IONIC MEDIUM EFFECT ON THE RATE OF HYDROLYSIS OF PYROPHOSPHATE IONS AT NEUTRAL PH AND 70-85-DEGREES-C

The rates of hydrolysis of pyrophosphate (diphosphate) ions with the P 2O74- skeleton have been studied in aqueous solutions containing vario us alkali-metal and tetraalkylammonium halides at neutral pH and 70, 8 0 and 85 degrees C. The apparent rate constant of hydrolysis at a give n pH and temperature increases in the order LiCl < NaCl < KCl < (CH3)( 4)NBr < (n-C3H7)(4)NBr. As the P2O74- ion is protonated to form HP2O73 - and H2P2O72-, the protonation constants at elevated temperature were evaluated on the basis of the van't Hoff equation using the protonati on constants and the corresponding enthalpies of formation at 25 degre es C. The protonation constants at elevated temperature increase almos t in the same order as that of the medium of 1:1 electrolytes at 25 de grees C, which suggests that the difference in the species distributio n of HP2O73- and H2P2O72- plays a primary role in the ionic medium eff ect. Their intrinsic rate constants of hydrolysis were then extracted on the basis of the protonation constants at each temperature. Interes tingly, the intrinsic rate constant of H2P2O72- exhibits practically n o dependence on the ionic medium, while the corresponding rate constan t of HP2O73- increases in the order Li+ < Na+ approximate to K+ approx imate to (CH3)(4)N+ < (n-C3H7)(4)N+. The latter is unexpected if we ta ke into account the acceleration effect upon formation of the metal-po lyphosphate complexes. Furthermore, the activation enthalpy is practic ally unchanged, while the activation entropy varies depending on the i onic medium. It is thus supposed that the change in the hydration stru cture around the HP2O73- ion owing to the overlap with that of medium cations plays an essential role in the ionic medium effect.