O-17 NMR-STUDY OF AQUEOUS POTASSIUM SILICATES

A quantitative study of the dependence of O-17 spectra (67.8 MHz) on t he 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 rap id intermolecular O-O exchange. At [OH-]:[Si-IV] = 1:1, exchange broad ening 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 sili cate anions as well as between silicate and solvent molecules (and aqu eous OH) is controlled by the silicate condensation/hydrolysis process . Classical band-shape analysis yields chemical exchange lifetimes tha t are in accordance with rates measured by Si-29 NMR; the average exch ange 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 repo rt of very slow O-O exchange.