With ADH = 0.511 for aqueous solutions at 25 °C (15, 16,
pp 251−261), the logarithmic form of eq 17 is expected to
give the slope of −2.04. The result of −2.17 is in agreement
with the one predicted by this equation. This result therefore
confirms the proposed mechanism (eq 6) with the reaction
between ions FeI2+ and I− as the rate-determining step. The
validity of the assumption used to simplify eq 15 into eq 17
can be easily verified by using K1° = 3.9 × 103 18 and [I−]0 =
0.02 mol L−1 used in the experiment (see the Supporting
Information).
To summarize this part, the slope of the line obtained from
the study of the dependence of the reaction rate on ionic
strength is ≈ −2 and not ≈ −5 because of the conditions used
(excess of iodide) and the method used to follow the kinetics
(the initial rates method). With these in mind, it is clear that
the result is in accord with the reaction mechanism.