Oxidation of oxalic acid

The oxidation of oxalic acid has been studied thoroughly by Allen et al. The stoichiometry of the reaction is 

The reaction is very sensitive to metal ion catalysis, particularly by Cu and Ag , and oxygen inhibits the reaction. Po and Allen studied the uncatalysed reaction in oxygen-free solutions containing 10 M EDTA to ensure that the concentrations of free metal ions were insignificant. Under these conditions the reaction is first order with respect to peroxodisulphate and the rate is essentially independent of oxalate concentration (there is a slight increase in the first-order rate coefficient with increase of oxalate concentration). Allyl acetate inhibits the reaction and reduces the rate to that observed in the absence of oxalate. In the range pH 0.5-10.3 a rate maximum occurs at pH 4.5. The first-order rate coefficient for the reaction using 0.08 M disodium oxalate is expressed by 

Provided that inequality (68) is assumed, the stationary-state approximation applied to the concentrations of the radicals leads to the rate equation 

From equation (67) it follows that the observed activation energy, E, is related to the activation energies of steps (61), (62), (65), and (66) by the equation 

El and 5 are known (Kolthoff and Miller, and Kalb and Allen ), and E must be very small, so Po and Allen deduced that E = 24.5 kcal.mole, indicating that reaction (62) is fairly slow, and that the steady-state concentration of sulphate radical-ions is relatively high. 

Catalysis of the oxidation of oxalic acid to carbon dioxide by oxygen (air) has been described by Warbnrg [99] and stndied in detail by Rideal and Wright 

It might be mentioned here that this conductometric determination of CO2 was also used successfully in the author s laboratory for the determination of small quantities of CO in an oxygen-containing gas stream when this was passed over a suitable oxidation catalyst such as Au/Ti02. 

The results confirm earlier reports [98,99]. The reaction rate is dependent on the concentration of oxalic acid. It has a maximum at c = 2 mmol/L. A nearly constant reaction rate is established when the pH of the solntion is set to pH 2.3 by addition of sulfuric acid. Warburg (cited in ref. 57) fonnd the oxidation rate to be proportional to the square root of oxygen pressnre this was confirmed in ref. 94. The catalytic activity of activated carbons followed the same order as in the oxidation of sulfurous acid [94], and it increased after heat treatment at 973 to 1073 K or NH3 treatment [39]. Carbon blacks were inactive. Also in this case, KNO3 had an inhibiting effect, even at a concentration of 1 mEq/L, and more at higher concentrations. Na2S04 had no effect on the oxidation rate. 

---

Here the change in oxidation number per atom of iron is from +2 to +3, or by 1 unit of oxidation, hence the equivalent of iron(II) sulphate is 1 mole. Another important reaction is the oxidation of oxalic acid to carbon dioxide and water ... 

Inverse dependences are seen with reactions with rapid prior equilibria. If they are rapid enough, a single denominator term is found over a wide range of the concentrations (even though this form, too, is the limit of a sum of two terms). One example is the oxidation of oxalic acid by chlorine according to the equations6... 

Early work of Dhar established that oxidation of oxalic acid by chromic acid occurs readily, but some of his kinetic data are unreliable as the substrate itself acted as the source of hydrogen ions. The reaction is first-order in oxidant and is subject to strong manganous ion catalysis (as opposed to the customary retardation), the catalysed reaction being zero-order in chromic acid. This observation is related to those found in the manganous-ion catalysed oxidations of several organic compounds discussed at the end of this section. 

The permanganate oxidation of oxalic acid has been studied exhaustively and has been reviewed by Ladbury and Cullis . It is characterised by an induction period and a sigmoid dependence of rate upon time. Addition of manganous ions eliminates the induction period and produces first-order decay kinetics . Addition of fluoride ions, however, practically eliminates reaction . ... 

Dhar noted that the oxidation of oxalic acid by chromic acid is markedly accelerated on adding manganous ions, the reaction order in Cr(VI) changing from one to zero. Bobtelsky and Glasner ° found the oxidation of bromide ions by chromic acid in aqueous sulphuric acid to follow kinetics... 

The oxidation of oxalic acid by mercuric chloride to give CO2 and mercurous chloride is a classic example of an induced reaction. This reaction is extremely slow unless small quantities of chromic acid and manganous ions are added, whereon facile reduction takes place Addition of permanganate or persulphate and some reducing agents is also effective and the oxidation proceeds readily under photo- or X-irradiation (Eder s reaction). The large quantum yield points to a chain mechanism , which could also function with an inducing oxidant, viz. 

The oxidation of oxalic acid by AuCl4. represents one of the few examples of a kinetic study of Au(IIl) oxidation falling within the present category, viz. 

The kinetics of the Ce(IV) sulphate oxidation of oxalic acid are simple second order although the rate coefficient is inversely proportional to both hydrogen and bisulphate-ion concentrations, and it is also reduced at very high oxalic acid concentrations. Values of the activation energy from 13.4+1.5 (ref. 411) to 16.5+0.4 (ref. 409) kcal.mole have been reported. An intermediate has been detected spectroscopically " this decays in first-order fashion with E = 10.5+0.5 kcal.mole" and with a rate independent of acidity. However, the extent of formation of this complex is reduced as the acidity is increased ", and it appears that a less reactive dioxalato complex is formed at higher substrate concentrations ". 

The oxidation by Mn(lII) chloride involves three complexes and the kinetic data of Taube " are summarised in Table 15. The greater thermal stability of the /m-complex is considered to result from the lowering of the free energy relative to the transition state as compared with bis- and mono-complexes. The study of MnC204 was based on the Mn(III)-catalysed chlorine oxidation of oxalic acid. ... 

The Np02 oxidation of oxalic acid in aqueous perchloric acid provides one of the few examples of redox kinetic studies of Np(VI) . The rate law is... 

The absence of retardation by V(IV) rules out a mechanism analogous to that of Mn(III) oxidation. Mn(II) ions strongly catalyse reaction , altering the kinetics to those observed for the Mn(II)-catalysed oxidation of oxalic acid by V(V) (preceding sub-section) except that the [V(V)] dependence has a Michaelis-Menten, form rather than being first-order. E is reduced from 19.7 to 6.9 kcal.mole , and a similar mechanism is believed to operate. 

It was observed by Gopala Rao and Sastri that the reaction between hydro-quinone and chromic acid leads to the induced oxidation of oxalic acid, glycerol, lactic acid, glucose, citric acid, and malic acid. If the concentrations of the above acceptors are cen times that of that of the hydroquinone inductor, the values of F found are, respectively, 0.51,0.46,0.35,0.27 and 0.17. The numerical values of the induction factor do not permit us to discuss the nature of coupling intermediate. 

Gopala Rao and Venkateswara Rao found that the oxidation of indigo to isatin by chromic acid is accelerated by the presence of oxalic acid, and at the same time the extent of the oxidation of oxalic acid by chromic acid is increased. This observation is an example of mutual induction. 

The effect of pH on the periodate oxidation of seven anilines has been investigated. " The kinetics of periodate oxidation of aromatic amines have been studied. " - " Periodate oxidation of oxalic acid is catalysed by Mn(II). " The reaction of ethane-1,2-diol with periodate has been investigated under a variety of conditions and the results compared with those of earlier work and analogous studies on pinacol. " The 104 ion is the primary reactant, with H5IO6 as a secondary reactant the reverse is true for pinacol. The complex observed in previous work is shown not to be an intermediate, but rather to deactivate the reactants. 

A differentiation in the activities of surfaces may likewise be witnessed in a variety of chemical and catalytic processes. Thus we find that charcoal will undergo slow autoxidation when exposed to air it will also cataljrtically accelerate the oxidation of a number of organic subtances such as oxalic acid. By processes of selective poisoning of the charcoal it can be demonstrated quite readily that the portion of the surface which can accelerate the oxidation of, oxalic acid is but a small portion of the surface which is available for say the adsorption of methylene blue and that but a minute fraction of the surface (less than 0 5 % foi a good sugar charcoal) is capable of undergoing autoxidation. 

Ghem. xxxi. 161, 1902) obtained 8 = 095 mm. from measurements on the rate of the electrolytic oxidation of oxalic acid. Fischer (Elektrochemie, p. 60) obtained the following values from data on the limiting current densities in the precipitation of copper at a rotating cathode ... 

In the field of organic ligands the oxidation of oxalic acid ligated to different central atoms provides us with almost any pattern of behavior of oxidized ligands. The oxidation of oxalate by permanganate in the presence of manganous ions (/, 68) proceeds according to Mechanism 10... 

Since the oxidation of oxalic acid involves the transfer of two electrons before forming a stable product, the oxidized intermediate may either break down to give a C2O4-- radical, or it may be long lived enough to undergo a second reaction with the oxidant. The latter mechanism has been demonstrated in the oxidation of chromic oxalate by ceric ions (88). 

Experimental observations indicate that the oxidation of cobalt (II) to cobalt (III) and the formation of ethylenediamine from N-hydroxyethylethylene-diamine occur simultaneously. This is quite the opposite to what is usually assumed in other instances of transition metal catalysis of organic reactions—for example, the catalytic effect of manganese in the oxidation of oxalic acid (7, 8), of iron in the oxidation of cysteine to cystine (22) and of thioglycolic acid to dithioglycolic acid (5, 23), of copper in the oxidation of pyrocatechol to quinone and in the oxidation of ascorbic acid (29, 30), and of cobalt in the oxidation of aldehydes and unsaturated hydrocarbons (4). In all these reactions the oxidation of the organic molecule occurs by the abstraction of an electron by the oxidized form of the metal ion. 

If compounds already react very fast with ozone, the addition of hydrogen peroxide is nearly ineffective, which was shown by Brunet et al. (1984) in the case of benzaldehyde and phthalic acid. The functional groups on the aromatic ring are relatively reactive towards molecular ozone. The advantage of this process lies in the removal of compounds relatively non-reactive with ozone. It was shown that the oxidation of oxalic acid, which is often an end product in the case of molecular ozone reactions, was significantly accelerated with the addition of hydrogen peroxide. 

Oxidation of oxalic acid with dimethyl-V,V-dichlorohydantoin and dichloroisocya-nuric acid is of first order with respect to the oxidant. The order with respect to the reductant is fractional. The reactions are catalysed by Mn(II). Suitable mechanisms are proposed.129 A mechanism involving synchronous oxidative decarboxylation has been suggested for the oxidation of a-amino acids with l,3-dichloro-5,5-dimethylhydantoin.130 Kinetic parameters have been determined and a mechanism has been proposed for the oxidation of thiadiazole and oxadiazole with trichloroiso-cyanuric acid.131 Oxidation of two phenoxazine dyes, Nile Blue and Meldola Blue, with acidic chlorite and hypochlorous acid is of first order with respect to each of the reductant and chlorite anion. The rate constants and activation parameters for the oxidation have been determined.132... 

In this reaction C02 can be produced at the anode by anodic oxidation of oxalic acid derivatives 369). In particular cases, the reaction can also be carried out in the presence of small amounts of water 370) ... 

Writing half-reaction for the oxidation of oxalic acid. Balancing (i) the atoms in the order carbon-oxygen-hydrogen and in (ii) Equalising the charge ... 

Wright and his co-workers [141] found mercury plus a small amount of aluminium (ca. 2%) and manganese (ca. 5%) to be a more efficient catalyst than mercury alone. The experiments have shown that mercury increases the reaction rate while manganese, though it has no influence on the principal reaction, assists in the complete oxidation of oxalic acid which would otherwise contaminate the reaction product. 

A scries of researches, concerning the relation between the oxidation of oxalic acid and the electrical conditions have been made. Oettel 1 discovered that the current consumption required for an oxidation process is greater when a smaller current density is used than when a higher density is employed. 

Ivandini, T. A., Rao, T. N., Fujishima, A. and Einaga, Y. (2006a), Electrochemical oxidation of oxalic acid at highly boron-doped diamond electrodes. Anal. Chem., 78(10) 3467-3471. 

The importance of the superficial state of diamond was also pointed out in a recent work by Ivandini et al. (2006) in which oxidation of oxalic acid was considered. 

Bergmann, H., Iourtchouk, T., Schops, K. and Bouzek, K. (2002) New UV irradiation and direct electrolysis - Promising methods for water disinfection. Chem. Eng. J. 85, 111-117 Bock, C., Smith, A. and MacDougall, B. (2002) Anodic oxidation of oxalic acid using WOx based anodes. Electrochim. Acta 48, 57-67... 

Ivandini, T.A., Naono, Y., Nakajima, A. and Einaga, Y. (2005) Gold-nanoparticle-dispersed boron-doped diamond electrodes for electrochemical oxidation of oxalic acid. Chem. Lett. 34, 1086-1087... 

Ivandini, T.A., Rao, T.N., Fujishima, A. and Einaga, Y. (2006) Electrochemical oxidation of oxalic acid at highly boron-doped diamond electrodes. Anal. Chem. 78, 3467-3471 Josephy, P. D. (1996) Molecular Toxicology, Oxford University Press, New York, NY Kraft, A., Stadelmann, M. and Blaschke, M. (2003) Anodic oxidation with doped diamond electrodes A new advanced oxidation process. J. Hazard. Mater. 103, 247-261 Kusic, H., Koprivanac, N. and Bozic, A.L. (2006) Minimization of organic pollutant content in aqueous solution by means of AOPs UV- and ozone-based technologies. Chem. Eng. J. 123, 127-137... 

Mendive, C.B., D.W. Bahnemann and M.A. Blesa (2005). Microscopic characterization of the photocatalytic oxidation of oxalic acid adsorbed onto Ti02 by FTIR-ATR. Catalysis Today, 101(3 1), 237-244. 

Palladous Salts likewise possess catalytic properties. For example, they accelerate the oxidation of oxalic acid by persulphates, and, to a less extent, by nitric acid. This is attributable to the alternate formation of a palladic salt by the oxidiser and its reduction by the oxalic acid. Ammonia is likewise oxidised by persulphates in the presence of a palladous salt.3... 

The heterogeneous catalysis of gas reactions has been extensively studied and indeed forms the subject matter of three previous volumes (19-21) of Comprehensive Chemical Kinetics. The heterogeneous catalysis of solution ractions has received far less systematic attention. This is surprising since the phenomenon has been known and utilised sporadically for almost 150 years. As long ago as 1845, Millon [1] found that the oxidation of oxalic acid by iodate... 

Early work on the kinetics of oxidation of oxalic acid by hypobromous acid is reviewed by Griffith et The latter workers showed that in solutions of... 

Abel and Hilferding studied the kinetics of the reaction, and showed that the oxidations of oxalic acid and its monoanion by hypoiodous acid are second-order. 

I lerrmann, J. M., M. N. Mozzanega, and P. Pichat (1983), Oxidation of Oxalic Acid in Aqueous Suspensions of Semiconductors Illuminated with UV or Visible Light, J. Photochem. 22, 333-343. 

---