Kinetic and mechanistic studies on the oxidation of hydroxylamine, semicarbazide, and thiosemicarbazide by iron(III) in the presence of triazines

In the presence of 3-(2-pyridyl)-5,6-bis(4-phenyl-sulphonicacid)-1,2,4-triazine disodium salt (PDTS), 3-(4-(4-phenylsulphonic-acid)-2-pyridyl)-5,6-bis(4-phenylsulphonic-acid)-1,2,4-triazine trisodium salt (PPDTS), or 2,4-bis(5,6-bis(4-phenylsulphonic-acid)-1,2,4-triazin-3-yl)pyridine tetra sodium salt (BDTPS), iron(III) oxidizes hydroxylamine to nitrogen gas, semicarbazide to CO2 and NH3 and thiosemicarbazide to a disulfide. The corresponding iron product is the 1:3 complex of iron(II) and PDTS, PPDTS, or BDTPS. The kinetics of these reactions was studied by monitoring the iron(II) product by conventional spectrophotometry. The reaction is first order in iron(III). Kinetic evidence was obtained for the formation of 1:1:2 ternary complexes of iron(III), substrate, and sulfonated triazine. Evidence for the ternary intermediate complexes was obtained by ion-exchange studies using 59Fe-labeled iron(III) solutions. The dissociation of the ternary complex is identified as the rate-determining step.