Kinetics of the methylene blue oxidation by cerium(iv) in sulphuric acid solutions

The oxidation of methylene blue (MB+) by cerium(IV) was studied in 0.1-5 M H2SO4. The reaction proceeds via MB radical (MB2+•) formed by one electron transfer to the oxidant. The radical is observed spectrophotometrically by a very intense absorbance at λmax = 526 nm and by the e.p.r signal at g = 2.000. The kinetics of the fast radical formation are two orders of magnitude slower than its decomposition, which were examined using a stopped-flow method at 298 K under pseudo-first order conditions. The rate laws for the both steps were determined and a likely mechanism reported.