A26 Effect of Temperature on a Luminol Clock Reaction

Description
First, luminol reacts with peroxide in the presence of a copper(II) ion catalyst to form a blue chemiluminescent species immediately. Next, cysteine, RS^-, chelates the copper(II) and prevents the catalysis of the oxidation of luminol until the cysteine has been oxidized to cystine, (RS)₂. This clock reaction with cysteine is run at two temperatures so that students can see the effect of temperature on the rate of reaction and the duration of the light. The cysteine oxidation, a free radical reaction which probably involves a copper(I) intermediate, is not a simple stoichiometric reaction because many pathways compete.

Equation
luminol + H₂O₂ + 4 OH^- --> 3-aminophthalate + 4 H₂O + N₂ + light

2 Cu²⁺ + 2 RS^-<--> (Cu₂(RS)₂)²⁺<--> (RS)₂ + 2 Cu⁺

v Cu₂(cysteine)₂²⁺ +w OH^- --> x H₂O₂ + y cystine + z Cu²⁺

Reference
B. Z. Shakhashiri, "Chemical Demonstrations", Vol. 1, The University of Wisconsin Press, Madison ,Wisconsin, (1983) p.170.

J. Peisach, P. Aisen, and W. E. Blumberg, Eds, "The Biochemistry of Copper", Academic Press, (1966) p. 18.