Temperature and pH Effects on the kinetics of 2-aminophenol auto-oxidation in aqueous solution

Article abstract of DOI:10.2478/bf02476226

The kinetics of the auto-oxidation of 2-aminophenol (OAP) to 2-amino-phenoxazin- 3-one (APX) was followed in air-saturated aqueous solutions and the influence of temperature and pH on the auto-oxidation rate was studied. The kinetic analysis was based on a spectrophotometric method following the increase of the absorbance of APX. The process follows rst order kinetics according to the rate law - d[OAP]/dt = k'[OAP]. The experimental data, within the pH range 4-9.85, were analyzed using both differential and incremental methods. The temperature variation of the overall rate constant was studied at pH = 9.85 within the range 25-50 °C and the corresponding activation energy was evaluated.

Full text of DOI:10.2478/bf02476226

CEJC 3 (2003) 233{241
Temperature and pH E®ects on the Kinetics of
-Aminophenol Auto-oxidation in Aqueous Solution
2
¤
y
Dumitru Oancea , Mihaela Puiu
Department of Physical Chemistry,
University of Bucharest,
-12 bd. Elisabeta, sector 3, 70346, Bucharest, Romania
4
Received 7 March 2003; accepted 23 May 2003
Abstract: The kinetics of the auto-oxidation of 2-aminophenol (OAP) to 2-amino-phenoxazin-
-one (APX) was followed in air-saturated aqueous solutions and the in®uence of temperature
3
and pH on the auto-oxidation rate was studied. The kinetic analysis was based on a
spectrophotometric method following the increase of the absorbance of APX. The process follows
­ rst order kinetics according to the rate law { d[OAP]/dt = k’[OAP]. The experimental data,
within the pH range 4{9.85, were analyzed using both di¬erential and incremental methods.
The temperature variation of the overall rate constant was studied at pH = 9.85 within the
range 25{50¸C and the corresponding activation energy was evaluated.
®
c Central European Science Journals. All rights reserved.
Keywords: auto-oxidation, aminophenol, phenoxazinone
1
Introduction
The oxidation of aqueous organic pollutants is an attractive removal method because of
its high e±ciency and simplicity [1,2]. The oxidation products are usually low molecular
weight oxygenated compounds that are easily biodegradable. Several new technologies
based on advanced oxidation processes (AOP) were used in order to convert the pollutants
into harmless chemicals [3,4]. Four major approaches to AOP are under development at
present:
²
a) homogeneous UV photolysis of H O or O used to generate OH and other radicals;
2
2
3
b) dark homogeneous oxidation involving the use of Fenton reaction [5], ozone at high
pH [6], and ozone or peroxide [7]; c) heterogeneous UV photolysis on TiO [8] when solid
2
¤
y
E-mail: doan@gw-chimie.math.unibuc.ro
E-mail: mpuiu@gw-chimie.math.unibuc.ro

2
34
D. Oancea, M. Puiu / Central European Journal of Chemistry 3 (2003) 233{241
²
particles of the semiconductor TiO absorb UV light and generate OH and other radicals
2
on the surface of the particles; d) radiolysis [9] using a source of high-energy radiation,
²
²
when OH, H , hydrated electrons and other radicals are generated during the radiolysis
of water.
Wet air oxidation { WAO [10] { is also a convenient and ecologically safe technology
to treat aqueous waste streams containing organic and inorganic pollutants. WAO in-
volves the liquid phase oxidation of organics at elevated temperatures (125{320 ºC) and
pressures (0.5 { 20 MPa) using a gaseous source of dioxygen (usually air). Enhanced sol-
ubility of dioxygen in aqueous solutions at elevated pressures provides a stronger driving
force for oxidation. The organic compounds are oxidized to CO and other innocuous
2
end products.
Recent research work proved that APX is an important antineoplastic agent called
questiomycin A [11]. This is one of the reaction products of OAP oxidation, a pollu-
tant that is often found in wastewaters. OAP is a toxic compound, and is only partly
biodegradable. There is therefore a justi¯ed need to develop e®ective methods for the
degradation of OAP to less harmful compounds such as APX. The cost of OAP oxidation
may be lowered if conventional oxidants are replaced by dioxygen from air, at ambient
temperature and pressure.
It was observed that OAP undergoes an oxidation reaction leading to the formation of
the corresponding o-quinone imine [12]. The ¯nal reaction product is not the o-quinone
imine itself since it is highly reactive, but APX.
This reaction can also occur in the presence of di®erent oxidizing agents such as
manganese dioxide [13], potassium ferricyanide or dichromate [14]. Some metalloproteins
like hemoglobin, tyrosinase [15] and catalase were found to be e±cient catalysts. A kinetic
model involving the so-called OAP \oxidative cascade" was also suggested and discussed
[16,17,18]. According to this model, APX formation was preceded by the appearance
of some active intermediates resulting in successive oxidation, conjugate addition, and
tautomerisation reactions. The global equation for OAP oxidation is shown in Fig. 1
[19]:
Fig. 1 Overall process of 2-aminophenoxazinone formation
2
Experimental
Materials: OAP was obtained from Aldrich Chemical Co. and APX was prepared by
the oxidation of 2-aminophenol with mercury oxide and recrystallization from ethanol,
according to literature methods [20]. UV-VIS spectra were measured on a JascoW530

D. Oancea, M. Puiu / Central European Journal of Chemistry 3 (2003) 233{241
235
spectrophotometer equipped with kinetic software. The reaction mixture was also ana-
lyzed on a HPLC Shimadzu 10 AVP system using Shim Pack VP-ODS 150 x 4.6. Oxygen
and Argon from Linde were 99.9% pure.
The oxidation was followed in an aqueous medium in a bubbling stirred tank reactor,
using dioxygen from air as oxidizing agent and OAP as substrate in 0.9{5mM solutions.
These concentrations were used due to the lower solubility in water of the substrate
and the main reaction product. The air stream was passed through two thermostated
saturation vessels containing water in order to avoid the evaporation of the solvent from
the reactor. Under these conditions the solution was saturated with air. All experiments
were performed at a constant total pressure of 100 kPa. The kinetic assays were run
at 25, 30, 40, 50 and 58ºC in a 0.2 M carbonate-bicarbonate bu®er at pH 9.85, and at
3
0ºC at pH 4.00, 6.00, 7.00, 7.45, 8.71, 9.37, 9.65, and 9.85. The other bu®ers used were
as follows: high pH range (8.71{9.85) 0.2 M carbonate-bicarbonate; mid pH range (6.00{
.45) 0.01 M monosodium phosphate-disodium phosphate and low pH (4.00) 0.2 M acetic
7
acid-sodium acetate. Since the reaction was very slow, the transformation was monitored
for conversions ranging from 8% to 20%, depending on solution pH and temperature. To
estimate the reaction order with respect to substrate, the initial rate method was used for
substrate concentration ranging from 0.9 to 5 mM. In order to estimate the reaction order
with respect to dioxygen, di®erent air-oxygen mixtures (with molar fraction of oxygen
equal to 0.21, 0.36, 0.68, 0.84 and 1) were used. The measurements were carried out at
2
5ºC and a pH of 7.45. The stability of OAP in the absence of oxygen was veri¯ed by
passing an argon stream through the reaction mixture, when no signi¯cant transformation
¡
1
¡1
of OAP was observed. APX formation was monitored at 434 nm (" = 23200 cm ¢M for
APX). The variation of molar absorption coe±cient with pH was veri¯ed by comparing
the absorbance at 434 nm of 1:1 diluted 0.018 mM APX samples with distilled water and
bu®er solutions (pH 4.00, 5.00, 7.00, 8.10, 9.00, 9.85). Within the 4.00{7.00 pH range, the
molar absorption coe±cient " was practically constant. At higher pH values (8.1{9.85)
an increase of " was observed. The di®erences between measured absorbencies of samples
diluted with distilled water and those diluted with bu®er solutions were within 5% -10 %
for this range.
3
Results
The chemical transformation of the substrate was studied by product (APX) analysis. For
each experiment, 14 samples were removed from the reactor at di®erent time intervals and
the spectra of the reaction mixture were scanned between 200 and 800 nm. The formation
of the product can be followed at 434 nm, where the substrate has no absorption (Fig.
2
).
The HPLC analysis of the reaction mixture indicated the formation of APX as the
main product and proved the appearance of a reaction product absorbing at ¸=340
nm, accumulated in an insigni¯cant amount with respect to OAP and APX in the pH
range 4.00{9.00. Until now we were not able to identify, isolate and characterize this

2
36
D. Oancea, M. Puiu / Central European Journal of Chemistry 3 (2003) 233{241
Fig. 2 UV/VIS spectra of the reaction mixture. Conditions: 0.01 M phosphate bu¬er, pH 6.00,
0
.00445 M substrate at 30¸C
compound. Its existence was previously reported in the literature [16]. At pH 9.85 and
at 40 - 50?C, the absorbance of APX, after having reached a maximum value, undergoes a
fast decrease and in a relatively short time the characteristic absorption band disappears,
being replaced by a new one, absorbing at ¸max = 340 nm (Fig. 3).
Fig. 3 UV/VIS spectra of the reaction mixture. Conditions: 0.2 M carbonate-bicarbonate
bu¬er, pH 9.85, 0.00445 M substrate at 40¸C (1:10 diluted samples)
4
Discussion and Conclusions
A simple rate law for the oxidation reaction is given by:

D. Oancea, M. Puiu / Central European Journal of Chemistry 3 (2003) 233{241
237
1
d[OAP ]
d[APX]
m
n
v = ¡ ¢
=
= k ¢ [OAP ] ¢ [O ]
(1)
R
2
2
dt
dt
This equation can be simpli¯ed to Eq. (2) under degenerated order conditions (con-
stant O concentration):
2
d[OAP ]
d[APX]
0
m
¡
= 2 ¢
= k ¢ [OAP ]
(2)
dt
dt
0
n
with k = 2 ¢ k ¢ [O ] (3)
2
The concentration of OAP was calculated from the mass balance using " = 23200
¡
1
¡1
cm ¢M for APX. From the linear dependencies of initial transformation rates (con-
version of substrate 0.5{1%) on initial concentrations, the reaction order with respect
to OAP was found to be one, with a correlation coe±cient r = 0.9991(Fig. 4), for a
signi¯cance level ® = 0.05.
Fig. 4 Plot of reaction rate of OAP vs. its initial concentration (pH = 9.85, T = 25¸C)
The experimental data, OAP concentration versus time at constant oxygen concentra-
tion, were used to ¯t di®erent kinetic equations. The best ¯t was found for the exponential
function:
0
c = c ¢ exp(¡k t)
(3)
0
corresponding to a pseudo-¯rst order process, which validates the ¯rst order reaction
with respect to substrate concentration. The rate constant can be determined using the
0
linear form of Eq.(3): ln c = ln c ¡ k t which yields biased parameter estimates. A non-
0
0
linear regression was subsequently performed to improve the estimates [21, 22]. The k
values for di®erent conditions, resulting from the integral kinetic analysis (dependent on
"
value), were practically the same with those calculated using both incremental [23] and
0
di®erential [24] methods (independent of " value). In Table I the values of k and the

2
38
D. Oancea, M. Puiu / Central European Journal of Chemistry 3 (2003) 233{241
corresponding statistical parameters r and Fstatistic are summarized (for a signi¯cance
level ®= 0.05). An increase of the auto-oxidation rate at higher pH values was observed.
5
0
¡1
pH
10 k (s )
r
Fstatistic
7
8
.0
.7
0.11 § 0.00 0.9995
1.70 § 0.16 0.9908
16499
377
9
9
.65 14.16 § 0.16 0.9983
.85 21.51 §1.68 0.9888
3114
3188
Table 1 Variation of pseudo-­ rst rate constant versus pH at 30¸C
According to similar models reported in the literature [25], the function:
h
i¡
1
0
+
k = A + B ¢ H
(4)
+
was ¯t to the experimental rate constant-[H ] data. Assuming that in aqueous solution
dioxygen reacts with both OAP molecules and aminophenolate ions (similar to auto-
oxidation of other phenols), an increase of the auto-oxidation rate at higher pH is ex-
¡6 ¡1
pected. The ¯tting parameters were r = 0.9977, A = (1.97§2.30)¢10 s and B =
¡
14
¡3 ¡1
(
3.04 § 0.05)¢10 mol¢dm ¢s (for a signi¯cance level ® = 0.05), as compared to A
¡
12 ¡1
¡11
¡3 ¡1
=
3.8¢10
s
and B = 5.8¢10
mol¢dm ¢s reported for phenol auto-oxidation at
2
0ºC [25], indicating a lower pH sensitivity of the auto-oxidation rate constant of OAP.
The reaction order with respect to dioxygen concentration was estimated from the
0
plot of k versus initial concentration of dissolved O . The concentration of dissolved
2
oxygen was calculated from the mole fraction solubility of oxygen from air in water [26],
using the molar fraction of oxygen in air and air-oxygen mixtures (see Table II).
7
0
¡1
¤
5
10 C0 mol ¢ dm¡3
1
0 k (s
)
X
O2
O2
0
0
0
3
8
.58§0.05 0.21
.07§0.11 0.36
2.60§0.46 0.68
7.07§0.23 0.84
1.58§1.04 1.00
1.27
2.29
4.36
5.34
6.36
1
1
2
Table 2 Variation of pseudo-­ rst order rate constant vs. initial concentration of O2 at pH 7.45
and 25¸C
*
Molar fraction of O2 in input gas
0
n
0
The equation k = 2k¢[O ] was ¯t to the experimental data. The non-linear regression
2
gave the reaction order n = 1.09§0.10 with r = 0.9949 and F = 400 (for ® = 0.05),
exceeding by far the critical value of F = 19.35. Similar values were obtained by using

D. Oancea, M. Puiu / Central European Journal of Chemistry 3 (2003) 233{241
239
the method of initial reaction rates, from the linear plot of reaction rate versus initial
concentration of dioxygen.
The temperature variation of the reaction rate was analyzed, taking into account the
solubility of dioxygen from air in water at di®erent temperatures [26] and the ¯rst order
dependence with respect to oxygen. The corrected rate constants were calculated using
the pseudo-¯rst order rate constants and the solubility of dissolved oxygen. The values
of the rate constants are summarized in Table III (for a partial pressure of oxygen in air
equal to 20.26 kPa). From these results, the overall activation energy of the oxidation
process was estimated using the Arrhenius equation. A nonlinear regression analysis gave
6
¡1
E = 56.82 § 3.17 kJ/mol and A = (1.15 § 1.43)¢10 s with a correlation coe±cient r
a
=
0.9974. No similar values of the activation energies were available in the literature for
auto-oxidation of phenols.
5
¡3
[
O2]T
105k (s 1)
0
¡
105k
0
(s 1)
¡
T(K) 10 [O ] mol ¢ dm
2
exp
corr
[
O2]298K
2
3
3
3
3
98.15
03.15
13.15
23.15
31.15
1.270
1.200
1.039
0.941
0.880
1.000
14.97§ 0.07
22.76 § 1.78
46.88§ 2.45
94.02 § 28.68
14.97 § 0.07
21.51 § 1.68
38.35 § 2.00
69.66 § 15.3
0.945
0.818
0.741
0.693
182.78 § 55.32 126.66§ 38.33
Table 3 Variation of pseudo-­ rst rate constant with temperature at pH 9.85
The overall kinetic equation can be consequently written as:
d [OAP ]
¡
= 2k ¢ [OAP ] [O2]
(5)
dt
The possible mechanisms reported in the literature for the catalytic and enzymatic
oxidation of OAP [17] take into account two pathways:
One pathway involves 2-electron oxidation of OAP to the highly reactive o¡quinone
imine, followed by several conjugate addition, 2-electron oxidation and tautomerisation
reactions to form APX. In support of this pathway it has been previously demonstrated
that chemically prepared quinone imine reacts with OAP to form APX [27] and that
quinone imine is an intermediate in the oxidation of OAP by tyrosinase [15].
Another pathway considers 1 electron oxidation of OAP to a phenoxy radical. The
OAP radicals undergo disproportionation to the o-quinone imine [19]. APX formation
can then follow the ¯rst pathway. Equation (5) suggests that molecular oxygen and OAP
are involved in the limiting reaction step and the phenoxy radical formation is a possible
step for this auto-oxidation.
The oxidation of OAP to APX is a natural process, which occurs very slowly in
air-saturated solutions, having an estimated half time of about 23 days under normal
conditions (neutral pH, 25{30ºC) and converts OAP from wastewaters into a less toxic
product without consumption of any other oxidizing reagents.

2
40
D. Oancea, M. Puiu / Central European Journal of Chemistry 3 (2003) 233{241
Acknowledgements
The ¯nancial support of this study by CNCSIS grant no. 192/2001 is gratefully acknowl-
edged.
We also thank Lucian Rotariu (Laboratory of Monitoring and Quality Control from
Faculty of Chemistry, Bucharest University) for running HPLC analysis of OAP, APX and
for helpful advice. The equipment was a donation from Japan International Cooperation
Agency { JICA, Technical Cooperation.
References
[
[
1] J. Kochany and J.R. Bolton: \Mechanism of photodegradation of aqueous organic
pollutants", J. Phys. Chem., Vol. 13, (1991), pp. 5116{5120.
2] C. Prat, M. Vicente, S. Esplugas: \Treatment of bleaching waters in the paper
industry by hydrogen peroxide and ultraviolet radiation", Water Res., Vol. 22, (1988),
pp. 663{668.
[3] W.H. Glaze and J.W. Kang: \The chemistry of water treatment involving ozone,
hydrogen peroxide and ultraviolet radiation", Ozone Sci. Eng., Vol. 9, (1987), pp.
3
52{353.
[
[
[
[
[
[
[
4] W.H. Glaze and J.W. Kang: \Advanced oxidation processes", Ind. Eng. Chem. Res.,
Vol. 28, (1989), pp. 1580{1587.
5] H.J. Fenton: \Oxidation of tartaric acid in the presence of iron", J. Chem. Soc., Vol.
6
5, (1894), pp. 899{902.
6] H.R. Eisenhauer: \Oxidation of phenolic wastes", J. Water. Pollut. Control. Fed.,
Vol. 36, (1964), pp. 1116{1120.
7] J. Hoign ¶e and H. Bader: \Ozonization of water: role of hydroxyl radicals as oxidizing
intermediates", Env. Sci. Technol., Vol. 109, (1975), pp. 4216{4225.
8] H. Paillard, R. Brunet, M. Dore: \Optimal conditions for applying an ozone-hydrogen
peroxide oxidizing system", Water Res., Vol. 22, (1988), pp. 91{103.
9] N. Geto®: \Radiation- and photoinduced degradation of pollutants in water", Radiat.
Phys. Chem., Vol. 37, (1991), pp. 673{678.
10] F.J. Rivas, S.T. Kolaczkowski, Beltr ¶a n, D.B. McLurgh: \Development of a model for
the wet air oxidation of phenol based on a free radical mechanism", Chem. Eng. Sci,
Vol. 53:14, (1998), pp. 2575{2586.
[
[
11] U. Hollstein: \Actinomycin. Chemistry and mechanism of action", Chem. Rev., Vol.
7
4, (1974), pp. 625{652.
12] C.E. Barry, G.N. Parmesh, T.P. Begley: \Phenoxazinone synthase: enzymatic
catalysis of an aminophenol oxidative cascade", J. Am. Chem. Soc., Vol. 110, (1988),
pp. 3333.
[
[
13] W. Prinz and N. Savage: \A simple, non-enzymatic synthesis of cinnabarinic acid
(2-amino-3H-phenoxazine-1, 9-dicarboxilic acid)", Hoppe-Seyler’s Z. Physiol. Chem.,
Vol. 358, (1977), pp. 1161{1163.
14] W. Sch Äa fer: \The chemistry of phenoxazones", Prog. Org. Chem., Vol. 6, (1964), pp.
1
35{163.

D. Oancea, M. Puiu / Central European Journal of Chemistry 3 (2003) 233{241
241
[
[
15] O. Toussaint and K. Lerch: \Catalytic oxidation of 2-aminophenols and ortho
hydroxylation of aromatic amines by tyrosinase", Biochemistry, Vol. 26, (1987), pp.
567{8571.
8
16] C.E. Barry, G.N. Parmesh, T.P. Begley: \Phenoxazinone synthase: mechanism for
the formation of the phenoxazinone chromophore of actinomycin", Biochemistry, Vol.
2
8, (1989), pp. 6323{6333.
[
[
17] L. Prati., M. Rossi, N. Ravasio: \Interaction of molecular oxygen with aminophenols
mediated by copper metal and copper compounds", J. Mol. Cat., Vol. 75, (1992), pp.
3
47{355.
18] A. Tomoda, E. Shirasawa, S. Nagao, M. Minami, Y. Yoneyama: \Involvement
of oxidoreductive reactions of intracellular hemoglobin in the metabolism of 3-
hydroxyanthranilic acid in human erythrocytes", Biochem. J. Vol. 22, (1984), pp.
7
55{760.
19] J. Kaizer, R. Csonka, G. Speier: \TEMPO-initiated oxidation of 2-aminophenol to
-aminophenoxazin-3-one", J. Mol. Cat., Vol. 180, (2002), pp. 91{96.
[
[
2
20] V. Stuzka and E. Sinderlarova: \Electronic spectra of phenoxazine dyestu®s and
substituent e®ects on the position of long-wave absorption bands", Collection Czech.
Chem. Commun., Vol. 42, (1977), pp. 13333{13337
[
[
[
[
[
[
[
21] K. Ebert, H. Ederer, T.L. Isenhour: Computer Applications in Chemistry, VCH,
Weinheim, 1989, pp. 359{387.
22] J.F. Rusling and T.F. Kumosinski: Nonlinear Computer Modeling of Chemical and
Biochemical Data, Academic Press, New York, 1996, pp. 3{31.
23] J.H. Espenson: Chemical Kinetics and Reaction Mechanisms, McGraw-Hill Inc, New
York, 1995, pp. 26{27.
24] M.J. Pilling and P.W. Seakins: Reaction Kinetics, Oxford Science Publications, 1995,
pp. 16{18.
25] C.H. Bamford and C.H.F. Tipper: Chemical Kinetics, Vol. 16, Elsevier, Amsterdam,
1
980, pp. 180{190.
26] IUPAC: Solubility Data Series { Oxygen and Ozone, Vol. 7, Pergamon Press, New
York, 1981.
27] T. Nogami, T. Hishida, M. Yamada, H. Milkawa, Y. Shirota: \Formation and
reactions of o-benzoquinone mono- and di- imines", Bull. Chem. Soc. Jpn., Vol.
4
8, (1975), pp. 3709{3714.