Ultrasonics Sonochemistry 8 (2001) 175±181
www.elsevier.nl/locate/ultsonch
Preliminary results on the e€ect of power ultrasound on
nitrogen oxide and dioxide atmosphere in nitric acid solutions
Ph. Moisy *, I. Bisel, F. Genvo, F. Rey-Gaurez, L. Venault, P. Blanc
Commissariat aÁ l'EÂnergie Atomique (CEA/Valrh^o), DRCP/SCPS/LPCA, BP 17171, 30207 Bagnols-sur-CeÁze Cedex, France
Abstract
The e€ect of ultrasound (20 kHz, 3 W cm 2 ) on the kinetics of HNO2 and H2 O2 formation was investigated in a 1 M HNO3
medium for NO2 ±Ar and NO±Ar gas mixtures in various volume fractions (f …NO2 † < 1:7 vol% and f …NO† < 1:1 vol%, respectively). The H2 O2 formation rate measured in 1 M HNO3 in the presence of N2 H5 NO3 was observed to be much lower than that of
HNO2 without N2 H5 NO3 , and was relatively independent of the NO2 or NO gas volume fractions in the argon atmosphere. The
HNO2 formation rate increased under ultrasound, and was higher with NO than with NO2 . The induction period observed without
ultrasound disappeared when ultrasound was applied. The ®rst step in the sonochemical mechanism of HNO2 formation in the
presence of NO2 involves thermal decomposition of NO2 into NO within the cavitation bubble. In the second step of HNO2 formation, NO reacts either with HNO3 in the cavitation bubble, or with NO2 in the cavitation bubble or at the bubble/solution
interface. Ó 2001 Elsevier Science B.V. All rights reserved.
Keywords: Ultrasound; Nitrous acid; Nitrogen oxide; Nitrogen dioxide
1. Introduction
Ultrasound has been successfully used over the last
two decades in a wide range of applied research ®elds [1±
4], including organic synthesis and catalysis, preparation of structured materials, surface depassivation and
electroanalysis, and environmental applications. Few
reports, however, have discussed the advantages of ultrasound for converting chemically active gases for liquid-phase reactions: most published work concerning
the e€ects of ultrasound according to the gas composition was carried out for fundamental research [5,6]. The
investigations have focused primarily on the noble gases
(He, Ne, Ar, Kr, Xe) or on the macro-constituents of air
(O2 and N2 ) in aqueous media (generally water) when
submitted to ultrasound. While the properties of gases
under ultrasound are suitably represented by the ratio of
their heat capacities …c ˆ Cp =Cv † and by their thermal
conductivity …C†, those of the chemically active gases
require a more complex approach [7,8]. Di€erent isotopes have been used [5,6] during sonolysis of water to
describe the role of the gaseous compounds O2 , H2 and
*
Corresponding author. Tel.: +33-466-791-611; fax: +33-466-796567.
E-mail address: [email protected] (Ph. Moisy).
N2 in the formation of H2 O2 , NO2 and NO3 in solution
and of gaseous NO, NO2 and N2 O. Although the e€ect
of N2 O during sonolysis of water has not been widely
investigated [9], sonolysis has been observed to form N2
and O2 at a maximum rate for a N2 O volume fraction of
about 15% in Ar; H2 O2 , NO2 and traces of NO3 tend to
accumulate in solution according to a decomposition
mechanism (Eqs. (1) and (2)) [9]:
2N2 O ! 2N2 ‡ O2
…1†
3N2 O ‡ H2 O ! 2HNO2 ‡ 2N2
…2†
The absence of H2 formation and the decreasing rate of
H2 O2 formation are justi®ed [9] by the fast kinetics of
the reactions between OH or H and N2 O (Eqs. (3) and
(4)):
H ‡ N2 O ! N2 ‡ OH
…3†
OH ‡ N2 O ! 2NO ‡ H
…4†
The maximum rate obtained with the N2 O volume
fraction …f …N2 O† ˆ 15%† is explained by the bubble
temperature drop when the N2 O concentration increases
in the argon [9]. To the best of our knowledge, the e€ect
of ultrasound on NO and NO2 diluted in a noble gas has
not been investigated.
1350-4177/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S 1 3 5 0 - 4 1 7 7 ( 0 1 ) 0 0 0 7 5 - X
176
Ph. Moisy et al. / Ultrasonics Sonochemistry 8 (2001) 175±181
The purpose of this preliminary study is to observe
the e€ect of ultrasound on gaseous mixtures of argon
and nitrogen oxides (NO and NO2 ) in a nitric acid
medium. NO and NO2 transfers by sparging in nitric
acid are known to result in the accumulation of HNO2
in solution.
2. Experimental
All the chemical reactants were analysis grade as
supplied by Prolabo and Merck, and were used without
additional puri®cation. Deionized water (Milli-Q/RO
System) was used to prepare the aqueous solutions. The
gases and initial gaseous mixtures (Ar, Ar ‡ 15% NO,
and Ar ‡ 5% NO2 ) were supplied by Air Products (Ar
55). The gaseous mixtures characterized by their volume percentage fraction (f(X)) were prepared from pure
argon and the initial NOx ±Ar mixtures. The solubility of
Ar and NO were estimated at 5.4 and 7.4 ml in 100 ml of
water at 0°C [10]. The heat capacity ratios …c ˆ Cp =Cv †
were 1.68 for Ar, 1.4 for NO and 1.3 for NO2 . The
thermal conductivity values (C: 106 cal cm 1 K 1 s 1 )
were 39.1 for Ar, 56.2 for NO and 400.6 for NO2 [10].
The experiments were performed with a thermostatically controlled batch sonoreactor equipped with an 18
cm2 piezoelectric transducer supplied by a 20 kHz generator (VibraCell 600). The transducer was secured to
the bottom of the glass sonoreactor by a watertight
PTFE coupling ring. The solution volume subjected to
ultrasound was 100 ml. The temperature in the reactor
was measured by a thermocouple immersed approximately 3 cm below the surface. Preliminary tests showed
that the solution temperature reached a constant value
about 10 min after the sonolysis began. The temperature
was maintained constant at 25°C for all the experiments
by a thermo-cryostat (Huber Unistat Tango). Gas was
supplied via a glass tube immersed in the solution, and
tipped with glass frit capable of forming very ®ne gas
bubbles at a rate of 140 ml min 1 . The solution was
saturated with Ar about 30 min before initiating the
sonolysis with appropriate gas. Preliminary tests con®rmed that the gas bubbling system considerably modi®ed the chemical kinetics of the system [4]. With a
constant gas ¯ow rate and ultrasonic intensity, for example, the H2 O2 sonochemical formation rate in 1 M
HNO3 in the presence of 0.1 M N2 H5 NO3 was about
5:4 0:1 lM min 1 when Ar was introduced through a
®ne glass tube ®xed at the surface of the solution, about
6:7 0:1 lM min 1 when the same ®ne glass tube was
immersed in the solution, and about 7:5 0:2 lM min 1
with a glass tube ®tted with a glass frit tip and immersed
in the solution. The gas was removed from the leaktight
sonoreactor through a glass tube extending directly to
the exterior. The use of a chemical trap in the reactor outlet line (sparging at a depth of about 10 cm of
solution) considerably modi®ed the chemical kinetics,
probably as a result of the pressure imposed in the
sonoreactor by the solution head in the chemical trap.
The acoustic power (W) transmitted to the system was
measured by calorimetry [11]. The ultrasonic intensity
(W cm 2 ) transmitted to the solution was calculated
from the P =S ratio, where S is the transducer surface
area (18 cm2 ). All the experiments were conducted at a
constant intensity of 3 W cm 1 (P ˆ 54 W).
Solution samples were taken during sonolysis without allowing air into the medium. The sample volumes
were small, and can be considered negligible compared
with the initial solution volume. The samples were analyzed immediately. HNO2 was monitored by initially
stabilizing the samples (as NO2 ) after mixing with 1 M
sodium hydroxide; the HNO2 concentration was determined by spectrophotometry (Shimadzu 3101 UVPC)
by the Griess method (k ˆ 530 nm; e ˆ 42 000 cm 1
M 1 ) [12]. The H2 O2 concentration in the HNO3 medium in the presence of N2 H5 NO3 was determined by
spectrophotometry by the Ti(IV) method in 0.5 M sulfuric acid (k ˆ 410 nm; e ˆ 600 cm 1 M 1 ) [12]. Under
these conditions no interference was observed from nitrate ions for a 1 M nitric acid concentration in the
sample.
3. Results
3.1. Sonolysis of NO2 ±Ar mixtures
Fig. 1 shows the evolution of the HNO2 concentration in solution during sonolysis (I ˆ 3 W cm 2 ) of a 1
M nitric acid solution saturated by a gaseous mixture
of argon and NO2 (0, 0.6, 0.9, 1.3 and 1.7 vol%). The
HNO2 concentration in solution increased over time to a
steady-state value due (as shown in a previous study
[11,13]) to the identical rates of HNO2 formation and
destruction. The slope of the linear HNO2 concentration
variation in solution at t ˆ 0 is used (Eq. (5)) to estimate
2
the HNO2 formation rate …xHNO
meas †. The results (Table 1)
indicate that the kinetics of HNO2 formation increase
with the NO2 volume fraction.
2
xHNO
meas ˆ
d ‰HNO2 Š
ˆ kfHNO2
dt
…5†
In order to take into account the rapid consumption
of HNO2 by hydrogen peroxide (Eq. (6)) formed under
ultrasound [11,13], the sonochemical H2 O2 formation
2 O2
rate …xH
meas † was determined in the HNO3 medium in the
presence of 0.1 M N2 H5 NO3 [14].
HNO2 ‡ H2 O2 ! HNO3 ‡ H2 O
…6†
Adding N2 H5 NO3 at an initial concentration of 0.1
M allowed H2 O2 to accumulate in solution, considering
the rapid decomposition kinetics of HNO2 [15]. Under
Ph. Moisy et al. / Ultrasonics Sonochemistry 8 (2001) 175±181
177
In order to discriminate between the e€ects of ultrasound and of simple sparging, the formation kinetics of
HNO2 in 1 M HNO3 were observed under the same
2
conditions …xHNO
chem †, but without ultrasound (Fig. 1b).
Contrary to the results obtained with ultrasound, the
kinetic law of HNO2 formation (Table 1) depended on
the NO2 volume fraction. With a NO2 concentration
above 1.3 vol%, the HNO2 formation kinetics estimated from the slope at t ˆ 0 were identical with those
observed under ultrasound. With lower NO2 volume
fractions (0.6 and 0.9 vol%) the rate of HNO2 formation
in solution exhibited an initial induction phase, then
accelerated. Regardless of the NO2 volume fraction,
however, a steady-state HNO2 concentration was
reached in solution.
These results can than be used to calculate the total
HNO2 concentration ([HNO2 ]T ) (Eq. (8)) and the HNO2
concentration formed under the e€ect of ultrasound
alone ([HNO2 ]US ) (Eq. (9)).
Fig. 1. (a) HNO2 concentration versus time in HNO3 (1 M; 25°C) with
ultrasound (20 kHz; 3 W cm 2 ) under NO2 ±Ar atmosphere (140
ml min 1 ): (1) f …NO2 † ˆ 1:7 vol%; (2) f …NO2 † ˆ 1:3 vol%; (3)
f …NO2 † ˆ 0:9 vol%; (4) f …NO2 † ˆ 0:6 vol%; (5) f …NO2 † ˆ 0 vol% and
(b) HNO2 concentration versus time in HNO3 (1 M; 25°C) without
ultrasound under NO2 ±Ar atmosphere (140 ml min 1 ): (1) f …NO2 † ˆ
1:7 vol%; (2) f …NO2 † ˆ 1:3 vol%; (3) f …NO2 † ˆ 0:9 vol%; (4) f …NO2 † ˆ
0:6 vol%.
these conditions, the H2 O2 concentration has been observed to increase in a linear manner with time, regardless of the NO2 volume fraction. The results (Table
1) can then be used (Eq. (7)) to calculate the total HNO2
2
formation rate …xHNO
† for a given volume fraction:
T
H 2 O2
2
2
xHNO
ˆ xHNO
T
meas ‡ xmeas
…7†
‰HNO2 ŠT ˆ ‰HNO2 Šmeas ‡ ‰H2 O2 Šmeas
…8†
‰HNO2 ŠUS ˆ ‰HNO2 ŠT
…9†
‰HNO2 Šchem
Fig. 2 shows the variation over time of the HNO2
concentration due to sonolysis of 1 M HNO3 with a
NO2 volume fraction of 0.6% ([HNO2 ]meas ); the H2 O2
concentration obtained under identical sonochemical conditions in the presence of N2 H5 NO3 ([H2 O2 ]meas ); the
HNO2 concentration obtained chemically by sparging
0.6 vol% NO2 without ultrasound ([HNO2 ]chem ); and the
HNO2 concentration ([HNO2 ]US ) formed under the effect of ultrasound alone (calculated from Eq. (9)). This
®gure shows that the induction period observed during
NO2 sparging without ultrasound is no longer observed
with ultrasound, and that the HNO2 formation rate is
higher with ultrasound. It is dicult to compare the
steady-state HNO2 concentrations obtained with and
without ultrasound, as the duration of the experiments
was not sucient to reach a steady-state value without
ultrasound. Moreover, H2 O2 was formed at a much
lower rate than HNO2 with ultrasound. The variation of
the HNO2 concentration formed with ultrasound alone
(Eq. (9)) appears to be much greater than without ultrasound.
Fig. 3 indicates the HNO2 concentration variation
actually due to the e€ect of ultrasound alone (calculated
Table 1
2 O2
Kinetic constants of formation of H2 O2 and HNO2 in HNO3 (1 M; 25°C) under NO2 ±Ar atmosphere (140 ml min 1 ) with ultrasound (xH
meas and
2 , respectively) and without ultrasound (xHNO2 )
xHNO
chem
meas
f(NO2 ) (% vol in Ar)
2
xHNO
meas …lmol l
0
0.6
0.9
1.3
1.7
7:7 0:4
65 3
59 2
120 5
165 5
1
min 1 )
2 O2
xH
meas …lmol l
7:5 0:2
6:5 0:1
5:8 0:2
5:2 0:4
5:1 0:4
1
min 1 )
2
xHNO
chem …l mol l
1
0
Induction period
Induction period
110 2
140 3
min 1 )
178
Ph. Moisy et al. / Ultrasonics Sonochemistry 8 (2001) 175±181
3.2. Sonolysis of NO±Ar mixtures
Fig. 2. HNO2 concentration versus time in HNO3 (1 M; 25°C) under
NO2 ±Ar atmosphere (f …NO2 † ˆ 0:6 vol%; 140 ml min 1 ): (1) with ultrasound (20 kHz; 3 W cm 2 ); (2) without ultrasound; (3) under the
e€ect of ultrasound alone, calculated from Eq. (9); (4) H2 O2 concentration versus time in HNO3 (1 M) ‡ N2 H5 NO3 (0.1 M) under NO2 ±
Ar atmosphere (f …NO2 † ˆ 0:6 vol%; 140 ml min 1 ).
Fig. 4a shows the evolution of the HNO2 concentration during sonolysis (I ˆ 3 W cm 2 ) of a nitric acid
solution (1 M) saturated with a gaseous mixture of
argon and NO (0, 0.3, 0.6 and 1.1 vol%). As with the
NO2 ±Ar mixtures, the linear HNO2 concentration
variation at t ˆ 0 can be used to determine the HNO2
formation kinetics (Table 2). For 0.3 vol% NO, the
HNO2 formation kinetics di€ered only slightly from
those observed during sonolysis of HNO3 (1 M) under
pure argon. A steady-state HNO2 concentration in solution was obtained for all three NO volume fractions.
The H2 O2 concentration was monitored in 1 M HNO3
(in the presence of 0.1 M N2 H5 NO3 ) to determine the
sonochemical formation kinetics of H2 O2 according to
the NO volume fraction (Table 2). As with the NO2 ±Ar
mixtures, the H2 O2 formation rate diminished very
slightly with the NO volume fraction.
Fig. 3. HNO2 concentration versus time in HNO3 (1 M; 25°C) under
NO2 ±Ar atmosphere (140 ml min 1 ) with the e€ect of ultrasound
alone, calculated from Eq. (9): (1) f …NO2 † ˆ 1:7 vol%; (2) f …NO2 † ˆ
1:3 vol%; (3) f …NO2 † ˆ 0:9 vol%; (4) f …NO2 † ˆ 0:6 vol%; (5) f …NO2 † ˆ
0 vol%.
from Eq. (9)) versus the NO2 volume fraction. The results show that for a volume fraction exceeding 1.3%,
ultrasound does not enhance the kinetics of HNO2
formation in solution or the steady-state HNO2 concentration above the values obtained by simple gas
sparging. The fact that the steady-state concentrations
were approximately the same with or without ultrasound shows that the HNO2 decomposition rate is the
same. For volume fractions below 1.3%, ultrasound has
an appreciable e€ect and the HNO2 formation rate is
signi®cantly higher than without ultrasound. It may
be noted that the rates obtained were signi®cantly
higher than measured during sonolysis (3 W cm 2 ) of a
nitric acid solution (1 M) under pure argon atmosphere
(Table 1).
Fig. 4. (a) HNO2 concentration versus time in HNO3 (1 M; 25°C) with
ultrasound (20 kHz; 3 W cm 2 ) under NO±Ar atmosphere (140
ml min 1 ): (1) f …NO† ˆ 1:1 vol%; (2) f …NO† ˆ 0:6 vol%; (3) f …NO† ˆ
0:3 vol%; (4) f …NO† ˆ 0 vol% and (b) HNO2 concentration versus
time in HNO3 (1 M; 25°C) without ultrasound under NO±Ar atmosphere (140 ml min 1 ): (1) f …NO† ˆ 1:1 vol%; (2) f …NO† ˆ 0:6 vol%.
Ph. Moisy et al. / Ultrasonics Sonochemistry 8 (2001) 175±181
179
Table 2
2 O2
Kinetic constants of formation of H2 O2 and HNO2 in HNO3 (1 M; 25°C) under NO±Ar atmosphere (140 ml min 1 ) with ultrasound (xH
meas and
2 , respectively) and without ultrasound (xHNO2 )
xHNO
chem
meas
f…NO† …% vol in Ar†
2
xHNO
meas …l mol l
0
0.3
0.6
1.1
7:7 0:4
72
93 9
125 7
1
min 1 †
Fig. 4b shows the evolution of the HNO2 concentration in 1 M HNO3 under NO±Ar atmosphere without
ultrasound. In the case of a 1.1% volume fraction, the
initial formation kinetics was determined from the linear
HNO2 concentration variation over time. With lower
volume fractions, however, the formation kinetics could
not be accurately determined either because of the initial
induction period (0.6 vol% NO) or the very low HNO2
accumulation rate (0.3 vol% NO). The calculated results
(Table 2) show that without ultrasound the HNO2 formation kinetics are slower and the steady-state concentrations lower in the presence of NO than for NO2 .
Fig. 5 shows the HNO2 concentration variation calculated from Eq. (9) ± i.e. actually formed under the
e€ect of ultrasound alone ± versus the NO volume
fraction: for 0, 0.6 and 1.1 vol% NO, ultrasound
enhanced the HNO2 formation kinetics and the steadystate concentration in solution compared with the results obtained by simple sparging of the gas mixture. As
with NO2 ±Ar mixtures, the rates obtained were signi®cantly higher than those measured during sonolysis (3
W cm 2 ) of a nitric acid solution (1 M) under pure argon
atmosphere. An induction period was again observed
with ultrasound for the 0.6% volume fraction. For the
0.3% volume fraction it is dicult to discuss the results
for several reasons: (1) the low HNO2 concentration
accumulated with ultrasound (on the same order of
magnitude as with pure argon); (2) the H2 O2 accumu-
Fig. 5. HNO2 concentration versus time in HNO3 (1 M; 25°C) under
NO±Ar atmosphere (140 ml min 1 ) with the e€ect of ultrasound alone,
calculated from Eq. (9): (1) f …NO† ˆ 1:1 vol%; (2) f …NO† ˆ 0:6 vol%;
(3) f …NO† ˆ 0 vol%.
2 O2
xH
meas …lmol l
1
min 1 †
7:5 0:2
6:1 0:3
4:1 0:1
4:0 0:1
2
xHNO
chem …l mol l
1
min 1 †
0
Induction period
Induction period
64 1
lation was comparable to that of HNO2 ; (3) the chemical accumulation of HNO2 by simple sparging was very
low. Comparing the HNO2 concentrations versus the
NO2 and NO volume fractions (Figs. 3 and 5) shows
that with ultrasound the HNO2 concentration was much
higher in the presence of NO than of NO2 , unlike the
behavior observed without ultrasound.
4. Discussion
Numerous parameters have already been identi®ed as
important for controlling the e€ect of ultrasound; the
speci®c heat ratio …c†, thermal conductivity …C† and gas
solubility are particularly important in controlling the
gaseous atmosphere. Considering the minor changes in
the speci®c heat ratio …c† and the thermal conductivity
…C† with NO and NO2 volume fractions well below 2%
in argon, the e€ect of ultrasound is probably chemical in
nature. It may thus be assumed that the temperature in
the cavitation bubble and the ultrasonic intensity are
similar to those obtained under pure argon. The problems encountered in determining the gas solubility under
the e€ect of ultrasound, particularly for NO and NO2
make it impossible to predict the role of this parameter
in the observed phenomenon, however, although these
gases are poorly soluble in aqueous media, and the
solubility diminishes as the temperature rises. The solubility of argon in water typically diminishes from 5.4
ml per 100 ml at 0°C to 4.5 ml at 25°C and 3.0 ml at
50°C. Moreover, it has been established [16] in the case
of NO2 that the dimerization equilibrium results in the
formation of N2 O4 (Eq. (10)), for which the solubility in
the aqueous phase is too low to measure. This low solubility is generally attributed [17] to dismutation of
N2 O4 into HNO2 and HNO3 (Eq. (11)).
2NO2 $ N2 O4
…10†
N2 O4 ‡ H2 O $ HNO2 ‡ HNO3
…11†
The equilibrium and kinetic constants are K…10† ˆ 6:54 f
104 l mol 1 and k…10†
ˆ 4:5 108 l mol 1 s 1 at 25°C for
the NO2 dimerization equilibrium [16], and K…11† ˆ
f
3:33 1011 mol l 1 and k…11†
ˆ 1:7 1010 l mol 1 s 1 for
the N2 O4 dismutation reaction [17]. NO2 is completely
dimerized in the gaseous phase considering the high
f
equilibrium constant: K…10†
ˆ 6:84 atm 1 at 25°C [18].
180
Ph. Moisy et al. / Ultrasonics Sonochemistry 8 (2001) 175±181
Considering the weakness of the N2 O4 bond, however,
NO2 is probably monomeric in the hot cavitation bubble. Although it is dicult to predict the impact of the
gas solubility on the ultrasonic e€ect, the temperature in
the cavitation bubble and the ultrasonic intensity are
thus probably the same as observed under argon atmosphere.
It is currently acknowledged [7,8] that the two principal mechanisms accounting for the sonochemical
phenomena observed in aqueous solution are a radical
mechanisms involving the primary radicals OH and H ,
and a thermolysis mechanisms inside the cavitation
bubble or at the bubble/solution interface. The fact that
the HNO2 formation and decomposition kinetics are
unchanged for NO2 volume fractions exceeding 1.3% in
Ar with and without ultrasound (Fig. 3) shows that the
mechanism probably consists in the generally accepted
reactions (Eqs. (10) and (11)) for HNO2 formation, and
Eq. (12) for its decomposition [11].
Another possible mechanism corresponds to the absorption of N2 O3 (Eq. (16)) formed from NO2 and NO
(formed in the cavitation bubbles) to yield HNO2 (Eqs.
(17) and (18)):
NO ‡ NO2 $ N2 O3
…16†
N2 O3 ‡ H2 O $ 2HNO2
…17†
NO ‡ NO2 ‡ H2 O $ 2HNO2
…18†
The equilibrium constant of Eq. (16) and of the overall
reaction (Eq. (18)), and the kinetic constants [23,24] at
f
25°C are: K…16† ˆ 5 104 l mol 1 , k…16†
ˆ 1:1 109
1
1
l mol s , and
K…18† ˆ
‰HNO2 Š2
ˆ 1 107 l mol
‰NOŠ‰NO2 Š
1
d‰NOŠ d‰NO2 Š
ˆ
ˆ k f ‰H2 OŠ‰NOŠ‰NO2 Š
dt
dt
k b ‰HNO2 Š
2
…12†
…19†
In the case of lower NO2 volume fractions, however,
ultrasound appears to accelerate the HNO2 formation
rate (Fig. 3) ± probably due to thermolysis of NO2 in the
cavitation bubble (Eq. (13)), where temperatures exceeding 1000°C are reached. NO2 is known to be totally
decomposed at temperatures above 620°C [19].
where k f ‰H2 OŠ ˆ 106 l mol 1 s 1 and k b ˆ 9:51
l mol 1 s 1 .
Considering the complexity of the equilibria involved, and of the redox chemistry of nitrogen, it is
dicult to favor one mechanism over the other to account for the results obtained in NO2 ±Ar atmosphere.
However, HNO3 reduction by NO in the cavitation
bubble (Eq. (14)) appears the most likely mechanism to
explain the results observed under NO atmosphere: NO2
is formed only by decomposition of HNO2 (just before
the steady state), since the equilibrium constant (Eq.
(20)) is very low [25] in 2 M HNO3 at 25°C (K…20† ˆ
2HNO2 ! NO ‡ NO2 ‡ H2 O
†††
2NO2 ! 2NO ‡ O2
…13†
Under these conditions ± small amounts of NO2 and
NO (production in the cavitation bubbles), several
probable mechanisms can account for the HNO2 formation kinetics with small NO2 volume fractions. One
possible mechanism involves the reduction of HNO3 by
the NO formed in the cavitation bubbles (Eq. (14)):
2NO ‡ HNO3 ‡ H2 O $ 3HNO3
…14†
The equilibrium constant has been estimated [17,18,
20] at about K…14† ˆ 3:3 10 2 atm 2 mol 1 l 2 (ambient
temperature). The kinetic law (Eq. (15)) established by
Abel and Schmidt [17] indicates the existence of an
autocatalytic phenomenon by nitrous acid:
d‰HNO2 Š
ˆ k f ‰HNO2 Š‰H‡ Š NO3
dt
kb
‰HNO2 Š4
2
PNO
…15†
where k f ˆ 1:6 l2 mol 2 min 1 and k b ˆ 46 l3 mol 3
min 1 . While this reaction is slow at room temperature,
quantitative NO absorption has been observed at temperatures above 60°C [21]. Moreover, work by Knox
and Reid [22] showed that a sustained high NO concentration shifts the equilibrium considerably toward
the formation of HNO2 . Moreover, the volatility of
undissociated nitric acid [11,13] is likely to allow HNO3
to accumulate in the cavitation bubbles, capable of reacting with NO.
3
PNO
a…H2 O†
a…HNO
2 ˆ 1:64 10
3†
with the acidity.
2
PNO
8
atm2 mol
NO ‡ 2HNO3 $ 3NO2 ‡ H2 O
1
l) and diminishes
…20†
The mechanisms of HNO3 reduction by NO formed
in the cavitation bubbles (Eq. (14)) is capable of accounting for the induction phase (by an autocatalytic
mechanism as per Eq. (15)) and for the higher steadystate HNO2 concentration following the rise in the NO
concentration in the cavitation bubble compared with
the result obtained without ultrasound. Given the similar steady-state HNO2 concentrations with and without ultrasound in NO2 atmosphere, and the increased
steady-state concentration in NO atmosphere, oxidation
of HNO2 (Eq. (21)) by the O2 formed according to Eq.
(13) is probably negligible. No experimental study
known to us has yet determined the kinetics of this reaction, and it is thus dicult to substantiate this hypothesis.
2HNO2 ‡ O2 $ 2HNO3
…21†
The main e€ect of ultrasound is thus probably the
thermolysis of NO2 , leading to the formation of NO.
Ph. Moisy et al. / Ultrasonics Sonochemistry 8 (2001) 175±181
The NO formed in the cavitation bubble can thus reduce
the HNO3 contained in the bubble or at the bubble/solution interface to form nitrous acid. As this reaction is
catalyzed by HNO2 , an induction period is observed at
low HNO2 concentrations. Additional work will be required to con®rm the mechanism of HNO3 reduction by
NO, and to assess its importance compared with the
N2 O3 absorption mechanism.
Moreover, in view of the slight drop in the rate of
H2 O2 formation depending on the NO2 volume fraction
(0±1.7 vol%) and of the NO volume fraction (0±1.1
vol%) in the argon, the scavenging power of NO2 and
NO appears to be low with respect to the free radicals
(OH and H ) released by homolytic decomposition of
water and nitric acid (Eqs. (22)±(24)) [11].
†††
H2 O ! OH ‡H
†††
HNO3 ! OH ‡NO2
†††
HNO3 ! H ‡NO3
…22†
…23†
…24†
It should also be noted (Fig. 2, Tables 1 and 2) that the
order of magnitude of the H2 O2 formation rate is considerably lower than the HNO2 formation rate by the
e€ect of ultrasound alone, as calculated from Eq. (9)
under NO and NO2 atmosphere. It has also been observed during sonolysis of a nitric acid solution in the
presence of hydrazinium nitrate (which acts as an HNO2
and NO2 scavenger) [14], that the H2 O2 formation rate
is independent of the nitric acid concentration (1 <
‰HNO3 Š < 6 M) and is signi®cantly lower than for
HNO2 . For example, the H2 O2 formation rate was estimated at about 12 lM min 1 in a 6 M HNO3 medium,
while the HNO2 formation rate under the same conditions is about 126 lM min 1 [14]. The HNO2 formation
mechanism under NOx atmosphere involving the free
radicals released by homolytic decomposition of water
and nitric acid is probably a minor contributor.
5. Conclusion
An investigation of the formation of HNO2 in a 1 M
HNO3 medium for various NO±Ar and NO2 ±Ar gas
mixtures with ultrasound revealed the following phenomena:
(1) The rate of HNO2 formation was signi®cantly
increased by ultrasound for NO and NO2 volume fractions below 1%. The induction period observed without
ultrasound for such low NO2 volume fractions disappeared under the e€ect of ultrasound.
(2) With ultrasound, the HNO2 formation rate was
higher under NO±Ar than under NO2 ±Ar atmosphere,
contrary to the behavior observed without ultrasound.
181
The rates increased with the NO and NO2 volume
fractions diluted in the argon under these experimental
conditions: f …NO2 † < 1:7 vol% and f …NO† < 1:1 vol%.
(3) The ®rst step in the sonochemical (20 kHz)
mechanism of HNO2 formation in HNO3 (1 M) under
NO2 ±Ar atmosphere (<1 vol% NO2 ) involves thermal
decomposition of NO2 into NO within the cavitation
bubble. The second step appears to be similar to the
phenomenon that occurs in NO±Ar atmosphere, and
corresponds either to reduction of HNO3 by NO in the
cavitation bubble, or to hydrolysis of N2 O3 (formed
from NO and NO2 ) in the cavitation bubble or at the
bubble/solution interface.
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