Decomposition of Thiourea Dioxide under Aerobic and Anaerobic Conditions in an Aqueous Alkaline Solution

Article abstract of DOI:10.1134/S0036024420100209

Abstract: The kinetics and mechanism of the decomposition of thiourea dioxide in an aqueous alkaline solution under aerobic and anaerobic conditions are established. It is discovered that along with the decomposition of thiourea dioxide molecules with C–S bond cleavage and the subsequent formation of sulfoxyl acid anions, there is a reversible stage of the formation of thiourea and peroxide anions. The rate constants of the indicated stages are determined via mathematical modeling using the experimental data.

Full text of DOI:10.1134/S0036024420100209

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2020, Vol. 94, No. 10, pp. 2038–2041. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Zhurnal Fizicheskoi Khimii, 2020, Vol. 94, No. 10, pp. 1505–1509.
PHYSICAL CHEMISTRY
OF SOLUTIONS
Decomposition of Thiourea Dioxide under Aerobic and Anaerobic
Conditions in an Aqueous Alkaline Solution
a
a,
a
K. S. Nikitin , Yu. V. Polenov *, and E. V. Egorova
a
Ivanovo State University of Chemistry and Technology, Ivanovo, 153000 Russia
e-mail: polyurij@yandex.ru
Received November 18, 2019; revised March 26, 2020; accepted April 17, 2020
*
Abstract—The kinetics and mechanism of the decomposition of thiourea dioxide in an aqueous alkaline solu-
tion under aerobic and anaerobic conditions are established. It is discovered that along with the decomposi-
tion of thiourea dioxide molecules with C–S bond cleavage and the subsequent formation of sulfoxyl acid
anions, there is a reversible stage of the formation of thiourea and peroxide anions. The rate constants of the
indicated stages are determined via mathematical modeling using the experimental data.
Keywords: thiourea dioxide, kinetic model, complex reaction mechanism, electronic absorption spectra
DOI: 10.1134/S0036024420100209
INTRODUCTION
which was listed in the NIST Chemistry WebBook
database. All reagents were of chemically pure grade.
Thiourea dioxide (TDO) or formamidine sulfinic
acid in aqueous solutions is used as a reducing agent
for both organic [1, 2] and inorganic [3–5] com-
pounds. In concentrated aqueous-alkaline solutions,
its activity is so high that it allows the reduction of cad-
mium cations to a metal with a fairly high negative
redox potential [6]. It is known from numerous litera-
ture sources that the high reducing activity of TDO
molecules is explained by their decomposition with
breaking of the weak bonds between carbon and sulfur
Spectrophotometry was used to study the kinetics
of TDO decomposition, since (according to the data
of [18]) the initial material (thiourea dioxide) displays
maximum absorption at λ = 270 nm in the electronic
spectrum, but the reaction products do not absorb in
this region of the spectrum. A LEKISS 2110 UV spec-
trophotometer (Finland) was used to measure elec-
tronic absorption spectra.
In our kinetic experiment, 1 g of NaOH was dis-
and the formation of intermediates: sulfoxyl acid solved in a 100 mL flask. In the second 50 mL flask,
2
2
−
i
−
2
8.6 mg of TDO was dissolved and adjusted to the mark
with distilled water. A reference NaOH solution was
anions (SO ), radical anions (SO ), and dithionite
2
4
−
anions (S O ) [7–12]. These particles that have deposited in one cell of the spectrophotometer, and an
strong reducing properties.
2
alkaline TDO solution prepared prior to the experiment
by adding NaOH solution to the aqueous TDO solution
was deposited in the other. It should be noted that TDO
does not decompose in a neutral solution. To obtain
kinetic curves, changes in the optical density of the solu-
tions over time were recorded automatically at constant
wavelengths. The absorption spectra of solutions of
thiourea dioxide of various concentrations were recorded
earlier at the wavelength of 270 nm corresponding to the
absorption maximum, and the linear dependence of
optical density on concentration was confirmed. The
coefficient of extinction, calculated on the basis of the
Bouguer–Lambert–Beer law, was ε = 563.41.
It should be noted that the stoichiometric mecha-
nism of decomposition of aqueous-alkaline solutions of
thiourea dioxide has been studied in a number of works
[
13–15]. However, they did not determine the rate con-
stants of the individual stages of the process nor perform
a comparative analysis of the kinetics of the decompo-
sition of TDO in air and inert atmospheres. One of the
aims of this work was to solve these problems.
EXPERIMENTAL
Thiourea dioxide was synthesized from thiourea by
oxidizing it with an aqueous solution of hydrogen per-
oxide, according to the procedure described in [16].
The content of the main substance in the final product,
determined iodometrically, was 99.1% [17]. The IR
absorption spectrum corresponded to that of amino-
The sodium hydroxide and TDO solutions were
preliminarily purged with argon under anaerobic con-
ditions, and a kinetic experiment was performed after
closing the cell tightly with a paraffin ribbon.
The presence of dithionite ions was determined
iminomethanesulfinic acid (another name for TDO), qualitatively on an Ecotest-VA device (Russia) using
2038

DECOMPOSITION OF THIOUREA DIOXIDE
2039
2
.5
.0
4
223
2
3
2
1
.5
1.0
270
315
3
2
0
.5
1
1
0
4
0
250
300
350
400
λ, nm
0
2000
4000
6000
τ, s
Fig. 1. Electronic absorption spectra of a solution of
thiourea dioxide under aerobic conditions at different
points in time, min: (1) 0, (2) 17, (3) 20, (4) 42; T = 301 K;
Fig. 2. Semilogarithmic transformation of the kinetic
curve A = f(τ). Aerobic conditions; λ = 270 nm; C
0.13 mol/L; T = 301 K.
=
NaOH
C
= 0.12 mol/L.
NaOH
an electrochemical cell with three electrodes: a
ceramic working electrode, a silver chloride reference
electrode, and a glassy carbon auxiliary electrode. To
obtain the current-voltage dependence in a solution of
thiourea dioxide, 1 mL of an aqueous alkaline solution
of it with a NaOH concentration of 0.25 mol/L and an
initial TDO concentration of 1.7 × 10 mol/L, was
mixed in a cell with 25 mL of Robinson–Britton buffer
solution (pH 9) after holding for 2 min.
2−
−
2
•
2
SO2 + O → SO + O ,
(2)
(3)
2
i
−
2−
2
SO ꢀ S O .
2 2 4
As is shown below, the formation of dithionite is not
observed under anaerobic conditions.
−3
In order to qualitatively confirm the formation of
dithionite anions, we obtained the current voltage
dependence for both the TDO solution and the sodium
dithionite solution. In both cases, the presence of a
maximum in the current–voltage dependence at the
potential of −0.7 V corresponding to the oxidation of
dithionite ions, is also confirmed by the data in [8].
Figure 2 shows the dependence of optical density on
time in semilogarithmic coordinates, which shows that
decomposition under aerobic conditions is described by
a first-order kinetic equation in terms of TDO. The
observed rate constant is 7.14 × 10 s , which is com-
parable to an order of magnitude with the value of the
rate constant indicated in the literature [14].
Figure 3 shows the time dependence of the optical
density obtained at the two wavelengths corresponding
to the absorption maxima of TDO and dithionite. The
kinetic curve with the extremum point for dithionite
confirms that it was an intermediate product of TDO
decomposition.
For comparison, the polarogram was taken of
sodium dithionite while preliminarily mixing a solu-
tion of it in 0.04 mL of distilled water with a concen-
−4
tration of 5.74 × 10 mol/L and 25 mL of Robinson–
Britton buffer solution (pH 9) inside an electrochemi-
cal cell. In all cases, the potential sweep rate was
5
0 mV/s, the initial potential was +200 mV, and the
final potential was ‒1700 mV. All polarograms were
recorded at room temperature.
−
4 −1
RESULTS AND DISCUSSION
Figure 1 shows the electronic absorption spectra at
different points in time during the decomposition of
TDO under aerobic conditions. We can see there was
a drop in the height of the maximum at λ = 270 nm,
which corresponds to the absorption of TDO. A max-
imum that, according to the literature [13], is associ-
It forms in stages (1)–(3), while the required
ated with the formation of dithionite anions, emerges amount of oxygen participates in the reaction due to
and grows at λ = 315 nm.
diffusion from the environment. Its consumption is
possible, due to the shift of the equilibrium of stage (3)
to the right and the interaction between anion radicals
−
Since no isobestic point is observed in the spectrum,
2
−
ions S O formed not from thiourea dioxide molecules
2
4
SO and superoxide according to the reaction
2
−
2
but from sulfoxyl acid anions SO upon interacting with
the oxygen in the air as a result of a sequence of stages:
2
−
2
•
2
−
2−
3
−
2
SO + O + OH → SO + HO .
(4)
–
We may assume that in the absence of atmospheric
1) oxygen (under anaerobic conditions), decomposition
proceeds according to reaction (1) and obeys the first-
(
NH ) CSO + 2OH
2 2 2
(
2
−
→
SO + (NH ) CO + H O,
2 2 2 2
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 94 No. 10 2020

2040
NIKITIN et al.
0
0
0
.6
0
0
0
.6
1
2
.4
.2
0
.4
.2
2
3
1
0
0
2000
4000
6000
τ, s
0
2000
4000
6000
8000
τ, s
Fig. 3. Dependence of optical density on time for TDO
(1) λ = 270 nm) and dithionite ((2) λ = 315 nm) under
anaerobic conditions: C = 0.13 mol/L; T = 301 K.
Fig. 4. Dependence of optical density on time for TDO ((1,
2) λ = 270 nm) and dithionite ((3) λ = 315 nm) under anaer-
(
obic conditions; curve 2, in the presence of a thiourea additive
NaOH
(C
= 5.7 × 10– mol/L) C
4
= 0.12 mol/L; T = 301 K.
NaOH
TM
order kinetic equation for the concentration of Decomposition does not slow under aerobic condi-
thiourea dioxide. However, this assumption is incor- tions, since thiourea can react with the superoxide
rect, as is shown by the experimental data in Fig. 4.
formed in stage (2) and turn into urea.
Using the above results, the considered process was
modeled mathematically under anaerobic conditions
without considering the initial stage of interaction
between sulfoxylate ions and the dissolved oxygen.
Our mathematical model with allowance for the
course of reversible stage (5) and irreversible stage (1) is
presented in the form of a system of differential equations:
First of all, the decomposition of TDO does not
proceed to the end as it does under aerobic conditions.
Second, the first order of TDO concentration is not
observed. Third, dithionite ions still form in the pro-
cess, but their concentration reaches a certain con-
stant value and then remains the same.
The latter can be explained by purging with argon
failing to completely remove all of the dissolved oxygen,
and it reacts according to (2) in the initial stage of the
process. In the future, oxygen does not enter from the
external environment, and the concentration of dithion-
ite ions remains constant. On the kinetic curve for TDO,
a kink is observed at the time of oxygen depletion.
dC(NH2)2CSO2
−
= (k + k )C
1 2 (NH2)2CSO2
dτ
−
k C(NH2)2CSCHO2
−
,
−1
dC(
NH ) CS
2
2
=
k1C(NH2)2CSO2 − k C(NH2)2CSCHO2
−
,
−1
dτ
dCHO2
With a high degree of probability, we may assume
that the drop in the rate of decomposition is associated
with the reversibility of primary stage (1) of the
decomposition of TDO molecules with C–S bond
cleavage, as is observed in neutral and acidic media
−
=
2k1C
− 2k−1C(NH2)2CSC
−
,
(NH2)2CSO2
HO2
dτ
where k is the rate constant of the forward stage (5);
1
k is the rate constant of reverse stage (5); and k is the
[
10]. However, adding urea does not inhibit the
−1
2
rate constant of stage (1).
decomposition process.
The concentration of NaOH was not included in
the system of equations, since it was in excess com-
pared to that of the remaining reagents. The inverse
kinetic problem was solved using the wkinet program
developed at Moscow State University, Department of
Chemistry, chair of Physical Chemistry. The concen-
trations of thiourea dioxide were calculated from the
values of optical density measured at a wavelength of
On the other hand, introducing a thiourea additive
slows the reaction and lowers the final degree of decom-
position of the initial material, as was discovered for the
first time. The decomposition of thiourea dioxide mol-
ecules proceeds in two parallel reactions: one with het-
erolytic cleavage of the C–S bond in the TDO molecule
and the formation of sulfoxylate anions (reaction (1)),
and one with no breaking of C–S bonds and the forma-
tion of thiourea in the reversible reaction
2
70 nm according to the methodology described
above. The initial approximation of rate constant k
1
k1
was estimated from kinetic curve CTDO = f (τ). The ini-
–
−
(
NH ) CSO + 2OH ꢀ (NH ) CS + 2HO . (5)
2
2
2
2 2
2
tial approximations varied for the other constants.
k−1
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 94
No. 10
2020

DECOMPOSITION OF THIOUREA DIOXIDE
2041
and the numerical values of the rate constants of indi-
vidual stages were calculated.
10
REFERENCES
. Yu. V. Polenov, E. V. Egorova, and A. V. Nikolaev, Izv.
Vyssh. Uchebn. Zaved., Khim. Khim. Tekhnol. 51 (5),
8
6
4
0
1
43 (2008).
2
3
4
5
. Yu. V. Polenov and V. V. Budanov, Izv. Vyssh. Uchebn.
Zaved., Khim. Khim. Tekhnol. 31 (8), 66 (1988).
. V. V. Kolesnik, S. V. Makarov, Yu. V. Polenov, et al.,
Radiokhimiya 24, 554 (1982).
. Yu. V. Polenov and V. V. Budanov, Izv. Vyssh. Uchebn.
Zaved., Khim. Khim. Tekhnol. 29 (5), 53 (1986).
. V. V. Budanov, S. V. Ermolina, Yu. V. Polenov, and
I. N. Terskaya, Russ. J. Gen. Chem. 70, 655 (2000).
0
2000
4000
6000
τ, s
Fig. 5. Results from modeling the kinetics of the decompo-
sition of thiourea dioxide under anaerobic conditions.
Points are experimental values; lines, calculations accord-
ing to the kinetic model.
6. Yu. V. Polenov, E. V. Egorova, and G. A. Shestakov,
Russ. J. Phys. Chem. A 92, 53 (2018).
https://doi.org/10.7868/S0044453718010193
7
. A. E. Miller, J. J. Bischoff, and K. Pae, Chem. Res.
Toxicol. 1, 169 (1988).
8
. Yu. V. Polenov, S. V. Makarov, and V. V. Budanov, Izv.
The optimum values of the rate constants were
sought using the gradient approach by finding the
minimum of the sum of the squares of the differences
between the experimental and calculated values of the
TDO concentrations.
Vyssh. Uchebn. Zaved., Khim. Khim. Tekhnol. 29
(
12), 30 (1986).
9. Yu. V. Polenov, A. V. Nikolaev, E. V. Egorova, et al.,
Izv. Vyssh. Uchebn. Zaved., Khim. Khim. Tekhnol. 52
(
5), 82 (2009).
The numerical values of the optimized rate con- 10. Yu. V. Polenov, E. V. Makarova, and E. V. Egorova, Ki-
−
4
−1
stants were equal: k = 3.73 × 10 s , k
=
net. Catal. 55, 566 (2014).
1
−1
−5
−1
https://doi.org/10.7868/S0453881114040121
1
25.6 mol/(L s), k = 4.39 × 10 s . The minimum
2
1
1. Y. V. Polenov, G. A. Shestakov, and E. V. Egorova, Izv.
−9
value of the optimized function was 6.8885 × 10 . A
comparison of the experimental and calculated values
of the concentrations of thiourea dioxide (Fig. 5) tes-
tifies to the adequacy of the proposed kinetic model
for experimental data.
Vyssh. Uchebn. Zaved., Khim. Khim. Tekhnol. 61
(
12), 87 (2018).
https://doi.org/10.6060/ivkkt.20186112.5835
1
2. Y. V. Polenov, E. V. Egorova, and K. S. Nikitin, Izv.
Vyssh. Uchebn. Zaved., Khim. Khim. Tekhnol. 62 (8),
9
5 (2019).
https://doi.org/10.6060/ivkkt.20196208.5961
CONCLUSIONS
1
3. S. A. Svarovsky, R. H. Simoiy, and S. V. Makarov,
J. Chem. Soc., Dalton Trans. 4, 511 (2000).
https://doi.org/10.1039/a907816i
In decomposing thiourea dioxide in an aqueous
alkaline solution, a slowing of the reaction with the
addition of thiourea was observed for the first time. It
was shown that the decay of TDO molecules proceeds
in two parallel directions: with and without carbon–
sulfur bonds. There were substantial differences
between the amounts of decomposition products
formed under aerobic and anaerobic conditions: in the
first case, the reaction proceeds to the end and the
main intermediate product is dithionite anions; in the
second, complete decomposition of TDO is not
observed, and dithionite is formed in negligible
amounts. The kinetics of the decomposition of TDO
under anaerobic conditions was modeled mathemati-
cally on the basis of the experimental kinetic curves,
1
4. Q. Gao, B. Liu, L. Li, and J. Wang, J. Phys. Chem. A
11, 872 (2007).
https://doi.org/10.1021/jp063956q
1
1
5. S. A. Svarovsky, R. H. Simoiy, and S. V. Makarov,
J. Phys. Chem. B 105, 12634 (2001).
https://doi.org/10.1021/jp0122474
1
6. V. V. Budanov, Chemistry and Technology of Sulfonic
Acid-Based Reducing Agents. Rongalit and Its Analogues
(Khimiya, Moscow, 1984) [in Russian].
1
7. Chemical Reagents and Preparations, Ed. by R. P. Lastovskii
(IREA, Moscow, 1963), p. 215 [in Russian].
1
8. Sodium Dithionite, Rongalite, and Thiourea Oxides:
Chemistry and Application, Ed. by S. V. Makarov
(World Scientific, New Jersey, 2017), Vol. 23.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 94 No. 10 2020