KINETIC AND EQUILIBRIUM STUDIES OF COMPLEXES OF AQUEOUS IRON(III) AND SQUARIC ACID

The kinetics and equilibria for the system of aqueous iron(III) and squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione) (H2Sq) have been studied in 1.00 M HClO4/LiClO4 with concentration ranges of H+ 0.02-0.10 M, total iron(III) (2.0-7.6) x 10(-3) M, and total squarate (0.5-2.0) x 10(-4) M. Spectrophotometric observations lead to the conclusion that two complexes are formed, described by the following reactions with equilibrium constants of 21.4 +/- 3.3 and (2.4 +/- 0.9) x 10(2) respectively at 23 +/- 1-degrees-C: Fe(OH2)6(3+) + HSq- reversible Fe(OH2)5(Sq)+ + H+ + H2O; (H2O)8Fe2(OH)2(4+) + HSq- reversible (H2O)6Fe2(OH)2(Sq)2+ + H+ + 2H2O. Formation of the iron dimer squarate complex causes the color of solutions to change from bluish purple to bluish red with an absorbance maximum shift from approximately 540 to lower wavelength as the iron(III) concentration is increased and/or the acidity is decreased. The kinetic observations (25.0 +/- 0.1-degrees-C) show a monophasic absorbance change at 545 nm, but a biphasic change at 460 nm, with a 3-5 times larger rate observed at 460 nm. These features are attributed to the formation of the monomeric and dimeric squarate complexes, with rate constants of 5 x 10(4) M-1 s-1 for Fe(OH2)5OH2+ + HSq- or Fe(OH2)6(3+) + Sq2- and 4.5 x 10(4) M-1 s-1 for (H2O)8Fe2(OH)2(4+) + HSq-.