The partial molar volume of the proton in water determined by laser-induced optoacoustic studies.

Claudio D. Borsarelli and Silvia E. Braslavsky^*

Max-Planck-Institut für Strahlenchemie, Postfach 10 13 65, D-45413 Mülheim and der Ruhr, Germany

Keywords: proton-transfer; proton partial molar volume; photoacoustics; time-resolved volume changes

*To whom correspondence should be addressed

Abstract

The structural volume change, DV_MLCT, associated with the formation and decay of the metal-to-ligand charge transfer (MLCT) state of (2,2'-bipyridine)tetracyanoruthenate(II) complex, Ru(bpy)(CN)₄^2-, was studied as a function of pH in dilute aqueous solutions ( < 0.4 M) using time-resolved laser-induced optoacoustic spectroscopy. The values of DV_MLCT for the MLCT formation of the complex strongly depend on pH, ranging from +3.0 ± 0.3 cm³/mol at pH = 0.35 to +15.0 ± 0.5 cm³/mol at pH = 6.63. The reduction of DV_MLCT at low pH values is assigned to the fast excited-state proton ejection undergone by monoprotonated Ru(bpy)(CN)₄^2- . With the DV_MLCT values and the Drude-Nernst equation for the electrostriction effect with an empirically determined constant, a value for the "absolute" partial molar volume of the hydrated proton in water of V°_H+ = (-5.5 ± 0.8) cm³/mol is obtained, in good agreement with literature data.