A preliminary in situ Raman spectroscopic study of the oxygen isotope exchange kinetics between H 2O and (PO 4) aq

Abstract

Studies on the mechanism and kinetics of the oxygen isotope exchange reaction between H2O and (PO4)aq are essential for the interpretation of natural variations in the oxygen isotope composition of biogenic and inorganic phosphate and utilization of the oxygen isotope composition of phosphates as a geochemical indicator. To gain a deeper understanding of the exchange kinetics we have started an in situ Raman spectroscopy study using an in-house-made Teflon-based fluid cell. Raman spectroscopy allows monitoring the exchange reaction because the substitution of O in aqueous phosphate species by O should cause a shift of those vibrational modes to lower frequencies which involve the motion of oxygen due to the different masses of both isotopes. First in situ exchange experiments have been carried out with a 2 M (NH4)2HPO4 solution (room temperature pH of 8) at 150(±1) °C and 175(±1) °C for 112 and 45 h, respectively. Speciation calculations show that under these conditions HPO4 is the main aqueous phosphate species. The phosphate solution was prepared with H2O enriched with 97 at.% O. At the beginning of the reaction at the experimental temperatures the Raman spectrum of the HPO4 phosphate solution is characterised by distinct bands near 310, 321, 366, 384, 398, 533, 867, 983, and 1076 cm. The latter three bands are the most intense bands and can be assigned to the ν(P–OH), νs(PO3), and the νas(PO3) vibration of the HPO4 molecule (based on a C3v symmetry), respectively. All bands involving oxygen motions are