Measurements of pKa of organic molecules using third-order nonlinear optics

We report the use of the Z-scan method, which is a nonlinear optical technique, as a general method to determine a chemical parameter, the pKa, which characterizes the equilibrium constant in acid–base reactions. The measurements were performed with picosecond pulses at 532 nm in aqueous solution of methyl orange, C14H14N3O3S−. The experimental results agree well with a phenomenological description of the molecule nonlinearity as well as with the results predicted using the Henderson–Hasselbalch equation.