Oxygen-17 NMR study of aqueous potassium silicates

A quantitative study of the dependence of O-17 spectra (67.8 MHz) on the temperature (275-425 K) and alkalinity ([OH-]:[Si-IV] = 1:1 to 2:1) of potassium silicate solutions has yielded six new, albeit tentative, peak assignments. Peak-widths are affected by quadrupolar relaxation (generally more significant for bridging oxygens, =SiOSi=) and by rapid intermolecular O-O exchange. At [OH-]:[Si-IV] = 1:1, exchange broadening is approximately uniform for all silicate nonbridging ((OSi)-O--=) and bridging (=SiOSi=) oxygen resonances as well as for the solvent resonance. This implies that intermolecular O-O exchange between silicate anions as well as between silicate and solvent molecules (and aqueous OH) is controlled by the silicate condensation/hydrolysis process. Classical band-shape analysis yields chemical exchange lifetimes that are in accordance with rates measured by Si-29 NMR; the average exchange lifetime for a potassium silicate solution with [OH-]:[Si-IV] = 1.1 is 0.3 s at 298 K. There was no evidence to support a previous report of very slow O-O exchange.