Kinetics of isoamyl alcohol and aniline inhibited uncatalysed and Ag (I) catalysed autoxidation of S (IV) in acidic Medium

 

A. K. Sharma¹*, R. Sharma², D. S. N. Prasad¹

¹Department of Chemistry, Govt. P. G. College, Jhalawar- 326001, Rajasthan (India)

²Department of Chemistry, S.P.C. Govt. College, Ajmer- 305001, Rajasthan (India)

*Corresponding Author E-mail: sharmaarun423@gmail.com

In this paper we report that  isoamyl alcohol and aniline are good inhibitor for SO₂ oxidation in pH range 4.02-5.25 in the presence of Ag(I). Based on the observed results following rate law given and a free radical mechanism has been proposed.

d[S(lV)]/dt       =    (k₁+ k₂[Ag(I)]) [S(lV)]/1 + B [Organics]

Experiments were carried out at 30≤T°C≤40, 4.02≤pH≤5.25, 1×10⁻³ mol/cm³≤ [S(lV)≤10×10⁻³ mol/cm³, 5×10⁻⁶ mol/cm³≤[Ag(I)]≤2.5×10⁻⁵ mol/cm³, 5×10⁻⁷ mol/cm³≤[ Isoamyl alcohol]≤8×10^-3 mol/cm³,5×10⁻⁷ mol/cm³≤[ Aniline]≤3×10^-4 mol/cm³ . Rate constants and order of reaction were calculated and found pseudo- first order in all cases. The effect of pH and temperature were also discussed. The value of apparent energy were found as 45.16 kJ mol^-1 and 26.43 kJ mol^-1in the presence of isoamyl alcohol and aniline respectively.

 

 

 

1. INTRODUCTION:

The study of air pollution has only been of major interest to the scientific community since the end of World War II. The earliest perceived problems were those related to the incomplete combustion of coal, soot and ash abounded in the major industrial cities of the world. (Dentener et al 1994)[1]. Studies in India and abroad have shown that anthropogenic sources in the atmosphere are the major contributors of SO₂ and NO_x which are transformed in to acids such as HNO₂, HNO₃, H₂SO₃ and H₂SO₄. Several reports on acid rain have shown the contribution of nitric acid and sulphuric acid to decrease the pH of rain water. (Acker et al 2001 [2], 2005 [3]; Brimblecombe1996 [4]).

 

A large number of laboratory and field studies on acid rain chemistry have been carried out globally[5-6]. Turning to our own country, field studies on rain water acidity have been carried out by several workers notably, kulshreshtha et al (1995,1996,2003)[7-9], Banerjee et al (2008)[10], Saxena et al (1991)[11], khemani et al (1989,1994)[12-13] and Gupta et al(2000)[14] The atmospheric oxidation of SO₂ by O₂ in aqueous media has been the subject of numerous studies, and the subject matter of several reviews, monographs and papers, notably by Kuo et al (2006) [15], Lee and Rochelle (1987)[16], Brandt and Van eldik, (1995) [17], Calvert and Stockwell, (1984) [18], Gupta (2002)[19], Huie and Peterson, (1983) [20], Hoffmann and Boyce (1983)[21], Hoffmann and Edward (1975) [22], Hermann, (2000) [23], Manoj et al (2000) [24], Rani et al (1992) [25], Manoj et al (1999) [26], Prasad et al (1991) [27] etc. Sharma et al. studied the inhibiting effect of  Isoamyl alcohol(2015)[28], aniline, (2017)[29],  in the presence of Ag (I) catalyzed autoxidation of SO₂ and report that  both are  influence the SO₂ oxidation in the atmosphere with moderate rates. The inhibiting effect of aliphatic alcohols (ethanol, isopropanol, secondary butanol and benzyl alcohol) on the oxidation of sodium sulphite was then investigated by Alyea and Backstrom (1929)[30] in a chain reaction theory-based study. Since in this paper we present the comparison of isoamyl alcohol and aniline to know better inhibitor for SO₂ oxidation in acidic medium in Ag (I) catalysed reaction. But still combined effect of organic compounds like alcohols and amines are not studied yet so more work is to be need in this area to understand SO₂ inhibition, Since it is planned to study the kinetics of Ag (I) catalysed oxidation of S(IV)  by O₂ in acidic medium  in the presence of two organics i.e.  isoamyl alcohol and aniline to examine  their  effect on the reaction rate.

 

2. EXPERIMENTAL:

The experimental procedure was exactly the same as described earlier [31]. All the chemicals used were AR grade. And their solutions were prepared in doubly distilled water.  The reaction were conducted in 0.15 L Erlenmeyer flask, open to air and follow to passage of atmosphere oxygen. The flask was placed in a beaker which had an inlet at a lower part and an outlet at an outer part for circulating thermostatic water for maintaining the desired temperature 30+1⁰C. The reaction was initiated by adding the desired volume of Na₂SO₃ solution to the reaction mixture containing other additive such as buffer and catalyst. The reaction mixture was stirred continuously and magnetically at 1600+10 rpm to allow the passage of atmospheric oxygen and to save the reaction from becoming oxygen mass transfer controlled. The kinetics was studied in acetate buffered medium in which the pH remained fixed throughout the entire course of reaction.  For this purpose 10 cm³ buffer made from sodium acetate (0.07 mol L^-1) and acetic acid (0.03 mol L^-1) for acidic medium were used (total volume 100 cm³) for obtaining the desired pH. The kinetics were followed by withdrawing the aliquot samples periodically and titrating the unreacted S(IV) iodometrically. The reproducibility of replicate measurements was generally better than 10+1 %. All calculations were performed in MS Excel. (Fig.1)

 

3. PRODUCT ANALYSIS:

The qualitative test shows sulphate to be only oxidation product. For quantitative analysis, the reaction mixture containing catalyst and S(IV) in appropriate buffered solutions were constantly stirred for a sufficiently long time so as to ensure complete oxidation of S(IV). When the reaction was complete then S (VI) estimated gravimetrically by precipitating sulphate ions as BaSO₄ using standard procedure. The product analysis showed the recovery of sulphate to be 98+1%., in all cases in agreement with eq. (1)

 

S(IV)+0.5O₂                                   S(VI)                      (1)

 

4. RESULTS:

4.1 Preliminary Investigation

The kinetics of both uncatalysed and Ag (I) Catalysed,  isoamyl alcohol and aniline inhibited reaction were studied in acidic medium in pH 4.95 and temperature 30 ^oC. In all the cases  the first order dependence of S (IV) was observed in the kinetics data treatment for the determination of  pseudo first order rate constant k₁was calculated from log [S(IV)] versus time, t. The plots were shown in fig 2. From the fig 2 it is observed that both the uncatalysed and Ag(I) catalysed autoxidation of S (IV) reaction are inhibited by isoamyl alcohol and aniline.

 

Fig 1: Schematic diagram of the experimental set up for the kinetic study of SO₂ – O₂ oxidation reaction. 1. Magnetic Stirrer, 2. Water Inlet, 3.Water outlet, 4. Erlenmeyer Flask (Reaction mixture), 5. Thermometer, 6.Thermostat.

 

Fig.2 The disappearance of [S(IV)] with time in air saturated suspensions at [S(IV)] = 2x10^-3 mol dm^-3 at pH = 4.95,        t = 30 ^oC with uncatalysed, Ag (I) catalysed,  isoamyl alcohol and aniline inhibited reaction.

 

 

4.2 Uncatalysed Reaction

Uncatalysed reaction was studied in the absence of Ag (I) and all the solutions were prepared in doubly distilled water.

 

4.3 Dependence of S (IV)

The detail dependence of the reaction rate on [S(IV)] was studied by varying it is in the range 1x10^-3 mol dm^-3 to 4x10^-3 mol dm^-3 at pH = 4.95, t = 30 ^oC in acetate buffered medium. The kinetics was found to be pseudo first order in [S(IV)] and values of k₁ was calculated from  log [S(IV)] v/s time plots which was  linear. The value of first order rate constant k₁ are given in Table- 1 The dependence of reaction rate on  [S(IV)] follows the rate law (2)

 

   -d [S(IV)] /dt= k₁[S(IV)]                                      (2)

 

Table 1 The values of k₁ for uncatalysed reaction at different [S(IV)] at pH = 4.95, t = 30 ^oC CH3COONa = 7x10-2 mol L-1 CH3COOH= 3x10-2 mol L-1

[S(IV)] mol dm-3	(10 3) k1 s-1
0.001	1.04
0.002	1.06
0.003	1.09
0.004	1.09

 

4.4 [Organics] Dependence

The major aim of this study was to examine the effect of  isoamyl alcohol and aniline which are known as organics on the autoxidation of S(IV) in acetate buffer medium and varying the [Organics] from 5x10^-7 mol dm^-3 to 8x10^-3 mol dm^-3 we observed  the rate of the reaction decreased by increasing [Organics] The results are given in Table 2 However the nature of the [S(IV)] dependence in presence of organics did not change and remains pseudo first order. The pseudo first order rate constant k_inh, in the presence of organics was defined by rate law (3)

 

-d [S(IV)] /dt = k_inh [S(IV)]                                                   (3)

 

The values of  k_inh  in the presence of  organics  decreased with  increasing [organics] are given in Table 2 which are in agreement with the rate law (4).

 

k_inh  = k₁/(1+B [organics] )                                                    (4)

 

Where B is inhibition parameter for rate inhibition by organics

 

The equation (4) on rearrangement becomes

1/ k_inh  = 1/ k₁+ B [organics] / k₁                                          (5)

 

In accordance with the equation (5) the plot of  1/ k_IAA  v/s  [Isoamyl alcohol] was found to be linear with non- zero intercept. The values of intercept (1/ k₁)  and slope (B/ k₁) were found to be 1.42x10³   mol dm^-3s and 7.27x10⁷ s at  pH = 4.95, t = 30 ^oC. From these values the value of inhibition parameter B was found to be 5.07x10⁴  mol dm^-3 Similarly  the plot of 1/ k_ANI  v/s  [Aniline] was found to be linear with non- zero intercept. The values of intercept (1/ k₁) and slope (B/ k₁) were found to be 1.53 x 10³ s and 2.86 x 10⁶ mol dm^-3s at  pH = 4.95, t = 30 ^oC. From these values the value of inhibition parameter B was found to be 1.86 x 10³ mol dm^-3

Table 2 The values of k_inh  at different [Organics] at pH = 4.95,  t = 30 ^oC  CH3COONa = 7x10-2  mol L-1 CH3COOH= 3x10-2 mol L-1.

 

4.5 Ag(I) Catalysed Reaction 

At first the kinetics of Ag(I) Catalysed reaction in the absence of inhibitor was studied.

 

4.6 [S(IV)] Variation 

The dependence of S (IV) on reaction rate was studied by varying  [S(IV)] from 1x10^-3 mol dm^-3 to 10x10^-3 mol dm^-3 at two different but fixed Ag(I) of 5x10^-6 mol dm^-3and 1x10^-5 mol dm^-3 at  pH = 4.95, t = 30 ^o The kinetics was found to be first order in [S(IV)] v/s time were linear as shown in Fig 2

 

4.7 Ag(I) variations 

The dependence of Ag(I) on the reaction rate was studied by varying Ag (I)from 5x10^-6 mol dm^-3 to 2.5x10^-5 mol dm^-3 at S(IV) = 2x10^-3 mol dm^-3 pH= 4.95, t= 30 ^o C in acetate buffer medium. The values of first order rate constant k_cat for S(IV) oxidation was determine are shown in fig 3 The nature of dependence of  k_cat on Ag(I) was indicated as two term rate law (6)

 

-d [S(IV)] /dt  = k_cat [S(IV)] =(k₁+k₂[Ag(I)] [S(IV)]            (6)                                                                                  

 

Or  k_cat    = k₁+k [Ag(I)]                                                        (7)

 

From the plot in fig 3 the values of intercept is equal to k₁ and slope is equal to k₂ were found to be 0.72x 10 ¹ s and 8.6 x 10 ^-3 mol  dm^-3 s respectively at pH = 4.5, t = 30 ^oC, in acetate buffered medium.

 

 

Fig.3 The dependence of catalyst concentration at [S(IV)] = 2x10^-3 mol dm^-3 pH = 4.95, t = 30 ^oC, in acetate buffered medium.

 

 

4.8 Variation of pH

Variation of pH was carried out from 4.02- 5.25 at different [S(IV)],  Ag (I),[Isoamyl alcohol], [Aniline]and temperatures. The rate decreases slightly by varying pH is inverse H⁺ ion dependence was observed. From the plot of log k₁ v/s log (H⁺). The order with respect to H⁺ is 0.20 which is a fractional order and can be neglected as shown in Table 3

 

 

4.9 [Organics] Dependence

To know the effect of  isoamyl alcohol and aniline on Ag (I) catalysed autoxidation of S(IV), isoamyl alcohol variation was carried out from 5x10^-7 mol dm^-3 to 3 x 10^-4 mol dm^-3 and  aniline variation was  from 5x10^-7 mol dm^-3 to 8 x 10^-3 mol dm^-3  at two different Ag (I) that is 5x10^-6 mol dm^-3 to 1 x 10^-5 mol dm^-3 but fixed S (IV) = 2x10^-3 mol dm^-3 at pH= 4.95 and temp 30 ^o C. The results indicated that by increasing organics the rate becomes decelerates.

 

 

 

Table 3 Rate of Ag (I) catalysed autoxidation in the presence of Isoamyl alcohol and Aniline

 

 

Fig. 4 Effect of catalyst at [S(IV)] = 2x10^-3 mol dm^-3  (Isoamyl alcohol)= 7.0x10^-5 mol dm^-3, [Aniline]= 5.0x10^-4 mol dm^-3 pH= 4.95 t= 30 ^oC in acetate buffered medium. Depending on the observed results the reaction follows the following rate law (8)

 

 

 

-d[S(IV)] /dt =

(k₁+k₂[Ag(I)] [S(IV)] /1 + B (Organics)                              (8)

 

Where k_inh =   (k₁+k₂[Ag(I)] / 1 + B (Organics)

= k_cat / 1 + B (Organics)                                        (9)

 

 

 

 

 

1/ k_{inh  =}  1 + B (Organics) / k_cat                                          (10)

 

1/ k_{inh  =} 1/ k_{cat   +}    B (Organics) / k_cat                              (11)

 

 

Fig 5 Effect of organics at [S(IV)] = 2x10^-3 mol dm^-3 Ag (I) = 5x10^-6 mol dm^-3  pH = 4.95, t = 30 ^oC, in acetate buffered medium. From this graph the value of B_iaa and B_ani are found as 1.24 x 10⁴  mol dm^-3 and 0.26 x10 ³mol dm^-3  respectively .

 

 

4.10 Effect of temperature

The values of k_obs were determined at three different temperatures in the range of 30 ^oC to 40 ^oC The results are given in Table 5.  By plotting a graph between log k v/s 1/t  yield us an apparent empirical energy of activation 45.16 kJ mol^-1 and 26.43  kJ mol^-1 in the presence of isoamyl alcohol and aniline respectively.

 

 

Table 5 Effect of temperature k_obs air saturated suspensions at [S(IV)] = 2x10^-3 mol dm^-3  at Ag (I) = 5 x10^-6 mol dm^-3   [Isoamyl alcohol]= 7.0x10^-5 mol dm^-3 [Aniline]= 5.0x10^-4 mol dm^-3 pH = 4.95.

t oC	10 3 kiaa S-1	10 3 kani S-1
30	0.373	0.686
35	0.458	0.781
40	0.605	0.915

 

5. DISCUSSION: 

SO₂ is present in four forms SO₂.H₂O, HSO₃^-1, SO₃^-2, S₂O₃^-2 In the experimental range of pH = 4.05-5.25 the following equilibrium operates

 

HSO₃^-1                         H⁺     +     SO₃^-2                       (12)

 

In this range of pH both species HSO₃^-1, SO₃^-2 are present but former one present predominantly. The order of reaction was 0.20 indicates that it is almost independent of pH which is co-relate with the work of Irena – Wilkkosz, (2008)[32]. Gupta et al(2008,2011-2012) [33-35] studied that ammonia which is present in atmospheric water in trace amount are also contribute in inhibition of SO₂. Q.Li et al (2014, 2017)[36-37] studied the magnesium sulphite oxidation by cobalt ions and multiwalled carbon nanotubes and reported that both are promoted the oxidation reaction. L. Wang et al (2013, 2015,2016) [38-40] studied the magnesium sulphite oxidation by ascorbic acid, transition metal catalyst, cobalt based molecular sieve and found ascorbic acid inhibited the reaction, transition metal catalysts is promoting the oxidation of solid sulfites in flue gas desulfurization and magnesium sulphite oxidation is promoted by a novel cobalt-based molecular sieve catalyst respectively. The rate of uncatalysed and Ag (I) catalysed reaction is decelerated by the addition of Formic acid and Isopropyl alcohol in the present study. Manoj et al (2008)[41] Sameena et al (2013)[42] reported that radical mechanism operate in those reaction in which the inhibition parameters lies 10³ - 10⁴ In the present study the value of inhibition parameter for uncatalysed and Ag (I) catalysed autoxidation of S (IV) by  isoamyl alcohol and aniline are found to be in the range. This is strongly support the radical mechanism in the present case too based on the observed results.

 

–l

By assuming long chain hypothesis and steady state approximation d[SO₃  ]/dt, d[SO₄  ]/dt,d[SO₅]/dt, to zero. It can be shown that rate of initiation is equal to rate of termination. (eq. 29)

 

 k₁[Ag(I)(SO₃^-2)(O₂)]    = 

{k₇[X] + k₈[Organics]} [SO₄^-1]                                   (29)

                 

Since the reaction is completely stopped in the presence of [Organics] = 2x10^-4 mol dm^-3, so the step (22) and (25) appear to be unimportant. The step (24 ) is ignored because the reaction is completely seized in the presence of higher concentration of organics by omission and substitution from the above mechanism the following rate law can be obtain (30)

 

R_cat

= k₁ [Ag(I)] [S(IV)] /   {k₉[x] + k₁₀[Organics]}         (30)                                                        

 

By comparing derived rate law with the experimental rate law we observe the similarity in these two. The calculated value of inhibition constant B is 1.24 x 10⁴  mol dm^-3 and 0.26 x 10 ³ mol dm^-3 by  isoamyl alcohol and aniline respectively which is in the range of 10³ -10⁴ and also coincide with the reported value of  B of Co₂O₃ catalysed autoxidation of S(IV) by formic  acid is 3.58 x 10 ³ mol dm^-3  So on the basis of calculated value of B we concluded that organics act as an free radical scavenger in Ag(I) catalysed autoxidation of aqueous SO₂ in acidic medium and a free radical mechanism can operate in this system.

 

6. CONCLUSIONS:

Ø     The role of organics  act as an inhibitor in  Ag(I)  catalysed autoxidation of SO₂ in acidic medium has been find, and based on the observed results rate law a free radical mechanism has been proposed.

 

-d [(SIV)/dt  =

(k₁+k₂[Ag]) [(SIV)]/ 1 +  B  [Organics]              Rate law

 

R_cat   = k₁ [Ag(I)] [S(IV)] /   {k₉[x] + k₁₀[organics]}

Based on the experimental results, rate constants and order of the reactions were determined. The reaction order in SO₂ was pseudo- first order for both reactions in the presence and absence of organics. 

 

The effect of pH on SO₂ oxidation in the presence of Ag(I) and organics has been studied and found rate of the SO₂ oxidation depends on the initial pH of the solution but it is independent of the pH change during the reaction.

 

The effect of temperature of solution on SO₂ oxidation catalysed by Ag(I) in presence of organics were discussed. By plotting a graph between log k v/s 1/T yield gives us an apparent empirical energy of activation which is found 45.16 kJ mol^-1 for isoamyl alcohol and 26.43 kJ mol^-1 for aniline respectively.

 

The value of inhibition factor (B) of both uncatalysed and Ag(I) catalysed autoxidation of SO₂ in the presence of isoamyl alcohol and aniline  study here found 1.24x10⁴ mol dm^-3  and 0.26x10³ mol dm^-3   which is confirm that isoamyl alcohol is best inhibitor compare to aniline which is also coincide by E_a higher for  isopropyl alcohol compare to aniline.

 

Environmental Importance -

Organics are able to inhibit the oxidation of SO₂ so rain water acidity can be controlled.

 

They increase the life span of SO₂ so vegetation, national buildings, monuments, fishes of water bodies and human being are not affected by rain water acidity.

 

The results are useful for modeling rain water acidity and therefore a great use of meteorology and atmospheric chemistry.

 

This study is important in understanding the mechanism of the atmospheric oxidation of S(IV) by O₂

 

7. ACKNOWLEDGEMENT:

The authors pay their sincere gratitude to Principal, Govt. P.G. College, Jhalawar- Rajasthan (India) for providing necessary research facilities to accomplish this study.

 

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Received on 19.03.2017         Modified on 20.05.2017

Accepted on 21.05.2017         © AJRC All right reserved