The oxidation of iodide by iron(III) ions in an acidic medium
(eq 1) is a reaction that can easily be transformed into a clock
reaction using thiosulfate as a limiting reagent and starch asan indicator for I3
− ions (2, pp 51−55). The complex Fe(S2O3)
+
is purple and the starch−pentaiodide complex is blue. The
reaction is started by mixing a colorless acidic solution of iron(III)
nitrate with a colorless solution containing sodium thiosulfate,
potassium iodide, potassium nitrate,13 and starch. The solution
immediately turns purple. The color gradually fades until the
solution becomes colorless. At that very moment, the solution
suddenly turns dark blue. The time elapsed from the mixing of
the two solutions to the sudden color change to blue, Δt, is
measured. The reaction, eq 1, is first order with respect to
iron(III) ions and second order with respect to iodide ions, as
expressed by eq 2.1