassium carbonate and hydrogencarbonate solu- tions in the slightly alkaline and alkaline pH range, the stability range of the HCO 3 − and CO 3 2− species. The vibrational spectra of CO 3 2− (aq) and HCO 3 − (aq) in such solutions have been measured and assigned. Quantitative Raman measurements of the hydrolysis of CO 3 2− (aq) in carbonate solutions (Na 2 CO 3 and K 2 CO 3 ) as a function of concentration and temperature have been carried out. Subsequently, the second dissociation constant of carbonic acid at 23 ◦ C has been determined from these data. Furthermore, the concentration of carbonate formed in hydrogencarbonate solu- tions by autoprotolysis has been measured by quantitative Raman spectroscopy. Using a Pitzer approach with parameters taken from the literature, equilibria in carbonate and hydrogencarbonate solutions were modelled thermodynamically and compared to the results derived from Raman spectroscopic data. a Institut f ¨ ur Virologie im MTZ, TU Dresden, Fiedlerstr. 42, 01307 Dresden, Germany. E-mail: wolfram.rudolph@mailbox.tu-dresden.de b Institut f¨ ur theoretische Physik, TU Bergakademie Freiberg, Leipziger Str. 23, 09596 Freiberg, Germany c School of Chemical and Mathematical Sciences, Murdoch University, Murdoch, WA 6150, Australia Experimental Preparation of solutions 7 Potassium hydrogencarbonate (KHCO 3 ) and sodium hydrogen- carbonate (NaHCO 3 ) were purchased from Merck, Darmstadt (pro analysi, >99.5%). The crystals were dried under a CO 2 atmosphere at 50 ◦ C. The KHCO 3 and the NaHCO 3 solutions have been prepared by weight and sealed in air tight bottles. Potassium and sodium carbonate (K 2 CO 3 and Na 2 CO 3 ) were purchased from Merck, Darmstadt (pro analysi, >99.5%) and dried at 150 ◦ C under a CO 2 atmosphere. K 2 CO 3 and Na 2 CO 3 stock solutions in degassed water were prepared by weight and sealed in air tight bottles. Two K 2 CO 3 solutions with an excess of KOH (Merck, Darmstadt, pro analysi, >99%) were also prepared to determine the scattering coefficient of the m 1 CO 3 2− with a known amount of NH 4 ClO 4 as the internal standard (m 1 ClO 4 − at 935 cm −1 ). The densities of the solutions have been measured with a pycnometer (2 ml volume) at a temperature of 23 ◦ C kept constant with a thermostat to ±0.05 ◦ C. With the densities at hand and the concentrations given in mol L −1 , the concentrations in mol kg −1 were calculated. Raman spectroscopic measurements Raman spectra were measured in the macro chamber of the T 64000 Raman spectrometer from Jobin Yvon in a 90 ◦ scattering geometry. The spectra were excited with a 514.5 nm line of an Ar + laser at power levels of ∼0.9 W. After passing the monochromator the scattered light was detected with a charge coupled device (CCD). I VV and I VH spectra were obtained with fixed polarisation of the laser beam passing the analyzer and a half-wave plate in suitable arrangement between the sample and the entrance slit to give the scattering geometries: I VV = I (Y[ZZ]X) = 45a  2 +4c  2 (1) 900 | Dalton Trans., 2008, 900–908 This journal is © The Royal Society of Chemistry 2008