Optical detection electric field jump study of dye dimerization in glacial acetic acid

Abstract

An optical detection electric field jump (E-jump) instrument was built by using either a xenon arc lamp or an argon ion laser source. The chemical system studied was the dimerization of methyl yellow (MeY). Its relaxation times in the concentration range 6.3-250 ..mu..M in acetic acid were best fit by a mechanism involving triple and quadrupole aggregations of the dye molecule. The forward and reverse rate constants for triple-ion formation were found to be k/sub f/ = 12.8 x 10/sup 10/ (Ms)/sup -1/, k/sub r/ = 2.8 x 10/sup 5/ s/sup -1/, while the equilibrium constants for triple and quadrupole formation were K/sub T/ = 138 M/sup -1/ and K/sub Q/ = 32 M/sup -1/. In the course of this study a light-induced relaxation process was observed which may be a photochemically induced cis-trans isomerization of the dye. In addition, an analysis of the factors which contribute to optimizing the signal-to-noise (S/N) ratio of the E-jump experiment has been carried out. It was found that an optimum concentration exists for the experiment, and the observed effect will have significant amplitude only in a limited region about this optimum.