Kinetic Studies on the Effect of Mn(II) on the Oxidation of α-Hydroxy Carboxylic Acids by Cr(VI)

Abstract

Mn(II) catalysis in the chromic acid oxidation of lactic acid (LA), malic
acid (MA), tartaric acid (TA) and citric acid (CA) in 50% acetic acid·50%
water medium has been studied. Mn(II) retards' oxidation at low
[Mn(II)]/[Cr(VI)] ratio but in H₂SO₄ (= 1.0 M) at high [Mn(II)]/[Cr(VI)] ratio
200 for lactic acid, 200 for malic acid and 500 for tartaric acid, rate of oxidation
increases while for citric acid it is very low ratio (= 10). Rate under the positive
catalysis is linearly proportional to [Mn(II)], [H₂SO₄] at ≥ 1.0 M, HCrO₄^- and
obey Michaelis-Menten kinetics. Addition of sodium pyrophosphate decreases
rate of oxidation. Energy of activation for uncatalysed, Cr(VI)-Mn(II) and
catalysed reactions have been given.