12.11 Chemiluminescence of Tris(2,2'-Bipyridyl) Ruthenium(II) Ion

Chemical Concept Demonstrated:  Luminescence occurring during a reduction reaction

Demonstration:

Solution A:
  • Add tris(bipy)Ru2+ ion to H2SO4 in a beaker.
  • Stir in PbO2.

SolutionB:

  • Add NaOH to a beaker of distilled water.
  • Stir in NaBH4.
  • Pour solution B into a flask.
Using a pipette, add solution A to B.

Observations:

    When solution A is added to the flask, orange flashes appear and the solution foams.

Explanation (including important chemical equations):

    2 Ru(bipy)32+(aq) + PbO2 + 4H+(aq) --> 2 Ru(bipy)33+(aq) + Pb+2 + 2H2O(l)

    When solution A is prepared, the Ru(II) is oxidized to Ru(III) in a hetergeneous reaction with PbO2.  The NaBH4 added to solution B reduces the Ru(III) back to Ru(II), but it generates Ru(II) in an excited state. When the electron drops into the ground state, a photon of light is emitted.  There is a 1:1 relationship between the number of electrons gained to photons released in this reaction.

    It is interesting to note that the tris(bipy)Ru3+ was a common indicator for redox titrations (this ion is so stable that it can be boiled in nitric acid).  The chemiluminescence that occurs when this complex is reduced was not noticed for many years, however, because titrations are seldom done in the dark.