Communication: Quantitative estimate of the water surface pH using heterodyne-detected electronic sum frequency...

Most chemical reactions in water are very sensitive to pH. Many environmentally important chemical reactions are known to take place at the water surface (i.e., air/water interface). However, the pH of the water surface is still controversial. Spectroscopic experiments and theoretical calculations indicate that the water surface is more acidic than the bulk, whereas electrophoretic experiments provide a contrary view. Here, we report that a novel nonlinear optical experiment with a surface-active pH indicator can quantitatively evaluate the pH of the water surface. The result clearly shows that the pH of the water surface is lower than that of the bulk by 1.7. This is the first study to apply a principle of bulk pH measurements to the water surface, and therefore provides a reliable experimental estimate for the pH difference between the water surface and bulk. It is considered that the higher acidity of the water surface plays a key role in marine and atmospheric chemical reactions.