Kinetic Studies of the Oxidation of L-Ascorbic Acid by Tris ( Oxalate ) Cobaltate in the Presence of CDTA Metal Ion Complexes

Realizaram-se estudos cinéticos envolvendo a reação de redução de tris(oxalato)cobaltato por L-ácido ascórbico, em diferentes valores de pH. A variação da concentração do complexo de Co(III) foi acompanhada pela absorbância em 600 nm, em condições de pseudo-primeira ordem: [H 2 A] = 3,0 x 10 mol L, [Co(C 2 O 4 ) 3 ] = 3,0 x 10 mol L, na presença de CDTA (3,0 x 10 mol L), em I = 1,0 mol L (NaCl) e a 25,0±0,1 C. Foram investigadas as atividades catalíticas dos complexos de CDTA com Fe(III), Ni(II), Cu(II), Cr(III) e Mn(II). Para Fe(III)/ CDTA, o melhor catalisador, os valores de constantes de velocidade observada de pseudoprimeira ordem foram proporcionais à concentração de ferro (1-10) x 10 mol L.


Introduction
The redox reactions of L-ascorbic acid are of fundamental interest in chemistry, biochemistry, pharmacology and several areas of medicine, since it is necessary in human diet in order to synthesize collagen and epinephrine, besides preventing scurvy.L-Ascorbic acid, H 2 A, has two acid protons (pK 1 = 4.04 and pK 2 = 11.34), and is a strong reducing agent (E 0 D/H 2 A = 0.390 V vs. N.H.E.) in aqueous solution.][5][6][7][8][9] The oxidation studies of L-ascorbic acid by [Co(C 2 O 4 ) 3 ] 3-were already performed in basic and acid aqueous solutions, and it was pointed out that the redox process in acidic medium (3.2 < pH < 4.7) produced Ldehydroascorbic acid, D (equation 1).L-dehydroascorbic acid, D, also includes the hydration and cyclization of D form, with formation of the bicyclic-L-dehydro species. 8,9hese studies indicated an outer-sphere electron-transfer process and no evidence of stable intermediate species formation.
1][12][13][14] This last study, performed in universal buffer medium, led to a linear relationship between the observed pseudo-first order constant and the iron(III)/EDTA complex concentration. 14he catalytic effect of iron(III)/EDTA could be explained by the much faster reactions of iron(III)/ EDTA, rather than [Co(C 2 O 4 ) 3 ] 3-, with L-ascorbic acid. 144][15][16][17] The reaction of Fe(II) with [Co(C 2 O 4 ) 3 ] 3-, which was investigated by some authors, has also to be considered.][20] In this work the catalytic effect of iron(III) on the oxidation of L-ascorbic acid by [Co(C 2 O 4 ) 3 ] 3-was studied in the presence of polyaminocarboxylic acid, CDTA (pK 1 = 2.42; pK 2 = 3.54; pK 3 = 5.84 and pK 4 = 9.22), in a universal buffer solution over a large pH region.This study provides information to improve an analytical method for iron(III) at pH = 7.0. 14,17For comparative studies, the catalytic effect of others transitions metal ions such as: Ni(II), Cu(II), Cr(III) and Mn(II) was also investigated.
Iron(III) perchlorate (Aldrich) stock solution (6.0x10 -2 mol L -1 ) was standardized by complexometric method with EDTA. 21The potassium tris(oxalate) cobaltate(III) salt was prepared as described elsewhere. 22It was dissolved in buffer solution just before use in order to get a solution 6.0 x 10 -3 mol L -1 .L-Ascorbic acid (Merck), L-C 6 H 8 O 6 , was also dissolved in buffer solution, just before use, in order to obtain a solution 6.0 x 10 -2 mol L -1 .

Working solutions and spectrophotometric measurements
Several universal buffers solutions were prepared containing H 3 PO 4 , H 3 C-COOH and H 3 BO 3 (0.18 mol L -1 of each).NaOH 1.768 mol L -1 standard solution was used to adjust the pH of the buffers solutions from 3 to 8. 23 NaCl 2.0 mol L -1 solution was use to make up the ionic strength 1.0 mol L -1 in all working solutions.
The working solutions were prepared by mixing equal volumes of the of L-C 6 H 8 O 6 6.0 x 10 -2 mol L -1 and [Co(C 2 O 4 ) 3 ] 3-6.0 x 10 -3 mol L -1 solutions containing CDTA 6.0 x 10 -3 mol L -1 .In the catalytic studies iron(III) was added as Fe(III)/CDTA in the L-C 6 H 8 O 6 solution just before mixing, once Fe(III) is reduced by L-C 6 H 8 O 6 .
A glass electrode, combined with an Ag/AgCl reference electrode, Metrohm AG Herisau, filled with 3.0 mol L -1 NaCl and a 654 pHMeter Metrohm instrument were used in the pH measurements at (25.0 ± 0.1) o C. Spectrophotometric measurements were performed in a Hewlett Packard 8452A diode-array spectrophotometer using a thermostated Tanden cell (optical path length = 0.875 cm).

Data treatment
The redox reaction was followed spectrophotometrically at 600 nm where the major absorbing species is the complex [Co(C 2 O 4 ) 3 ] 3-(molar absorptivity of 150 ± 10 mol -1 L cm -1 ). 14 ten times excess of L-C 6 H 8 O 6 over [Co(C 2 O 4 ) 3 ] 3-was kept in all experiments in order to have pseudo first order conditions.As the reaction was not affected by dissolved oxygen it was not necessary to eliminate the dissolved air before the kinetic runs.
The kinetic curves were analysed with the OLIS KINFIT set of programs.All the observed rate constants values presented in this work are the mean of at least three determinations and have an average error smaller than 5%. 24e pH influence on the uncatalysed reaction The variation of [Co(C 2 O 4 ) 3 ] 3-concentration with time from the reduction by L-C 6 H 8 O 6 , in the absence of any CDTA metal ion complexes showed a pseudo-first-order behaviour.
These experiments were carried out over a large pH range in universal buffer solution containing CDTA, which was added to avoid precipitation of cobalt(II) and cobalt(III) oxalate at high pH.No experiments at pH lower than 3.0 were carried out, due to the [Co(C 2 O 4 ) 3 ] 3- decomposition and precipitation of CDTA. 25,26he dependence of k obs with pH (Figure 1) suggests that the H 2 A specie is much less reactive than the HA - species.In the 6.0 < pH < 7.5 range, where HA -is the predominant species, the k obs value is almost constant.It was observed that at pH higher than 10 the oxidised species, D, decomposes rapidly. 1,4,14e sequence of reactions represented below, may describe the mechanism.
The reduction rate of the tris(oxalate)cobaltate(III) complex concentration is given by the rate law described in equation 8. Using the experimental data obtained at pH range from 3 to 5 the rate law under pseudo-first-order conditions can be written by equation 9, which results in equation 10 (in all equations C H 2 A is the total concentration of ascorbic acid).(8)   In this pH range the contribution A 2-is very small (see Figure 1) and the term k c [A 2-] can be ignored.( 9) (10)   By working under limiting conditions, such as k a is much smaller than k b because H 2 A (equation 4) reacts much slower than HA -(equation 5), the term 2k a [H + ] can be neglected and the equation 10 can be represented as equation 11, as follows: (11)   Taking the equation 11, in the pH region from 3 to 5, a plot of 1/ k obs vs. [H + ] provides a linear relationship and from the slope and intercept the values of k b and K 1 can be, respectively, obtained (Figure 2).The least squares regression (Y= 5465 + 6.382x10 6 X, r = 0.97) showed some dispersion of the experimental data in spite of the k obs values have been obtained with the average error smaller than 5%.The values found were k b = (3.2± 1.5) x10 -3 mol -1 L s -1 and K 1 = (0.83 ± 0.09) x 10 -4 mol L -1 and the order of magnitude of these data is in good agreement with the literature: k b = 4.1x10 -3 mol -1 L s -1 , K 1 = 1.12x10 -4 mol L -1 , k b = 7.0x10 -3 mol -1 L s -1 and K 1 = 0.71x10 -4 mol L -1 . 9,14 By using the experimental data, which were obtained in the pH range from 6.0 to 8.0, the rate law can be described according to the following equations: (12) (13)   Taking into account the second ionisation step for ascorbic acid K 2 = 4.6 x 10 -12 mol L -1 , the k c = 8.5 mol -1 L s -1 was obtained from the slope of the k obs vs. 1/[H + ] plot.The k c values found in the literature were 20 mol -1 L s -1 and 10.8 mol -1 L s -1 , in universal buffer solution and in ionic strength kept with NaClO 4 and NaCl, respectively. 8,14,23e iron(III) catalytic effect The variation of the [Co(C 2 O 4 ) 3 ] 3-concentration by the reduction with L-C 6 H 8 O 6 , in the presence of Fe(III)/ CDTA complex, also revealed a pseudo-first-order rate behaviour.The observed rate constant is proportional to the Fe(III) concentration and dependent of the pH medium (Figures 1 and It can be also noted that at pH lower than 5.0, where H 2 A and HA -species are present, the reaction is slower even in the presence of Fe(III).

Figure 2 .
Figure 2. Determination of k b considering the equation (11).K 1 and the k obs calculated in the range of pH 3-5.