A KINETIC STUDY OF MONOHYPOPHOSPHITE COMPLEX OF IRON(3) IN PERCHLORIC ACID SOLUTIONS

The acid dissociation quotient of hypophosphorous acid is 0.135 ± 0.006 M at 25.0° and ionic strength 1.00 M (nitric acid-lithium nitrate medium), evaluated by pH measurements at a glass electrode. Under the same conditions the first acid dissociation quotient of phosphorous acid is 0.107 ± 0.005 M. The rate of formation and aquation of the monohypophos-phitoiron(III) ion follow the expression d[FeH2PO22+]/dt = {a + (b/[H-]}[Fe3+][H3PO2] - {c + d[H+]}[FeH2PO22+] with the values (M and sec units) a = 35 ± 3, b = 13 ± 3, c = 0.23 ± 0.02, and d = 0.083 ± 0.010 at 25.0° in a perchloric acid-lithium perchlorate medium of ionic strength 1.00 M. The stability constant derived from the kinetic data for the reaction Fe3+ + H3PO2 = FeH2PO22+ + H+ is 152 ± 10 M- and that for the reaction Fe3+ + H2PO2- = FeH2PO22+ is (1.1 ± 0.1) × 103 M-1. © 1969, American Chemical Society. All rights reserved.