Spontaneous decomposition of industrially manufactured sodium hypochlorite solutions

Article abstract of DOI:10.1007/s11167-005-0336-6

Spontaneous decomposition of industrially manufactured aqueous solutions of sodium hypochlorite was studied. The rate constants of the decomposition reactions were calculated for different pH values.

Full text of DOI:10.1007/s11167-005-0336-6

Russian Journal of Applied Chemistry, Vol. 78, No. 4, 2005, pp. 541 545. Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 4, 2005,
pp. 548 552.
Original Russian Text Copyright
2005 by Sizeneva, Kondrashova, Val’tsifer.
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
Spontaneous Decomposition of Industrially Manufactured
Sodium Hypochlorite Solutions
I. P. Sizeneva, N. B. Kondrashova, and V. A. Val’tsifer
Institute of Technical Chemistry, Ural Division, Russian Academy of Sciences, Perm, Russia
Received December 16, 2004; in final form, November 2004
Abstract Spontaneous decomposition of industrially manufactured aqueous solutions of sodium hypochlorite
was studied. The rate constants of the decomposition reactions were calculated for different pH values.
In view of the exceedingly high toxicity and
The choice in developing the demercurizer was
made in favor of sodium HC, taking into account its
availability as a solution, high content of active chlo-
rine, and high stability against decomposition in
an alkaline medium (compared with calcium HC) [9].
The behavior of the solutions was analyzed at two
temperatures, 25 and 35 C, in view of the regional
climatic conditions (the maximum summer tempera-
ture is not, as a rule, higher than 35 C).
volatility of metallic mercury (MPC for habitats
3
0
.0003 mg m ) [1] and its wide use, the problem of
efficient salvation in tackling with possible mercury
spillage events remains a matter of current interest.
For this purpose, new demercurization methods are
being developed and the already existing techniques
are being improved. At present, the most widely used
methods are those in which mercury is oxidized with
strong oxidizing agents [2, 3]. In developing a de-
mercurizer, with industrially manufactured sodium
hypochlorite (HC) solutions as an oxidizing agent, it
became necessary to assess the stability of solutions
of this kind in storage and to study the oxidizing
activity of these industrial solutions at various pH
values.
EXPERIMENTAL
As object of study served industrially manufactured
(Berezniki Soda Plant Open Joint-Stock Company)
sodium HC solution with a concentration of 180
00 g l 1 in terms of active chlorine. Model solutions
2
HC solutions are nonequilibrium systems that
undergo spontaneous decomposition. The problem of
spontaneous decomposition of HC solutions has been
extensively studied. The influence exerted by the con-
centration, temperature, and acidity on decomposition
processes has been analyzed and the possible mech-
anisms and kinetic characteristics of decomposition
have been reported [4 8]. In the known studies, so-
lutions of both calcium and sodium HC were ex-
amined. Model solutions have been mostly studied,
with the HC solutions having low concentrations:
were prepared by passing at (0 5 C) Cl obtained by
reacting chemically pure hydrochloric acid with
2
KMnO through a NaOH solution containing no car-
4
bonates. The excess amount of the alkali was about
1
20 g l . The reaction vessel was cooled with a mix-
ture of NaCl and ice. The pH values of the solutions
were measured with an EV-74 ionometer with glass
and silver chloride electrodes.
In stability studies, the alkaline (pH 13.5) HC so-
lutions under study were thermostated in hermetically
sealed polyethylene jars for 3 months. The pH value
of the solutions remained virtually constant, irrespec-
tive of the duration and temperature of experiments,
because of the presence of an excess amount of NaOH
in the HC solutions.
0
.01 0.20 M in terms of active chlorine.
The aim of this study was to assess the stability of
industrially manufactured concentrated HC solutions
in order to determine the maximum shelf life of so-
lutions of this kind. Another goal was to determine
the kinetic characteristics of real solutions used to
prepare the demercurizing agent.
The dependence of the degree of HC decomposi-
tion on the pH value was studied at 25 and 30 C.
1
070-4272/05/7804-0541 2005 Pleiades Publishing, Inc.

542
SIZENEVA et al.
Table 1. Decomposition of industrially manufactured sodium HC in solutions of different concentrations at 25 and 35 C
Variation of c
in storage, M
k,
Variation of c
in storage, M
k,
T,
C
t, h
mol l 1 h 1
mol l 1 h 1
2
5
0
cin = 5.20 M
cin = 2.10 M
4
4
4
4
4
4
4
4
1
2
3
3
4
6
44
16
12
84
80
96
5.10
4.86
4.50
4.39
4.19
3.90
3.20
2.70
0.2618 10 4
0.6229 10 4
2.09
2.08
2.07
2.00
1.97
1.92
1.80
1.64
0.2060 10
0.2120 10
0.4579 10
0.6200 10
0.6547 10
0.6414 10
0.6239 10
0.6253 10
4
0.9588 10
0.9240 10 4
0.9658 10 4
4
0.9210 10
1
2
272
040
0.9449 10 4
0.9078 10 4
kav (312 2040 h) = (0.9371 0.0225) 10 4
kav (384 2040 h) = (0.6296 0.0155) 10 4
3
5
0
cin = 5.20 M
cin = 2.10 M
4
1
2
3
3
4
6
9
44
16
12
84
80
96
60
4.50
4.20
3.65
3.45
3.23
2.76
2.12
1.84
1.30
0.2077 10 4
2.00
1.95
1.89
1.85
1.80
1.69
1.58
1.46
1.20
0.1653 10
0.1695 10
0.1696 10
0.1676 10
0.1653 10
0.1660 10
0.1633 10
0.1641 10
0.1672 10
0.2119 10 3
3
4
3
3
3
3
3
3
3
0.2617 10
0.2540 10 3
3
0.2444 10
0.2443 10 3
0.2910 10 3
3
1
2
272
136
0.2761 10
0.2649 10 3
kav = (0.2507 0.0266) 10 3
kav (384 2040 h) = (0.1668 0.021) 10 3
The experiments were performed with solutions con-
taining the same amounts of active chlorine (1.24,
w1/w2
n =
.
log (c /c )
1
2
1
.29 M) at a virtually constant pH value, which was
This formula makes it possible to determine the re-
action order n from the initial rates w and w , i.e.,
from the reaction rates at different initial concentra-
tions c and c of the starting HC.
continuously monitored during the experiments.
The acidity of the medium was adjusted by adding
an H SO solution. The degree of decomposition was
1
2
2
4
determined after 10 min.
1
2
The calculations demonstrated that spontaneous
decomposition of sodium HC in alkaline solutions
occurs as a second-order reaction at both 25 and 35 C.
The second reaction order was also obtained by graph-
ical differentiation from the slope of the dependence
of the rate of HC decomposition on the logarithm of
the concentration, log( c/ t) log c. The rate con-
stants of the decomposition reaction were calculated
using second-order kinetic equations
To study the kinetics of decomposition at different
acidities, HC solutions were placed in hermetically
sealable glass jars, with the required acidity adjusted
by introduction of an HCl solution. Changes in the
acidity of the medium and in the concentration were
determined at regular intervals of time. In all the ex-
periments, the variation of the HC concentration was
monitored iodometrically.
The experiments in which the stability of sodium
HC solutions was studied were carried out with in-
dustrial and model solutions of close concentrations.
The data obtained were used to calculate the order of
the reaction by which the alkaline solution of sodium
HC decomposes, using the known Van’t Hoff formula
c0
c c
c
k = 1/t
.
0
The results obtained are listed in Table 1.
The second order of the reactions and the formal
rate constants of decomposition were obtained using
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SPONTANEOUS DECOMPOSITION OF SODIUM HYPOCHLORITE SOLUTIONS
543
the above-described procedures for a model HC so-
lution with a concentration of 2.26 M at two tempera-
4
tures: kav = 0.2060 10 M at 25 C and kav
0
=
3
.1064 10 M at 35 C.
The activation energies calculated using the Ar-
rhenius equation
log (k /k )
T2 T )/T T
1 2 1
2
1
E =
,
(
were 82.33 kJ mol 1 (19.67 kcal mol ) for a sodium
1
HC solution with a concentration of 5.2 M and
9.53 kJ mol 1 (19.00 kcal mol ) for that with a con-
1
7
centration of 2.1 M. The results obtained are in
an agreement with published data for alkaline HC
solutions [10].
The decomposition of HC consists of two concur-
rent processes, oxygen and chlorate decompositions,
which can be expressed by the following reactions
Fig. 1. Stability of industrially manufactured sodium hypo-
chlorite solutions with a concentration of (a) 5.2 and
b) 2.1 M. (c) Concentration and (t) time; the same for
(
Fig. 2. Temperature ( C): (1) 25 and (2) 35.
ClO + ClO = 2Cl + O2,
ClO + ClO = ClO2 + Cl ,
ClO2 + ClO = ClO3 + Cl .
(I)
(II)
(III)
The rate-determining reactions are (I) and (II),
whereas reaction (III) is considerably faster than
the first two [11].
As can be seen in Table 1, a certain deviation from
the average kinetic parameters (reaction orders, reac-
tion rate constants) is observed in the initial stage of
the process ( 216 h) at 25 C, which can be attributed
to predominance of the oxygen decomposition [11].
At 35 C, the process is considerably faster: the chlo-
rate decomposition predominates. The model solu-
tions decompose in a similar way.
Fig. 2. Stability of (1) model sodium hypochlorite solutions
with a concentration of 2.26 M and (2) industrially manu-
factured sodium hypochlorite solutions with a concentration
of 2.1 M at (a) 35 and (b) 25 C.
The experimental data were used to compare
the decomposition rates of sodium HC in industrially
manufactured and model solutions at 25 and 35 C.
Figures 1 and 2 show kinetic curves that describe
the self-decomposition of alkaline solutions of sodi-
um HC. It can be seen that that the processes of spon-
taneous decomposition are autocatalytic at 25 C,
with the process rates dependent on the initial concen-
tration. The calculated rate constants of HC decom-
position were used to compare the decomposition
rates of HC in solutions with different concentrations
at this temperature.
tion of 2.1 M. The temperature dependence of the de-
composition rate is particularly strong. The decom-
position is approximately 2.8 times faster at 35 C,
compared with 25 C, for the solution with a sodium
HC concentration of 5.2 M, and 2.65 times faster at
a concentration of 2.1 M.
Comparison with model solutions shows that de-
composition of a diluted industrially manufactured
sodium HC solution with a concentration of 2.1 M at
25 C is approximately 3 times faster than that of
For example, sodium HC decomposes at 25 C in
the solution with a concentration of 5.2 M approx-
imately 1.5 times faster than in that with a concentra-
a model solution with a close concentration (c =
2.26 M). This can be attributed to presence in the in-
dustrial solution of mostly colloid particles of iron
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 78 No. 4 2005

544
SIZENEVA et al.
hydrogen ions in solution. Figure 3 shows how the
degree of HC decomposition depends on the pH value
at 25 and 30 C. It can be seen that the maximum
degree of HC decomposition is observed at pH close
to 7.
It is known that not only the mechanisms of HC
decomposition in alkaline, neutral, and acid media,
but also the rates of this process are different: the de-
composition rate is at a minimum in a strongly acidic
medium (pH < 1), increases with the pH value to
reach a maximum in a nearly neutral medium, and
then again decreases in an alkaline medium. Thus,
the activity of hydrogen ions is one of the key fac-
tors affecting the composition and stability of HC
solutions and, consequently, their oxidizing power
Fig. 3. Degree of sodium hypochlorite decomposition, A,
vs. the pH value. (A) Ratio of the number of decomposed
moles of HClO + ClO to their initial number. c (M):
0
(
(
1) 1.24 and (2) 1.29. Temperature ( C): (1) 30 and
2) 25.
[
13 15].
The decomposition kinetics of industrial sodium
HC solutions was studied at pH 6.5 8.0 (solution
no. 1, c = 1.188 M) and pH 4.8 (solution no. 2, c =
hydroxide Fe(OH) . These particles have an excess
3
energy and accelerate the decomposition of HC, with
the catalytic action of Fe(OH) becoming less pro-
0
0
3
0.941 M) at room temperature (21 22 C). A fast
nounced in the course of time, which is commonly ac-
counted for by a decrease in dispersity, named aging
12]. At 35 C, the decomposition rate of the industrial
solution exceeds that of the model solution only by
approximately a factor of 1.5, because the accelerating
effect of temperature equally tells upon the decom-
position of both HC solutions. At the same time,
change in both the pH of the medium and the HC
concentration was observed at pH 6.5 8.0. The ex-
perimental data obtained are listed in Table 2.
[
The order of the reaction of HC decomposition was
determined from the variation of the HC concentration
with time, and the rate constants of the reaction were
calculated. The reaction order was found using the
graphical variety of the method of substitution of
experimental data (HC concentrations at different in-
stants of time in the course of the reaction) into first-,
second-, and third-order kinetic equations. For this
purpose, time dependences of various functions of
concentration were constructed. Each kinetic equation
gives a straight line only in the appropriate coordi-
nates [16]. Both for pH 6.5 8.0 and for pH 4.8, a lin-
ear plot was obtained for the 1/c t dependence, i.e.,
for a second-order kinetic equation. The reaction or-
der was confirmed, as in the case of an alkaline me-
dium, by graphical differentiation. The rate constants
were calculated for a second-order kinetic equation:
the catalytic action of Fe(OH) on the decomposition
3
of the industrial becomes less pronounced as temper-
ature increases, because a higher temperature favors
coagulation, i.e., leads to coarsening of colloid par-
ticles and, consequently, to a decrease in their total
surface area and excess energy.
The stability and oxidizing power of sodium HC
solutions are strongly dependent on the activity of
Table 2. Dynamics of variation of the pH value and
concentration of sodium HC solutions
Solution no.
1
t, h
c, M
pH
0
1.188
0.689
0.437
0.255
0.207
0.185
0.941
0.323
0.182
0.132
0.105
8.06
7.84
7.52
6.09
6.05
6.45
4.83
5.01
5.30
5.25
5.22
0
1
2
3
4
0
6
.5
k, l 1 mol 1 h 1
pH
.0
.0
.0
.0
4.80
8.0 6.5
13.5
(2.12 0.60) 10 2
1.34 0.20
(0.63 0.02) 10 4
2
9
It was noted in [8] that the reaction at pH 6.0 8.0
is so fast that the rate of self-decomposition increases
by several orders of magnitude to become comparable
with that of fast reactions.
1
3
5
68
84
28
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SPONTANEOUS DECOMPOSITION OF SODIUM HYPOCHLORITE SOLUTIONS
545
The same dependence was observed in the present
study.
2. Mel’nikov, S.M., Tekhnika bezopasnosti v metallurgii
rtuti (Safety Measures in Metallurgy of Mercury),
Moscow: Metallurgiya, 1974.
The decomposition of sodium HC solutions at
pH < 4.8 was not considered because preparation of
solutions of this kind is accompanied by vigorous
3
. Zakharov, L.N., Tekhnika bezopasnosti v khimiches-
kikh laboratoriyakh (Sfety Measures in Chemical
Laboratories), Leningrad: Khimiya, 1985.
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2
4. Flis, I.E. and Bynyaeva, M.K., Zh. Prikl. Khim.,
957, vol. 30, no. 3, pp. 339 345.
ficient use of HC.
1
5
. Flis, I.E., Zh. Prikl. Khim., 1963, vol. 36, no. 8,
pp. 1669 1675.
CONCLUSIONS
6
. Grigor, T.T., Tumanova, T.A., Mishchenko, K.P.,
and Shalanki, L., Zh. Prikl. Khim., 1967, vol. 40,
no. 9, pp. 2039 2044.
(
1) An assessment of the stability of industrially
manufactured sodium hypochlorite solutions revealed
that dilute sodium hypochlorite solutions are the most
stable in storage at temperatures not exceeding 25 C.
It was shown that model hypochlorite solutions pre-
pared with distilled water are more stable in storage
than the industrially manufactured solutions.
7. Nikol’skii, B.P., Krunchak, V.G., L’vova, T.V., et al.,
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(2) It was demonstrated that the spontaneous de-
9
. Krunchak, V.G., Zh. Prikl. Khim., 1974, vol. 47,
composition of industrial sodium hypochlorite solu-
tions in a wide range of pH values (4.8 13.5) is a sec-
ond-order reaction. The rate constants of the decom-
position process, calculated at different pH values,
confirmed the extremal type of the dependence of
the decomposition rate on the pH value. It was shown
that the decomposition pattern of dilute hypochlorite
solutions is also valid for concentrated solutions.
no. 9, pp. 2112 2113.
1
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1
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1
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