A study of mercury dissolution in aqueous solutions of sodium hypochlorite

Article abstract of DOI:10.1007/s11167-005-0337-5

Dissolution of mercury in aqueous solutions of sodium hypochlorite at pH 5.9-8.5, the corresponding reaction order, and the mechanism of this process were studied. The ratio of the rates of mercury oxidation and sodium hypochlorite decomposition was analyzed.

Full text of DOI:10.1007/s11167-005-0337-5

Russian Journal of Applied Chemistry, Vol. 78, No. 4, 2005, pp. 546 548. Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 4, 2005,
pp. 553 555.
Original Russian Text Copyright
2005 by Sizeneva, Val’tsifer, Strel’nikov.
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
A Study of Mercury Dissolution in Aqueous Solutions
of Sodium Hypochlorite
I. P. Sizeneva, V. A. Val’tsifer, and V. N. Strel’nikov
Institute of Technical Chemistry, Ural Division, Russian Academy of Sciences, Perm, Russia
Received December 16, 2004
Abstract Dissolution of mercury in aqueous solutions of sodium hypochlorite at pH 5.9 8.5, the cor-
responding reaction order, and the mechanism of this process were studied. The ratio of the rates of
mercury oxidation and sodium hypochlorite decomposition was analyzed.
Salts of hypochlorous acid, hypochlorites, find
wide application as strong oxidizing agents, their ox-
idizing power being largely determined by the acidity
of the medium.
centration was determined at regular intervals of time
by a selective spectrophotometric technique with di-
thizone [1]. The optical density of the solutions was
determined with a KFK-2 photoelectric colorimeter.
The aim of this study was to assess the possibility
of using a sodium hypochlorite (NaClO) solution at
pH 5.9 8.5 for oxidation of mercury and to determine
the rate constants of mercury dissolution via its ox-
idation to Hg2+ ions in the above-mentioned acidity
range.
Mercury is a metal that is liquid at room temper-
ature. The high surface tension allows even com-
paratively large mercury drops to take the shape of
a sphere, the body with the smallest relative sur-
face area. Dissolution of mercury is a heterogeneous
process that occurs at the interface and depends on
the area and state of the surface. Kinetic character-
istics of the process of mercury dissolution in sodium
hypochlorite solutions were determined using the for-
mula established experimentally by A.N. Shchukarev
for the rate of dissolution of a solid body in a liquid
[2, 3]. This formula made it possible to calculate
the rate constant k of mercury dissolution:
EXPERIMENTAL
Mercury was oxidized with an aqueous solution of
NaClO at pH 5.9, 6.5, 6.9, and 8.5 and temperatures
of 25 and 50 C; the concentration in terms of active
chlorine was 0.3 0.8 M. The required acidity was
created with a buffer phosphate solution obtained by
adding orthophosphoric acid, potassium dihydrophos-
phate, and sodium hydrophosphate to the NaClO so-
lution. The pH value was monitored with an EV-74
ionometer with glass and silver chloride electrodes.
w = kS(cs
c),
where w is the dissolution rate; S, area of contact be-
tween the solid and liquid; c, concentration of the sub-
stance being dissolved in the bulk of the liquid; cs,
concentration of a saturated solution; and k, constant
for the given experimental conditions.
The NaClO solution was produced by passing chlo-
rine through a carbonate-free solution of sodium hy-
droxide at a temperature of
5 0 C. The reaction
vessel was cooled with a mixture of sodium chloride
and ice. Chlorine was obtained by reacting chemically
pure hydrochloric acid with potassium permanganate.
The concentration of the hypochlorite was determined
iodometricaly.
As the concentration of a saturated solution was
taken the maximum possible concentration of Hg2+
ions. The latter was determined from the mass of
the mercury drop and the volume of the hypochlorite
solution in which it dissolved. The rate of mercury
dissolution was found from the concentration of mer-
cury(II) at certain instants of time. The contact area
was calculated from the mass of a spherical drop of
mercury and its density [4] at a given temperature.
A mercury drop of mass 0.05 0.15 g was placed
in a hermetically sealable glass jars thermostated at
25 and 50 C, and 50 ml of a sodium hypochlorite so-
lution with certain acidity was added. The Hg2+ con-
1070-4272/05/7804-0546 2005 Pleiades Publishing, Inc.

A STUDY OF MERCURY DISSOLUTION IN AQUEOUS SOLUTIONS OF SODIUM HYPOCHLORITE 547
The order of the dissolution reaction was deter-
mined for two mercury drops with different masses
by the Van’t Hoff formula
n = (logw1
logw2)/(logc1
logc2),
which includes reaction rates for two concentrations
of the starting substance.
The parameters of the reaction of mercury dissolu-
tion at pH 5.9 8.5 are listed in the table. The kinet-
ic curves plotted as the Hg2+ concentration c vs. time t
(Fig. 1a) and logc vs. t (Fig. 1b) confirm the first re-
action order obtained by calculation, because the time
dependence of the concentration of Hg2+ ions is ex-
ponential, and that of logc, linear (see Fig. 1). The ac-
tivation energies were calculated using the Arrhenius
equation from the dissolution rate constants for 25
and 50 C. The results obtained are listed in the table.
The dissolution of mercury is a heterogeneous pro-
cess in which a kinetic region, with the process rate
determined by the chemical reaction, and a diffusion
region, with the process rate determined by diffusion,
can be distinguished. As a rule, the reaction occurs
in the kinetic mode at low temperatures. As the tem-
perature is raised, the rate constant of the chemical
process rapidly increases and, at a certain temperature
at which the rate constant of the chemical reaction
becomes higher than that of diffusion, the heteroge-
neous process passes from the kinetic to the diffusion
region. It has been noted in the literature [2] that, if
Fig. 1. Kinetic curves of mercury dissolution at 25 C.
2+
(c) Concentration of Hg ions and (t) time. Variation of
2+
the Hg concentration at pH: (1) 6.5, (2) 6.9, (3) 5.9,
and (4) 8.5.
of 2.6 4, which points to a substantial influence ex-
erted by the diffusion process on the dissolution of
mercury. The same is indicated by the temperature
coefficients of mercury dissolution, calculated for
different pH values. The range pH 6 7 corresponds
to the maximum decomposition rate of hypochlorite
and, as a result, to a high rate of mercury oxidation by
active chlorine evolved in the decomposition. There-
fore, the key role in the dissolution process is played
in the given case by diffusion. On passing to the al-
kaline region, the rate of hypochlorite decomposition
markedly decreases and the chemical reaction of ox-
idation starts to exert a noticeable influence on the
process at pH 8.5. The observed increase in the dis-
solution rate on raising the temperature to 50 C is
small as compared with that expected. This is prob-
ably accounted for by a rise in the instability of the
hypochlorite solution with increasing temperature,
which is accompanied by a decrease in the concentra-
1
the activation energy is 5 20 kJ mol , the process
can be identified as occurring in the diffusion mode.
1
If the activation energy is 50 200 kJ mol , then the
heterogeneous process occurs in the kinetic region. In
the intermediate region, the rate of the heterogeneous
process is determined both by diffusion and by the
chemical process at the phase boundary. The calcu-
lated activation energies suggest that the dissolution
of mercury proceeds in the intermediate region, with
its rate determined both by the rate of the chemical
reaction and by the diffusion process.
The temperature dependence of the reaction rate
is characterized by the temperature coefficient defined
as the amount by which the rate increases on raising
the temperature by 10 C. The temperature coefficients
Parameters of the reaction of mercury dissolution
1
k, cm 1 h
E,
kJ mol
Reaction
order
*
are 2 4 for the rate of the chemical reaction,
(kch)T + 10 /(kch)T, and 1.1 1.4 for that of the diffu-
sion process, = (kD)T + 10 /(kD)T, [2]. According to
=
pH
ch
1
25 C
50 C
D
3
3
2
5.90 2.17 10
1.003
1.008
1.010
this rule, an increase in the reaction rate by at least
a factor of 6 would be observed on raising the tem-
perature by 25 C if the dissolution of mercury pref-
erentially occurred in the kinetic region. However,
the reaction rate actually increases by only a factor
2
2
3
6.50 1.18 10 3.65 10
1.24
1.04
4.00
36.15
30.58
44.15
6.90 2.07 10 5.38 10
3
8.50 6.75 10 2.68 10
*
, temperature coefficient of the dissolution process.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 78 No. 4 2005

548
SIZENEVA et al.
1
tion of active chlorine and the resulting decline in the
oxidizing power.
ergies of 30 44 kJ mol indicate that the heteroge-
neous dissolution occurs in the intermediate region,
in which both the chemical reaction and diffusion
affect this process.
It was established experimentally that the reaction
of self-decomposition of hypochlorite occurs at the
maximum rate at pH 6 8.5 and is first-order with
respect to the concentration of hydrogen ions, i.e.,
the self-decomposition rate changes by a factor of
10 upon a unity change in the pH value. Therefore,
the self-decomposition rate increases at pH 6 8.5 by
several orders of magnitude, to become comparable
with the rate of fast reactions [5 7]. The maximum
rate of mercury dissolution is observed at pH 6.5 6.9,
which correlates well with published data for the
maximum rate of hypochlorite decomposition (pH 7)
[8]. The rate constant of mercury dissolution at pH
(3) It was found that the rate of mercury oxidation
by sodium hypochlorite at pH 5.9 8.5 is considerably
lower than the decomposition rate of the hypochlorite
itself.
REFERENCES
1. Unifitsirovannye metody analiza vod (Unified Methods
for Analysis of Water), Lur’e, Yu.Yu., Ed., Moscow:
Khimiya, 1973.
2. Stromberg, A.G. and Semchenko, D.P., Fizicheskaya
khimiya (Physical Chemistry), Moscow: Vysshaya
Shkola, 1973.
2
5.9 8.5 is (0.2 2.0) 10 , i.e., the decomposition
rate of the hypochlorite is many times that of mercury
oxidation. Therefore, in the authors’ opinion, use of
sodium hypochlorite for oxidation of mercury in this
pH range is inadvisable because most part of active
chlorine is wasted without having enough time for
entering into the reaction.
3. Golikov, G.A., Rukovodstvo po fizicheskoi khimii
(Manual of Physical Chemistry), Moscow: Vysshaya
Shkola, 1988.
4. Mel’nikov, S.M., Metallurgiya rtuti (Metallurgy of
Mercury), Moscow: Metallurgiya, 1971.
5. Nikol’skii, B.P., Krunchak, V.G., Pal’chevskii, V.V.,
and Sosnovskii, R.I., Dokl. Akad. Nauk SSSR, 1971,
vol. 197, no. 1, pp. 140 142.
CONCLUSIONS
(1) The rate constants of mercury dissolution in
hypochlorite solutions at pH 5.9 8.5 were calculated.
It was shown that the maximum rate of mercury dis-
solution is observed at pH 6.5 6.9.
6. Nepenin, N.N., Tekhnologiya tsellyulozy (Technology
of Cellulose), Moscow: Goslesbumizdat, 1956.
7. Pozin, M.E., Tekhnologiya mineral’nykh solei (Tech-
nology of Mineral Salts), Moscow: Goskhimizdat, 1961.
(2) It was established that the reaction of mercury
dissolution is first-order. The apparent activation en-
8. Nikol’skii, B.P. and Flis, I.E., Zh. Obshch. Khim.,
1952, vol. 22, no. 8, pp. 1298 1307.
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