Enthalpy and volume changes on the pH jump process studied by the transient grating technique

A pH jump reaction of o-nitrobenzaldehyde is studied by using the time-reso lved transient grating method. 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 m olar volume of the proton can be determined accurately. In an alkali soluti on, the hydroxyl ion is observed as the main species of the grating signal. The enthalpy changes of the reaction in pH=6.5 and 8.2 are determined to b e -131.2 and -133.6 kJ/mol, respectively, indicating that the reaction is e xothermic. The negative volume change at the lower pH solutions is due to t he electrostriction by the creation of the ionic species. At the higher pH, the reaction volume changes to positive, which is mainly due to the water formation. This pH jump reaction was used to study the unfolding process of apomyoglobin.