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

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   Fe   III= 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)   4+ 2(=Fe 2 H −2 ), FeSO   + 4 , and Fe(SO 4 )   − 2with formation constants of log β   FeH   −1= −2.84, log β   Fe   2 H   −2= −2.88, log β   FeSO   4 += 2.32, and log β   Fe(SO   4 )   2  −= 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   + 4 ⇌ Fe 3+ + Fe(SO 4 )   − 2equilibrium is established well before the system as a whole reaches its equilibrium state, and the main path of the formation of Fe(SO 4 )   − 2is 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