A pH-jump reaction studied by the transient grating method:: Photodissociation of o-nitrobenzaldehyde

A proton-releasing photodissociation reaction of o-nitrobenzaldehyde is investigated by using the time-resolved transient grating method. The grating signal is very sensitive to the pH of the solution. The chemical species existing in the solution are identified from the diffusion constant of the species. For low-pH solutions, the grating signal due to the proton is observed clearly without any pH indicator. The origin of the signal is attributed to a pure volume grating, from which the partial molar volume of the proton can be determined accurately. The diffusion constant of the proton decreases with increasing the pH. This change is explained by the electrostatic interaction between the proton and the anion product, o-nitrosobenzoic anion. In an alkali solution, the hydroxyl ion is observed as the main species of the gating signal. By the quantitative measurement of the grating signal and the photoacoustic signal, the enthalpy change and the volume change of the reaction and the partial molar volumes of the proton are determined. This proton signal can be used to quantitatively measure the proton concentration change without any pH indicator.