Monatshefte f€ ur Chemie 133, 1363–1372 (2002) DOI 10.1007/s00706-002-0488-5 Kinetics of the Formation of the Blue Complex CrO(O 2 ) 2 Formed by Dichromate and H 2 O 2 in Acid Solutions. A Stopped-Flow Investigation Using Rapid-Scan UV-VIS Detection Gu ¨ nter Grampp  , Stephan Landgraf, Tomasz Wesierski a , Beata Jankowska a , Ewa Kalisz a , Dana-Maria Sabou b , and Boryana Mladenova c Graz University of Technology, Institute of Physical and Theoretical Chemistry, A-8010 Graz, Austria Received January 30, 2002; accepted (revised) June 5, 2002 Published online September 2, 2002 # Springer-Verlag 2002 Summary. The kinetics of the CrO(O 2 ) 2 formation by H 2 O 2 and Cr 2 O 7 2  in aqueous acidic media was measured at 293  2 K in a pH range between 2.5 and 3.3. Using the stopped-flow method with rapid scan UV-VIS detection, the rate law of the formation of CrO(O 2 ) 2 was determined. For the media HClO 4 , HNO 3 and CH 3 COOH, the reaction order in the Cr 2 O 7 2  concentration was found to be 0.5. For [H 2 O 2 ] as well as for [H þ ], the reaction was first order in all acids used. In HCl and H 2 SO 4 media the reaction was first order in Cr 2 O 7 2  . At T ¼ 293  2K the rate constant for the formation of Cr(O)(O 2 ) 2 was found to be (7.3  1.9)  10 2 M  3=2 s  1 in HClO 4 . Keywords. Chemical kinetics; Stopped-flow technique; Cr(O)(O 2 ) 2 complex; Reaction order. Introduction In strong acidic media the blue CrO(O 2 ) 2 complex (I) is formed by the reaction between Cr 2 O 7 2  and H 2 O 2 . Stabilized by organic solvents like ether, alcohols or pyridine, this complex became an analytical tool for the identification of chromium  Corresponding author. E-mail: grampp@ptc.tu-graz.ac.at On leave of: a A. Mickiewicz University, Faculty of Chemistry, Dept. of Physical Chemistry, Poznan, Poland b ‘‘Babes-Bolyai’’ University Cluj-Napoca, Faculty of Chemistry, Dept. of Physical Chemistry, Cluj, Romania c Bulgarian Academy of Science, Institute of Catalysis, EPR-Laboratory, Sofia, Bulgaria