Bromide mediated oxidation of antimony(III) by cerium(IV) in aqueous sulphuric acid medium

Bromide mediated oxidation of antimony(III) by cerium(IV) in aqueous sulphuric acid medium

D S Munavalli, K A Thabaj, S A Chimatadar & S T Nandibewoor*

P.G. Department of Studies in Chemistry, Karnatak University, Dharwad 580 003, India Email: stnandibewoor@yahoo.com

Received 18 May 2007; revised 9 August 2007

The micro amount (10⁻³ mol dm⁻³) of bromide mediated oxidation of antimony(III) by cerium(IV) in an aqueous sulphuric acid medium has been studied spectrophotometrically at 25°C and I = 3.10 mol dm⁻³. In the presence of bromide, one mole of antimony(III) requires two moles of cerium(IV). The reaction is first order with respect to [cerium(IV)] and [bromide], whereas the order with respect to [antimony(III)] is less than unity. Increase in [sulphuric acid] accelerates the reaction rate. The order with respect to [H⁺ ion] is less than unity. Added products, cerium(III) and antimony(V) do not have any significant effect on the rate of reaction. The active species of oxidant is identified as H3Ce(SO4)4−, whereas that of reductant as SbBr₂⁺. The possible reaction mechanism is proposed and the activation parameters determined and discussed.

IPC Code: Int. Cl.⁸ C07B33/00

Halides play an important role in our daily life.

Antimony(III) is stable in high acid concentration and its oxidation by different oxidants is reported in acid media¹. The reduction potential² of the couple Sb^V/Sb^III in dilute acid is –0.581 V. In presence of Br⁻, antimony(III) forms several bromide complexes, such as SbBr²⁺, SbBr2

+, SbBr3 , SbBr4

−, SbBr5

2− and SbBr6 3−, similar to that of chloride³, but their role is not understood clearly. Cerium(IV) is a well known oxidant in acid media⁴ having reduction potential² of the couple Ce^IV/Ce^III (1.70 V) and is stable only in high acid concentration. In sulphuric acid and sulphate media, cerium(IV) forms several sulphate complexes⁵, but their role has not received much attention so far. The slow reaction between cerium(IV) and antimony(III) is facilitated by minute amounts⁶ of I⁻ (10⁻⁷ mol dm⁻³), and Cl⁻ (10⁻² mol dm⁻³), and also by Br^– (10⁻³ mol dm⁻³) (present work) in aqueous sulphuric acid. We report herein the Br⁻ mediated oxidation of antimony(III) by cerium(IV) in aqueous sulphuric acid medium.

Materials and Methods

The preparation of solutions and the method of following kinetics were as reported in our earlier study⁶. During the kinetic studies, it was observed that under the present experimental conditions in the absence of Br⁻, the oxidation of antimony(III) by cerium(IV) occurs very slowly, though in a measurable quantities. Hence, during the calculation

of the pseudo-first order rate constants (kc), the uncatalysed rate (ku) has also to be taken in to account. Thus,

kt = kc + ku

and, kc = kt – ku

The rate constants, kc (average of at least four independent kinetic runs) obtained in this way were reproducible within ±5% (Table 1). Regression analysis of experimental data was performed with Microsoft Office Excel-2003 programme.

Results and Discussion

Different sets of concentrations of reactants in 1.0 mol dm⁻³ sulphuric acid, at constant ionic strength, I = 3.10 mol dm⁻³ and at constant catalyst concentrations were kept over 10 h in a closed container under nitrogen atmosphere at 25°C and were analyzed. The analysis indicated that in presence of Br⁻, two moles of cerium(IV) reacted with one mole of antimony(III), as given in Eq. (1).

2Ce (IV) + Sb (III) 2Ce (III) + Sb (V) …(1) The orders with respect to cerium(IV) and antimony(III) concentrations were determined as in earlier work⁶ and found to be unity and less than unity (0.70), respectively.

Br-

The effect of initially added products, cerium(III) and antimony(V) did not have any significant effect on the rate of reaction. At fixed concentrations of catalyst (other conditions remaining constant), the rate constant, kc was found to increase with increasing [sulphuric acid] (Table 2). The in situ [H⁺ ion] in the sulphuric acid – sulphate media was calculated using the known ionization constant⁷ of acid sulphate as reported in an earlier study^4,6. The order with respect to [H⁺] was found to be less than unity in presence of Br⁻. Cerium(IV) is known to form several complexes in acid sulphate media^4,7 such as CeSO4

2+, Ce(SO4)2, Ce(SO4)2HSO4

− and H3Ce(SO4)4

− as shown in equilibria (3)-(6).

Ce⁴⁺+H2O Ce(OH)³⁺+H⁺ KOH …(2) Ce⁴⁺+SO4

2− Ce(SO4)²⁺ K1 …(3) CeSO4

2++SO42−

Ce(SO4)2 K2 …(4)

Table 1 — Effect of variation of cerium(IV) and antimony(III) concentrations on the Br⁻ mediated oxidation of antimony(III) by cerium(IV) at 25°C ([Br⁻] = 5.0×10⁻³, I = 3.10/mol dm⁻³)

10⁴ [Ce(IV)] 10³ [Sb(III)] [H₂SO₄] k_u×10⁴ k_t ×10³ k_c×10³ (mol dm⁻³) (mol dm⁻³) (mol dm⁻³) (s⁻¹) (s⁻¹) (s⁻¹)

0.5 2.0 1.0 6.46 5.04 4.39

1.0 2.0 1.0 6.52 5.12 4.47

2.0 2.0 1.0 6.52 5.05 4.40

3.0 2.0 1.0 6.48 5.15 4.50

4.0 2.0 1.0 6.41 5.03 4.39

5.0 2.0 1.0 6.43 5.02 4.38

1.0 0.50 1.0 3.61 1.88 1.52

1.0 1.0 1.0 4.92 2.97 2.48

1.0 2.0 1.0 6.52 5.12 4.47

1.0 3.0 1.0 7.53 6.61 5.86

1.0 4.0 1.0 8.92 7.50 6.61

1.0 5.0 1.0 9.99 8.67 7.67

Ce(SO4)2+HSO4− HCe(SO4)3− K3 …(5) Ce(SO4)2HSO4−+ HSO4−+H⁺ H3Ce(SO4)4−

K4 …(6)

The Ce(OH)2

2+ species might also be present in these solutions and its concentration varies with acidity. The formation of Ce(OH)2

2+ occurs to a much lesser extent in comparison with others and is therefore neglected. The total [cerium(IV)] is the sum of different cerium(IV) species concentrations, [Ce⁴⁺], [Ce(OH)³⁺], [CeSO4

2+], [Ce(SO4)2], [HCe(SO4)3−

] and [H3Ce(SO4)4−

], the complexes having the cumulative equilibrium constants^4,7, KOH, β1, β2, β3 and β4 as shown in Eq. (7):

[Ce⁴⁺]t =[Ce⁴⁺]f {1+ + β1[SO42−]+ β2[SO42−]² + β3[SO4

2−]²[HSO4−] + β4[SO4

2−]²[HSO4−]²[H⁺]} ... (7) where, KOH = 15, β1 = 3.85×10², β2 = 1.69×10², β3 = 1.01×10² and β4 = 2.03×10².

The approximate concentrations of cerium sulphate complexes can be calculated from the concentrations of dissolved Ce⁴⁺, H⁺, HSO4−

and SO42−

from the equilibria and their constants^4,7 (Table 2). From the Table 2, it is clear that among the concentrations of different cerium(IV) species, the variation of H3Ce(SO4)4−

with [H⁺] shows only parallelism with the variation of rate with [H⁺ ion].

At constant oxidant, reductant acid and at constant ionic strength, [Br⁻] was varied (Table 3). As the [Br⁻] increased, the rate of the reaction also increased.

The order with respect to [Br⁻] was found to be unity.

In presence of Br⁻, antimony(III) is known to form different bromide complexes similar to that of

Table 2 — Variation of different cerium(IV) species* with [H⁺] on Br⁻ mediated oxidation of antimony(III) by cerium(IV) in sulphuric acid medium at 25°C ([Ce(IV)] = 1.0×10⁻⁴; [Sb(III)] = 2.0×10⁻³; [Br⁻] = 5.0×10⁻³; I = 3.10/mol dm⁻³)

H₂SO₄ [H⁺] [SO₄²⁻] [HSO₄⁻] 10² α_o 10 α_OH 10 α₁ 10² α₂ 10² α₃ 10² α₄ k_u ×10⁴ k_t×10³ k_c ×10³ (mol dm⁻³) (mol dm⁻³) (mol dm⁻³) (mol dm⁻³) (s⁻¹) (s⁻¹) (s⁻¹)

0.5 0.287 0.30 0.713 .518 2.71 5.97 7.89 3.36 1.75 3.60 1.48 1.12

0.6 0.415 0.228 0.785 0.711 2.59 6.24 6.28 2.94 2.10 4.15 1.93 1.52

0.7 0.565 0.178 0.835 0.937 2.49 6.42 5.04 2.51 2.39 4.77 2.62 2.14

0.8 0.730 0.144 0.870 11.9 2.43 6.54 4.14 2.15 2.74 5.42 3.23 2.69

0.9 0.906 0.119 0.894 14.5 2.40 2.62 3.47 1.85 3.02 6.03 4.15 3.55

1.0 1.088 0.101 0.912 17.2 2.37 6.68 2.97 1.62 3.23 2.52 5.12 4.47

*αo, αOH,α1, α2, α3, and α4, are the fractions of total cerium(IV) of the species Ce⁴, Ce(OH)³⁺, Ce(SO4)²⁺, Ce(SO4)2 ,HSO4−

and H3Ce(SO4)4−

, respectively.

K_OH [H+ ]

antimony chloride complexes^8,6 having general formula SbBrn+3−n

, n being the number of bromide complexed.

Sb³⁺+Br⁻ SbBr²⁺ K5 …(8) SbBr²⁺+Br⁻ SbBr2+

K6 …(9)

SbBr2

++Br⁻ SbBr3 K7 …(10) SbBr3+Br⁻ SbBr4

− K8 …(11)

SbBr4

- +Br⁻ SbBr5

2− K9 …(12)

SbBr52−+Br⁻ SbBr63− K10 …(13) Since the antimony bromide complexes behave similar to that of antimony chloride complexes, the values of cumulative stability constants of antimony chloride complexes are taken to calculate the approximate concentrations of antimony bromide complexes. The cumulative stability constants β5,β6, β7, β8, β9 and β10 of complexes of equilibria (8) to (13) have values, 1.8×10², 3.1×10³, 1.5×10⁴, 5.3×10⁴, 5.2×10⁴, and 1.3×10⁴, respectively at 25°C.

Approximate concentrations of antimony bromide complexes containing 1 to 6 bromides were calculated using Eq. (14) from the concentrations of the dissolved antimony(III) and bromide with their competing equilibria⁹.

[Sb³⁺]t = [Sb³⁺]f {1+ β5[Br⁻] + β6[Br⁻]² + β7[Br⁻]³ + β8[Br⁻]⁴ + β9[Br⁻]⁵ + β10[Br⁻]⁶} …(14)

Fig. 1 — Effect of Br⁻ ion concentration on Sb(III) species and the rate constant of the Br⁻ mediated oxidation of Sb(III) by Ce(IV) in acid medium at 25°C (Conditions as in Table 3).

Variation in the concentrations of such species with increase in [Br⁻] is shown in Table 3 together with the rates of reaction. These results were utilized to draw Fig. 1 and a parallel is found only between rate of reaction, kc and [SbBr2

+].

At constant concentration of reactants and with other conditions constant, the ionic strength was varied between 1.6 and 3.5 mol dm⁻³ in presence of Br⁻. The rate was found to decrease with increase in ionic strength. A plot of log kc versus I^½ was linear with negative slope. As the acetic acid content of the medium increases from 0-40% (v/v), the rate of the reaction increased in presence of Br⁻. The plot of log kc versus 1/D was linear with positive slope.

Table 3 — Effect of Br⁻ ion concentration on antimony(III) species* and rate constant of the Br⁻ mediated oxidation of antimony(III) by cerium(IV) in acid medium at 25°C ([Ce(IV)] = 1.0×10⁻⁴; [Sb(III)] = 2.0×10⁻³; [H₂SO₄] = 1.0; I = 3.10/mol dm⁻³)

10³[Br⁻] 10² α₅ 10¹ α₆ 10² α₇ 10³ α₈ 10⁴ α₉ 10⁶ α₁₀ 10⁹ α₁₁ k_u × 10⁴ k_t × 10³ k_c ×10³

(mol dm⁻³) (s⁻¹) (s⁻¹) (s⁻¹)

0.0 - - - - - - - 6.50 - -

1.0 0.845 1.52 0.26 0.013 4.48×10⁻⁴ 4.39×10⁻⁵ 1.09×10⁻⁵ 6.50 1.89 1.24

2.0 0.729 2.62 0.9 0.087 6.18×10⁻³ 1.21×10⁻³ 6.06×10⁻⁴ 6.50 2.79 2.14

3.0 0.638 3.44 1.77 0.258 2.74×10⁻² 8.06×10⁻³ 6.04×10⁻⁴ 6.50 3.70 3.05

4.0 0.565 4.07 2.8 0.542 7.66×10⁻² 3.01×10⁻² 3.01×10⁻² 6.50 4.29 3.64

5.0 0.505 4.55 3.92 0.947 0.167 8.21×10⁻² 0.103 6.50 5.12 4.47

6.0 0.456 4.92 5.08 1.47 0.313 0.184 0.276 6.50 6.00 5.35

7.0 0.414 5.21 6.28 2.13 0.526 0.362 0.633 6.50 6.78 6.13

8.0 0.378 5.44 7.5 2.9 0.820 0.643 1.29 6.50 7.46 6.81

9.0 0.347 5.62 8.71 3.79 1.20 1.07 2.41 6.50 8.05 7.40

10 0.320 5.76 9.92 4.80 1.69 1.66 4.16 6.50 8.68 8.03

*α5, α6, α7, α8, α9, α10 andα11 are the fractions of total antimony(III) of the species Sb³⁺ f, SbBr²⁺, SbBr2+

, SbBr3, SbBr4−

, SbBr52−

and SbBr63−

, respectively.

The kinetics were studied at four different temperatures. The rate constant, kc×10³(s⁻¹) was obtained as 3.72, 4.47, 7.11 and 9.45 at 20, 25, 30, and 35°C, respectively. These values lead to the activation parameters, Ea = 51±2 kJ mol⁻¹, ∆H^# = 48±2 kJ mol⁻¹, ∆S^# = -129±4 JK^-1 mol⁻¹, ∆G^# = 86±4 kJ mol⁻¹ and log A = 7.0±0.1.

It has been pointed out by Moelwyn-Hughes¹⁰ that in presence of the catalyst bromide, the uncatalyzed and catalyzed reactions proceed simultaneously:

kt = ku + KC [catalyst]^x

where kt is the observed pseudo-first order rate constant in the presence of bromide; ku is the pseudo- first order rate constant in the absence of the catalyst, bromide; KC is the catalytic constant and ‘x’ is the order with respect to bromide.

In the present study, ‘x’ is unity for Br⁻. The catalytic constants, KC for Br⁻ at 20, 25, 30 and 35°C were found to be 1.41, 1.61, 2.61 and 3.57 dm³mol⁻¹s⁻¹, respectively and their corresponding activation parameters are Ea = 50.3±2.0 kJ mol⁻¹, ∆H^#

= 47.9±2.0 kJ mol⁻¹, ∆S^# = -80±3 JK⁻¹mol⁻¹, ∆G^# = 72±3 kJ mol⁻¹ and log A = 9.0±0.2

The cerium(IV) oxidation of antimony(III) is slow in aqueous sulphuric acid. However, the oxidation occurs with reasonable rates in the presence of Br⁻ ion in aqueous sulphuric acid, and has a stoichiometry of 2:1. The order with respect to cerium(IV) and antimony(III) concentrations were found to be unity and less than unity. In the presence of catalyst bromide, as the [sulphuric acid] increased, the rate of reaction also increased and the order with respect to [acid] was found to be less than unity. As discussed already, cerium(IV) forms several sulphate complexes in sulphuric acid and sulphate media^4,7. Among the different sulphate complexes of cerium(IV), only H3Ce(SO4)4

− parallels the rate (Table 2) which indicates that the H3Ce(SO4)4

− is the active species of cerium(IV) in presence of Br⁻. In the presence of bromide, initially added products did not have any significant effect on the rate of reaction.

The increase in [Br⁻] resulted in increase in the rate of reaction also. It has been found here that order with respect to [Br⁻] concentration is unity. As discussed earlier, in the presence of bromide, antimony(III) forms different antimony bromide complexes similar to that of antimony chloride complexes⁸. From the plot of [Br⁻ complexes] versus [Br⁻] and from the plot of kc versus [Br⁻], it is clear that, only the SbBr2

+

species concentration parallels the rate (Fig. 1).

Hence, the species SbBr2

+ is considered as the active species. Thus, the following mechanism (Scheme 1) is envisaged:

β6

SbBr²⁺+Br⁻ SbBr2 +

β4

Ce⁴⁺+2SO42−

+2HSO4−

+H⁺ H3Ce(SO4)4−

SbBr2

++H3Ce(SO4)4

− H3Ce(SO4)4

2- + SbBr³⁺ +Br⁻ {Ce(III)} {Sb(IV)}

SbBr³⁺+H3Ce(SO4)4

− H3Ce(SO4)4

2−+SbBr⁴⁺

{Ce(III)} {Sb(V)}

Scheme 1

In presence of bromide, antimony(III) forms the active species SbBr2

+; cerium(IV) in presence of sulphuric acid–sulphate media forms H3Ce(SO4)4−

as active species in prior equilibrium steps. The formed active species SbBr2

+ and H3Ce(SO4)4

− react together to give the product cerium(III), an intermediate antimony(IV) with the regeneration of bromide ion in a rate determining step. Then in a fast step, the formed intermediate antimony(IV) reacts with another mole of H3Ce(SO4)4

− to give the final products, cerium(III) and antimony(V). Scheme 1 leads to the rate law (15):

−d[Ce(IV)]

k β4 β6[Sb(III)][Ce(IV)] [H⁺]

×[Br⁻] [SO42−

]² [HSO4−

]² Rate =

dt =

{1+β4[SO42−

]² [HSO4−

]²[H⁺]}

×(1+ β6[Sb(III)]) or

Rate

k β4 β6 [Sb(III)] [H⁺] [Br⁻] [SO4 2−]²

×[HSO4−]² [Ce(IV)] = kc = 1+ β4 [SO42−

]² [HSO4−

]² [H⁺] + β6[Sb(III)] + β4 β6(Sb(III) [SO42−

]²

×[HSO4−

]² [H⁺]

…(15) In view of the low concentration of bromide used here, the term β6 [Br⁻] can be neglected in Eq. (15).

Hence, the rate law (15) can be rearranged to Eq.(16):

[Br⁻] 1

kc

= k β4 β6 [Sb(III)] [H⁺] [SO42−

]² [HSO4−

]² + 1

k β6[Sb(III)]

1 1

+ kβ4[SO42−

]² [HSO4−

]²[H⁺] +

k …(16) k

slow fast

According to Eq.(16), the plot of [Br⁻]/kc versus 1/[H⁺] and [Br⁻]/kc versus 1/[Sb(III)] should be linear which has been found to be true (Fig. 2).

The increase in the ionic strength decreases the rate, which qualitatively explains the reaction between two oppositely charged ions. The effect of solvent on the reaction rate is already described in detail¹¹. The increase in the acetic acid content in the reaction medium leads to an increase in rate of reaction, which also supports the reaction between positively and negatively charged ions (Scheme 1).

The oxidation of antimony(III) by cerium(IV) is a non-complementary reaction, and may occur by the intervention of reactive antimony(IV) species. The formation of intermediate antimony(IV) is also in accordance with earlier work¹². No experimental evidence for the formation of antimony(IV) is available, but the experimental data rule out a single two equivalent step.

The Br⁻ ion modifies the reaction path by lowering the energy of activation. A bridge theory is proposed for Br⁻ mediated cerium(IV)–antimony(III) reaction.

The importance of bridging anions in facilitating electron transfer in inorganic reactions is well known.

A chloride ion bridge accelerated mechanism has been found in many exchange reactions¹³ including Ce(IV)–Ce(III) exchange. In all such cases, there are two possibilities for the mechanism. The electron may first be transferred and the anion may then move in the opposite direction; alternatively, chloride atom transfer may take place. Whatever the mechanism, the

oxidation–reduction probably takes place through the formation of bridge. Taube and co workers¹⁴ have demonstrated the transfer of large number of univalent atoms and groups. Apart from the bridge theory, the great tendency of the chloride ion to complex with the reaction product Sb(V) makes conditions more favorable¹⁵ for the reaction. Similar things might also happen in presence of bromide as catalyst. The present reaction is slow in the absence of bromide ion in 1 mol dm⁻³ sulphuric acid, whereas in the presence of bromide the rate is very fast. In the absence of bromide ion, the electron transfer might take place through sulphate bridge¹⁶, which is known to be not very effective. This group is also not transferred, because Sb(V) has no tendency to co- ordinate with SO4

2− and the rate is slow. In the absence of bromide ion, Sb(III) in sulphuric acid is known to exist as SbO⁺. Columbic repulsion between various ions present in solution and SbO⁺ might be another important factor leading to a slower rate in the absence of bromide ion. The activation parameters evaluated for the presence and absence of halides explain the catalytic effect on the reaction. The catalyst, Br- modifies the reaction path by lowering the energy of activation.

Comparison of catalytic effect

The catalytic constant Kc (dm³ mol⁻¹ s⁻¹) for bromide catalysed cerium(IV)–antimony(III) reaction at 25°C is 1.61 and for chloride and iodide catalysed reactions⁶ and the catalytic constants at 25°C are 6.04×10⁻² and 55.11×10⁵, respectively. Thus, the catalytic activity increases in the order of I⁻ > Br⁻ >

Cl⁻ and I⁻ is more efficient catalyst than Br⁻ and Cl⁻. According to bridge theory, the electron transfer or effectiveness of the bridge decreases in the order of I⁻

> Br⁻ > Cl⁻. The difference in the rates of halogen transfer is expected, as the most polarizable ligand must transfer most easily. This type of behavior is already known¹⁷.

Conclusions

The bromide ion mediated cerium(IV) oxidation of antimony(III) in sulphuric acid medium is reported.

The main active species of cerium(IV) and antimony(III) are considered as H3Ce(SO4)4

− and SbBr2

+,respectively, although other species might be active but to much lesser extent. The roles of hydrogen ions are crucial to the reaction. The mechanism is consistent with all the observed experimental results.

Fig. 2 — Verification of rate law (Eq. 15) in the form of Eq. (16) ([Ce(IV)] = 1.0 × 10⁻⁴; [Sb(III)] = 2.0 × 10⁻³; [Br⁻] = 5.0 × 10⁻³ ; I = 3.10 mol dm⁻³).

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