Investigation of lead thiosulfate photolysis in aqueous solutions

Article abstract of DOI:10.1134/S0018143914010056

A model of photolysis of PbS₂O₃ aqueous solutions has been proposed on the basis of identified photolysis products and semiempirical quantum-chemical calculations. The degradation of PbS₂O₃ starts with the dissociation of the sulfur-sulfur bond in the thiosulfate group via photochemical excitation and transition of the system a whole to the activated state, which is decomposed by the solvent. The interaction of the primary photolysis products with PbS₂O₃ results in the formation of final products.

Full text of DOI:10.1134/S0018143914010056

ISSN 0018ꢀ1439, High Energy Chemistry, 2014, Vol. 48, No. 1, pp. 37–41. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © N.B. Egorov, 2014, published in Khimiya Vysokikh Energii, 2014, Vol. 48, No. 1, pp. 57–62.
PHOTOCHEMISTRY
Investigation of Lead Thiosulfate Photolysis in Aqueous Solutions
N. B. Egorov
Tomsk Polytechnic University, pr. Lenina 30, Tomsk, 634050 Russia
eꢀmail: egorov@tpu.ru
Received August 7, 2013
Abstract—A model of photolysis of PbS O aqueous solutions has been proposed on the basis of identified
2
3
photolysis products and semiempirical quantumꢀchemical calculations. The degradation of PbS O starts
2
3
with the dissociation of the sulfur–sulfur bond in the thiosulfate group via photochemical excitation and
transition of the system a whole to the activated state, which is decomposed by the solvent. The interaction
of the primary photolysis products with PbS O results in the formation of final products.
2
3
DOI: 10.1134/S0018143914010056
The photolysis of crystalline lead thiosulfate The procedure of the precipitate formation was
(
PbS O3) results in the formation of lead sulfite repeated many times to accumulate an amount suffiꢀ
2
(
PbSO3) and elemental sulfur (S) [1], whereas the cient for analysis. The precipitate formed was sepaꢀ
photolysis of aqueous solution of the lead thiosulfate rated by centrifugation, washed three times with water
complexes (Na2(x – 1)[Pb(S O ) ], where the number of and ethanol, and dried in a vacuum desiccator.
2
3 x
the ligands х varies from two to four depending on the
Xꢀray diffractograms were recorded on a D8 DISꢀ
COVER diffractometer with CuK radiation (
solution concentration and on the thiosulfate to lead
ion molar ratio taken for synthesizing the compound)
yields lead sulfide (PbS) and S [2–4]. These findings
suggest that water is an immediate participant, not an
inert medium, in the photochemical degradation of
lead thiosulfate.
λ
=
α
0.154056 nm).
IR spectra were recorded on a Nicolet 6700 Fouꢀ
–1
rierꢀtransform IR spectrometer in the 400–4000 cm
range. Samples were prepared by pelletizing with KBr.
The IR spectra of PbS O3 solutions were recorded in
2
–1
The objective of this work was to study the photolꢀ the range of 600–4000 cm using an ATR attachꢀ
ysis products of PbS O aqueous solutions and to ment.
2
3
reveal the role of water in the photochemical transforꢀ
mation of lead thiosulfate.
Geometry optimization and molecular dynamics
calculations were performed with the use of the
HyperChem software. The molecular mechanics
method and the PM3 semiempirical quantum chemꢀ
istry method were used. By molecular mechanics calꢀ
EXPERIMENTAL
Lead thiosulfate PbS O was prepared as described culation, an optimized model of PbS O3 was obtained
2
3
2
in [5]. PbS O is sparingly soluble in water and alcohol
2
3
by, which was next surrounded with 27 water moleꢀ
cules (“periodic box” option) and calculated by the
and insoluble in ether. The solubility in water is 6.32
×
–
4
–7
1
0
mol/L (Ksp
=
4
×
10 ) [6]. Saturated aqueous PM3 method.
solutions of PbS O were used in the photolysis experꢀ
2
3
iments.
RESULTS AND DISCUSSION
The source of UV radiation was a highꢀpressure
mercury lamp.
Electronic absorption spectra of the initial and
photolyzed solutions of PbS O3 are presented in
2
Photolysis, and recording of electronic absorption
spectra, and determination of a particle size were conꢀ
ducted in a fusedꢀsilica cell with an optical path length
of 1 cm using an Evolution 600 spectrophotometer.
The particle size and particle size distribution were
determined with a Delsa Nano C analyzer.
An inoLab pH/Ion 740 WTW ionmeter with a Senꢀ
Tix 81 combination electrode was used for pH meaꢀ
surements.
Fig. 1. The spectrum of the initial PbS O3 solution is
2
characterized by the presence of two absorption bands
in the ultraviolet region: the 215ꢀnm band due to
charge transfer from thiosulfate ions to solvent moleꢀ
cules and the thiosulfate to lead ion chargeꢀtransfer
band of the at 256 nm [7]. The irradiation of a PbS O
solution results in a decrease in the intensity of these
bands, which is accompanied by the emergence of
absorption in the region of 300–900 nm without a disꢀ
2
3
A solution (20 mL) was placed under the mercury tinct maximum, with the solution taking a brown
lamp and irradiated for 10 min at room temperature. color.
37

38
EGOROV
A
pH
5.5
A
1.8
1.2
0.6
0
3
2
1
2
'
5.0
1'
4.5
4.0
3.5
230
345
λ
, nm
5
4
3
2
1
0
2
00
400
600
800
λ
, nm
0
2
4
6
8
10
t, min
Fig. 1. Electronic absorption spectra of the PbS O soluꢀ
2
3
tions: (
) 6 min. Inset: electronic spectra of (
ꢀhexane, and ( ') compounds extracted with
from the solid product of PbS O photolysis.
1
) initial; and irradiated for (
2
) 1, (
') elemental sulfur
ꢀhexane
3) 2, (4) 4, and
(
in
5
1
n
2
n
Fig. 2. Changes in pH of the PbS O solution during its
2 3
photolysis.
2
3
The formation of colored solutions with a broad
Thus, the identified products of the photolysis of
0
absorption band in the visible spectral region indicates PbS
that photolysis results in a PbS colloid solution [2]. and
According to the particleꢀsize distribution curves, a
fraction of particles of a ~240 nm size forms a peak
O
H
aqueous solutions are PbS, PbSO
.
, PbSO
, S ,
2
3
4
3
+
To elucidate the structure of PbS O3 in aqueous
solutions, we measured its IR spectrum and compared
2
during the initial time (10 s) of the photolysis. As the
photolysis time increases (5 min), the average particle
size increases to ~550 nm. At a longer irradiation time
with the IR spectrum of crystalline PbS O3 (Fig. 4,
2
curve
4,
5). As shown in [7] the
ν
a(SO) vibration mode
is the most useful for determining the structure of thioꢀ
(
10 min), the photolysis is characterized by a broad
size distribution of particles from 500 nm to 10
m,
which have no aggregation stability and precipitate 1130 cm (Sꢀcoordinated), ~1130 cm (ionic
–
1
sulfate compounds: >1175 cm (Sꢀbridging), 1175–
µ
2
3
−
–1
–1
S
O
2
);
–1
from the bulk of the solution.
and <1130 cm (Oꢀcoordinated). A displacement of
–
1
the
ν
s(SO) vibration frequency above 1000 cm indiꢀ
The initial PbS O solutions are slightly acidic.
2
3
cates coordination through the sulfur atom, and a shift
Photolysis results in buildup of hydrogen ions in the
solution (Fig. 2).
–1
below 1000 cm suggests coordination through the
oxygen atom.
The treatment with nꢀhexane of the PbS O3 phoꢀ
2
tolysis product isolated from the solution by centrifuꢀ
gation showed that the electronic absorption spectrum
The frequency values in the PbS O3 IR spectra
2
indicate that the thiosulfate group is coordinated to
the lead atom through the sulfur and oxygen and manꢀ
ifests itself as a bidentate ligand. The results are conꢀ
of the extract contain bands characteristic of dissolved
0
S
[8] (Fig. 1, inset).
firmed by the data on the PbS O3 crystal structure as
2
reported in [9]. It was found that the lead atom in
Figure 3 presents the Xꢀray diffraction pattern of
the solid product obtained after 10ꢀmin irradiation of
PbS O3 coordinates six oxygen atoms and three sulfur
2
atoms.
PbS O aqueous solutions. There are reflections
2
3
attributed to PbS, PbSO4, and PbSO3
.
The thiosulfate group does not undergo significant
changes upon dissolution of PbS O3 in water as shown
2
The IR study of the precipitate (Fig. 4, curve
1)
by the positions of its stretching vibration bands
showed the presence of absorption bands at 1169,
ν
a(SO) and s(SO), which change only slightly. This
ν
1
056, and 597 cm–1 characteristic of PbSO4 (Fig. 4, indicates that PbS O3 in water exists in the form of
2
–1
curve
2
) and bands at 959, 882, 629, and 480 cm
). These results [10]. It is also noteworthy that the splitting of the antiꢀ
of the IR study are consistent with the Xꢀray diffracꢀ symmetric stretching vibrations in the IR spectrum of
tion data. the PbS O3 aqueous solution becomes more proꢀ
neutral molecule, in agreement with published data
characteristic of PbSO3 (Fig. 4, curve
3
2
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2014

INVESTIGATION OF LEAD THIOSULFATE PHOTOLYSIS IN AQUEOUS SOLUTIONS
00
39
1
50
0
2
0
40
60
2
θ
Fig. 3. Diffractogram of the solid product of 10ꢀmin photolysis of an aqueous PbS O solution.
2
3
4
1
2
5
3
1500
1000
500
ν
, cm–1
1200
1000
ν
, cm–1
Fig. 4. IR spectra of (1) the product of 10ꢀmin photolysis of an aqueous PbS O solution, (2) PbSO , (3) PbSO , (4) PbS O
2 3 4 3 2 3
aqueous solution, and (5) crystalline PbS O .
2 3
HIGH ENERGY CHEMISTRY
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2014

40
EGOROV
Table 1. S–S bond lengths (
pounds
d
) in some thiosulfate comꢀ reveal possible routes of its photodegradation in aqueꢀ
ous solutions and in the solid state. In the quantumꢀ
chemical experiment, the results of calculation of the
bond lengths in the suggested structures were comꢀ
pared. The results of the IR study—coordination of
the lead atom by the sulfur and oxygen atoms of the
Compound
PbS O
dS–S, Å
2.110 [9]
2
3
Na S O
⋅
5H O
2.024 [12]
2.015 [13]
2.056 [14]
1.964 [15]
2.050 [16]
2.060 [17]
1.750 [18]
2.045 [19]
thiosulfate group—were used for building the spatial
model of PbS O3
2
2
3
2
.
2
NiS O · 6H O
2
3
2
Consideration of the sequence of transformations
was based on the assumption that the S S bond is the
CdS O · 2H O
2
3
2
−
BaS O · H O
weakest bond in the thiosulfate group and it is from its
2
3
2
dissociation that the degradation of PbS O begins
2
3
NaAgS O · H O
2
3
2
upon photochemical excitation. This assumption is
Na Au(S O ) · 2H O
indirectly confirmed by the fact that aqueous PbS O
3
2
3 2
2
2
3
solutions undergo partial decomposition yielding PbS
during their longꢀterm storage in the light or in the
dark. The formation of PbS by hydrolysis would be
unlikely if the S–S bond were strong. In addition, it
Tl S O
2 3
2
Pb (S O ) (CH COO)
2
3
2
3 2
3
was taken into account that the PbS O3 decomposition
2
to the final products is determined by the reactivity of
the solvent, since water molecules can add to the S–S
nounced, which may be related to the enhancement of
nonequivalence of the three thiosulfate oxygen atoms.
6+
2–
bond dissociation sites, both
Figure 5 presents the scheme of transformation of
. The calculated bond lengths are
S and S .
Calculation of the force constants [11] showed that
the S–S bond is the weakest in the thiosulfate ion.
Analysis of the data on the crystal structure of thiosulꢀ hydrated PbS O
2 3
fate compounds showed that the S–S bond in PbS O3 given in Table 2.
2
has a large length (Table 1) and is evidently easy to
break upon excitation.
The calculations showed that the elongation of the
Pb–O bond with simultaneous shortening of the other
The stretching vibrations in the IR spectrum of the bonds take place after the rupture of the S–S bond in
PbS O aqueous solution are slightly shifted toward the thiosulfate group (Fig. 5, structure II). Thus, the
2
3
lower frequencies in comparison with the IR spectrum distance between the sulfur (+6) and sulfur (–2) atoms
of crystalline PbS O (Fig. 4, curves 4, 5). Such a shift and water molecules decreases from 3.454 to 1.703 Å,
2
3
of vibration frequency indicates an increase in the indicating the possible addition of water molecules to
length of bonds between the atoms in the thiosulfate the S–S bond dissociation sites.
group (S–O and S–S) upon transition from the crysꢀ
The addition on waters molecule binding to the
talline to the dissolved state and, hence, their further
central sulfur (+6) and the sulfide sulfur (–2) (Fig. 5,
weakening.
structure III) results in further elongation of the Pb–O
Computational methods were used to explain the bond (from 2.862 to 5.897 Å) and shortening of the
effect of water on the photolysis of PbS O and to Pb–S bond (from 2.304 to 2.243 Å), which actually
2
3
H
O
O
H
O
O
O
Pb
O
S
S
S
O
O
S
O
S
S
Pb
S
O
Pb
I
II
III
Fig. 5. Scheme of the transformations of hydrated PbS O during photolysis: (I) PbS O structure before S–S bond dissociation,
2
3
2 3
(
II) PbS O structure after S−S bond dissociation, and (III) PbS O structure after the S–S bond dissociation and addition of
2
3
2
3
6+
2–
water molecules to S and S
.
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INVESTIGATION OF LEAD THIOSULFATE PHOTOLYSIS IN AQUEOUS SOLUTIONS
41
Table 2. Bond lengths (
tures, which change during photolysis, as calculated by the
semiempirical PM3 method
d) of the suggested PbS O strucꢀ ing a strong bond between the lead and sulfur atoms
2 3
after the dissociation of the S–S bond in the thiosulꢀ
fate group, since the conditions for the formation of
the strong Pb–S bond are less favorable in the solid
state because of the rigidity of the crystal lattice. The
presence of negatively charged oxygen and sulfur ions
d
, Å
d
, Å
d, Å
Bond
in structure I in structure II in structure III
in crystalline PbS O3 creates a field that prevents the
lead and sulfur atoms from approaching one another,
thereby ultimately resulting in enhancement of the
2
Pb–S
2.839
2.098
1.619
1.484
1.465
2.146
2.304
2.862
1.495
1.464
1.482
–
2.243
5.897
1.415
1.431
1.432
–
Pb–O
Pb–O bond and the rupture of the Pb–S bond to give
S–O (1)
S–O (2)
S–O (3)
S–S
0
PbSO3 and
S
, the experimentally proved products [1].
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the lead atom to give PbS and H SO4.
Thus, proceeding from the general concepts of the
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2
3
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2
3
4. Ichimura, M., Narita, T., and Masui, K., Mater. Sci.
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tion of an activated complex, which is hydrolyzed to
5
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2
6. Yatsimirskii, K.B., Zh. Fiz. Khim., 1951, vol. 25, no. 4,
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*
PbS O + h
3
ν
PbS O ,
(1)
(2)
2
2
3
7
. Egorov, N.B., Russ. J. Inorg. Chem., 2010, vol. 55,
no. 2, p. 179.
*
PbS O3 + H O
PbS + H SO .
2 4
2
2
8. Maurice, M.J., Anal. Chim. Acta, 1957, vol. 16, p. 574.
The results obtained by the computational methꢀ
ods explain only the formation of the primary photolꢀ
ysis products PbS and H SO . It is obvious that the
9
. Christensen, A.N., Hazell, R.G., Hewat, A.W., and
O’Reilly, K.P.J., Acta Chem. Scand., 1991, vol. 45,
p. 469.
2
4
0
presence of PbSO , PbSO3, and
S in the photolysis
4
10. Lur’e, Yu.Yu., Spravochnik po analiticheskoi khimii
Analytical Chemistry Handbook), Moscow: Khimiya,
1979.
products is due to secondary reactions.
(
It is known that acidification of Na S O solutions
2
2
3
results in their decomposition with the formation of 11. Kharitonov, Yu.Ya., Knyazeva, N.A., and Goeva, L.V.,
thiosulfuric acid H S O , which is very unstable and
Opt. Spektrosk., 1968, vol. 24, no. 4, p. 639.
12. Lisensky, G.C. and Levy, H.A., Acta Crystallogr. B,
2
2
3
0
can decompose to give
S
and H SO [20].
2
3
+
1978, vol. 34, p. 1975.
With allowanced for the buildup of
H
during the
photolysis in our case (Fig. 2), the following reactions 13. Elerman, Y., Uraz, A.A., and Armagan, N., Acta Crysꢀ
are possible:
tallogr. B, 1978, vol. 34, p. 3330.
1
4. Baggio, S., Pardo, M.I., Baggio, R., and Gonzalez, O.,
PbS O + H SO
2
PbSO + H S O ,
2
(3)
(4)
(5)
3
2
4
4
2
3
Acta Crystallogr. C, 1997, vol. 53, p. 1521.
H S O
2
S + H SO ,
2 3
2
3
15. Aka, Y., Armagan, N., and Uraz, A.A., Z. Kristallogr.
980, vol. 151, p. 61.
,
1
PbS O + H SO
2
PbSO + H S O ,
2
2
3
3
3
2
3
16. Cavalca, L., Mangia, A., Palmieri, C., and Pelizzi, G.,
H SO
2
4
Inorg. Chim. Acta, 1970, vol. 4, p. 299.
7. Ruben, H., Zalkin, A., Faltens, M.O., and Templeꢀ
PbS O
2
PbSO + S,
3
(6)
(7)
3
1
PbS + H SO
2
PbSO + H S.
4 2
4
ton, D.H., Inorg. Chem., 1974, vol. 13, no. 8, p. 1836.
Since the photolysis was run in the presence of air 18. Andersson, J.E. and Bosson, B., Acta Crystallogr. C
oxygen, H2S could be oxidized according the followꢀ
,
1
976, vol. 32, p. 2225.
ing reaction:
19. Christensen, A.N. and Hazell, R.G., Acta Chem.
Scand., 1990, vol. 44, p. 1077.
0
H S + 1/2O
2
S + H O.
2
(8)
2
2
0. Volynskii, N.P., Tiosernaya kislota. Politionaty.
Reaktsiya Vakenrodera (Thiosulfuric Acid. Polythionꢀ
ates. Wackenroder’s Reaction), Moscow: Nauka, 1971.
It is likely that the difference in the photolysis
products between crystalline PbS O and PbS O3
2
3
2
aqueous solutions is due to the impossibility of formꢀ
Translated by L. Brovko
HIGH ENERGY CHEMISTRY
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2014