Autocatalytic Reaction Pathway on Manganese Dioxide Colloidal

Aug 13, 2009 - resulting in an inhibition of the autocatalytic reaction pathway by phosphate ... involving the amino acid as ligand, the autocatalysis...
0 downloads 0 Views 806KB Size
Download PDF
15982

J. Phys. Chem. C 2009, 113, 15982–15991

Autocatalytic Reaction Pathway on Manganese Dioxide Colloidal Particles in the Permanganate Oxidation of Glycine Joaquin F. Perez-Benito* Departamento de Quimica Fisica, Facultad de Quimica, UniVersidad de Barcelona, Marti i Franques, 1, 08028 Barcelona, Spain ReceiVed: February 16, 2009; ReVised Manuscript ReceiVed: June 24, 2009

The progress of the reaction of permanganate ion with the amino acid glycine in near-neutral aqueous solutions was monitored with a UV-vis spectrophotometer at two different wavelengths in order to observe the decay of the oxidant (MnO4-, at 526 nm) and the formation of one of the reaction products (colloidal MnO2, at 418 nm). The use of a phosphate buffer resulted in the stabilization of MnO2 as a soluble colloid during the kinetic runs. The experimental data are consistent with the hypothesis that of the two conjugated buffer ions H2PO4-/HPO42- the acidic one (H2PO4-) is the predominant species responsible for that stabilization, suggesting that the formation of hydrogen bonds between the MnO2 oxygen atoms and the hydrogen atoms from phosphate ions might be involved in the process. An increase of the phosphate concentration resulted in a notable decrease of both the molar absorption coefficient and the size of the MnO2 colloidal particles. The reaction showed an autocatalytic behavior, but the kinetic plots deviated from the differential rate law usually employed for the study of autocatalytic reactions. The experimental data suggest that these deviations might be caused by the competitive adsorption of both phosphate ions and glycine on the surface of the MnO2 colloidal particles, with the adsorption of the two species being too slow for the quasi-equilibrium approximation to hold, and resulting in an inhibition of the autocatalytic reaction pathway by phosphate ions. Introduction Permanganate ion has been for a long time one of the most versatile and widely employed oxidizing agents.1,2 Among its applications, we can detail the use of permanganate as an in situ scavenger of organic contaminants in the purification of water.3,4 However, its efficiency has shown some limitations because of the formation of an insoluble manganese dioxide precipitate as reduction product.5,6 Deposition of MnO2 particles may prevent effective oxidant distribution and contact with contaminants.7 The applicability of permanganate in green chemistry as an environmentally friendly oxidant has been enhanced by the development of a method for the recycling of the product manganese dioxide, which can be converted back into permanganate or, alternatively, used in organic synthesis as a mild oxidant or in the manufacture of catalysts, batteries, or pigments.8 As recently reported, manganese(IV) compounds might also find some applications as new chemiluminescence reagents.9 In the biological field, permanganate ion is used as a chemical probe in many biochemical essays.10 Its reactions with amino acids are of particular interest because they show autocatalytic patterns both in acidic11,12 and in neutral media.13 Although the autocatalysis found in acidic solutions is usually explained in terms of the formation of either Mn(II)14 or Mn(III)15 complexes involving the amino acid as ligand, the autocatalysis observed when the same reactions are performed in neutral solutions is explained in terms of a heterogeneous reaction pathway taking place on the surface of the colloidal particles belonging to a soluble form of colloidal manganese dioxide.16 In fact, soluble forms of colloidal MnO2, formed either as long-lived intermediates or as reaction products and constituted by molecular * Fax: 34 93 4021231. E-mail: [email protected].

aggregates with sizes in the nanoparticle range,17,18 are known to be involved in many permanganate reactions.19 In several cases this species plays an important kinetic role because of its involvement as a heterogeneous autocatalyst for the reaction.20 This autocatalytic role usually takes place when colloidal MnO2 is one of the reaction products, as happens in the permanganate oxidation of the amino acid L-threonine in neutral phosphatebuffered solutions,21 but it can also occur when the colloid is formed as an intermediate, as in the permanganate oxidation of formic acid in perchloric media.22 Moreover, for the permanganate oxidation of oxalic acid in sulfuric solutions, arguably the most important and most studied of the permanganate reactions,23 although its mechanism has proven so far to be rather elusive,24,25 it has been recently reported that its autocatalysis cannot be explained if the Mn(II)-oxalate complex formed as reaction product is considered the only autocatalyst, and that an essential reaction pathway to explain its kinetic behavior takes place on the surface of the MnO2 colloidal particles formed as a long-lived intermediate, hence providing a positive feedback loop for the reaction.26 It thus seems a plausible hypothesis that soluble forms of colloidal MnO2 might play the role of a heterogeneous autocatalyst in many permanganate reactions, not only when formed as a reaction product (in neutral solutions) but also when formed as a long-lived intermediate (in acidic solutions). A chemical model suitable for studying the applicability of this hypothesis might be the permanganate oxidation of amino acids. New features of the kinetic behavior of these important autocatalytic reactions in neutral media are now presented using as reducing agent for permanganate ion the simplest and the only nonchiral of the proteinogenic biological monomers, the amino acid glycine (NH3+CH2CO2-).27 The stoichiometry of the reaction is

10.1021/jp9014178 CCC: $40.75  2009 American Chemical Society Published on Web 08/13/2009

Permanganate Oxidation of Glycine

J. Phys. Chem. C, Vol. 113, No. 36, 2009 15983

2MnO4- + 3NH3+CH2CO2- + 2H+ ) 2MnO2 + 3NH3 + 3CH2O + 3CO2 + H2O (1) In this study, some deviations from the kinetic model generally used for autocatalytic reactions28-31 have been observed, and an interpretation of those deviations has provided useful information on the mechanism of the autocatalytic reaction pathway. Experimental Section Materials and Instruments. The solvent was twice-distilled water. All the reactants [KMnO4, NH2CH2CO2H, KH2PO4, K2HPO4 · 3H2O, KCl, BzEt3NCl, Ca(NO3)2, and gum arabic] were of analytical-grade quality (Merck). The reducing agent, glycine, was used in large excess with respect to the oxidant, potassium permanganate. A KH2PO4-K2HPO4 buffer mixture was used in all the experiments. The pH measurements were done with a Metrohm 605 pH meter provided with a glass-calomel combined electrode. The kinetic runs were followed with a Varian Cary 219 spectrophotometer, using thermostatized glass cells (optical path length 1 cm), and measuring both the decay of MnO4- at 526 nm and the formation of colloidal MnO2 at 418 nm. Kinetic Method. In order to obtain the values of the permanganate concentration at different instants during each kinetic run from the absorbance at 526 nm, it was necessary to discount the contribution due to the colloidal reaction product. This was attained from the absorbance reading at 418 nm (wavelength at which permanganate ion is almost transparent to radiation) by means of the following equation:

[MnO4-]t )

A(526)t - [εP(526)/εP(418)]A(418)t εR(526)l

(2) where the numbers in parentheses indicate wavelengths, εR and εP are the molar absorption coefficients of the reactant (MnO4-) and product (colloidal MnO2), respectively, and l is the optical path length. Equation 2 does not require that εP(418) and εP(526) be constant during the kinetic runs, but only their ratio, and it is thus useful for correcting absorbances in a reaction involving a colloidal product such as MnO2. The reaction rates at different instants during the course of the reaction were obtained from the permanganate concentration data by an approximate derivation method:

V)-

d[MnO4-]t ∆c ≈dt ∆t

(3)

applied at short time intervals (∆t ) 1-3 min), and where c represents the permanganate concentration at time t.

Figure 1. Plots of A(526)t - A(526)r against the absolute value of A(418)t - A(418)r at different instants t during the course of the reaction of KMnO4 (5.12 × 10-4 M) with glycine (0.160 M) in KH2PO4 (0.080 M)-K2HPO4 (0.080 M) buffer at ionic strength 0.320 M (KCl), pH 6.64, and 25.0 °C. Fixed references: either first absorbances [A(418)r ) 0.034, A(526)r ) 1.185, filled circles] or last absorbances [A(418)r ) 0.234, A(526)r ) 0.465, open circles] recorded.

easily implemented graphical method can be applied.33 If only two absorbing species are present in the solution, the difference of absorbances at a certain wavelength A(λ1)t - A(λ1)r should lead to a straight line passing through the origin when plotted against the difference A(λ2)t - A(λ2)r corresponding to a second wavelength, where the subscripts denote a variable instant t during the course of the reaction and a fixed instant r taken as reference. As shown in Figure 1, under experimental conditions such that the reaction product behaved as a stable colloid [so that its εP(418) and εP(526) values are constant during the progress of the reaction], a plot of A(526)t - A(526)r vs A(418)t - A(418)r led to a straight line passing through the origin whether the first absorbances or the last absorbances recorded were taken as reference. This allows us to conclude that only two absorbing species (MnO4- and colloidal MnO2) were present in the reacting system, and that the existence of longlived intermediates could be discarded. Double-Absorbance Plots. Assuming that (i) permanganate ion is transparent to radiation at 418 nm [εR(418) = 0] whereas both reactant (MnO4-) and product (colloidal MnO2) absorb radiation at 526 nm, (ii) Beer’s law is fulfilled at both wavelengths, and (iii) as confirmed by Figure 1 all the manganese intermediates are present in negligible concentrations (in steady state), we can infer the following relationship between the absorbances at 526 and 418 nm:

A(526)t ) A(526)o +

εP(526) - εR(526) A(418)t εP(418)

(4)

Results Number of Absorbing Species. Complex algebraic methods have been developed for the determination of the number of independent absorbing species present in a chemical system studied by absorption spectroscopy.32 However, in systems subjected to the restriction of the sum of the concentrations of absorbing species being constant (in the present case, the sum of the concentrations of the different manganese species equals the initial concentration of permanganate ion), a convenient,

where A(526)o is the initial absorbance at 526 nm. According to eq 4, a linear relationship between the absorbances at the two wavelengths with a negative slope [εP(526) < εR(526)] is expected provided that the reaction product behaves as a stable colloid [both εP(418) and εP(526) constant]. In fact, three types of A(526) vs A(418) plots were found; some of them were linear, whereas others showed either an upward-concave or a downwardconcave curvature, depending on the experimental conditions.

15984

J. Phys. Chem. C, Vol. 113, No. 36, 2009

Figure 2. Dependence of the absolute value of the slope of the A(526) vs A(418) plot on the initial permanganate concentration for its reaction with glycine (0.152 M) in KH2PO4 (0.040 M)-K2HPO4 (0.040 M) buffer, at pH 6.78 and 25.0 °C. Inset: A(526) vs A(418) plot at [KMnO4]o ) 7.68 × 10-4 M.

Perez-Benito

Figure 4. Dependence of the absolute value of the slope of the A(526) vs A(418) plot on the benzyltriethylammonium chloride concentration for the reaction of KMnO4 (5.00 × 10-4 M) with glycine (0.120 M)) in KH2PO4 (0.040 M)-K2HPO4 (0.040 M) buffer at pH 6.78 and 25.0 °C. Inset: A(526) vs A(418) plot at [BzEt3NCl] ) 0.100 M.

TABLE 1: Effect of Initial Permanganate Concentration on Physical Properties of Manganese Dioxide Colloidal Particlesa [MnO4-]o/10-3 M

εP(418)b/M-1 cm-1

Nc/Noc

Dc/Dod

0.000 0.512 0.640 0.768 0.896 1.020

600.1 ( 4.6e 650.0 ( 0.8 661.3 ( 1.0 671.7 ( 1.0 676.2 ( 1.0 681.2 ( 0.9

1.000 ( 0.000 1.083 ( 0.010 1.102 ( 0.010 1.119 ( 0.010 1.127 ( 0.010 1.135 ( 0.010

1.000 ( 0.000 1.027 ( 0.003 1.033 ( 0.003 1.038 ( 0.003 1.041 ( 0.003 1.043 ( 0.003

a [glycine] ) 0.152 M, [KH2PO4] ) [K2HPO4] ) 0.040 M, pH 6.78, and 25.0 °C. b Molar absorption coefficient of colloidal MnO2 at 418 nm. c Ratio between the estimated values of the number of MnO2 molecules per colloidal particle at a certain initial permanganate concentration and at [MnO4-]o ) 0. d Ratio between the estimated values of the diameter of the colloidal particles at a certain initial permanganate concentration and at [MnO4-]o ) 0. e Extrapolated value.

Figure 3. Dependence of the absolute value of the slope of the A(526) vs A(418) plot on the total phosphate concentration ([KH2PO4] + [K2HPO4]) for the reaction of KMnO4 (5.12 × 10-4 M) with glycine (0.160 M) in phosphate buffer ([KH2PO4] ) [K2HPO4]), at ionic strength 0.320 M (KCl), pH 6.64, and 25.0 °C. Inset: A(526) vs A(418) plot at [KH2PO4] ) [K2HPO4] ) 0.048 M.

In three series of experiments the plots were linear (Figures 2-4, insets); the absolute value of the corresponding slope decreased with increasing initial concentration of permanganate (Figure 2), increased with increasing phosphate concentration approaching asymptotically a maximum value at high buffer concentrations (Figure 3), and increased again with increasing concentration of benzyltriethylammonium chloride (Figure 4). From those linear plots, it has been possible to infer some information on the physical properties of the colloidal particles. First, the molar absorption coefficient of MnO2 at 418 nm was obtained from the slope of eq 4. If we assume that in the 418nm region the main contribution to the spectrum of MnO2 is that of light scattering by the colloidal particles, a directproportionality relationship can be established between εP(418) and the number of MnO2 molecules per colloidal particle (N).19 This allowed us to make an estimation of the ratio between the

values of N at a certain concentration of a chemical additive (Nc) and in its absence (No) as

εP(418)c Nc ) No εP(418)o

(5)

and of the corresponding ratio between the values of the diameter of the colloidal particles as

()

Dc Nc ) Do No

1/3

(6)

since the symmetry of those particles is roughly spherical.17 The values of εP(418), Nc and Dc all increased as the initial permanganate concentration increased (Table 1) and decreased as the concentration of either phosphate buffer (Table 2) or benzyltriethylammonium chloride (Table 3) increased. Under the different experimental conditions used in this study the molar absorption coefficient varied in the range 600 < εP(418) < 785 M-1 cm-1, whereas in a given series of experiments the other

Permanganate Oxidation of Glycine

J. Phys. Chem. C, Vol. 113, No. 36, 2009 15985

TABLE 2: Effect of Total Phosphate Concentration on Physical Properties of Manganese Dioxide Colloidal Particlesa [phosphate]T/M εP(418)b/M-1 cm-1 0.000 0.032 0.064 0.096 0.128 0.160

784.4 ( 4.6 712.5 ( 4.0 645.3 ( 1.1 630.3 ( 0.9 610.2 ( 1.4 618.7 ( 1.0

e

Nc/Noc

Dc/Dod

1.000 ( 0.000 0.908 ( 0.010 0.823 ( 0.006 0.804 ( 0.006 0.778 ( 0.006 0.789 ( 0.006

1.000 ( 0.000 0.968 ( 0.004 0.937 ( 0.002 0.930 ( 0.002 0.920 ( 0.002 0.924 ( 0.002

a [MnO4-]o ) 5.12 × 10-4 M, [glycine] ) 0.160 M, [KH2PO4] ) [K2HPO4] ) [phosphate]T/2, ionic strength ) 0.320 M (KCl), pH 6.64, and 25.0 °C. b Molar absorption coefficient of colloidal MnO2 at 418 nm. c Ratio between the estimated values of the number of MnO2 molecules per colloidal particle at a certain total phosphate concentration and at [phosphate]T ) 0. d Ratio between the estimated values of the diameter of the colloidal particles at a certain total phosphate concentration and at [phosphate]T ) 0. e Extrapolated value.

TABLE 3: Effect of Benzyltriethylammonium Chloride Concentration on Physical Properties of Manganese Dioxide Colloidal Particlesa [BzEt3NCl]/M εP(418)b/M-1 cm-1 0.000 0.050 0.100 0.150 0.200

663.9 ( 0.9 657.8 ( 0.8 647.0 ( 0.8 638.1 ( 1.0 627.9 ( 0.7

Nc/Noc

Dc/Dod

1.000 ( 0.000 0.991 ( 0.003 0.974 ( 0.002 0.961 ( 0.003 0.946 ( 0.002

1.0000 ( 0.0000 0.9969 ( 0.0008 0.9914 ( 0.0008 0.9868 ( 0.0009 0.9816 ( 0.0008

Figure 5. A(526) vs A(418) plots for the reaction of KMnO4 (5.00 × 10-4 M) with glycine (bottom, 0.080 M; top, 0.240 M) in KH2PO4 (0.040 M)-K2HPO4 (0.040 M) buffer, at pH 6.78 and 25.0 °C. The dashed line is the tangent to the curve at t ) 0.

a [MnO4-]o ) 5.00 × 10-4 M, [glycine] ) 0.120 M, [KH2PO4] ) [K2HPO4] ) 0.040 M, pH 6.78, and 25.0 °C. b Molar absorption coefficient of colloidal MnO2 at 418 nm. c Ratio between the estimated values of the number of MnO2 molecules per colloidal particle at a certain benzyltriethylammonium chloride concentration and at [BzEt3NCl] ) 0. d Ratio between the estimated values of the diameter of the colloidal particles at a certain benzyltriethylammonium chloride concentration and at [BzEt3NCl] ) 0.

two parameters had only minor changes of up to 22% (Nc) and 8% (Dc). Of the three variables studied, it seems that the concentration of phosphate was the one affecting most the value of those parameters. Although the molar absorption coefficient of the product was obtained only for those series of experiments in which MnO2 behaved as a stable colloid, thus leading to A(526) vs A(418) linear plots (Figures 2-4), since in the other cases the values of both εP(418) and εP(526) changed with time (eq 4), a comparison of the data given in Tables 1 and 3 suggests that other variables, such as the concentration of glycine, might have also an effect on the molar absorption coefficient of colloidal MnO2 [the value of εP(418) decreased as the amino acid concentration increased]. On the other hand, at low amino acid concentration the A(526) vs A(418) plot was linear (Figure 5, bottom), whereas at high concentration the plot showed a downward-concave curvature (Figure 5, top). When the A(526) vs A(418) plots led to upwardconcave curves, addition of Ca(NO3)2 at relatively low concentration (5.00 × 10-4 M) caused a dramatic increase of the plot curvature (Figure 6), and at higher concentrations of that additive precipitation of brown manganese dioxide occurred during the kinetic runs. The upward-concave curvature of the plots increased also with increasing temperature (Figure 7). Rate Plots. The reaction of permanganate ion with glycine in near-neutral aqueous solutions is strongly autocatalytic, as evidenced by the occurrence of bell-shaped rate vs time plots.27

Figure 6. A(526) vs A(418) plots for the reaction of KMnO4 (5.00 × 10-4 M) with glycine (0.080 M) in KH2PO4 (0.040 M)-K2HPO4 (0.040 M) buffer in the absence (squares) and presence (triangles) of Ca(NO3)2 (5.00 × 10-4 M) at pH 6.78 and 25.0 °C.

Assuming now that (i) both the noncatalytic and the autocatalytic reaction pathways are of first order with respect to the reactant in defect (MnO4-), (ii) the autocatalytic reaction pathway is of first order in the autocatalytic product (colloidal MnO2), and (iii) as confirmed by Figure 1 all the manganese intermediates are present in negligible concentrations (in steady state), we can infer the following rate law:

V ) k1c + k2c(co - c)

(7)

where k1 and k2 are the pseudo-first-order and pseudo-secondorder rate constants (glycine was present in large excess) of the noncatalytic and autocatalytic reaction pathways, respec-

15986

J. Phys. Chem. C, Vol. 113, No. 36, 2009

Perez-Benito

Figure 8. Dependence of the V/c ratio on the permanganate concentration during the reaction of KMnO4 (5.00 × 10-4 M) with glycine (0.160 M) in KH2PO4 (0.008 M)-K2HPO4 (0.008 M) buffer at pH 6.90 and 25.0 °C. The dashed line is the tangent to the curve at t ) 0. Figure 7. A(526) vs A(418) plots for the reaction of KMnO4 (5.00 × 10-4 M) with glycine (0.080 M) in KH2PO4 (0.040 M)-K2HPO4 (0.040 M) buffer, at pH 6.78 and 20.1 (bottom) or 40.4 (top) °C. The dashed lines are the tangents to the curves at t ) 0.

tively, whereas co is the initial permanganate concentration. Integration of eq 7 leads to

ln

k1 + k2(co - c) k1 ) ln + (k1 + k2co)t c co

(8)

Most kinetic studies on the autocatalytic reactions of permanganate ion with different organic reductants such as formic acid,22 amines,34-38 and amino acids39-56 have been carried out by the above integrated method. However, the use of eq 8 to obtain the values of the rate constants k1 and k2 requires an iterative procedure. An alternative, easier to implement differential method could be proposed simply by rewriting eq 7 in the more convenient linear form

V ) (k1 + k2co) - k2c c

(9)

Equation 9 predicts that a plot of V/c vs c should be linear, and from its intercept and slope the values of k1 and k2 can be obtained directly without the requirement of any iterative method. Actually, V/c vs c linear plots have been reported for the autocatalytic reactions of permanganate ion with formic acid,22 with dimethylamine30 and triethylamine,20 and with the amino acids L-alanine13 and L-threonine.21 However, in the present study the V/c vs c plots for the permanganate-glycine reaction showed in most kinetic runs a definite curvature. At very low phosphate buffer concentrations the V/c vs c plots showed an upward-concave curvature (Figure 8). On the contrary, at high phosphate buffer concentrations the V/c vs c plots showed a downward-concave curvature (Figures 9-15). The latter curvature increased with increasing permanganate initial concentration (Figure 9), decreased with increasing amino acid concentration (Figure 10), increased with increasing phosphate buffer

Figure 9. Dependence of the V/c ratio on the c/co ratio during the reaction of KMnO4 (bottom, 5.12 × 10-4 M; top, 1.02 × 10-3 M) with glycine (0.152 M) in KH2PO4 (0.040 M)-K2HPO4 (0.040 M) buffer, at pH 6.78 and 25.0 °C. The dashed lines are the tangents to the curves at t ) 0.

concentration (Figure 11), decreased in the presence of either the cationic surfactant benzyltriethylammonium chloride (Figure 12) or the polymeric protective colloid gum arabic (Figure 13), and decreased with increasing either pH (Figure 14) or temperature (Figure 15). Phosphate ions provoked an inhibition of the autocatalytic reaction pathway, as evidenced by the effect of an increase in their concentration on the profile of the bell-shaped rate vs time plot (Figure 16). Kinetic Data. The values of the apparent rate constants for the noncatalytic and autocatalytic reaction pathways were obtained at different total phosphate concentrations either by the integrated method, from the intercepts and slopes of the

Permanganate Oxidation of Glycine

J. Phys. Chem. C, Vol. 113, No. 36, 2009 15987

Figure 10. Dependence of the V/c ratio on the permanganate concentration during the reaction of KMnO4 (5.00 × 10-4 M) with glycine (bottom, 0.080 M; top, 0.240 M) in KH2PO4 (0.040 M)-K2HPO4 (0.040 M) buffer, at pH 6.78 and 25.0 °C. The dashed lines are the tangents to the curves at t ) 0.

Figure 12. Dependence of the V/c ratio on the permanganate concentration during the reaction of KMnO4 (5.00 × 10-4 M) with glycine (0.120 M) in KH2PO4 (0.040 M)-K2HPO4 (0.040 M) buffer, in the absence (bottom) and presence (top) of benzyltriethylammonium chloride (0.150 M), at pH 6.78 and 25.0 °C. The dashed lines are the tangents to the curves at t ) 0.

Figure 11. Dependence of the V/c ratio on the permanganate concentration during the reaction of KMnO4 (5.12 × 10-4 M) with glycine (0.160 M) in KH2PO4 (bottom, 0.016 M; top, 0.080 M)-K2HPO4 (bottom, 0.016 M; top, 0.080 M) buffer, at ionic strength 0.320 M (KCl), pH 6.64, and 25.0 °C. The dashed lines are the tangents to the curves at t ) 0.

Figure 13. Dependence of the V/c ratio on the permanganate concentration during the reaction of KMnO4 (5.00 × 10-4 M) with glycine (0.080 M) in KH2PO4 (0.040 M)-K2HPO4 (0.040 M) buffer, in the absence (bottom) and presence (top) of gum arabic (0.640 g dm-3), at pH 6.78 and 25.0 °C. The dashed lines are the tangents to the curves at t ) 0.

ln{[k1 + k2(co - c)]/c} vs t linear plots (eq 8), or by the differential method, from the intercepts and slopes of the tangents to the V/c vs c curves at t ) 0 (eq 9). The value of rate constant k1 obtained by the integrated method increased strongly as the total phosphate concentration increased, whereas that obtained by the differential method was almost independent of that variable (Figure 17, bottom). Likewise, the value of rate

constant k2 obtained by the integrated method decreased strongly as the total phosphate concentration increased, whereas that obtained by the differential method showed a weaker dependence on that variable (Figure 17, top). However, extrapolation of the experimental rate constants to zero phosphate concentration led to values for both k1 and k2 from the two methods

15988

J. Phys. Chem. C, Vol. 113, No. 36, 2009

Perez-Benito KI

NH3+CH2CO2- {\} NH2CH2CO2- + H+

(10)

kII

MnO4- + NH2CH2CO2- 98 HMnO4- + NH•CH2CO2slow

(11) MnO4- + NH•CH2CO2- f MnO42- + NHdCH2 + CO2 MnO42- + H+ a HMnO4HMnO4- + NH2CH2CO2- f H2MnO42- + NHdCH2 + CO2

Figure 14. Dependence of the V/c ratio on the permanganate concentration during the reaction of KMnO4 (5.00 × 10-4 M) with glycine (0.120 M) in phosphate buffer ([KH2PO4] + [K2HPO4] ) 0.064 M), at pH 6.80 (bottom) or 7.03 (top) and 25.0 °C. The dashed lines are the tangents to the curves at t ) 0.

(12) (13)

(14)

H2MnO42- + H+ f MnO2 + H2O + HO-

(15)

NHdCH2 + H2O f NH3 + CH2O

(16)

It has been reported that a mixture of permanganate and L-serine can initiate the polymerization of acrylamide.57 This is consistent with a mechanism for the permanganate oxidation of amino acids involving a one-electron transfer in the rate-determining step, leading to the formation of Mn(VI) and an organic free radical (eq 11). The reduction product, colloidal MnO2 (eq 15), was identified by its characteristic UV-vis spectrum,19 whereas the oxidation products, CO2 (eq 14), NH3, and CH2O (eq 16) are consistent with the ones reported for the permanganate oxidation of amino acids in both acidic14 and neutral39,43 media. On the other hand, considering the experimental results previously reported27 along with the new results now presented, the following mechanism can be proposed for the autocatalytic reaction pathway: KIII

MnO2 + NH2CH2CO2- {\} MnO2-NH2CH2CO2-

(17) KIV

MnO2-NH2CH2CO2- + H2PO4- {\} MnO2-H2PO4- + NH2CH2CO2-

(18)

kV

Figure 15. Dependence of the V/c ratio on the permanganate concentration during the reaction of KMnO4 (5.00 × 10-4 M) with glycine (0.080 M) in KH2PO4 (0.040 M)-K2HPO4 (0.040 M) buffer, at pH 6.78 and 20.1 (bottom) or 40.4 (top) °C. The dashed lines are the tangents to the curves at t ) 0.

(integrated and differential) that could be considered identical within the experimental error (Table 4). Discussion Mechanism. Based on experimental results previously reported,27 the following mechanism can be proposed for the noncatalytic reaction pathway:

MnO2-NH2CH2CO2- + MnO4- 98 MnO2 + HMnO4- + NH•CH2CO2-

(19)

Adsorption of the basic form of the amino acid (generated from the zwitterionic form, predominant in near-neutral aqueous solutions58a) on the MnO2 colloidal particles takes place in eq 17. Competition of dihydrogen phosphate ions for the same active sites displaces glycine from the colloid surface (eq 18). Alternatively, the glycine molecules remaining on that surface can react with permanganate ions from the solution surrounding the colloidal particles (eq 19). After that, the intermediates Mn(VI) and the organic free radical formed in the latter step

Permanganate Oxidation of Glycine

J. Phys. Chem. C, Vol. 113, No. 36, 2009 15989 TABLE 4: Apparent Rate Constants for the Noncatalytic and Autocatalytic Reaction Pathways Extrapolated to Zero Phosphate Concentrationa method

k1b/10-5 s-1

k2c/M-1 s-1

integrated differentiald

4.2 ( 0.6 3.7 ( 0.3

2.52 ( 0.10 2.42 ( 0.15

a [MnO4-]o ) 5.12 × 10-4 M, [glycine] ) 0.160 M, ionic strength ) 0.320 M (KCl), pH 6.64, and 25.0 °C. b Apparent rate constant for the noncatalytic reaction pathway. c Apparent rate constant for the autocatalytic reaction pathway. d Values extrapolated at t ) 0.

Figure 16. Rate vs time plots for the reaction of KMnO4 (5.12 × 10-4 M) with glycine (0.160 M) in KH2PO4 (circles, 0.032 M; triangles, 0.080 M)-K2HPO4 (circles, 0.032 M; triangles, 0.080 M) buffer, at ionic strength 0.320 M (KCl), pH 6.64, and 25.0 °C.

Figure 17. Dependence of the apparent rate constants for the noncatalytic (bottom) and autocatalytic (top) reaction pathways, obtained either by the integrated method (empty circles) or by the differential method with extrapolation at t ) 0 (filled circles), on the total phosphate concentration during the reaction of KMnO4 (5.12 × 10-4 M) with glycine (0.160 M) in KH2PO4-K2HPO4 buffer ([KH2PO4] ) [K2HPO4] ) [phosphate]T/2), at ionic strength 0.320 M (KCl), pH 6.64, and 25.0 °C.

would suffer further rapid reactions as in eqs 12-16 of the noncatalytic reaction pathway. Interpretation of the Double-Absorbance Plots. The linearity of the A(526) vs A(418) plots observed in three series of kinetic runs (Figures 2-4) indicates that under those particular experimental conditions the molar absorption coefficients of the colloidal reaction product at both 418 and 526 nm remained constant during the progress of the reaction (eq 4). Thus, processes affecting the size of the colloidal particles (increase either by flocculation or by deposition of new MnO2 molecules

produced in the autocatalytic reaction pathway taking place at the colloid surface, and decrease by reduction of MnO2 molecules with adsorbed amino acid) were much slower than the permanganate-glycine reaction. However, although the colloidal particle size remained essentially independent of time in those kinetic runs, it showed a certain dependence on the initial experimental conditions: the particles were larger at high initial permanganate concentrations (Table 1) and smaller at high concentrations of either phosphate buffer (Table 2) or benzyltriethylammonium chloride (Table 3). We can conclude that the colloidal particle size increased with increasing values of the [MnO2]/[stabilizer] ratio, with both phosphate and quaternary ammonium ions playing the role of colloid stabilizers. The appearance of a downward-concave curvature in the A(526) vs A(418) plot as a consequence of an increase in the amino acid concentration (Figure 5) can be clearly ascribed to a decrease in the colloidal particle size as a consequence of the MnO2-glycine reaction taking place at the colloid surface, thus resulting in a decrease in the values of both εP(418) and εP(526) (caused by light scattering rather than by light absorption, and so strongly dependent on the colloidal particle size) and in an increase in the absolute value of the slope of the plot (eq 4). On the contrary, the upward-concave curvature in the A(526) vs A(418) plot caused by addition of Ca2+ ion (Figure 6) can be ascribed to an increase in the colloidal particle size as a consequence of the increase in the rate of flocculation of those particles, since divalent cations are known to strongly adsorb on colloidal MnO2 and decrease the negative electrostatic charge that is the reason for its stability in solution.17,59 The increase in the upward-concave curvature of the A(526) vs A(418) plots caused by an increase in the temperature (Figure 7) indicates that the rate of growth of the MnO2 colloidal particles was more sensitive toward temperature than the rate of the permanganate-glycine reaction. Although the growth of those particles can take place through two different mechanisms, either by flocculation or by the autocatalytic reaction pathway, the activation energies reported in the literature (62.0 and 64.5 kJ mol-1 for the autocatalytic and noncatalytic reaction pathways, respectively,27 and 82-128 kJ mol-1 for the flocculation of colloidal MnO2 at different phosphate concentrations60) are consistent with the flocculation process being the main route for the growth of the colloidal particles. Thus, it seems that the new MnO2 molecules formed as a consequence of the autocatalytic reaction pathway taking place on the colloid surface are involved in the generation of new particles rather than in the growth of the preexisting ones. Interpretation of the Rate Plots. If we assume that the quasiequilibrium approximation can be applied to eqs 17 and 18, then eq 19 would be the rate-determining step of the autocatalytic reaction pathway and the rate law obtained from the proposed mechanism (eqs 10-19) would be consistent with that usually applied to autocatalytic reactions (eq 7) with

15990

J. Phys. Chem. C, Vol. 113, No. 36, 2009

k1 )

k2 )

KIkII[NH3+CH2CO2-]T KI + [H+]

Perez-Benito

(20)

KIKIIIkV[NH+ 3 CH2CO2 ]T KIKIII[NH+ 3 CH2CO2 ]T

+ (KI + [H+]) (1 + KIIIKIV[H2PO4 ])

(21) where [NH3+CH2CO2-]T ) [NH3+CH2CO2-] + [NH2CH2CO2-] is the total concentration of glycine (in large excess with respect to permanganate). Actually, it is often proposed that the adsorption of amino acids, such as L-cysteine, on the surface of the MnO2 colloidal particles takes place in a rapid quasi-equilibrium step.61 However, the finding that under a very low concentration of phosphate buffer ([glycine]/[phosphate]T ) 10) the V/c vs c plot showed an upward-concave curvature (Figure 8) suggests that the adsorption of glycine on those colloidal particles (eq 17) is a rather slow process, since the apparent value of the rate constant k2 corresponding to the autocatalytic reaction pathway (eq 9) increased as the reaction advanced due to the increase with time of the number of glycine molecules adsorbed per colloidal particle. The present results also suggest that the adsorption of phosphate ions on the colloid surface (eq 18) is also a slow process. This would explain that at high concentrations of phosphate buffer ([glycine]/[phosphate]T ) 1-5) the V/c vs c plots consistently showed downward-concave curvatures (Figures 9-15), since the apparent value of the rate constant k2 corresponding to the autocatalytic reaction pathway (eq 9) decreased as the reaction advanced due to the increase with time of the number of phosphate ions adsorbed per colloidal particle. Hence, phosphate ions provoked an inhibition of the autocatalytic reaction pathway (Figure 16), probably by competing with the glycine molecules for the same active sites on the colloid surface. Therefore, the experimental data suggest that the rapid quasiequilibrium approximation cannot be safely applied to eqs 17 and 18 of the proposed mechanism, since both steps seem to take place at a rate similar to that of the redox step (eq 19), usually invoked as the rate-determining step for the autocatalytic reaction pathway. The increase in the downward-concave curvature of the V/c vs c plot caused by an increase in the initial permanganate concentration (Figure 9) suggests that the adsorption of phosphate ions measured per mole of MnO2 is slower as the concentration of colloidal particles increases (at constant phosphate concentration). The pronounced decrease in the downward-concave curvature of the V/c vs c plot caused by an increase in the glycine concentration (Figure 10) and the pronounced increase caused by an increase in the phosphate concentration (Figure 11) can be explained by the competitive slow adsorptions of glycine (causing an upward-concave curvature) and phosphate (causing a downward-concave curvature) on the colloid surface. An increase in the [glycine]/ [phosphate] ratio caused an increase in the upward-concave curvature of the V/c vs c plot, whereas a decrease in that ratio caused an increase in the downward-concave curvature of the plot. The decrease in the downward-concave curvature of the V/c vs c plot caused by addition of either the cationic surfactant benzyltriethylammonium ion (Figure 12) or the polymeric protective colloid gum arabic (Figure 13) suggests that these two species also adsorb on the surface of the MnO2 colloidal

particles and that they might compete with phosphate ions for the same active sites, although the effect on the curvature of the V/c vs c plot caused by the first additive might be alternatively explained by an increase in the rate of adsorption of phosphate ions caused by the previous adsorption of the unlike-charged quaternary ammonium ions. The decrease in the downward-concave curvature of the V/c vs c plot caused by an increase in the pH (Figure 14) suggests that, of the two phosphate ions predominant in near-neutral aqueous solutions (H2PO4- and HPO42-),58b it is the acidic one (H2PO4-) that adsorbs preferentially on the surface of colloidal MnO2, and thus the species predominantly responsible for the stabilization in solution of the colloidal particles. The difference in the adsorption behavior of the two phosphate ions suggests that hydrogen bonds between oxygen atoms belonging to the surface of the MnO2 colloidal particles and hydrogen atoms belonging to H2PO4- ions might play an important role in the adsorption process. Dihydrogen phosphate, arsenate, and vanadate ions have all been reported to stabilize colloidal Mn(IV) against flocculation.62 The decrease in the downward-concave curvature of the V/c vs c plot caused by an increase in the temperature (Figure 15) suggests that the adsorption of phosphate ions on the surface of the MnO2 colloidal particles is more sensitive toward changes in the temperature than the permanganate-glycine reaction, so that an increase in the temperature caused an acceleration of the adsorption process of phosphate ions relative to the redox reaction. Hence, it should be considered as a likely hypothesis that the activation energy associated with the adsorption of phosphate ions on colloidal MnO2 is higher than that associated with the permanganate-glycine reaction. This high activation energy might be caused by the electrostatic repulsion that must be overcome when an H2PO4- ion approaches the negatively charged surface of the MnO2 colloidal particles. Determination of Kinetic Data for Autocatalytic Reactions. The use of a differential method (eq 9) might present some advantages over the use of an integrated method (eq 8) with respect to the determination of correct values for the rate constants of the noncatalytic (k1) and autocatalytic (k2) reaction pathways in permanganate reactions. Given that the autocatalyst is usually a colloidal form of MnO2, the apparent value of rate constant k2 shows a tendency to change during the course of the reaction. This temporal dependence of parameter k2 can be more easily observed in a differential rate plot (such as those shown in Figures 8-15) than in an integrated rate plot (as typically used in the chemical literature). This is so because, whereas the slope of eq 8 (integrated method) involves both rate constants (k1 and k2), the slope of eq 9 (differential method) involves only the autocatalytic rate constant (k2), which is usually the one prone to present a temporal dependence (due to phenomena such as the slow adsorption of phosphate ions and amino acid molecules on the surface of the MnO2 colloidal particles). The temporal dependence of rate constant k2 might lead to a systematic error in the value of rate constant k1 if the integrated method is used. For instance, in the series of experiments in which the dependence of the kinetic parameters on the total phosphate concentration at constant pH was analyzed, whereas the decrease of the value of rate constant k2 determined by the integrated method with increasing buffer concentration (Figure 17, top) can be easily interpreted as caused by the inhibition provoked by phosphate ions on the autocatalytic reaction pathway (Figure 16), the strong increase of the value of rate constant k1 obtained by the same method should be considered as due to a systematic error, since the ap-

Permanganate Oxidation of Glycine proximately constant value of k1 obtained when the differential method was applied (Figure 17, bottom) suggests that the true value of k1 is independent of the phosphate concentration at constant pH. Although extrapolation of the k1 and k2 data to zero phosphate concentration led to values that were independent of the method applied (either integrated or differential) within the experimental error (Table 4), those extrapolated values were very close to the ones obtained directly at each buffer concentration by the differential method, but were much lower (k1) or much higher (k2) than those obtained by the integrated method (Figure 17). Correct time-independent values of the rate constants for MnO2-autocatalyzed reactions might be obtained from the intercept and slope of the initial portion of the V/c vs c plots, using the tangents to the curves at time zero. Conclusions The experimental data indicate that H2PO4- ions have a greater contribution to the stabilization of the MnO2 colloidal particles than HPO42- ions. This suggests that the adsorption of phosphate takes place by hydrogen binding of the hydrogen atoms of the H2PO4- ions with oxygen atoms of the MnO2 molecules belonging to the colloid surface. Phosphate ions have a notable inhibition effect on the autocatalytic reaction pathway of the permanganate-glycine reaction. Some deviations from the integrated rate law generally used for the kinetic study of autocatalytic permanganate reactions have been observed, and they are probably caused by the fact that the adsorption of both glycine and phosphate ions on the surface of colloidal MnO2 do not take place as rapid quasi-equilibrium steps, as usually accepted in the published literature; rather adsorption of those species takes place at a time scale comparable to that of the redox reaction itself. The use of a differential method might help to obtain correct time-independent values of the rate constants associated with the noncatalytic and autocatalytic reaction pathways. References and Notes (1) Freeman, F. ReV. React. Species Chem. React. 1976, 1, 179. (2) Fatiadi, A. J. Synthesis 1987, 85. (3) Lee, D. G. In Oxidation in Organic Chemistry, Part D; Trahanovsky, W. S., Ed.; Academic Press: New York, 1982; p 147. (4) Crimi, M. L.; Siegrist, R. L. J. EnViron. Eng.sASCE 2004, 130, 562. (5) Seol, Y.; Zhang, H.; Schwartz, F. W. EnViron. Eng. Geosci. 2003, 9, 37. (6) West, M. R.; Grant, G. P.; Gerhard, J. I.; Kueper, B. H. AdV. Water Resour. 2008, 31, 324. (7) Crimi, M.; Quickel, M.; Ko, S. J. Contam. Hydrol. 2009, 105, 69. (8) Singh, N.; Lee, D. G. Org. Process Res. DeV. 2001, 5, 599. (9) Brown, A. J.; Francis, P. S.; Adcock, J. L.; Lim, K. F.; Barnett, N. W. Anal. Chim. Acta 2008, 624, 175. (10) Bui, C. T.; Rees, K.; Cotton, R. G. H. Nucleosides, Nucleotides Nucleic Acids 2003, 22, 1835. (11) Khan, F. H.; Ahmad, F. Oxid. Commun. 2004, 27, 869. (12) Bahrami, H.; Zahedi, M. Int. J. Chem. Kinet. 2006, 38, 1. (13) De Andres, J.; Brillas, E.; Garrido, J. A.; Perez-Benito, J. F. J. Chem. Soc., Perkin Trans. 2 1988, 107. (14) Verma, R. S.; Reddy, M. J.; Shastry, V. R. J. Chem. Soc., Perkin Trans. 2 1976, 469. (15) Hassan, R. M. Can. J. Chem. 1991, 69, 2018. (16) Perez-Benito, J. F.; Rodrı´guez, R. M.; De Andres, J.; Brillas, E.; Garrido, J. A Int. J. Chem. Kinet. 1989, 21, 71. (17) Perez-Benito, J. F.; Brillas, E.; Pouplana, R. Inorg. Chem. 1989, 28, 390. (18) Perez-Benito, J. F.; Arias, C.; Amat, E. J. Colloid Interface Sci. 1996, 177, 288.

J. Phys. Chem. C, Vol. 113, No. 36, 2009 15991 (19) Perez-Benito, J. F.; Arias, C. J. Colloid Interface Sci. 1992, 152, 70. (20) Perez-Benito, J. F.; Arias, C. Int. J. Chem. Kinet. 1991, 23, 717. (21) Rodrı´guez, R. M.; De Andres, J.; Brillas, E.; Garrido, J. A.; PerezBenito, J. F. New J. Chem. 1988, 12, 143. (22) Perez-Benito, J. F.; Arias, C.; Brillas, E. Int. J. Chem. Kinet. 1990, 22, 261. (23) Stewart, R. In Oxidation in Organic Chemistry, Part A; Wiberg, K. B., Ed.; Academic Press: New York, 1965; p 65. (24) Pimienta, V.; Lavabre, D.; Levy, G.; Micheau, J. C. J. Phys. Chem. 1994, 98, 13294. (25) Pimienta, V.; Lavabre, D.; Levy, G.; Micheau, J. C. J. Phys. Chem. 1995, 99, 14365. (26) Kovacs, K. A.; Grof, P.; Burai, L.; Riedel, M. J. Phys. Chem. A 2004, 108, 11026. (27) Perez-Benito, J. F.; Mata-Perez, F.; Brillas, E. Can. J. Chem. 1987, 65, 2329. (28) Capellos, C.; Bielski, B. H. J. Kinetic Systems; Wiley: New York, 1972; p 63. (29) Andres-Ordax, F. J.; Arrizabalaga, A. An. Quim. 1985, 81, 431. (30) Mata-Perez, F.; Perez-Benito, J. F. J. Chem. Educ. 1987, 64, 925. (31) Perez-Benito, J. F.; Arias, C.; Brillas, E. An. Quim. 1991, 87, 849. (32) Peintler, G.; Nagypal, I.; Jancso, A.; Epstein, I. R.; Kustin, K. J. Phys. Chem. A 1997, 101, 8013. (33) Coleman, J. S.; Varga, L. P.; Mastin, S. H. Inorg. Chem. 1970, 9, 1015. (34) Mata-Perez, F.; Perez-Benito, J. F.; Arranz, A. Z. Phys. Chem. (Neue Folge) 1983, 135, 141. (35) Mata-Perez, F.; Perez-Benito, J. F. Z. Phys. Chem. (Neue Folge) 1984, 141, 213. (36) Mata-Perez, F.; Perez-Benito, J. F. An. Quim. 1985, 81, 79. (37) Mata-Perez, F.; Perez-Benito, J. F. An. Quim. 1987, 83, 459. (38) Mata-Perez, F.; Perez-Benito, J. F. Can. J. Chem. 1987, 65, 2373. (39) Andres-Ordax, F. J.; Arrizabalaga, A.; Martinez, J. I. An. Quim. 1984, 80, 531. (40) Garrido, J. A.; Perez-Benito, J. F.; Rodrı´guez, R. M.; De Andres, J.; Brillas, E. J. Chem. Res., Synop. 1987, 380. (41) Brillas, E.; Garrido, J. A.; Perez-Benito, J. F.; Rodrı´guez, R. M.; De Andres, J. Collect. Czech. Chem. Commun. 1988, 53, 479. (42) De Andres, J.; Brillas, E.; Garrido, J. A.; Perez-Benito, J. F.; Rodrı´guez, R. M. Gazz. Chim. Ital. 1988, 118, 203. (43) Andres-Ordax, F. J.; Arrizabalaga, A.; Ortega, K. An. Quim. 1989, 85, 218. (44) Insausti, M. J.; Mata-Perez, F.; Alvarez-Macho, M. P. An. Quim. 1991, 87, 821. (45) Andres-Ordax, F. J.; Arrizabalaga, A.; Peche, R.; Quintana, M. A. An. Quim. 1991, 87, 828. (46) Insausti, M. J.; Mata-Perez, F.; Alvarez-Macho, M. P. An. Quim. 1991, 87, 877. (47) Insausti, M. J.; Mata-Perez, F.; Alvarez-Macho, M. P. Int. J. Chem. Kinet. 1991, 23, 593. (48) Andres-Ordax, F. J.; Arrizabalaga, A.; Casado, J.; Peche, R. React. Kinet. Catal. Lett. 1991, 44, 293. (49) Andres-Ordax, F. J.; Arrizabalaga, A.; Peche, R.; Quintana, M. A. An. Quim. 1992, 88, 440. (50) Insausti, M. J.; Mata-Perez, F.; Alvarez-Macho, M. P. Int. J. Chem. Kinet. 1992, 24, 411. (51) Insausti, M. J.; Mata-Perez, F.; Alvarez-Macho, M. P. Int. J. Chem. Kinet. 1995, 27, 507. (52) Arrizabalaga, A.; Andres-Ordax, F. J.; Fernandez-Aranguiz, M. Y.; Peche, R. Int. J. Chem. Kinet. 1996, 28, 799. (53) Arrizabalaga, A.; Andres-Ordax, F. J.; Fernandez-Aranguiz, M. Y.; Peche, R. Int. J. Chem. Kinet. 1997, 29, 181. (54) Bahrami, H.; Zahedi, M. Can. J. Chem. 2004, 82, 430. (55) Zahedi, M.; Bahrami, H. Kinet. Catal. 2004, 45, 351. (56) Bahrami, H.; Zahedi, M. J. Iran. Chem. Soc. 2008, 5, 535. (57) Bajpai, U. D. N.; Bajpai, A. K. Macromolecules 1985, 18, 2113. (58) (a) Coetzee, J. F.; Ritchie, C. D. Solute-SolVent Interactions; Dekker: New York, 1969; p 17. (b) Coetzee, J. F.; Ritchie, C. D. Solute-SolVent Interactions; Dekker: New York, 1969; p 20. (59) Perez-Benito, J. F. Colloids Surf. A 2003, 225, 145. (60) Mata-Perez, F.; Perez-Benito, J. F. Can. J. Chem. 1985, 63, 988. (61) Al-Thabaiti, S. A.; Al-Nowaiser, F. M.; Obaid, A. Y.; Al-Youbi, A. O.; Khan, Z. J. Dispersion Sci. Technol. 2008, 29, 1391. (62) Doona, C. J.; Schneider, F. W. J. Am. Chem. Soc. 1993, 115, 9683.

JP9014178