Initial Reaction Products of Chlorate Ion with Iron(II) Ion in Perchloric Acid Solution

Abstract

The absorbance change at 380 and 240 nm with time during the reaction of sodium chlorate and iron (II) perchlorate in 2 mol⋅dm^-3 perchloric acid was measured by stopped-flow spectrophotometry. The absorbance change with time at 380 nm showed the apparent and irregular formation and decay of chlorine dioxide, and that at 240 nm showed the formation of iron (III )ion. These spectral changes are confirmed to be due to the consecutive reactions involving chlorine dioxide as an intermediate.
The rate constant for the formation of chlorine dioxide in the reaction of chlorate ion with iron (II) ion, k_c, was calculated by Koutecky's equation for catalytic current measured by D. C. polarography, and that of reduction of chlorine dioxide with iron (II), k, was measured by stopped-flow spectrophotometry. In 2 mol. dm^-3 perchloric acid at 25±0.1°C, the values of k_c and k were found to be 4.04 and 6.83±10³dm³⋅mol^-1⋅s^-1 respectively. The rate law for chlorine dioxide was given by
-d[ClO₂]/dt= k⋅ [ClO₂]⋅[Fe²⁺] On the basis of these results (k_c/k≅6×10^-3) and stoichiometric relation, the rate-determining step in the reaction between chlorate ion and iron (II) ion was confirmed to be the formation reaction of chlorine dioxide, i. e.
ClO₃^-+Fe²⁺ H⁺→ClO₂+ Fe³⁺+OH^-