Experimental acidity of the water surface

Abstract

The acidity experienced by gases colliding with the water surface is herein investigated in expts. where the protonation of gaseous trimethylamine on water microjets is monitored by online electrospray mass spectrometry as a function of pH of the bulk liq. (pHBLK). Trimethylammonium (TMAH+) signal intensities track pHBLK along a titrn. curve displaying an equivalence point at pHBLK ~ 3.8 that is significantly smaller than the acidity const. pKA(TMAH+) = 9.8 in bulk soln. The protonation of TMA above pHBLK 4 is strongly enhanced by the addn. of up to ~ 1 mM inert Li+ or Na+, and weakly inhibited at higher concns. Enhanced TMA protonation is always accompanied by direct kinetic solvent hydrogen isotope effects. Since TMA(g) can be protonated by bare H3O+, and by H2O only upon extensive solvent participation, we infer that H3O+ emerges and becomes available at the surface of neat water below pHBLK 4.