A Study of the Kinetics of the Permanganate—Oxalate Reaction. I. The Effect of Various Salts upon the Rate of Reaction¹

Abstract

Since the classical work of Harcourt and Essona the kinetics of the reaction between permanganate and oxalate ions in acid solution have been studied by several investigator^.^^^^^ The development of a sound theory of kinetic salt effect by Bronsted6 offers another method of investigating the mechanism of this reaction. Lamersa has shown that a positive salt effect is found when the reaction occurs in the presence of relatively large concentrations of oxalate and of added manganous ion.A mechanism for the reaction is presented here as a basis for discussion. rMn04-$. yMn++ + 8xH+ + (Sum of series of rapid reactions) Mn++++ -b Mn++ Mn+++ + 3c~o4' Mn++ + 2CzOh Mn++++ -k c 2 0 4 ' --f Mn+++ 4-(20% + Cog-Mn++++ 4-COz-+ Mn+++ f COa (rapid) Mn+++ + Mn+++ + CG-+ Mn++ + COa zMn++++ + [ (x 4y)z]Mn+++ + 4xHz0 (1) 2Mn++* (rapid, reversible) Mn(GO&' (rapid, reversible) (rapid, slower than (6)) (8) + Mn++ + COS + C0,-If the reduction of the permanganate ion to the intermediate valences occurs in a series of rapid bi-molecular reactions, as seems probable to the authors, both Mnf+++ and Mn+++ ions are produced. Due to the existence of the equilibrium represented by equation (2), this point is not easily demonstrated and is not essential to the rest of the mechanism.The formula of the manganic oxalate complex ion in equation (3) differs from that proposed by Evidence for the formula Mn(Ca0,)p will be presented in another paper. The existence(1) The material in this paper is taken from a thesis submitted by Bryan C. Redmon to the Graduate School of the Massachusetts State College in partial fulfilment of the requirements for the degree of (6) Bronstcd, Z. physik. Cham., 10%. 169 (1922); 118. 337 (l92K). Yost, ibid., 66,2K71 (1934). of the equilibrium represented in equation (4) is shown by the work of Scholder and Linstrom.' Equations (5), (6), (7) and (8) are those proposed by Launer and Yost.6b The existence of reaction (7) is ditficult to prove because of the existence of the equilibrium represented by equation (2) and reaction (5).From an inspection of the mechanism it is evident that, if the rate-determining equation is either (5) or (7), the salt effect should be negative according to the Bronsted theory. LauneP in his investigation of the salt effect used a relatively high concentration of the oxalate ion and added manganous ion. Under these conditions, the reactions from left to right in the equilibria represented by equations (2) and (3) are favored, and the rate of reaction becomes inversely proportional to the concentration of the manganic oxalate complex ion. The effect of added salts on this equilibrium would be to increase the dissociation of the manganic oxalate complex ion and so increase the rate of reaction. From these considerations the positive salt effect found by Launer is apparently not a kinetic salt effect, but a salt effect on the equilibrium represented by equation (3) or a secondary salt effect according to the Bronsted theory.If the reaction is carried out in the presence of relatively low concentr...

Autocatalytic Chemiluminescence Sheds New Light on the Classic Permanganate–Oxalate Reaction

“…The formation of intermediates (C 2 O 4 •– and/or CO 2 •– ) in the title reaction is widely acknowledged, ,− but their fate is the source of much conjecture. Numerous researchers have proposed the reduction of manganese species (Mn III and/or Mn IV ) by radical intermediates. , Kinetic analysis with radical scavengers have suggested that such reactions do occur, but in recent discussions, , these pathways have been overlooked. Similarly, Perez-Benito et al reported autocatalysis in the reduction of oxalic acid by colloidal MnO 2 (formed in neutral solution), which they attributed to a pathway involving adsorption of both oxalic acid and [Mn II (C 2 O 4 )] on the particle surface.…”

Kinetics And Mechanism Of Oxidation By Permanganate

“…which gives for the rate of carbon dioxide formation: While the differences in the exponents of the permanganate-and manganous-ion concentrations may reasonably be neglected, the dependence of rate on hydrogen-ion concentration is in disagreement with the experiments of Launer (85), Launer and Yost (87), and Duke (46). Fessenden and Redmon (49) have proposed another mechanism, but their scheme is in conflict with the radiochemical work, for although the scheme involves both Mn4+ and Mn3+ ions, the reaction would permit the spread of radioactivity from Mn2+ to .…”

“…Their scheme is very similar to that proposed by Launer and Yost (87). Fessenden and Redmon (49) have also studied the effect of various salts on the rate of the permanganateoxalate reaction. In the oxalate-concentration range in which their studies were made, they find the maximum rate of oxidation to be approximately inversely proportional to the oxalate concentration.…”

“…This indicates that the reaction between permanganate and manganous ions in the presence of oxalate ions is not instantaneous. A very rapid reaction between permanganate and manganous ions is an essential requirement of the mechanisms put forward by Launer and Yost (87) and Fessenden and Redmon (49). Malcolm and Noyes (96) propose the following mechanism, which obviates the requirement of a rapid reaction between manganous and permanganate ions: Mn2+ + C204_ MnC204 (rapid equilibrium) (15) MnC20< + -> Mn(III) + Mn04~(fast relative to (16) reaction 19) + 3Mn(II) + 8H+ -> 4Mn(III) + 4H20 (17) (very fast series of steps) + MCsO + 8H+ -> Mn(III) + 3C02 + 4H20 (18) (fast series of steps) Mn(III) -» Mn(II) + 020 (measurable) (19) Mn(III) + 020 -> Mn(II) + 2C02 (very fast) (20) Mn(II) and Mn(III) refer to any divalent and trivalent species, respectively.…”

Requirements for Primary Redox Standards

“…Consider a reaction mechanism somewhat modified from that proposed by Launer and Yost (30): 2MnOt-+ 3Mn(II) + 16H + ->5Mn(IV) +8H20 (rapid) (1) Mn(IV) + Mn(II) <=i2Mn(III) (rapid, reversible) (2) Mn(IV) + C204 Mn(III) + C02 + C02-(measurable) (3) Mn(IV) + C02--C02 + Mn(III) (rapid) (4) Mn(III) + 3C204--<=i [Mn(III)(C204)3]--(reversible) (5) Mn(III) + C204 -Mn(II) + C02 + C02_ (measurable) (6) Mn(III) + C02~-> Mn(II) + C02 (fairly rapid) (7) C02~+ 02 <=t02C02-(rapid, reversible) (…”