Measurements of pK(a) of organic molecules using third-order nonlinear optics

We report the use of the Z-scan method, which is a nonlinear optical techni que, as a general method to determine a chemical parameter, the pK(a), whic h characterizes the equilibrium constant in acid-base reactions. The measur ements were performed with picosecond pulses at 532 nm in aqueous solution of methyl orange, C14H14N3O3S- The experimental results agree well with a p henomenological description of the molecule nonlinearity as well as with th e results predicted using the Henderson-Hasselbalch equation. (C) 2000 Else vier Science B.V. All rights reserved.