STRUCTURE OF CONCENTRATED AQUEOUS SODIUM FORMATE SOLUTIONS

Time-of-flight neutron and X-ray diffraction as well as Raman spectros copic measurements were carried out on an aqueous 15 mol% HCOONa solut ion in order to investigate the hydration structure of both the format e and sodium ions in the concentrated aqueous solution. The intramolec ular bond distances of DCOO- (tau(CD) = 1.07+/-0.01 Angstrom, tau(CO) = 1.272+/-0.007 Angstrom, and tau(OO) =2.18+/-0.02 Angstrom) were dete rmined through a least-squares fit of the observed neutron interferenc e term in the high-Q region. The average hydrogen-bond distances betwe en the D2O molecules in the solution (tau(O...D)=1.92+/-0.02 Angstrom and tau(D...D)=2.42+/-0.02 Angstrom) were also obtained. The hydration parameters on the formate ion were estimated to be tau(formate)...H2O =2.82+/-0.01 Angstrom and n(Oformate)...H2O=4.4+/-0.2 from a least-squ ares fit of the X-ray intermolecular interference term. A preferred or ientation between the formate ion and hydrated water molecules was sug gested from the present X-ray data. The hydration number of Na+ in the solution was given to be 4.6+/-0.2, besides the intermolecular distan ce (tau(Na+)...H2O=2.37+/-0.01 Angstrom). The symmetrical stretching v ibrational band of the hydrated sodium ion, Na+(H2O)n, in a 15 mol% HC OONa solution was observed at 225 cm(-1) in the present isotropic Rama n spectrum.