Acid-base properties and stability of sulfoxylic acid in aqueous solutions

Article abstract of DOI:10.1134/S0036023610020269

A comparative study of decomposition of sulfoxylic acid S(OH)₂ and sulfoxylate anion and their reactions with formaldehyde in aqueous solutions with different acidity was carried out. Using the obtained results, the pK ₁ value of S(OH)₂ was determined for the first time. Pleiades Publishing, Ltd., 2010.

Full text of DOI:10.1134/S0036023610020269

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2010, Vol. 55, No. 2, pp. 301–304. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © S.V. Makarov, D.S. Sal’nikov, A.S. Pogorelova, 2010, published in Zhurnal Neorganicheskoi Khimii, 2010, Vol. 55, No. 2, pp. 309–312.
PHYSICAL CHEMISTRY
OF SOLUTIONS
Acid–Base Properties and Stability of Sulfoxylic
Acid in Aqueous Solutions
S. V. Makarova, D. S. Sal’nikova, and A. S. Pogorelovab
a Ivanovo State University of Chemistry and Technology, pr. F. Engel’sa 7, Ivanovo, 153000 Russia
b Institute of Solution Chemistry, Russian Academy of Sciences, Akademicheskaya 1, Ivanovo, 153045 Russia
Received November 17, 2008
Abstract—A comparative study of decomposition of sulfoxylic acid S(OH)2 and sulfoxylate anion and their
reactions with formaldehyde in aqueous solutions with different acidity was carried out. Using the obtained
results, the pK1 value of S(OH)2 was determined for the first time.
DOI: 10.1134/S0036023610020269
The chemistry of compounds with low molecular experiments, the ionic strength was 0.1. Sulfoxylate
weight has experienced rapid progress in recent years. was prepared by keeping solutions of TDO in 0.5 M
International conferences are held considering the NaOH an under inert atmosphere for 3 h (TDO is
results of studies in this field, monographs are pubꢀ completely decomposed over this period [3]). Since
lished, the most recent being by Ghosh [1]. This interꢀ sulfoxylate is the only sulfurꢀcontaining product
est is due to the frequent occurrence of these comꢀ formed upon decomposition of thiourea dioxide in
pounds in nature and the important role they play in highly alkaline media [3, 8], it was assumed that its
biochemical processes. The properties of oxygenꢀconꢀ concentration in TDO solutions after the storage is
taining compounds XO and XO2 are exceptionally equal to the initial concentration of thiourea dioxide.
Solutions with different pH values were obtained by
mixing sulfoxylate solutions in 0.5 M NaOH with
buffer solutions with different acidity. The TDO
decomposition rate in solutions was determined by
spectrophotometry (Specord M40 spectrophotomeꢀ
ter) at a 269 nm wavelength, corresponding to the
absorption maximum of TDO.
diverse. Among these compounds, there are both
rather inert, such as CO2, and highly reactive subꢀ
2−
SO2
stances. The latter are exemplified by sulfoxylate
,
an intermediate product of hydrogen sulfide oxidation
[2]. The most convenient way for preparing sulfoxylate
is decomposition of thiourea dioxide ((NH2)2CSO2
,
TDO) in argonꢀsaturated alkaline aqueous solutions
[3, 4] (the nitrogenꢀcontaining reaction product, i.e.,
urea, does not affect the kinetic parameters of the
redox reactions involving sulfoxylate). The use of sulꢀ
foxylate, which is a very potent reducing agent [5], in
reactions with inert molecules and for generation of
metals and complexes of metals in low oxidation states
2−
RESULTS AND DISCUSSION
The pK1 value of sulfoxylic acid was determined in
this study using the known reaction of sulfoxylate with
formaldehyde [9] to give hydroxymethane sulfinate
ꢀ
stable in alkaline media
HOCH2SO2 :
is especially promising [6]. In particular,
is able
SO2
to reduce the methyl viologen cation to its neutral
form [3]. The
K1 value of the SO2H– ion was deterꢀ
NH2NHCSO–2
+CH O
SO2H–(SO22–)
HOCH2SO–2 .
p
(1)
2
mined from the dependence of the rate of reaction of
the methyl viologen monocation with sulfoxylate on
the pH (13.55 0.02 for ionic strength of 1) [7]. Howꢀ
–CH O
2
First, complete decomposition of alkaline soluꢀ
tions of TDO under anaerobic conditions gave a soluꢀ
tion of sulfoxylate with a specified concentration (the
sulfoxylate concentration was taken to be equal to the
initial TDO concentration). Then, a sample of this
solution was added, under anaerobic conditions, to an
alkaline solution containing a tenfold excess of formꢀ
aldehyde. It was shown in special experiments that this
ever, the pK value for sulfoxylic acid S(OH)2 has
remained unknown. This work was performed to fill
this gap. In addition, stability of the sulfoxylic acid in
aqueous solutions was studied.
EXPERIMENTAL
Commercial thiourea dioxide (Aldrich) was used. excess suffices for virtually complete binding of sulfoxꢀ
Solutions with constant ionic strength were prepared ylate. The hydroxymethane sulfinate thus formed was
using reagent grade KCl and/or NaOH. In all kinetic quantifued by iodometric titration in acidic medium
301

302
MAKAROV et al.
Table 1. Dependence of [HMS]exp/[HMS]theor on pH at
298 K
place but also decomposition of the acid does. In
alkaline media, decomposition of sulfoxylate under
anaerobic conditions is very slow; therefore, the
[HMS]exp/[HMS]theor value approaches unity. It is sigꢀ
nificant that, even in weakly acidic medium, the rates
of sulfoxylic acid decomposition and its reaction with
pH
4.3
Time, min
[HMS]exp/[HMS]theor
0.58
5
60
formaldehyde are comparable ([HMS]exp/[HMS]theor
=
120
15
0.58); i.e., sulfoxylic acid is not so highly unstable as it
was believed before [9]. It was noted above that sulfoxꢀ
ylic acid is an intermediate of hydrogen sulfide oxidaꢀ
tion. In recent years, vigorous studies of the possible
role of hydrogen sulfide as a signaling molecule in livꢀ
ing organisms have been carried out [10]. The hydroꢀ
gen sulfide concentrations in brain tissues and in blood
8.1
10.1
13.3
0.81
0.83
0.83
0.75
0.98
0.99
0.87
0.95
0.99
60
120
5
15
are 50–160 and 10–100 mol/L, respectively [10],
µ
120
5
which is much higher than the CO and NO concentraꢀ
tions. Nevertheless, as compared with carbon oxides
and especially nitrogen oxides, the biological activity
of hydrogen sulfide has been little studied. The results
obtained in this work show that the formation of sulꢀ
foxylates should be taken into account for hydrogen
sulfide oxidation.
15
120
[9]. The product of sulfoxylate decomposition, sulfite,
reacts with formaldehyde to give hydroxymethane sulꢀ
fonate, which does not react with iodine in acidic
medium. To eliminate the influence of another possiꢀ
ble sulfoxylate decomposition product, sulfide, a
waterꢀsoluble zinc salt was added to the solution
before titration.
It should also be emphasized that variation of the
timeꢀindependent maximum [HMS]exp/[HMS]theor
value vs. pH is Sꢀshaped (Fig. 1). Using this depenꢀ
dence and procedure [11], the
acid was determined to be 7.97 0.03 at
p
K1value for sulfoxylic
I
= 0.1. It is
significant that this value is close to pH (8.5) at which
the band at 918 cm–1 typical of alkaline solutions of
sulfoxylates disappeared in the IR spectrum [12]. Sulꢀ
foxylic acid is known [13] to exist as the oxyacid
S(OH)2 both in the gas phase and in solution. Hence,
the known rule concerning the ratio of successive
deprotonation constants is applicable:
Using the results of titration, the ratio of experiꢀ
mentally determined and theoretical concentrations
of hydroxymethane sulfinate ([HMS]theor = [sulfoxyꢀ
late]init) was determined (Table 1).
It can be seen from Table 1 that, with an increase in
pH, the [HMS]exp/[HMS]theor ratio and, hence, the
stability of theSII oxygen compounds increase. Eviꢀ
dently, in weakly acidic medium, not only the reacꢀ
tion between formaldehyde and sulfoxylic acid takes
(2)
(3)
HnXOm
Hn−1XOm + H+,K1,
Hn−2XOm + H+,K2.
Hn−1XOm
For sulfoxylic acid, m – n = 0. In this case,
[14]. The difference between the successive
Kvalues is 4–5. Our data confirm the validity of these
pK1 ≈
8.5
p
1
[HMS]exp/[HMS]theor
ratios. Since sulfoxylic acid is the only oxyacid with
m = n = 2 (the table of oxyacids presented in the book
by Cotton and Wilkinson [14] does not at all contain
such compounds), the results obtained in this work are
of obvious interest for the development of acid and
base theory.
1.0
0.9
0.8
0.7
0.6
0.5
On the basis of the obtained and published data on
the
pK of sulfoxylic acid, the following scheme can be
written:
pK1 = 7.97
pK2 = 13.55
(4)
−H+
+H+
−H+
ꢀ
2−
2
ꢀꢀꢀꢁ
ꢂꢀꢀꢀ
ꢀꢀꢀꢁ
ꢂꢀꢀꢀ
+H+
H2SO2
SO H
SO .
2
4
6
8
10
12
14
pH
In the next series of experiments, the rate constants
for sulfoxylate decomposition were obtained. The
experiments were carried out in the following
sequence.
Fig. 1. [HMS] /[HMS]
vs. pH at 298 K.
theor
exp
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 2 2010

ACID–BASE PROPERTIES
303
II
ln([S ])
[SII]
1.65
×
103, mol/L
–6.4
–6.6
–6.8
2
1.50
1.35
1.20
1.05
–7.0
0
60
120
, min
τ
1
2
0
25
50
75
100
125
τ, min
II
Fig. 2. Concentration of sulfoxylic acid and sulfoxylate anions vs. time during decomposition at different pH at 298 K. [S ] =
[S(OH) ] + [SO H ] + [SO22– ]; pH is (
1) 8.11, (2) 7.94.
–
2
2
An alkaline solution of sulfoxylate with a specified
concentration (solution no. 1) was prepared under
anaerobic conditions by a known procedure [3]. Then,
a sample of solution no. 1 was introduced into a buffer
solution (solution no. 2). After some period of time, a
sample of solution no. 2 was added under anaerobic
conditions to 0.1 N NaOH containing a 10ꢀfold excess
of formaldehyde with respect to sulfoxylate (solution
no. 3). Then, solution no. 3 was kept for 20 min, and
the content of the hydroxymethane sulfinate formed
was determined by titration with iodine in acidic
medium in the presence of a zinc salt according to a
known procedure [10]. Since the hydroxymethane
sulfinate concentration does not change during the
titration, the results of titration can be used to consider
the decomposition of sulfoxylic acid and its anions in
solution no. 2.
It was shown that decomposition of sulfoxylic acid
and its anions is adequately described by a firstꢀorder
equation (Fig. 2).
The TDO decomposition is also described by a
firstꢀorder equation. The rate constants for decompoꢀ
sition of TDO and sulfoxylic acid and its anions are
given in Table 2.
From this, one can draw an important and unexꢀ
pected conclusion: at pH 9 under anaerobic condiꢀ
tions, TDO decomposes faster than sulfoxylate. Thus,
TDO decomposition results in accumulation of sulꢀ
foxylate not only in highly alkaline media, as was conꢀ
sidered before [4], but also in weak alkaline solutions
(pH ≈ 9).
ACKNOWLEDGMENTS
This work was supported by the Russian Foundaꢀ
tion for Basic Research and the Romanian Academy
of Sciences (project no. 07ꢀ03ꢀ91687ꢀRAꢀa).
Table 2. Rate constants for decomposition of TDO, sulꢀ
foxylic acid, and sulfoxylate anions as a function of pH
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k
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