Kinetics and mechanism of complex formation reactions in the iron(III)-phosphate ion system at large iron(III) excess. Formation of a tetranuclear complex

The kinetics and mechanism of the iron(III)-phosphate ion reaction were stu died at large iron(III) excess using the stopped-flow method at 10.0 degree s C in 1.0 M NaClO4. In the first few hundred milliseconds of the reaction, the formation of a novel tetranuclear complex was confirmed. The following composition is proposed for the new species: Fe-4(PO4)(OH)(2)(H2O)(16)(7+) . According to detailed kinetic studies, the formation of this species is f irst order with respect to Fe-2(OH)(2)(H2O)(8)(4+) and H2PO4- and presumabl y proceeds via a dinuclear intermediate Species. At longer reaction times s low dissociation of the tetranuclear complex controls the formation of the thermodynamically favored Fe(PO4)(H2O)(5) complex. The overall reaction was interpreted in terms of the following reactions: Fe-2(OH)(2)(H2O)(8)(4+) r eversible arrow 2Fe(mn)(3+); Fe-2(OH)(2)(H2O)(8)(4+) + P(V) reversible arro w Fe2PV; Fe2PV + Fe-2(OH)(2)(H2O)(8)(4+) reversible arrow Fe(4)p(V); Fe-mn( 3+) + P(V) reversible arrow Fe(PO4)(H2O)(5). (Fe-mn(3+) = Fe(H2O)(6)(3+) Fe(OH)(H2O)(5)(2+); P(V) = H3PO4 + H2PO4-; Fe2PV = Fe-2(HPO4)(OH)(H2O)(8)(3 +); Fe4PV = Fe-4(PO4)(OH)(2)(H2O)(16)(7+)) The pH dependence and relevant r ate and equilibrium constants are reported for the individual reaction step s.