Kinetic study on hydrolysis and oxidation of formamidine disulfide in acidic solutions

Article abstract of DOI:10.1007/s11426-011-4378-8

Hydrolysis and oxidation of formamidine disulfide in acidic medium were investigated using high-performance liquid chromatography (HPLC) and mass spectrometry (MS) at 25 °C. By controlling the slow reaction rate and choosing appropriate mobile phase, HPLC provides the unique advantages over other methods (UV-Vis, chemical separation) in species tracking and kinetic study. In addition to thiourea and formamidine sulfinic acid, two unreported products were also detected in the hydrolysis reaction. Mass spectrometry measurement indicates these two products to be formamidine sulfenic acid and thiocyanogen with mass weights of 92.28 and 116.36, respectively. In the oxidation of formamidine disulfide by hydrogen peroxide, besides thiourea, formamidine sulfenic acid, formamidine sulfinic acid, thiocyanogen and urea, formamidine sulfonic acid and sulfate could be detected. The oxidation reaction was found to be first order in both formamidine disulfide and hydrogen peroxide. The rate constants of hydrolysis and oxidation reactions were determined in the pH range of 1.5-3.0. It was found both rate constants are increased with the increasing of pH. Experimental curves of different species can be effectively simulated via a mechanism scheme for formamidine disulfide oxidation, including hydrolysis equilibrium of formamidine disulfide and irreversible hydrolysis of formamidine sulfenic acid.

Full text of DOI:10.1007/s11426-011-4378-8

SCIENCE CHINA
Chemistry
February 2012 Vol.55 No.2: 235–241
doi: 10.1007/s11426-011-4378-8
• ARTICLES •
Kinetic study on hydrolysis and oxidation of formamidine disulfide
in acidic solutions
HU Ying, FENG JiaMin, LI YanWei, SUN YanYan, XU Li, ZHAO YueMin & GAO QingYu*
College of Chemical Engineering, China University of Mining and Technology, Xuzhou 221008, China
Received July 28, 2010; accepted April 8, 2011; published online September 9, 2011
Hydrolysis and oxidation of formamidine disulfide in acidic medium were investigated using high-performance liquid chro-
matography (HPLC) and mass spectrometry (MS) at 25 °C. By controlling the slow reaction rate and choosing appropriate
mobile phase, HPLC provides the unique advantages over other methods (UV-Vis, chemical separation) in species tracking
and kinetic study. In addition to thiourea and formamidine sulfinic acid, two unreported products were also detected in the hy-
drolysis reaction. Mass spectrometry measurement indicates these two products to be formamidine sulfenic acid and thiocyan-
ogen with mass weights of 92.28 and 116.36, respectively. In the oxidation of formamidine disulfide by hydrogen peroxide,
besides thiourea, formamidine sulfenic acid, formamidine sulfinic acid, thiocyanogen and urea, formamidine sulfonic acid and
sulfate could be detected. The oxidation reaction was found to be first order in both formamidine disulfide and hydrogen per-
oxide. The rate constants of hydrolysis and oxidation reactions were determined in the pH range of 1.5–3.0. It was found both
rate constants are increased with the increasing of pH. Experimental curves of different species can be effectively simulated via
a mechanism scheme for formamidine disulfide oxidation, including hydrolysis equilibrium of formamidine disulfide and irre-
versible hydrolysis of formamidine sulfenic acid.
formamidine disulfide, hydrolysis, oxidation, HPLC-MS
frequency quasi-periodic and other complex oscillatory
1 Introduction
phenomena can be observed [18,19]. To uncover the core
oscillatory mechanism underlying such phenomena, exten-
sive efforts [5,6,10,20] were devoted to the study of these
reactions, indicating that formamidine disulfide (FDS,
H2N(=NH)CS-SC(=NH)NH2) is an important intermediate
in the oxidation of TU. Marshall [21] investigated the sta-
bilities of FDS in aqueous solution. The results suggested
that FDS is instable in weak acid medium and could under-
go significant hydrolysis in a short period of time when
pH > 3.0. In 1996, Stedman et al. [22] studied the hydroly-
sis of FDS by UV-Vis spectroscopy and proposed some
hydrolysis products including TU, cyanamide and element
sulfur. Simoyi et al. [20] studied the kinetics of the oxida-
tion reaction between bromate and thiourea, which was
traced by determining Br2 forming in the reactions using the
diode-array detector. They concluded that the reaction pro-
Oxidation of sulfur(-II) species has attracted considerable
attention in the past decades due to their ability in support-
ing extremely rich nonlinear phenomena. As a typical sulfur
-containing compound, thiourea (TU, (H2N)2C=S) can react
with various oxidants such as oxyhalogen compounds
[110], hydrogen peroxide [1114] and OH radicals [15], or
via electrochemical methods [16,17]. Investigations during
the past couple of decades indicated that the oxidation pro-
cesses displayed extremely rich nonlinear phenomena in-
cluding autocatalysis, autoinhibition, bistability, oligooscil-
lations, oscillations, birhythmicity, chaos, and spatiotem-
poral pattern [110]. When TU was oxidized on a platinum
electrode, period-doubling bifurcation, two- and three-
*Corresponding author (email: gaoqy@cumt.edu.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2011
chem.scichina.com www.springerlink.com

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Hu Y, et al. Sci China Chem February (2012) Vol.55 No.2
ceeds through an initial cleavage of the S-S bond to give the
unstable sulfenic acids which are then rapidly oxidized
through the sulfinic acids and sulfonic acids to give sulfate.
However, sulfur species can’t be tracked during their reac-
tion process.
Previous studies have shown that HPLC is an effective
method in the study of the oxidation of thiourea [13,23]. Its
combination with mass spectrum (MS) can determine the
molecular structure of separated species. From the above
statements, investigations on stability and oxidation of
formamidine disulfide in acidic medium are expected to get
further insight into the thiourea oxidation, through the
tracking of different sulfur species.
permanganate solution. Formamidine disulfide (Aldrich,
97%), cyanamide (Fluka, 99%), formamidine sulfinic acid
(FSIA, H2N(=NH)CSO2H, Fluka, > 98%), and thiourea (>
99%) were used without further purification. Formamidine
sulfonic acid (FSOA, H2N(=NH)CSO3H) was prepared ac-
cording to ref. [24].
3 Results and discussion
3.1 Kinetics of FDS hydrolysis
During the oxidation of FDS by hydrogen peroxide, hy-
drolysis of FDS, which may consume reactant FDS, is ex-
pected to occur, and the resulting products would further be
oxidized by H2O2. Therefore, the hydrolysis reaction of
FDS could not be neglected. To evaluate the effect of hy-
drolysis on the oxidation dynamics, kinetics of FDS hy-
drolysis was investigated first.
Figure 1 illustrates the chromatograms recorded during
the hydrolysis of FDS, in which the initial concentration of
FDS is 0.5 mM and pH equals 2.0. As shown in Figure 1(a),
10 min after the reaction started, in addition to the reactant
FDS at the retention time of ca. 4.50 min, another peak
with retention time of ca.7.1 min was observed and identi-
fied to be TU by comparing its retention time to the stand-
ard substance. It was observed that the peak height of TU
significantly increased when the reaction proceeded. As can
be seen in Figure 1(b), three more peaks with characteristic
retention times of 4.80 min, 5.50 min and 6.10 min,
2 Experimental section
The HPLC analysis was conducted on the Agilent 1100
system (Agilent, Santa Clara, CA), which includes one
Model 1379A degasser, a G1311A quaternary pump, a
G1316 column thermostat, a G1315A multiple wavelength
UV-Vis detector and a 20 µL sample loop. The separation
was performed on a Phenomenex Gemini C18 column (5 µm,
250 × 4.6 mm i.d.). In this study, binary mobile phase and
ternary mobile phase were used for HPLC and HPLC-MS,
respectively. For better separation, the mobile phase was
prepared by mixing methanol and 2.5 mM tetrabutylammo-
nium hydroxide (TBAOH) solution in a volumetric ratio of
:
5 95. Its pH was adjusted by [H3PO4]/[NaH2PO4], and the
flow rate was set to be 0.5 mL/min. When the column ef-
fluence was introduced into a mass spectrometer, the mobile
phase composition was changed to methanol, acetonitrile
and buffer (hydrochloric acid) with the volume ratio of
: :
6 26 68. The flow rate was changed to 0.4 mL/min. For
each analysis, the injection volume was 10 µL, and the de-
tection wavelengths for five channels were set to be 200,
214, 220, 235 and 254 nm, respectively. In the HPLC-MS
measurement, the LC column effluent was interfaced with
an LCQ Advantage ion-trap mass spectrometer (Thermo
Finnigan, Waltham, MA) equipped with the electrospray
ionization (ESI) ion source. Both positive- and negative-ion
modes were used to detect different substances. The flow
rates of sheath gas and auxiliary/sweep gas were set to be
40 and 10 arbitrary units (arb), respectively, and the tem-
perature of ion-transfer capillary was 300 °C. The probe
voltage was kept at +3.0 kV in positive-ion mode and 2.0
kV in negative-ion mode. The reaction was conducted in an
enclosed glass vessel. The reaction temperature was kept at
25 °C in a water bath. The pH and ion strength in the mobile
phase were kept the same as the reaction solution. For the
anaerobic reaction, the buffered solutions were bubbled
with nitrogen gas for two hours, followed by sampling un-
der nitrogen atmosphere.
Figure 1 HPLC chromatograms recorded during the hydrolysis of FDS at
different sampling time. Sampling time = 10 min (a) and 76 min (b). UV
detection wavelength = 220 nm, [FDS]0 = 5.0 x 104 M, pH 2.0.
Hydrogen peroxide was titrated with standard potassium

Hu Y, et al. Sci China Chem February (2012) Vol.55 No.2
237
were observed at the sampling time of 76 min. To assign the
three peaks, we injected all the previously reported products
associated with this reaction. However, only the peak at
6.10 min was confirmed to be formamidine sulfinic acid.
Element sulfur has been reported to be an important product
[22]. In our experimental condition of the low initial con-
centration of FDS, there was yellow precipitate formed in
the solution during the reaction which suggests that sulfur
could be produced during this process. In order to observe
the S precipitate formed in the reaction without ambiguity,
the initial concentration of FDS may need to be increased.
To further confirm the hydrolysis intermediates and as-
sign the two unidentified products mentioned above,
HPLC-MS experiment was conducted as well. Figure 2
presents the total ion chromatograms and mass spectra. As
shown in Figure 2(a) and (b), in the positive-ion mode and
negative-ion mode, five chromatographic species were ob-
tained. The mass spectra indicate that components eluted at
4.76, 5.33, 5.87, 6.25 and 6.95 min should be attributed to
formamidine disulfide ([M + H]+ = 151.34, [M + MeOH +
H]+ = 183.49 and [M + ACN + H]+ = 191.87), formamidine
sulfenic acid (FSEA, H2N(=NH)CSOH, [M + H]+ = 93.28
Figure 2 Total ion chromatogram (TIC) of FDS hydrolysis products and the corresponding mass spectra of HPLC-MS. Sampling time = 150 min, [FDS]0
= 1.0 x 103 mol L1, pH 2.0.

238
Hu Y, et al. Sci China Chem February (2012) Vol.55 No.2
and [M + ACN + H]+ = 134.12), thiocyanogen ((SCN)2, [M
+ H]+ = 117.38 and [M + ACN + H]+ = 158.40),
formamidine sulfinic acid ([M +CI] = 143.27), thiourea
([M + H]+ = 77.24, [M + MeOH + H]+ = 109.21 and [M +
ACN + H]+ = 118.03), respectively. By comparing Figures
1 and 2, the same retention sequences for five species (FDS
→ FSEA → (SCN)2 → FSIA → TU) were obtained,
although different compositions of mobile phase were used
in HPLC (UV-Vis detector) and HPLC-MS.
It is noted that formamidine sulfinic acid was clearly ob-
served in the above chromatograms and mass spectra. Con-
sidering the O2 effect in solution, the experiments in nitro-
gen and air atmosphere were performed and no difference
was observed. At different time during FDS hydrolysis, the
determined ratio of [TU] / ([FDS]0-[FDS]) was kept in the
range of 1.251.30, indicating that formamidine sulfinic
acid and Thiourea are produced from disproportionation of
formamidine sulfenic acid. The formation of intermediates
can be explained by the following reactions.
[22], i.e. concentrated HSOH could decompose to sulfur. In
addition, the thiocyanogen could result from FDS deamina-
tion in the acid solution. Figure 3 shows kinetic curves and
typical plots of ln{[FDS]0/[FDS]t} against t. The linear rela-
tionship suggests that FDS hydrolysis undergo as a first
order reaction for a given pH at 25 °C. The rate constants
are summarized in Table 1. These results are consistent with
the previous UV spectroscopy measurements [22]. In sum-
mary, the advantage of the HPLC method over other meth-
ods is that it could simultaneously track different sulfur
species during the FDS hydrolysis.
3.2 Oxidation kinetics of FDS
The oxidation reactions between FDS ([FDS]0 = 0.50 mM)
and H2O2 ([H2O2]0 = 10.00 mM) were investigated at 25 °C
at different pH from 1.50 to 3.00. Figure 4 presents the
chromatograms collected at different reaction time with
fixing pH 2.00. As shown in Figure 4(a), in the initial stage,
two observed peaks were attributed to FDS and H2O2, re-
H2N(NH)CSSC(NH)NH2 + H2O ⇌
(NH2)2CS + H2N(NH)CSOH R1
H2N(NH)CSOH+ H2O → (NH2)2CO + HSOH
R2
Table 1 FDS hydrolysis rate constants at different pH
2H+ + H2N(NH)CSSC(NH)NH2 →NCSSCN + 2NH4+ R3
kh (s1) (HPLC)
6.00 × 106
8.07 × 106
2.83 × 105
7.17 × 105
3.25 × 104
kh (s1) [22]
pH
pH
2H2N(NH)CSOH → H2N(NH)CSO2H + (NH2)2CS R4
The hydrolysis scheme consists of four reactions R1-R4,
in which R1 and R2 are consecutive reactions. Because of S
precipitation during the reaction, we believe R2 is also an
indispensable reaction which agrees with the previous study
1.50
1.82
2.00
2.50
3.00
3.58 × 105
3.98 × 105
9.15 × 105
6.50 × 104
1.91
2.02
2.43
3.38
Figure 3 Kinetic curves of sulfur species and plots of concentration logarithms versus time during the hydrolysis of FDS. (a) Time series of chromatogram
peak-areas at pH 2.00; (b, c) Integrated rate plots of first order rate equation: ln{[FDS]0/[FDS]t} versus time at pH 1.82 (b) and 2.00 (c). UV detection wave-
length = 214 nm and [FDS]0 = 5.0 x 104 M.

Hu Y, et al. Sci China Chem February (2012) Vol.55 No.2
239
Figure 5 Kinetic curves of the reaction and plots of concentration loga-
rithms versus time during the oxidation of FDS. (a) Measured (symbols)
and simulated (lines) time series; (b) integrated rate plots of first order rate
equation: ln{[FDS]0/[FDS]t} versus time. The other conditions are the
same with Figure 4.
Figure 4 HPLC chromatograms collected at different time of the oxida-
tion reaction between hydrogen peroxide and FDS. Sampling time = 2 min
(a) and 53 min (b). UV detection wavelength = 220 nm, [FDS]0 = 5.0 x
104 M and [FDS]0 [H2O2]0 = 1 20, pH 2.0.
:
:
spectively. As the reaction proceeded, several intermediates
including TU, FSEA, FSIA, FSOA and (SCN)2, were de-
tected. Figure 4(b) shows the chromatogram of the reaction
solution sampled at 53 min, in which a larger amount of
FSIA was observed. Obviously, it could be attributed to
direct oxidation of FDS and further oxidation of sulfur spe-
cies produced from the hydrolysis of FDS. The same result
can also be concluded from Figure 5(a), in which concen-
trations of the main species versus reaction time were plot-
ted at pH 2.00. It is noted that the concentration of TU de-
creases gradually after its maximum (about 0.08 mM),
which could be caused by the oxidation of TU by hydrogen
peroxide. With the excess amount of hydrogen peroxide, the
final oxidation product, sulfate, was identified by adding
BaCl2 solution to the acidified reaction solution.
stant was studied as well, and the constants were measured
in the ratio range of 20 to 60. The overall rate constant for
FDS k can be expressed as k = kh + ko[H2O2]n, where kh and
ko are hydrolysis rate constant and oxidation rate constant,
respectively, and n is the order for H2O2. With large excess
of H2O2, the concentration of H2O2 can be mathematically
treated as a constant for limited reaction time, i.e., [H2O2]
n
≈ [H2O2]0 . If we denoted k1 = ko[H2O2]n = ko[H2O2]0 , the
expression of k can be converted to k = kh + k1 in which k1 is
the pseudo-first order apparent rate-constant for FDS oxida-
tion. In the plots of ln(k – kh) versus ln[H2O2]0, the value of
n is determined to be 0.97 from the slope, indicating reac-
tion is also the first order in H2O2. The values of k1 and ko at
different pH can be further calculated according to the
above formula, which are listed in Table 2.
As illustrated in Tables 1 and 2, the pseudo-first order
oxidation rate constant is larger than that of hydrolysis,
Figure 5(b) shows a linear relationship between ln{[FDS]0/
[FDS]t} and reaction time with the slope of k = 2.42 × 104
s1, suggesting the reaction is first order in FDS. With
Table 2 Rate constants of hydrogen peroxide–FDS reaction at different pH a)
:
:
[FDS]0 = 0.50 mM and [FDS]0 [H2O2]0 = 1 20, the values
of k are 0.62 × 104 s1, 1.32 × 104 s1, 2.42 × 104 s1, 3.04
× 104 s1 and 5.82 × 104 s1 at the pH of 1.50, 1.82, 2.00,
2.50 and 3.00, respectively. The results illustrate that the
reaction rate increases with the increase of pH.
k0 (102 M1s1
0.57
k1 (104 s1) b)
0.57
b)
)
pH
1.50
1.82
2.00
2.50
3.00
1.24
1.24
2.14
2.14
Through HPLC-MS analysis of species in the oxidation
of FDS by hydrogen peroxide, urea (NH2)2CO was detected
([M + H]+ = 61.2), consistent with the previous study [19].
The influence of [H2O2]0/[FDS]0 on the reaction rate con-
2.32
2.32
2.57
2.57
a) [H2O2]0 = 0.01 M; b) k1 and k0 are pseudo-first order rate constant
and second order rate constant, respectively.

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Hu Y, et al. Sci China Chem February (2012) Vol.55 No.2
Table 3 The mechanism and rate constant of oxidation of formamidine disulfide by hydrogen peroxide at pH 2.00
No.
R1
Reaction
Rate constants
3.30 × 104 s1
H2N(=NH)CS-SC(=NH)NH2+ H2O ⇌ (H2N)2C=S + H2N(=NH)CSOH
4.80 s1
1.94 × 104 s1
R2
R5
R6
R7
R8
R9
R10
H2N(=NH)CSOH+ H2O → (NH2)2CO + HSOH
H2O2 + (H2N)2C=S → H2N(=NH)CSOH + H2O
H2N(=NH)CS-SC(=NH)NH2+H2O2 → 2H2N(=NH)CSOH
H2N(=NH)CSOH + H2O2 → H2N(=NH)CSO2H + H2O
H2O2 + H2N(=NH)CSO2H → H2N(=NH)CSO3H + H2O
H2N(=NH)CSO3H + H2O → HSO3 + (H2N)2CO + H+
HSO3 + H2O2 → SO42 + H2O + H+
1.50 × 102 M1 s1
2.14 × 102 M1 s1
4.44 × 102 M1 s1
4.27 × 104 M1 s1
7.42 × 106 s1
1.48 × 105 M1 s1
which means that oxidation of FDS is the main reaction in
the presence of excess H2O2 at pH 1.502.50. However,
when the pH is greater than 3.00, the hydrolysis reaction
rate increases greatly with increasing pH and becomes
comparable with that of oxidation reaction.
4 Conclusion
In addition to TU, FSIA and FSOA, the formation of FSEA
and thiocynogen was newly detected by HPLC-MS during
hydrolysis and oxidation of FDS, and the HPLC technique
also provides real time tracking of the above sulfur species
for kinetic analysis. With a 10-step reaction mechanism
including hydrolysis equilibrium of formamidine disulfide
and irreversible hydrolysis of formamidine sulfenic acid,
experimental curves of different species can be effectively
simulated for FDS oxidation. The new results help to un-
cover the complicated mechanism of spatiotemporal
self-organization in the oxidation of TU. It is also helpful to
control the FDS stability in aqueous solution for practical
applications such as precious metal leaching, chemical syn-
thesis, and corrosion protection.
3.3 Mechanistic analysis and simulation
Several intermediates and the final products during the hy-
drolysis and oxidation of FDS were identified using HPLC
and MS. The oxidation of FDS by hydrogen peroxide was
accompanied by hydrolysis reaction, which produced TU,
FSIA, FSOA, two previously unreported intermediates,
FSEA and (SCN)2, and finally (H2N)2CO, SO42 and sulfur.
Since the FSEA peak during oxidation was higher than that
during hydrolysis as shown in Figure 4 in contrary to Figure
1, FSEA was produced not only from hydrolysis but also
from oxidation of FDS and TU. In our separate experiments,
we found that the consumption rates of FSOA in decompo-
sition and oxidation by hydrogen peroxide are of the same
when pH < 3, indicating that direct oxidation of FSOA can
be ignored. Table 3 gives the mechanism scheme for oxida-
tion of FDS, where R1 and R2 are hydrolysis equilibrium of
FDS and irreversible hydrolysis of FSEA, respectively. The
scheme does not include R3 and R4 because concentrations
of thiocynogen and FSEA were very low in FDS hydrolysis.
R5, R6, R7 and R8 are oxidation reactions that produce
FSEA, FSIA and FSOA, respectively. R9 is hydrolysis re-
action of FSOA followed by sulfite oxidation reaction R10.
The rate constants k1, k-1 and k2 have been fitted. Especially,
our fitted k1 value agrees with the values reported by Rábai
et al. [6] and Gao et al. [13] respectively. k5 and k7 were
reasonably adjusted according to pH difference between this
work (at pH 2.0) and the reference (at pH 1.5). k8 and k9
were determined with HPLC [25]. k10 were obtained from
Rabai et al. [26]. By pre-equilibrium approximation, the
rate constants of hydrolysis reactions (R1, R-1 and R2) were
calculated which are consistent with Table 1.
This work was partly supported by the National Natural Science Founda-
tion of China (21073232 & 50921002) and the Fundamental Research
Fund from the Chinese Central University (2010LKHX02).
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