Kinetics and Mechanism of the Oxidation of Thiourea Dioxide by

Article pubs.acs.org/IC

Kinetics and Mechanism of the Oxidation of Thiourea Dioxide by Iodine in a Slightly Acidic Medium Li Xu,†,¶ László Valkai,† Alena A. Kuznetsova,‡ Sergei V. Makarov,‡ and Attila K. Horváth*,† †

Department of Inorganic Chemistry, University of Pécs, Ifjúság útja 6, Pécs H-7624, Hungary Department of Food Chemistry, State University of Chemical Technology, 7 Sheremetevsky Avenue, Ivanovo 153000, Russia

‡

S Supporting Information *

ABSTRACT: The thiourea dioxide (TDO)−iodine reaction was investigated spectrophotometrically monitoring the consumption of total amount of iodine at 468 nm, at T = 25.0 ± 0.1 °C, and at 0.5 M ionic strength in buffered slightly acidic medium. The nitrogen- and carbon-containing products were found to be ammonium ion and dissolved carbon dioxide, respectively, while from sulfur part sulfate ion was exclusively detected, when fresh TDO solution was used. The stoichiometry of the reaction was + − established as 2I2 + TDO + 4H2O → SO2− 4 + 2NH4 + 4I + CO2 + + 4H indicating a strict 2:1 stoichiometric ratio. However, using aged TDO solution this stoichiometric ratio is shifted to lower values suggesting the formation of elementary sulfur augmented + − by the 2TDO + I2 + 4H2O → S + SO2− 4 + 4NH4 + 2I + 2CO2 hypothetical limiting stoichiometry. We also confirmed experimentally that in aqueous solution TDO slowly rearranges into an unindentified species. This species then produces elementary sulfur at a later stage of the aging process via subsequent reactions accounting for a loss of reducing power. The direct reaction between TDO and iodine was found to be relatively rapid and completed within seconds in absence of initially added iodide ion. Formation of the latter ion, however, strongly inhibits the oxidation process; hence, the system is autoinhibitory with respect to iodide ion. Furthermore, increase of pH markedly accelerates the reaction as well. These observations suggest that a short-lived steady-state intermediate (iodinated TDO) is produced in a rapid pre-equilibrium, where iodide and hydrogen ions are also involved. A nine-step kinetic model, to be able to describe the most important characteristics of the experimental curves with four fitted parameters, is proposed and discussed.

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may also form from TDO;12 thus, in principle, it might serve as an oxidizing agent as well. This possibility was later confirmed by theoretical calculations.13 It has recently been shown that under the same condition the chemiluminescence intensity is 23 times higher in case of a luminol−thiourea dioxide system than that of the luminol−H2O2 system.14 All these examples suggest that TDO in alkaline conditions is a versatile compound and is connected to many interesting features. In contrast to this, however, TDO is reported to be quite stable at acidic conditions and decomposes vanishingly slowly.1 Without observing any significant change in its UV spectrum over a couple of hours after dissolving TDO in an aqueous solution gradual drop of pH was observed.15 One of the possible explanations of this effect is the relatively slow tautomerism of TDO to form aminoiminomethanesulfinic acid (NH2NHCSO2H, AIMSA), which has stronger acidic properties than TDO. Further direct evidence that thiourea dioxide solution ages at acidic aqueous conditions and remarkable changes should be encountered during the course of aging

INTRODUCTION Thiourea dioxide ((NH2)2CSO2, TDO) has been extensively used in chemistry and chemical technology as an effective reducing agent despite the fact that its transformation in redox reactions is not yet fully understood.1−3 Even nowadays new applications of this versatile compound have been reported that span a wide range of different areas in chemistry and chemical technology. Without providing an exhaustive survey the following important fields should be emphasized to support this statement: reduction of graphene4 and graphite oxides,5 preparation of nanometer-sized metal powders,6 organocatalysis,7,8 polymerization reactions,9 and even in demonstrating nonlinear dynamical phenomena in chemical kinetics.10,11 Majority of these new fields usually exploits rich chemistry of TDO such as its unusual structure, rearrangement in aqueous solutions, as well as its complex decomposition in different solvents. Focusing on the application of TDO in aqueous solution it is well-known that strong reducing feature of TDO in alkaline condition stems from a controlled release of sulfoxylic acid or sulfoxylate ion by splitting the unusually long C−S bond of the target molecule.1 Despite its well-known reducing capability in certain experimental conditions dioxygen © 2017 American Chemical Society

Received: February 6, 2017 Published: March 24, 2017 4679

DOI: 10.1021/acs.inorgchem.7b00326 Inorg. Chem. 2017, 56, 4679−4687

Article

Inorganic Chemistry

run contained more than 200 absorbance−time data pairs; therefore, it was necessary to reduce the number of time points (∼55−65) to avoid unnecessary and time-consuming calculations. The essence of this method has already been described elsewhere.23 Altogether almost 6500 experimental points from 101 kinetic curves were used for simultaneous evaluation. Our quantitative criterion for an acceptable fit was that the average deviation for the relative fit approached 2%, which is close to the experimentally achievable limit of error of the spectrophotometer.

process has recently been published by Csekö et al. studying the chlorine dioxide−thiourea dioxide reaction.16 Although the mechanism of this reaction was found to be extremely complex17 the most important characteristics of the experimental results could be explained by a slow rearrangement of TDO into a more reactive form followed by a special action of key intermediates bisulfite, hypochlorous acid, and chlorite resulting in an autocatalytic removal of chlorine dioxide. It was clearly demonstrated that these forms of TDO must have significantly different reactivity. Because that reaction was found to be very complex, herein, we report the results of the less complicated thiourea dioxide−iodine reaction. Though the reactions of AIMSA with iodine and iodate18 were already studied, different reactivity of the two forms of thiourea dioxide was completely overlooked in those reports. Furthermore, the reaction of AIMSA with iodate18 exhibits the main characteristics of autocatalysis-driven clock reaction;19,20 therefore, understanding the nonlinear feature of this system eagerly requires the exact knowledge of a firmly established mechanism of the iodine−TDO reaction.

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RESULT Stoichiometry. As a starting point, it is worthwhile to emphasize that sulfinic part of TDO takes place in the redox process; therefore, first, determination of the consumed iodine−TDO ratio was placed in the center of our interest without focusing the fate of the nitrogen- and carboncontaining products. The stoichiometry of the reaction was established by visible spectroscopy from the end of kinetic measurements in an excess of iodine. The calculation indicated that the ratio of consumed iodine and TDO (afterward it is abbreviated as SR) is very close to 2 suggesting the formation of sulfate and iodide ions as products. The results are indicated in

EXPERIMENTAL SECTION

Materials and Buffers. All the chemicals including iodine, TDO (Sigma-Aldrich), acetic acid, sodium acetate, and potassium iodide were of the highest purity commercially available and were used without further purification. Stock solutions were freshly prepared each day from twice ion-exchanged and double-distilled water. Argon gas was bubbled through the solution to protect the stock solution from oxygen except as otherwise stated. The ionic strength was adjusted to 0.5 M in all measurements with sodium acetate as a buffer component. The pH was regulated between 3.6 and 4.4 by adding the appropriate amount of acetic acid taking the pKa of acetic acid as 4.55.21 Temperature of the reaction vessel was maintained at 25.0 ± 0.1 °C. Initial concentrations of TDO and iodine were varied in the ranges of 0.300−3.00 and 0.15−2.9 mM, respectively, and the initial concentration of iodide in all kinetic experiments was kept constant at 0.10 M, except for those studies when effect of initially added iodide ion was investigated at the concentration range between 0.01 and 0.1 M. Effect of the age of stock TDO solutions was also studied in subsequent experiments; the age of TDO solution was varied between 6 min to 5 d. Altogether by this experimental setup the title reaction was investigated at 101 different experimental conditions. Methods and Instrumentation. The reaction was followed by Zeiss S10 and Analytik Jena Specord S600 diode array spectrophotometers within the wavelength range of 400−800 nm without using the deuterium lamp of the photometers. The reaction was performed in a standard quartz cuvette equipped with magnetic stirrer and a Teflon cap having 1 cm optical path. The buffer components followed by the iodine and the potassium iodide solutions were delivered from a pipet. Spectrum of the solution was always recorded just before initiation of the reaction to determine the exact initial iodine concentration. The reaction was started by a rapid addition of the necessary amount of thiourea dioxide stock solution from a fast delivery pipet. The Raman spectrum of the end products was recorded by an NXR FT-Raman spectrometer. 13C NMR spectra of carboncontaining end product and excess of TDO were recorded by a Bruker Avance III 500 spectrometer.

Table 1. Calculated (SR) Defined as ([TI2]0 − [TI2]t)/ ([TDO]0 − [TDO]t) in an Excess of Iodine [TDO]0, mM

[TI2]0, mM

pH

[TDO]t, mM

At

SR

ages of TDO, s

0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.60 0.60 0.60 0.60

1.24 1.24 1.24 1.23 1.22 1.22 1.18 1.21 2.41 2.41 1.80 2.90

4.0 4.0 4.0 4.0 4.0 4.0 4.4 4.0 4.0 3.8 3.8 3.6

0.00288 0.00323 0.00239 0.00233 0.00217 0.00204 1.46 × 10−5 0.00318 0.00779 3.39 × 10−4 0.00805 34.50 × 10−4

0.4745 0.4820 0.4886 0.4876 0.4867 0.4920 0.4254 0.4526 0.9112 0.8962 0.4539 1.2753

2.01 1.97 1.94 1.92 1.88 1.86 2.03 2.01 2.00 2.03 1.99 1.99

362 78 289 170 892 255 062 349 479 431 574 362 362 362 362 362 362

Table 1. It appears to suggest the following governing stoichiometry: (NH 2)2 CSO2 + 2I 2 + 4H 2O → 2NH+4 + CO2 + 4I− + SO24 − + 4H+

(1)

Although at this moment fate of the nitrogen- and carboncontaining products is ambiguousurea or cyanamide may equally be used as well instead of ammonium ion and carbon dioxidewe shall later prove experimentally that ammonium ion and carbon dioxide are the final products in question. One may easily check that the same SR value may be obtained if urea or cyanamide is used as products instead of ammonium ion and carbon dioxide. The stoichiometric ratio mentioned above is, however, valid only when fresh TDO solution is used. As seen SR decreases gradually with an increase of aging time of TDO stock solution indicating an appearance of a side reaction. As it was mentioned above this decrement should be approached from the sulfurcontaining end products. A conceivable explanation of this fact

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DATA TREATMENT In the visible range only iodine and triiodide ion were found to absorb the light; therefore, the isosbestic point of the iodine− triiodide system (λ = 468 nm) was selected for the parameter estimation by ChemMech/Zita program package developed to fit basically unlimited number of experimental series.22 Molar absorbance of iodine and triiodide ion was set to be 750 M−1 cm−1 for the whole calculation process. Originally, each kinetic 4680


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Figure 1. Initial rate studies of the iodine−TDO reaction. Initial conditions are as follows: (A) [TI2]0 = 1.2 mM and [I−]0 = 0.1 M. pH = 4.4 (black), 4.2 (blue), 4.0 (green), 3.8 (cyan), 3.6 (red). (B) [TDO]0 = 0.6 mM and [I−]0 = 0.1 M. pH = 4.4 (black), 4.2 (blue), 4.0 (green), 3.8 (cyan), 3.6 (red). (C) [TDO]0 = 0.8 mM, [TI2]0 = 1.2 mM and pH = 3.8 for the black curve; and [TDO]0 = 0.3 mM, [TI2]0 = 1.2 mM and pH = 3.6 for the blue curve. (D) [TDO]0 = 0.8 mM; [I−]0 = 0.1 M and [TI2]0 = 1.2 mM for the black curve; [TDO]0 = 1.2 mM; [I−]0 = 0.1 M and [TI2]0 = 1.2 mM for the blue curve; [TDO]0 = 0.6 mM; [I−]0 = 0.1 M and [TI2]0 = 2.4 mM for the green curve.

is a preferred formation of aminoiminomethanesulfonic acid (AIMSOA) over sulfate between the aged TDO and iodine that would require SR to be unity.18 A tentative background of this possibility would be the lack of direct reaction between AIMSOA and iodine confirmed by Makarov et al.24 AIMSOA, however, slowly hydrolyzes into sulfite, which then reacts with iodine in a rapid well-known reaction meaning that eventually it gives back again the 2:1 ratio. It indirectly rules out the possibility of formation of AIMSOA as a product. Furthermore, all of our 13C NMR experiments were completely negative for presence of AIMSOA with its characteristic chemical shift. Therefore, it appears that the only remaining feasible possibility is the parallel formation of elementary sulfur and sulfate in the aged TDO solution, since elementary sulfur cannot be oxidizied further by iodine. Majority of our experiments was performed at a relatively low initial TDO concentration (couple of mM or even less), meaning that there was no way to notice directly the appearance of elementary sulfur in those solutions during the course of aging. Therefore, in subsequent experiments we followed the aging process in a more concentrated TDO solution. Figure S1 of the Supporting Information clearly shows that colloidal sulfur precipitation occurs during the aging process. At an almost saturated TDO solution elementary sulfur deposit appears within a couple of hours that can even be seen by naked eye. We thus concluded that eventually TDO produces elementary sulfur in part as a result of the aging process, meaning that the following limiting equation must play a marginal role in determining the overall stoichiometry, when aged TDO solution is used.

2(NH 2)2 CSO2 + I 2 + 4H 2O → 4NH+4 + 2CO2 + 2I− + SO24 − + S

(2)

As seen eqs 1 and 2 establish ammonium ion and carbon dioxide as nitrogen- and carbon-containing end products of the reaction. It seems to be, however, in complete contradiction with the major message of Mambo and Simoyi’s paper stating explicitly that both urea and AIMSOA must be the end product of the reaction.18 As it was already mentioned AIMSOA was ruled out as a conceivable possibility. If urea is really an end product both in iodine and in TDO excesses as well it should be detected by 13C NMR. However, all of our efforts to identify the presence of urea with its characteristic 162.9 ppm chemical shift were unsuccessful, from which we concluded that urea cannot appear (see Figure S2 of the Supporting Information) as a final product. Recently, cyanamide was shown to be a major product of the chlorine−dioxide−thiourea dioxide reaction;17 therefore, we tried to identify it as a possible product in the TDO−iodine reaction as well. Unfortunately, its characteristic 13 C chemical shift at 118.7 ppm has never been found. Since we know that cyanamide readily hydrolyzes in acidic solution to produce urea,25,26 it appears to prove that neither of them is a product of the title reaction (see Figure S2 of the Supporting Information). The only possibility remaining is the formation of carbon dioxide that can easily escape from the solution at pH = 3.0−4.5 explaining the lack of carbon peak of 13C NMR spectrum in excess of TDO besides the characteristic shift of TDO. It clearly supports the idea that under acidic condition oxidation of TDO results in the formation of carbon dioxide and ammonium ion.16 4681


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Inorganic Chemistry Initial Rate Studies. Figure 1 shows the formal kinetic orders of the reactants obtained from log−log plots of initial rates versus the corresponding reactant concentrations. As it is clearly seen the order of the reaction with respect to both TDO and iodine is definitely unity. However, the slope of the straight line in case of log(v0) − log([I−]0) plot is perfectly −2 indicating a very strong inhibitory effect of the product iodide ion within the concentration range studied. In case of hydrogen ion we found the formal kinetic order to be close to −1.5 indicating that increase of pH significantly accelerates the title reaction. These results appear to suggest the following empirical rate law: −

dTI2 dt

= (k[H+] + k′)

[TDO]TI2 − 2

+ 2

[I ] [H ]

= kobs

[TDO]TI2 − 2

Figure 3. Effect of aging the stock TDO solution on the kinetic runs (color dots) at the same initial concentration of the reactants. Initial conditions are as follows: [TDO]0 = 0.3 mM, [TI2]0 = 1.2 mM, [I−]0 = 0.1 M, and pH = 4.0. Age of the stock TDO solutions are as follows (min): 6 (blue); 1305 (green); 2848 (cyan); 4251 (red); 5825 (magenta); 7193 (brown). Corresponding solid lines represent the calculated absorbance−time curves obtained by the kinetic model.

+ 2

[I ] [H ]

(3)

where T I2 is defined as the sum of the equilibrium concentration of triiodide and free iodine. On the basis of eq 3 all the measured kinetic curves can individually be fitted to obtain the apparent rate coefficient kobs. Therefore, we determined kobs for all the kinetic curves by an individual parameter estimation and calculated its average values at each pH. Plotting kobs against the [H+] thus allows us to calculate k and k′ for eq 3 as seen in Figure 2. As a result we obtained k = (3.10 ± 0.10) × 10−7 s−1 and k′ = (5.86 ± 0.13) × 10−11 M s−1, respectively.

Figure 4. Stability of TDO (2.0 mM) in aqueous acidic solution at pH = 4.5 adjusted by acetic acid/acetate buffer. Spectra were acquired at every 2 h (black) within the first day of the aging process, daily (blue) between the age of 2 and 50 d, and every 2 d between 52 and 74 d. The last spectrum was registered (indicated by red line) at 77 d.

Figure 2. Plot of kobs against [H+]. Error bars indicate the standard deviation of the average of kobs values at each pH. The slope of the fitted straight line is (3.17 ± 0.10) × 10−7, and the intercept of that is (5.86 ± 0.13) × 10−11.

spectral changes of TDO solution in buffered medium at pH = 4.5 followed up to 77 d. At a first glance the following important observations should be taken into consideration. As seen in Figure 4 within the first day of aging process two isosbestic points appear at 246 and 281 nms, respectively. At the same time the maximum of this characteristic band is gradually shifted from 269 nm to longer wavelengths. These changes appear to suggest a relatively slow transformation of the initial form of TDO into another species whose reactivity must be different toward the oxidizing agents applied. At this time we may offer at least three possibilities to explain this fact. It is reported that TDO is present in oligomeric form in aqueous solution and that its breakup into monomers takes time due to strong intermolecular hydrogen bonds.27 The second possibility is the slow tautomeric rearrangement of the dioxide form into the aminoimino form.16 Furthermore, it may also be easily conceivable that the stable, unreactive zwitterionic form of aminoiminomethanesulfinic acid slowly rearranges into a more reactive carbenoid structure,28 which then leads to the formation of sulfoxylate ion. Any of these possibilities occurs,

Effect of Aging of TDO in Aqueous Acidic Solution. As it was already pointed out aging of TDO in aqueous acidic solution shifts the stoichiometry via formation of colloidal sulfur precipitation. We also learned from our previous study16 that the species, being formed gradually as the stock TDO solution ages, is much more reactive toward chlorine dioxide, and similar effect may also be involved in the title reaction. Figure 3 describes parallel kinetic runs at exactly the same initial reagent concentrations performed at differently aged TDO stock solution. As seen not only the final absorbance values indicate a notable trend but also the beginning phase of reaction is significantly affected. Intense drops of the initial absorbance upon mixing the reactants appear to suggest a rapid reaction between iodine and the species formed during the course of aging. To establish a feasible model first independent kinetic information is required about the beginning stage of the aging of TDO in aqueous acidic solution. Figure 4 indicates the 4682


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Inorganic Chemistry

procedure with more than 60 different fitted kinetic parameters, and the steps found to be insensitive to the average deviation were eliminated stepwisely. As a result we arrived at the following minimal kinetic model to describe quantitatively the experiments:

the transition between these forms should be relatively slow and must be accompanied by a slight spectral change indicated in Figure 4. Our present kinetic experiments, however, do not provide a solid basis to distinguish between these conceivable options; therefore, in the kinetic model we consider TDOinit and TDOaged as the two different forms of TDO being present right after dissolution and in the aged solution, respectively. For the sake of completeness it should be emphasized that the spectrum having an absorption maximum at 284 nm does not uniquely belong to TDOaged. It is probably a mixture of spectra of different species involved in the aging process of TDO at a longer time scale. Experiments studying the aging of TDO solution were repeated between pH = 3 and 5 as well, to establish the pH dependence of the transition process. All these experiments suggested a similar feature that was observed at pH = 4.5, that is, an appearance of isosbestic points at around 246 and 281 nms at the beginning stage of the aging process; hence, spectral changes at this time period at the maximum of characteristic band of TDO (269 nm) are suitable to characterize the pH dependence of the

CH3COOH ⇌ H+ + CH3COO− −

I2 + I ⇌

(E1)

I−3

(R1)

I 2 + H 2O ⇌ HOI + H+ + I−

(R2)

TDOinit → TDOaged

(R3)

TDOinit + I 2 ⇌ TDOI + H+ + I−

(R4)

TDOaged + I 2 → TDO2 + + 2I−

(R5)

TDOI + 3H 2O → I− + HSO−3 + CO2 + 2NH+4

(R6)

TDO2 + + 3H 2O → HSO−3 + CO2 + 2NH+4 + H+ (R7)

kage

TDOinit ⎯→ ⎯ TDOaged

(4)

TDO2 + + TDOinit + 4H 2O

process. As Figure 5 indicates, in slightly acidic conditions (3 < pH < 5), this reaction seems to be totally independent of pH, and the half-life of TDOinit is ∼5 d.

→ 4NH+4 + 2CO2 + SO24 − + S

(R8)

HSO−3 + I 2 + H 2O → SO24 − + 2I− + 3H+

(R9)

Rate coefficients determined by the nonlinear simultaneous parameter estimation are illustrated in Table 2. Table 2. Fitted and Fixed Rate Coefficients of the Proposed Kinetic Model step R1 R2 R2′ R3 R4 −R4 R5 R6 R6′ R7 R8 R9

Figure 5. Plot of the obtained initial rate of aging the TDO solution at different [H+]. v0 is defined as the initial absorbance change at 269 nm. The intercept of the straight line suggests that kage = 1.55 × 10−6 s−1.

Proposed Kinetic Model. To establish the kinetic model, the following species should be taken into account: the reactants (TDO and iodine), the products (sulfate, elemental sulfur, ammonium, and iodide ion as well as dissolved carbon dioxide). As we have seen previously two different forms of TDO should definitely be involved, TDOinit and TDOaged, whose reactivities are considerably different. It is also evident that reactivity difference between iodine and triiodide ion cannot explain the very strongsecond-orderinhibitory effect of iodide ion. Therefore, we suggest the formation of a short-lived intermediate iodinated-TDO (TDOI) that is formed in a rapid preequilibrium. A similar behavior was observed in case of the polythionate−iodine and arsenous acid−iodine reactions.29−34 We then considered all the conceivable reactions of this species to account for the stoichiometry of the reaction. The rate of the subsequent mono- and bimolecular reactions of TDOI was then supposed to be proportional to the concentration of hydrogen-, hydroxide-, and iodide ion as well. We started the fitting

rate equation −

kR1[I2][I ] k−R1[I3−] kR2[I2] k−R2[HOI][I−][H+] kR2 ′ [I2][H+]−1 k−R2 ′ [HOI][I−] kR3[TDOinit] kR4[TDOinit][I2] k−R4[TDOI][H+][I−] kR5[TDOaged][I2] kR6[TDOI] k′R6[H+]−1[TDOI] kR7[TDO2+] kR8[TDO2+][TDOinit] kR9[HSO3−][I2]

parameter valuea 5.66 × 109 M−1 s−1 8.5 × 106 s−1 0.0552 s−1 1.023 × 1011 M−2 s−1 1.98 × 10−3 M s−1 3.67 × 109 M−1 s−1 (1.0 ± 0.1)×10−6 s−1 1 × 103 M−1 s−1 1 × 109 M−2 s−1 ≫1 × 104 M−1 s−1 114 ± 7 s−1 0.0495 ± 0.0031 M s−1 ≫1 × 103 s−1 kR8/kR7 = 1230 ± 50 M−1 3 × 109 M−1 s−1

a No error indicates that the given parameter is fixed during the calculation process.

The average deviation was found to be 1.4% by a relative fitting procedure. Altogether, only four fitted parameters were used, and the rest of the parameters was either fixed or directly taken from previous reports. Figures 6−9 demonstrate the quality of the fit for representative examples. Supporting Information is also available to depict the sound agreement between the measuread and calculated data in case of all the measured kinetic curves. Discussion. As indicated, Step E1 is only an auxiliary process, necessary to take the slight pH change into account during the course of reaction. The ratio of rate coefficients of 4683


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Figure 6. Experimental (symbols) and calculated (solid lines) kinetic curves. Initial conditions are as follows: TI20 = 1.2 mM, pH = 3.6, [I−]0 = 0.1 M. [TDO]0, mM = 0.3 (black), 0.6 (blue), 0.8 (green), 1.2 (cyan), 1.6 (red), 2.1 (magenta), 3.0 (orange). Age of the TDO stock solution is 6 min.

Figure 9. Experimental (symbols) and calculated (solid lines) kinetic curves. Initial conditions are as follows: [TDO]0 = 0.8 mM, TI20 = 1.2 mM, pH = 3.8. [I−]0, M = 0.01 (black), 0.02 (blue), 0.04 (green), 0.06 (cyan), 0.08 (red), 0.1 (magenta). Age of the TDO stock solution is 6 min.

106 s−1 to give log βI−3 = 2.83, where βI−3 is the formation constant of triiodide ion.21 Step R2 is the hydrolytic equilibrium of iodine in aqueous solution. The rate coefficients of the forward and reverse reactions including the H+-inhibited pathway were determined previously. Therefore, we used these values as fixed ones throughout the whole fitting procedure.37−39 Step R3 is responsible for taking into consideration the aging of TDO stock solution. As it was already pointed out our experiments do not provide a solid basis to identify unambiguously the species involved in this process. It, however, does not prevent us to build a suitable kinetic model, where the initial form of TDO after dissolution is assigned as TDOinit, and TDOaged is considered as a kinetically different form of TDO present in the aged solution. The rate coefficient kR3 was determined to be (1.0 ± 0.1) × 10−6 s−1 based on fitting the experimental kinetic curves. This is in a reasonable agreement with the value (kage = 1.55 × 10−6 s−1) obtained independently from the absorbance−time curves registered during the course of aging the TDO solution at pH = 4.5. For the sake of completeness it should also be mentioned that the value calculated here was found to be an order of magnitude less than the one obtained in the presence of phosphoric acid/ dihydrogen phosphate buffer studying the chlorine dioxide− TDO reaction.23 It appears to suggest that aging of TDO must be affected by the quality of buffers applied. This opportunity will be checked and analyzed in a subsequent paper. Step R4 is a rapidly established equilibrium between the reactants to produce a short-lived intermediate proposed to be iodinated TDO. Since iodide and hydrogen ions are products of this process the inhibitory effect of hydrogen and iodide ions is taken into account by this step. The equilibrium constant of this process cannot be calculated from our experiments; therefore, we set a sufficiently small equilibrium constant (KR4 = 1 × 10−6 M) to provide a low steady-state concentration level for TDOI. Step R5 is an instantaneous reduction of iodine by the kinetically more reactive form of TDO to produce TDO2+ and iodide ions. Formation of ion pairs of different halide ion and trihalide ion with thiourea was recently suggested in case of the thiourea−iodine reaction.40 As an analogy, here, we suggest that similar ion pairs may form in case of the TDO−iodine reaction, but this possibility occurs only in aged TDO solution. Moreover, Biesiada et al. also showed that in part derivatives

Figure 7. Experimental (symbols) and calculated (solid lines) kinetic curves. Initial conditions are as follows: [TDO]0 = 0.6 mM, pH = 4.0, [I−]0 = 0.1 M. TI20, mM = 2.4 (black), 1.8 (blue), 1.2 (green), 0.8 (cyan), 0.6 (red), 0.3 (magenta), 0.15 (orange). Age of the TDO stock solution is 6 min.

Figure 8. Experimental (symbols) and calculated (solid lines) kinetic curves. Initial conditions are as follows: [TDO]0 = 1.2 mM, [I−]0 = 0.1 M, TI20 = 1.2 mM. pH = 4.4 (black), 4.2 (blue), 3.8 (green), 3.6 (red). Age of the TDO stock solution is 6 min.

the rapid forward and reverse reactions was adjusted to give the pKa of acetic acid to be 4.55.21 Step R1 is the well-known fast equilibrium of the formation of triiodide ion studied by several individual research groups.35,36 The rate coefficients of the forward and reverse reactions were set to kR1 = 5.66 × 109 M−1 s−1 and k−R1 = 8.5 × 4684


Article

Inorganic Chemistry ⎛ k6′ ⎞ r = ⎜k 6 + ⎟[TDO][HOI] ⎝ [H+] ⎠

of thiadiazole may form.40 As a result we also sought experimental evidence for possible formation of derivatives of thiadiazole-dioxide with no success. Analysis of 13C NMR spectrum gave no evidence for an appearance of new chemical shifts that may be contributed to this compound. In the proposed model Step R5 is responsible for the rapid drop of iodine concentration at the initial stage of the reaction. Our calculation provided only a lower limit (1 × 104 M−1 s−1) for this rate coefficient. Step R6 is the hydrolysis of the short-lived iodinated-TDO formed in step R4. The rate law of this process was found to be ′ were found vR6 = (kR6 + kR6′[H+]−1)[TDOI], where kR6 and kR6 to be 114 ± 7 s−1 and (4.95 ± 0.31) × 10−2 M s−1, respectively. More explicitly, our kinetic experiments provide a solid basis to calculate KR4kR6 = 1.14 × 10−5 M s−1 and KR4k′R6 = 4.95 × 10−8 M2 s−1. As we shall see later this reaction along with its rate equation explains readily the inhibitory effect of iodide and hydrogen ions. Steps R7 and R8 are rapid transformations of the short-lived intermediate TDO2+. As we can see step R7 produces bisulfite, while step R8 is responsible for the parallel formation of sulfate ion and elementary sulfur. While Step R7 along with steps (R3), (R5), and (R9) leads to the governing stoichiometry indicated by eq 1, the other route slightly shifts the stoichiometric ratio from two to a lower value, as the initial stock solution ages. Since TDO2+ cannot appear during the course of the reaction in detectable amount, the individual rate coefficients kR7 and kR8 cannot be determined from our experiments, only the ratio of kR8/kR7 = 1230 ± 50 M−1 can be calculated. Step R9 is the well-known fast, essentially diffusioncontrolled, reaction between bisulfite and iodine studied thoroughly by Yiin and Margerum.41 The rate coefficient kR9 = 3 × 109 M−1 s−1 was determined in their work, and we fixed it throughout the whole calculation process. A word is also in order here to clarify that although there is no clear information about the real chemical structures of the proposed short-lived intermediates like TDOaged, TDOI, and TDO2+, the steps proposed here compose a reasonable minimal kinetic core model of the system, since elimination of any of these steps would lead to a complete contradiction with the experimental result. A better insight into a molecular level of this system, however, requires further investigation of the chemical processes involved in the aging process of an aqueous TDO solution in a subsequent paper. We must emphasize as well that the present model is working properly if the age of the stock TDO solution is not longer than 3−4 d, since kinetic runs were performed with shorter than 5 d aged TDO stock solution. To extend the model certainly more intimate details of the chemical processes occurring during the course of aging should be enlightened. Last but not least it is also worthwhile to examine whether an alternative kinetic model exists that is also capable of good description of the kinetic data presented. In aqueous iodine solution hypoiodous acid is always present as a result of iodine hydrolysis, and its reactivity toward TDOinit might also play a significant role. Therefore, we tried to remove Step R4 and replace Step R6 with the following reaction:

(6)

The quality of the fit was found to be quite sound with an average deviation of 1.8%. k6 = (5.5 ± 0.1) × 106 M−1 s−1 and k6′ = 1100 ± 50 s−1 parameter values obtained are also plausible ones. We therefore concluded that this kinetic model is also feasible, but the larger average deviation rather suggests that our proposed model is more probable. Thus, Steps R4 and R6 are more preferred indicating that iodine is the kinetically active species. Formal Kinetics. Because steady-state approximation can be applied to all the short-lived intermediates a rate law may easily be derived from the proposed model. Taking into consideration that eqs R3, (R5), (R7), and (R8) altogether play a crucial role to account for the slight stoichiometric shift between eq 1 and eq 2 with respect to the age of stock TDO solution as well as the rapid drop of the initial phase of the kinetic curves, the absorbance−time curves at 468 nm are governed by the following differential equation: −

dTI2 dt

= kR4[TDOinit ][I 2] − k −R4[TDOI][H+][I−] ′ + kR6[TDOI] + kR6

[TDOI] [H+]

(7)

Steady-state approximation for the intermediate TDOI leads to the following expression [TDOI] =

kR4[TDOinit ][I 2] k −R4[H+][I−] + kR6 +

′ kR6 [H+]

(8)

and substitution of eq 8 into eq 7 followed by some algebraic rearrangement yields the following differential equation: −

dTI2 dt

(

2kR4 kR6 + =

′ kR6 [H+]

)[TDO

init ][I 2]

k −R4[H+][I−] + kR6 +

′ kR6 [H+]

(9)

Since all the kinetic curves were recorded in the presence of a large excess of iodide ion, the [I2] = TI2/(1 + KR1[I−]) ≈ TI2/ KR1[I−] approximation can be applied. Substituting it into eq 9 followed by a simplification approved by the inequality of k−R4[H+][I−] ≫ kR6 + kR6 ′ /[H+] we arrive at the final form of the differential equation as −

dTI2 dt

=

′ ) [TDOinit ]TI2 2KR4(kR6[H+] + kR6 KR1 [H+]2 [I−]2

(10)

Comparison of eq 10 with eq 3 clearly explains the hydrogen dependence of the apparent rate coefficient obtained from the individual evaluation of the kinetic curves. Moreover it also reflects to the chemical meaning of k and k′. Calculating the value of k and k′ from the rate coefficients determined by simultaneous evaluation of the kinetic curves leads to k = 2KR4kR6/KR1 = 3.42 × 10−7 s−1 and k′ = 2KR4kR6 ′ /KR1 = 1.49 × 10−10 M s−1, respectively, indicating a sound agreement with the previously calculated values.

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CONCLUSION The work presented here can be considered as a first approach to determine the kinetics and mechanism of the TDO−iodine reaction. It was clearly demonstrated that the main products are

TDOinit + HOI + 2H 2O → I− + HSO−3 + CO2 + 2NH+4 (5)

having the rate equation indicated below 4685


Article

Inorganic Chemistry

(3) Makarov, S. V.; Horváth, A. K.; Silaghi-Dumitrescu, R.; Gao, Q. Sodium Dithionite, Rongalite and Thiourea Oxides: Chemistry and Application; World Scientific Press: Singapore, 2016. (4) Chua, C. K.; Pumera, M. Chemical Reduction of Graphene Oxide: A Synthetic Chemistry Viewpoint. Chem. Soc. Rev. 2014, 43, 291−312. (5) Wang, J.; Zhou, T.; Deng, H.; Chen, F.; Wang, K.; Zhang, Q.; Fu, Q. An Environmentally Friendly and Fast Approach to Prepare Reduced Graphite Oxide with Water and Organic Solvents Solubility. Colloids Surf., B 2013, 101, 171−176. (6) Wang, H.; Li, Q.; Gao, C. Preparation of Nanometer Nickel Powder from Spent Electroless Nickel Plating Baths by Using Thiourea Dioxide as a Green Reductant. J. Cleaner Prod. 2014, 84, 701−706. (7) Sambher, S.; Baskar, C.; Dhillon, R. S. Novel Chemoselective Reduction of Aldehydes in the presence of Other Carbonyl Moieties with Thiourea Dioxide. Synth. Commun. 2008, 38, 2150−2157. (8) Verma, S.; Singh, R.; Tripathi, D.; Gupta, P.; Bahuguna, G. M.; Jain, S. L. Thiourea Dioxide with TBHP: A Fruitful and Greener Recipe for the Catalytic Oxidation of Alcohols. RSC Adv. 2013, 3, 4184−4188. (9) Percec, V.; Popov, A. V.; Ramirez-Castillo, E. Single-ElectronTransfer/Degenerative-Chain-Transfer Mediated Living Radical Polymerization of Vinyl Chloride Catalyzed by Thiourea Dioxide/Octyl Viologen in Water/Tetrahydrofuran at 25 °C. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 287−295. (10) Jones, J. B.; Chinake, C. R.; Simoyi, H. R. Oxyhalogen-Sulfur Chemistry: Oligooscillations in the Formamidinesulfinic Acid− Chlorite Reaction. J. Phys. Chem. 1995, 99, 1523−1529. (11) Yuan, L.; Yang, T.; Liu, Y.; Hu, Y.; Zhao, Y.; Zheng, J.; Gao, Q. pH Oscillations and Mechanistic Analysis in the Hydrogen PeroxideSulfite-Thiourea Reaction System. J. Phys. Chem. A 2014, 118, 2702− 2708. (12) Burgess, E. M.; Zoller, U.; Burger, L. J. Conversion of Thiourea Dioxide to Dioxygen. J. Am. Chem. Soc. 1984, 106, 1128−1130. (13) Kis, Z.; Makarov, S. V.; Silaghi-Dumitrescu, R. Computational Investigations on the Electronic Structure and Reactivity of Thiourea Dioxide: Sulfoxylate Formation, Tautomerism and Dioxygen Liberation. J. Sulfur Chem. 2010, 31, 27−39. (14) Gao, W.; Qi, W.; Lai, J.; Qi, L.; Majeed, S.; Xu, G. Thiourea Dioxide as a Unique Eco-Friendly Coreactant for Luminol Chemiluminescence in the Sensitive Detection of Luminol, Thiourea Dioxide and Cobalt Ions. Chem. Commun. 2015, 51, 1620−1623. (15) Makarov, S. V.; Kudrik, E. V. Tautomerization of Thiourea Dioxide in Aqueous Solution. Russ. Chem. Bull. 2001, 50, 203−205. (16) Csekö, G.; Hu, Y.; Song, Y.; Kégl, T. R.; Gao, Q.; Makarov, S. V.; Horváth, A. K. Kinetic Evidence of Tautomerism of Thiourea Dioxide in Aqueous Acidic Solutions. Eur. J. Inorg. Chem. 2014, 2014, 1875−1879. (17) Hu, Y.; Horváth, A. K.; Duan, S.; Csekö, G.; Makarov, S. V.; Gao, Q. Mechanism Involving Hydrogen Sulfite Ions, Chlorite Ions, and Hypochlorous Acid as Key Intermediates of the Autocatalytic Chlorine Dioxide−Thiourea Dioxide Reaction. Eur. J. Inorg. Chem. 2015, 2015, 5011−5020. (18) Mambo, E.; Simoyi, R. H. Kinetics and Mechanism of the Complex Oxidation of Aminoiminomethanesulfinic Acid by Iodate in Acidic Medium. J. Phys. Chem. 1993, 97, 13662−13667. (19) Xu, L.; Horváth, A. K. A Possible Candidate to be Classified as an Autocatalysis-Driven Clock Reaction: Kinetics of the Pentathionate−Iodate Reaction. J. Phys. Chem. A 2014, 118, 6171−6180. (20) Xu, L.; Horváth, A. K. Autocatalysis-Driven Clock Reaction II: Kinetics of the Pentathionate−Periodate Reaction. J. Phys. Chem. A 2014, 118, 9811−9819. (21) IUPAC Stability Constant Database. Royal Society of Chemistry: London, England, 1997. (22) Peintler, G. ZiTa, A Comprehensive Program Package for Fitting Parameters of Chemical Reaction Mechanisms, Versions 2.1− 5.0, 2001; http://www.staff.u-szeged.hu/~peintler/enprogs.htm.

sulfate and carbon dioxide, ammonium and iodide ions, and the reaction has a strict stoichiometry indicating a 2:1 iodine−TDO ratio, when fresh TDO solution is used. In absence of iodide ion the reaction is very fast. Unfortunately, when TDO solution ages the stoichiometry of reaction is shifted from the theoretical 2:1 ratio to a lower value due to the formation of elementary sulfur produced from the slow decomposition of TDO in acidic conditions. The reaction was found to be strongly inhibited by the iodide ion formed during the course of reaction. We have also provided an experimental evidence that upon dissolving TDO in water a rearrangement starts immediately and proceeds slowly. In agreement with our previous study we found again the reactivity of the original form of TDO and that of the rearranged one significantly differs from each other.23 The chemistry behind this process is still unclear and requires further investigations, but at the same time it evidently indicates that kinetic studies of any reactions of TDO, even in acidic aqueous solution, should be performed with special circumspection, and the reactivity difference between these forms must clearly be taken into consideration. It probably means that reactions of TDO with chlorite, iodate, and bromate should be revisited to clarify the effect of the aging process as well.18,42,43 It subsequently means that the aging effect must be also considered in case of those reactions of thiourea, where TDO, as an intermediate, plays a substantial role.44,45

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00326. Figures containing the measured and fitted kinetic curves, NMR spectra, spectral changes of TDO solution (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Attila K. Horváth: 0000-0002-1916-2451 Present Address ¶

Department of Chemical Engineering and Technology, School of Chemisty, Biology and Materials of Science, East China University of Technology, Nanchang 330013, P. R. China.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support of the Hungarian Research Fund NKFIH-OTKA Grant No. K116591. This work was supported by the GINOP-2.3.2-15-2016-00049 grant and Russian Foundation for Basic Research, Grant No. 16-03-00162. The present scientific contribution is dedicated to the 650th anniversary of the foundation of the University of Pécs, Hungary.

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REFERENCES

(1) Makarov, S. V. Recent Trends in the Chemistry of SulfurContaining Reducing Agents. Russ. Chem. Rev. 2001, 70, 885−895. (2) Makarov, S. V.; Horváth, A. K.; Silaghi-Dumitrescu, R.; Gao, Q. Recent Developments in the Chemistry of Thiourea Oxides. Chem. Eur. J. 2014, 20, 14164−14176. 4686


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Kinetics and Mechanism of the Oxidation of Thiourea Dioxide by

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