Peculiar kinetics of the complex formation in the iron(III)–sulfate system

✍ Scribed by Balázs Kormányos; Gábor Peintler; Andrea Nagy; István Nagypál

• Publisher: John Wiley and Sons
• Year: 2008
• Tongue: English
• Weight: 447 KB
• Volume: 40
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/kin.20300

✦ Synopsis

Abstract

The results of comprehensive equilibrium and kinetic studies of the iron(III)–sulfate system in aqueous solutions at I = 1.0 M (NaClO~4~), in the concentration ranges of T = 0.15–0.3 mM, and at pH 0.7–2.5 are presented. The iron(III)–containing species detected are FeOH^2+^ (=FeH~−1~), (FeOH) (=Fe~2~H~−2~), FeSO, and Fe(SO~4~) with formation constants of log β = −2.84, log β = −2.88, log β = 2.32, and log β = 3.83. The formation rate constants of the stepwise formation of the sulfate complexes are k~1a~ = 4.4 × 10^3^ M^−1^ s^−1^ for the \documentclass{article}\pagestyle{empty}\begin{document}${\rm Fe}^{3+} + {\rm SO}_4^{2-},\stackrel{k_{1a}}{\rightleftharpoons}, {\rm FeSO}_4^+$\end{document} step and k~2~ = 1.1 × 10^3^ M^−1^ s^−1^ for the \documentclass{article}\pagestyle{empty}\begin{document}${\rm FeSO}_4^+ + {\rm SO}_4^{2-} \stackrel{k_2}{\rightleftharpoons}, {\rm Fe}({\rm SO}_4)_2^-$\end{document} step. The mono‐sulfate complex is also formed in the \documentclass{article}\pagestyle{empty}\begin{document}${\rm Fe}({\rm OH})^{2+} + {\rm SO}_4^{2-} \stackrel{k_{1b}}{\longrightarrow} {\rm FeSO}_4^+$\end{document} reaction with the k~1b~ = 2.7 × 10^5^ M^−1^ s^−1^ rate constant. The most surprising result is, however, that the 2 FeSO⇌ Fe^3+^ + Fe(SO~4~) equilibrium is established well before the system as a whole reaches its equilibrium state, and the main path of the formation of Fe(SO~4~) is the above fast (on the stopped flow scale) equilibrium process. The use and advantages of our recently elaborated programs for the evaluation of equilibrium and kinetic experiments are briefly outlined. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 114–124, 2008