Thermodynamics and Kinetics of Reduction of Fe Acetohydroxamic Acid Complex by Ascorbic Acid

• Published in: Pakistan journal of scientific and industrial research. Series A: physical sciences 2018-01, Vol.61 (1), p.8
• Main Authors: Perviaz, Muhammad, Nisar, Shazia, Kazmi, Syed Arif, Imad, Saima
• Format: Article
• Language: English
• Subjects: Analysis; Dynamics (Mechanics); Magnetite; Organic acids; Thermodynamics; Vitamin C
• ISSN: 2221-6413

Kinetic and thermodynamic studies of reduction of Fe(III) acetohydroxamic acid ([Fe(III)-AHA]) complex by ascorbic acid (AA) was performed at pH values ranging from 3.00-4.50 [+ or -]0.1 (ionic strength 0.2) and temperature 05.0 - 25.0 [+ or -] 0.5 [degrees]C. The redox reactions were studied, spectrophotometrically, under pseudo first order conditions of [AA] over the [Fe(III)-AHA] under the experimental conditions. The redox reaction was found to be highly pH dependent. The values of 2nd order rate constants ([k.sub.2]), at 25 [degrees]C and pH 3.0, 3.5,4.0 and 4.5 were found out to be 2.3920, 2.1550,2.0122, 1.7596 [M.sup.-1][s.sup.-1], respectively. At 20 [degrees]C the values were 1.7290, 1.4000, 1.3400, 1.2650 [M.sup.-1][s.sup.-1] at pH 3.0, 3.5, 4.0 and 4.5, respectively. The results at15[degrees]C and pH 3.0, 3.5, 4.0 and 4.5 were 1.1410, 0.9459, 1.0390, 0.9126 [M.sup.-1][s.sup.-1], respectively. While, the values of rate constants at 10 [degrees]C were 0.7847, 0.7697, 0.6810, 0.7096 [M.sup.-1][s.sup.-1] and at 5 [degrees]C were 0.6167,0.5106,0.4775,0.4833 [M.sup.-1][s.sup.-1], respectively. In addition to that, the following rate law was evaluated. Rate = dp/dt = [k.sub.2] [Fe(III)-AHA] [AA]. Moreover, the thermodynamic activation parameters of the reaction were also determined. The values of [DELTA]Ea# at pH 3.0, 3.5,4.0 and 4.5 were found out to be 22.4058,16.3243, 19.9636, 14.6050 J/mol, respectively. While, the values of [DELTA]H# were 58.5056, 42.6258, 52.1288, 38.1363 J/mol, respectively at pH 3.0 to 0 4.5. Values of [DELTA]S# were also evaluated as -23.0297, -68.7567, -37.8287, -84.1377 J/mol/K at pH 3.0, 3.5, 4.0 and 4.5, respectively. Keywords: acetohydroxamic acid, ascorbic acid, hydroxamate, siderophore, thermodynamics