Indian Journal of Chemistry

Vol.39A, May 2000, pp. 507-521

Kinetics and mechanism of heterogeneous cadmium sulphide and homogeneous manganese(II) catalysed oxidation of sulphur(IV)

by dioxygen in acetate buffered medium

S V Manoj, R Singh, M Sharma & K S Gupta* Atmospheric Chemistry Laboratory,

Department of Chemistry, University of Rajasthan, Jaipur 302 004, India.

Received 4 June, 1999; revised 27 July 1999

Cadmium sulphide and Mn(II) catalysed autoxidation of aqueous sulphur(IV) has been studied in detail. The CdS

catalysed reaction obeys rate law (i), with k3 = 1.l7xlO-12 mol L -Is-I

-d [S(IV)] Idt = k3 [S(IV)f [Wr2 + k4 [CdS] [S(lV)]2 [H1-1

and k4 = 4.5 X 10-7 L~ mol-2 g-I s-I at 30°C in acetate buffered medium. For manganese(II) catalysed reaction,

the overall rate law is given by Eq. (ii) with ke =2 X 10-4 L mol-1s-1 and kf = 6.9 x 10-2 L2 mol-2 s-I

Robs = Ro + ke [Mn(II)] [S (IV) ] [Hi-I + kf [Mn(II)] [S(IV)f [Wr l

at 30°C in acetate buffered medium. Ro tern:t represents the uncatalysed and lor trace metal ion catalysed rate. Ethanol strongly inhibits both the reactions indicating the operation of a radical chain mechanism. A study of the effect of ethanol shows that whereas the non-transition metal oxide catalysed sulphur(IV) autoxidation is severely inhibited, the transition metal oxide catalysed autoxidation is inhibited only slightly. This has been interpretated in terms of the operation of a radical chain mechanism in the case of former and a non-radical mechanism in the case of the latter.

... (i)

... (ii)

The aqueous phase homogeneous transition metal ion catalysed 1-3 and heterogeneous particle surface

I d h 'd . . 145 f cata yse aqueous p ase autoxl atIOn reactIOns " 0

sulphur dioxide are known to contribute significantly to the acidification of rain water6 and other forms of atmospheric aqueous systems. We, in this paper, report the kinetics of the autoxidation of sulphur(IV) catalysed by cadmium sulphide particles and manganese(II) ions because of their strong atmospheric connections.

reaction are far from settled. In the previous investigations, a buffer was not used for maintaining pH, and, in many cases as a consequence, the pH

varied as t.he reaction pro/ress.ed .. It may be mentioned that Conmck and Zhang mamtamed constant pH by adding the base periodically. Berglund et al? adjusted the pH in the beginning with dilute perchloric acid. In the present investigation, the kinetics have been studied in acetate buffered solutions and this appears to be the first study in a buffer medium.

Manganese(II) catalysed autoxidation of sulphur(IV) has been the subject of numerous studies7

.

Most detailed investigation on this system are due to Connick and Zhang8

, Berglund and co-workers7•9 and

Huss et al. 1O Despite the plethora of studies the kinetic rate law as well as the reaction mechanism for this

Cadmium sulphide, which IS an n-type semiconductor has been used as a photocatalyst for production of hydrogen by splitting of water in sodium sulphite solutions 11. During the study of photocatalytic oxidation of sulphite in aerated suspensions using CdS as catalyst, it was indicated 12 that the reaction occured

508 INDIAN J CHEM, SEC. A, MA Y 2000

in the dark also to a significant extent ("" 30%).

However, no detailed investigation of the reaction in

dark was undertaken. This particular finding has environmental implication because the most important

use of this sulphide is in the paints and pigments for variety of uses. The humid or aquated CdS particles are

expected to act as catalysts for the air oxidation of

atmospheric sulphur dioxide resulting in the deposition

of sulphates on surfaces.

Materials and Methods

All chemicals used were of reagent grade, and all

the solutions were prepared in doubly distilled water.

The reactions were conducted in Erlenmeyer flasks

open to air and in room light using the procedure 'b . I 13-15 F d . 0 descn ed prevIous y or stu ymg 2-

dependence of reaction, a special type of reactor which

had a bubbler for passing N2 , 02 mixtures was used. A narrow side tube allowed the withdrawal of aliquots. The rotameters were used for controlling the ratio of

oxygen in N2-02 mixtures. The reactions were initiated by the simultaneous addition of sodium

sulphite and cadmium sulphide! manganese(II) to the

reaction mixture maintained at the desired pH and temperature. Acetate buffer was used for maintaining

pH. For this purpose, 10 ml buffer containing 1M

CH3COOH and 1M CH3COONa m different proportions were used iil a total reaction mixture

volume of 100m!' The reaction mixture was stirred

continuously, magnetically at a speed of 1600 ± 100

rpm to save the reaction from becoming oxygen mass transfer controlled. The kinetics were followed by

analysing the aliquots periodically for unreacted

sulphur(IV) iodimetrically using sodium thiosulphate as titrant, and starch as an indicator. Since CdS interferes in the iodimetric estimation of sulphur(lV),

CdS was removed from the aliquot sample with the

help of glass wool before adding to pre-determined quantity of iodine. The treatment of kinetic results is based on initial rates. The reaction rates ' were

reproducible within ±IO%. All calculations were made

using curve fitter and scientific plotter programmes of

Interactive Microwave Inc. USA. In the presentation of rate data the statistical parameters have been

abbreviated as follows, CD = coefficient of determination, CC :: coefficient of co-relation and SEE

:: standard error of estimate.

Product analysis

Qualitative tests showed sulphate to be the only oxidation product of S(IV) in both the cases . Dichromate method 16 indicated the absence of dithionate. The quantitative analysis for sulphate by precipitating it as barium sulphate showed _98% recovery of sulphur(VI) in CdS catalysed and -97% in the case of Mn(II) catalysed autoxidation m accordance with stoichiometric Eq. (I).

S(IV) + 0.5 02 ---~) S(VI) . .. (I)

Results

CdS catalysed autoxidation

The reaction profiles, shown in Fig.l, are seen to be almost linear, from the slope of which, the rate of reaction, Robs, was determined. A study of the effect of [buffer], by keeping the ratio [acetate] ! [acetic acid] fixed, showed the rate to decrease by about 20% when [buffer] was increased 2.5 times. The rate of reaction increased with increase in pH and from the plots of log Robs vs log [H+] at several [CdS] and [S(lV)], an order

of -1.5 ± 0.21 was determined in [H+] .

The dependence of Robs on [S(lV)] was examined at different [CdS] and at pH 5.25, 5.37, 5.5 and 5.68 at 30°C. From the plots of log Robs vs log [S(IV)] an

order of 1.9 ± 0.07 was determined in sulphur(IV) .

Thus the reaction rate has a second order dependence in sulphur(IV). Robs values under different experimental conditions are collected in Table I.

The dependence of Robs on [CdS] at different [S(lV)] and pH is shown in Fig. 2 which is in agreement with the rate law (2).

R"hs = R" + k2 [CdS] . .. (2)

The variation of Ro with [S(IV)] and [H+] is shown in Fig. (3), k2 had a second order dependence on [S(IV)] and an inverse first order dependence on [H+] (Table 2). The kinetics results fitted rate law (3).

-d [S(IV))/d/ = k, [S(IV)]2 [H+r2 + k4 [CdS] [S(IV)]2 [Hl- ' .... (3)

The values of k3 and k4 are 1.17x 10-12 L mol-I

s -I (CD = 0.91, CC :: 0.96, SEE = 2.3 x 10-7) and 4.51

x 10-7 L2mol-1g-1 s-I (CD =0.96, CC = 0.98, SEE = 1.5 x 10-7) respectively at 30°C. A study of oxygen

dependence using four different N2 and 02 gas mixtures showed the reaction to be independent of [02].

MANO) et. at.: KINETICS OF S(IY) BY D10XYGEN 509

Table I-Values of Robs at different [S(lV)] , [CdS] and pH at 30°C

103 [S(IY)]. 10 [CdS] .

mol L-1 gL-1

2.0 1.0

2.0 2.0

2.0 5.0

3.0 1.0

4.0 2.0

2.0 1.0

2.0 2.0

3.0 2.0

3.0 3.0

4.0 1.0

4.0 2.0

5.0 1.0

2.0 1.0

2.0 2.0

3.0 3.0

4.0 2.0

2.0 2.0

2.0 3.0

3.0 3.0

3.0 4.0

Ethanol is a known inhibitor of sulphite autoxidation reactions8.17-21 . In line with this, the addition of ethanol strongly inhibited the rate of the reaction and

an ethanol concentration of 5.1 x 10-2 L mol-I

completely stopped the reaction; indeed no reaction occured for one hour. Lower ethanol concentrations introduced induction period and thereafter the reaction proceeded at a reduced rate (Table 3). The addition of

EDTA also ((1-3) x 10-4 mol L -I) reduced the rate slightly, as ha\ been noted by Connick et a[l7 .

M anganese( 1I) catalysis

The reaction profiles are shown in Fig. 4 and these were utilised for obtaining the initial rates, R obs. The rate increased with increase in pH and from the log Robs vs log [H+] plot an order of -1 .0 in [H+] was determined. The variation of Robs with [S(lV)] indicated an order of more than one in [S(IV)] . In fact, from the log - log plot between Robs and [S(IV)] , an order of 1.4 in [S(lV)] was determined. The values of

pH 107

Robs. mol L-1 s-I

5.25 1.7

5.25 2.1

5.25 3.3

5.25 4.2

5.25 10.4

5.37 4.2

5.37 4.6

5.37 9.7

5.37 11.1

5.37 12.5

5.37 14.6

5.37 25.0

5.50 5.0

5.50 5.8

5.50 16.7

5.50 20.8

5.68 13.5

5.68 15.6

5.68 29.2

5.68 31.3

Robs under different experimental conditions are given in Table 4. The variation of Robs with [Mn(II)] (Fig. 5) shows the reaction to consist of a manganese(II) catalysed pathway and an uncatalysed pathway Ro. A detailed analysis of the dependence of Mn(II) catalysed pathway on sulphur(lV) indicated the overall kinetics results to be in agreement with rate law (4).

Robs = Ro + kc (S(IV)] (Mn(lI)] + kd (Mn(lI)] (S(IV)I. . . (4)

The values of 107 Ro are 1.4, 2.0, 3.9 and 6.1 mol L-1 s- I when 103 [S(IV)] = 1.5,2.5, 3.5 and 4.5 mol L -I respectively under the conditions pH = 5.32 and temp. = 30oC. The values of kc and kd (Table 5) depended inversely on [H+]. On this basis rate law (4) changes to (5).

R"hs = R" + Ice [Mn(Il)] [S(IV)] [H+r l + kr [S(IV)]2 [Mn(II)] [H"r l

. . . (5)

510 INDIAN J CHEM, SEC. A, MAY 2000

Table 2 - Yalues of Ro and k2 at different [S(IY)] and pH at 30°C

103 [S(IY)] , pH 107 Ro . 107 k2. Statistical Parameters

moiL-I mol L-1 s-I -I -I mol g s

2.0 5.25 1.2

3.0 5.25 3.1

4.0 5.25 6.8

2.0 5.37 3.4

3.0 5.37 6.9

4.0 5.37 10.4

5.0 5.37 22.5

2.0 5.50 4.2

3.0 5.50 12.6

4.0 5.50 16.5

2.0 5.68 11.1

3.0 5.68 22.9

The values of ' ke and kf are. 2.0

x 10-4 and 6.9 x 10-2 L2mol-2s-1 with CD, CC and

SEE of 0.97, 0.98, 6.67 and 0.99, 0.99 and 973.9 respectively. The values of Ro , determined from [S(IV)] and [Mn(II)] variation, showed a large variation between them. It must be understood that uncatalysed reaction makes only a small contribution towards total rate and, therefore, one does not expect to

CD CC SEE

4.3 0.99 0.99 3.8 x 10-9

9.0 0.98 0.99 2.2 x 10-8

17.7 0.98 0.99 3.3 x 10-8

6.3 0.97 0.98 1.9 x 10-8

13.9 0.99 0.99 3.6 x 10-10

20.8 0.99 . 0.99 3.1 x 10-10

29.2 0.98 0.99 7.6 x 10-8

8.3 0.99 0.99 3.2 X 10-10

15.6 0.96 0.98 5.0 x 10-8

25.0 0.95 0.97

13.5

20.8

0.98 0.99 3.3 x 10-8

0.99 0.99 3.2 X 10- 10

obtain Ro values with high precision. We, therefore, did not attempt to find out the functional dependence of Ro on [S(IV)] and [H+].

The dependence of Robs on [buffer] was studied by increasing the [buffer] and by keeping the ratio [CH3COO- ] / [CH3COOH] and hence the pH fixed. During these kinetics run, the acetate concentration varied in the range (0.4 - 3.4)10-1 mol L -I. The results

MANO] et. al.: KINETICS OF S(IV) BY DIOXYGEN 511

7~----------------------------_'

. ~ . a

2

1~---L--~----L---~---L--~ __ ~ o 10 20 30 40. 50. 60 70

Fig. 1 - The rate profiles for the CdS catalysed autoxidation of S(lV) at pH = 5.37, [CdS) = 0.2 g L-I, [S(lV») = 3

x 10-3 mol L -I and t = 300C

[ethanol), 0 = nil ; • = 1.36 X 10-2 mol L -I ; tJ. = 5.1

X 10-2 mol L-1

40r-------------------------~

30

.. -" ~

~ 20 ;,

" 0

10

O~--~ __ _L __ ~ ____ ~ __ _L __ ~

o 0..1 0 .2 0.3 0 .4 0.5 0 .6

rCd.~ , I.-I

Fig. 2 - The dependence of Robs on the concentration of CdS at -] -I' 0

[S(IV») = 3 x 10 . mol Land 30 C. pH, 0 = 5.25; tJ. = 5.37;

• = 5.5 and'" = 5.68

indicated virtually no effect of [buffer]. The reaction rate was found to be independent of [02] and the study of the effect of ethanol showed it to be strongly inhibited and indeed an [ethanol] of 0.17 mol L-1

completely stopped the reaction.

Effect of ethanol on other suiface catalysed autoxidation reactions

26r-----------~------__ --_.

• •

20

• .. 15

~ 1 • ,j rS 10

• •

• oL----L----~ __ ~ ____ ~ __ ~

o 10 15 '20 26

Fig. 3 - The dependence of rate constant Ro on [S(IV)] and [H+] at 300 C. '

x

'"'

O~--~--~--~----L-__ ~ __ _L __ ~

0. 10. 20 3D 40. 50. 60 10.

iimr .. min.

Fig. 4 - The rate profiles for manganese(II) catalysed autoxidation

of S(IV) at [S(lV)] = 2.5 x 10-3 mol L -I, [Mn(iI)] = 1.2

x 10-5 mol L-1, pH = 5.32 and 30°C [ethanol], 0 = nil ; • = 1.7

X 10-3 mol L-'; tJ. = 5.1 x 9-3 mol L-'

Kinetics of autoxidation of aqueous sulphur(IV) catalysed by CuO(ref. 14), Si02(ref. 14), CdO(ref. 22),

MgO(ref. 23), Mn02(ref. 24), CU20(ref. 25), C0203

(ref. 26) etc. have been reported earlier, but the effect of ethanol was not studied. A study of the effect of alcohol now revealed the reactions to fall in two categories. While the reactions catalysed by MgO, CdO, Si02, etc. were completely inhibited by addition of 0.17 mol L -I ethanol and no reaction was observed

512 INDIAN J CHEM. SEC. A. MAY 2000

12~----------------------~

10

a

!. : ~ : .

. 1 J,

4

2

• 8 U ~ R 24 ~-'-'------'

Fig. 5 - The dependence of Robs on [manganese(I1)] at pH = 5.32 and 30°C [S(lV)]. 0 = 1.5 X 10-3 mol L- I

• D. = 2.5 X 10-3 mol L -I •• = 3.5 x lO-:l mol L -I; • = 4.5 X 10-3 mol L-I

for a period of one hour, the other reactions catalysed by transition metal oxides suffered only a slight retardation in rate (Table 3).

Discussion

The results of CdS catalysed autoxidation are quite similar to those obtained for catalysis by nontransition

metal oxies like MgO(ref. 23), Si02(ref. 14) and

CdO(ref. 22) and anthropogenic materials like carbon particles13 and flyash27. All these reactions display second order in [sulphur(IV)], first order in the [catalyst] and zero order in [oxygen]. In order to explain the observed kinetic results non-radical

. d22-26 h' h .. d h mechamsms were propose w IC enVlSlOne t e . formation of complexes on the particle surface. However, these non-radical mechanisms fail to explain the strong inhibitory effect·of ethanol observed for CdS and other non-transition metal oxide catalysed reactions. Since ethanol inhibition is an outcome of the

. f I h d' I 81718 h . scavengmg 0 oxysu p ur ra Ica s' . ,t e operation of the free radical mechanism in all these cases must be considered.

Currently, the most favoured route for explaining metal ion catalysed autoxidation of sulphur(IV)

involves oxysulphur radicals viz. S03', SOs', S04 ' etc. The role of these radicals has recently been

. 178172829· exammed by several workers'" , . . A cham 1!lechanism is easily constructed if the catalyst metal

0·8

• :0-6'

•

. ,

. :::

°o~--~--~----L----L--~----L-~ 0·1 0·2 0·3 0.4 o-s 0.8

'10-3 (1101«>1101). LmoF'

Fig. 6 - The dependence of Robs on the concentration of ethanol at [CdS] = 0.2 g L -I. [S(IV)] = 3 x 10-3 mol L -I. pH = 5.37 and 30°C

ion is reducible30-33 by sulphur(IV) as in the case of iron(III)30. However, the difficulty arises when the catalyst is a metal ion like manganese(II) which cannot

be reduced by sulphur(IV) to generate S03 ' radicals. For explaining the initiation of the reaction in Mn(II) catalysed autoxidation two approaches have been made. In the first, Berglund et ai.7,9 have ascribed the

initial formation of S03" radical to the reaction of trace Fe(III) ions, present as an impurity in the solution, with S(IV). As an alternative suggestion,

Connick et al. 17 suggested that some HSOs may be present in the reaction mixture and its reaction with

HS03' may produce S04' and S03" radicals to initiate

the reaction. Since HSOs was not present initially, it was' surmised that probably the slow direct reaction

between HS03' and 02 produces a small concentration

of HSOs which permits the build up of the chain reaction autocatalytically to the steady state17.

Assuming step (6) to be initiation reaction, Connick et al. 17 proposed the following mechanism for uncatalysed autoxidation of sulphur(IV).

k~

HSOs + HS03' ) S04 ' + S03 . + H20

.. . (6)

k7 S03" + D2 ) sOs ' . .. (7)

MANOJ et. al.: KINETICS OF S(lY) BY DlOXYGEN

Table 3 - The effect of ethanol on the particle surface catalysed autoxidation of sulphur(IY) at 30°C.

[S(lY)] = 3 x 10-3 mol L -I, pH = 5.5. acetate buffer. 10 [catalyst] = 2.0 g L -I .

103 [Alcohol] , Induction

molL"'l period min.

CdS catalysed reactiona

0

1.7 5

5.1 5

8.5 10

10.2 10

13.6 15

51.0 60

MgO catalysed reaction

0

170 No reaction

CdO catalysed reaction

0

170 No reaction

Si02 catalysed reactionb

0

170 No reaction

CU20 catalysed reactionC

0 6

170 6

Mn02catalysed reaction

0 0

170 0

COO catalysis

0

170

Ni203 catalysis

0

170

CuO catalysis

0

170 6

a pH = 5.37, acetate buffer, b [S(IY)] = 4 x 10-3 mol L -I and to [catalyst] = 5.0 g L-1

, C unbuffered medium.

107 Robs, mol L-1 S-I

9.7

8.3

7.1

4.6

4.2

3.3

0.0

19.4

55.6

7.9

4.3

4.3

62.5

41.7

4.2

3.5

2.1

1.7

4.2

1.4

513

514 INDIAN J CHEM, SEC. A, MAY 2000

0.09

0·08

': 7

t o.O ~:

rl

, ­o

: ;-::'

•

• •

O.04L---L...--....I----L3--~----L--L-....J o 1 2 .. 5 6

Fig. 7 - The dependence of R obs on the concentration of ethanol at

[manganese(II)] =1 x 10-5 mol L-', [S(lY)] = 3 x 10-3 mol L-' ,

pH = 5.32 and 30°C kg

SOs' + S(lV) ) HSOs + S03' ... (8)

k9 SOs + S(JV) ) .soj- + S04 ' + H+ _ ... (9)

kw 504' + S(IV) ) S03 ' +' soa- + H+

. .. (10)

kl l HSOs + S(lV) ) products .. . (II )

kl2 SOs ' + SOs' ) S20~- + 02 . .. (12)

These mechanistic steps (7-12) are well known 17,29,31,32,34-37 and the individual rate constants

for most of these are available29,38. Although we have studied the kinetics in acetate buffered medium, the possibility of the chain termination through the reaction of S04 radicals with CH3COOH ICH3COO­is ruled out by the fact that buffer concentration has little effect on the rate of reaction for both CdS and Mn(II) catalysed reactions25. The observed kinetics in the case of CdS catalysed reaction can be explained by adding the following steps to the mechanism proposed by Connick et at

17.

CdS + S(lV) [CdS . S(lV)] ... (13)

SOs ' + [CdS . S(lV)] ---t) CdS + HSOs + S03' .. . (14)

The formation of the complex at the surface [CdS.S(IV)] is s'imilar to that proposed earlier in the particle surface catalysed oxidation of sulphur(IV) 1,6 .

Assuming long chain hypothesis and that a very small steady state concentration of HSOS was controlled 17 through reactions (8, 11 & 14), from the mechanistic steps (6-14) using the procedure of Connick et at.17

and ignoring [H+] ion dependence the rate law (15) can be derived.

l-d[S(IV)] l dt l = l(kR +ky) (k6kR12 kll kl2) [S(lV)]2j +

1(2 kg + k9) (k6 kl4 K13 / 2 kll k12) [CdS] [S(lV)]2 j

+ ~k6 k~4 Kh 1 2 kll k1 2) [CdS]2 [S(IV)]2 j ... (15)

Since, under our reaction conditions, the term

corresponding to [CdSf [S(IV)p is not observed, it

must be negligible as compared to the other two terms and then the rate law (15) reduces to (16).

1(2 kg + k9) (k6 kl4 K13 / 2 kll k l2) [CdS] [s(lv)f j . .. (16)

which, at a fixed pH, is the same as experimental rate law (3).

Besides the pathways mentioned by Connick et al. 17

HSOS, in our case, can result from the reaction of

HS03' with pre-adsorbed oxygen on the surface of CdS particles. The role of pre-adsorbed oxygen on particle surface has been noted by Brodzinsky et at. 13 .

Qualitatively, the experimentally observed [H+] dependence can be deri ved from the proposed mechanism if one assumes pathways (8,9) to be inversely dependent on [H+] and the reaction steps (13,14) to be independent of [H+]. This in effect means

the replacement of S(IV) by S01- in steps (8,9) and by

HS03' in steps (13,14). The former stipulation is in agreement with the observed reactivity pattern 16

S01- > HS03'. For steps (8,9), a similar [H+]

dependence has been assumed by Connick et at. 17 and Martin et at?l . The former workers considered step (11) to be almost independent of [H+] . The recent 39 ., detailed study . ·shows the reaction (11) to have only small [H+] dependence in the pH region 4.7 - 5.65.

MANOJ et. al.: KINETICS OF S(IV) BY DlOXYGEN 515

Table 4 - The values of Robs for Mn(II)-catalysed autoxidation at 30°C

103 [S(IV)] , 106 [Mn(II)] ,

moiL-I mol L-1

1.5 8

1.5 16

2.5 8

2.5 16

4.5 12

1.5 4

2.5 8

2.5 20

3.5 8

3.5 20

1.5 4

1.5 12

2.5 8

2.5 16

3.5 4

4.5 4

4.5 12

2.5 12

4.5 2

4.5 10

Regarding Mn(II) catalysis, it may be pointed out that Mn(III) formed from the oxidation of Mn(II) has b . l' d . . d' I 89 31 0 een Imp Icate as a reactive mterme tate. ' , . ur

pH 106 Robs,

mol L-1 s- I

4.7 0.21

4.7 0.33

4.7 0.42

4.7 0.81

4.7 1.42

5.1 0.24

5.1 1.34

5.1 2.34

5.1 2.12

5.1 4.32

5.32 0.62

5.32 1.58

5.32 1.90

5.32 3.62

5.32 1.86

5.32 2.70

5.32 6.68

5.65 5.92

5.65 3.02

5.65 11.0

results for Mn(II) catalysed study can be explained by adding steps 17 to 19 to the mechanism proposed for uncatalysed reaction (6-12). While the steps (17) and

516

pH

4.7

5.1

5.32

5.32*

5.32*

5.65

INDIAN J CHEM. SEC. A. MAY 2000

Table 5 - The values of kc and kd at different pH and 30°C.

kc . 103 kd. Statistical parameters

L mol-I s-I L2 mol-2 s-I CD CC

6.5 3.4 0.93 0.96

25 8.5 0.99 0.99

50 16.0 0.93 0.96

49 0.96 0.98

16.5 0.99 0.99

88 30.8 0.96 0.98

* [S(IV)) variation

Table 6 - Comparison of uncatalysed rates and rate constants for CdS and

Mn(II) catalysed reactions

Ro • kll· Reaction conditions Remarks

mol L-1 s-I L mol-I s-I

0.72 x 10-7 " pH =4.5 Mn(1I) catalysed study

1 = 25°C

[S(IV)] = 4.5 x 10-3 moIL-I

0.79 x 10-7 b pH =4.5

uncatalysed study

t= 25°C

[S(lY)] = 4.5 x 10-3 moIL-I

0.24 x 10-7 pH =4.5 CdS catalysed study 1= 30°C

[S(IY)] = 4.5 x 10-3 moiL-I

3.3 x 10-7 pH = 5.37 CdS catalysed study 1= 30°C

[S(lY)] = 2.5 x 10-3 moiL-I

2.0x 10-7 pH = 5.32 Mn(I1) catalysed study 1= 30°C

[S(IV)] = 2.5 x 10-3 moiL-I

1.23 pH =4.5 Mn(1I) catalysed study 1= 25°C

6.3 pH =4.5 Mn(lI) catalysed study 1= 30°C

SEE

1.5

2.6

6.8

3.2 X 10-3

2.4 x 10-3

10.7

Ref.

(8)

(17)

This work

This work

This work

(8)

This work

a calculated from ka = 3.6 x 10-3 L mol-I s -I at 25°C and its inverse square power dependence on [H+] in reference (8).

b estimated from Fig. 3 in ref. (17).

MANOJ el. al.: KINETICS OF S(lY) BY DIOXYGEN 517

(18) find place in the mechanism proposed by Connick and Zhang8,

SOs' + MnHSO! I Mn(lI) + HSOs + SO) . . .. (17)

klx

Mn(lI) + SOs . I Mn(lII) + HSOs ... (18)

kl9

Mn(lII) + HS03 I Mn(lI) + S03' + W .. . (19)

all the three steps are part of the mechanism proposed by Berglund et al.7.

In the proposed mechanism, Mn(II) includes Mn2+ ions as well as [Mn(00CCH3)]+. The stability constant for the latter complex has been reported to be 25 at 25°C by Archer and Monk40. A speciation of manganese(II) using the reported value of the stability

constant40 at [acetate] = 8 x 10-2 mol L-1, used in most of the experiments, shows nearly two-thirds of total man,r,anese(II) to be present as acetate complex. Berglund ,9 assumed the formation of the complex

MnHSO! and estimated a very high value of 3 x 104

for its stability constant. Connick and Zhang8 disputed this and suggested the complex to be weak having a low value of the stability constant. In any case, the formation of the complex is not in doubt and has been assumed to be more reactive than the uncomplexed sulphur(IV) 7,9. Following the procedure already indicated, the mechanistic steps (6-12) and steps (17-19), will lead to rate law (20).

l-d [S(lV)] I d/l = j(k6 kx 12 kll kl2) (kx + k9) [S(lv)]2 l

+ j(k6 klx 12 kll k12) (2 kx + k9) [Mn(II)] [S(lV)] l

+ j(k6 kl7l2 kll k12) K (2 kx + k9) [Mn(II)] [S(lV)]2 l

+ j(k6 kl7 kl X I kll k12) K [Mn(II»)2 [S(ly») l

+ j(k6 kt712 kll k12) K2 [Mn(II»)2 [S(lV)]21

+ j(k6 ktK 12 kll' k1 2) [Mn(II)]2l

. .. (20)

where K is stability constant for the formation of

MnHSO; from Mn2+ and HS03'.

In the experimental rate law (5), the terms correspondin~ to [Mn(II)]2[S(IV)]2, [Mn(II)]2 [S(IV)] and [MnQII)] are absent. These terms probably have negligible contribution to the total rate due to low

[Mn(II)] used and hence are not experimentally observed. On neglecting these terms, rate law (20) reduce to (2 J) .

j-d[S(IV)]/d/l = j(k6kg/2k11 k12) (kx+k9) [S(lV»)2l

+ j(k6 klx 12 kll k12) (2 kg + k9) [Mn(II)] [S(lV)]l

+ j(k~ kl7l2 kll k12) K (2 kx + k9) [Mn(lI)] [S(lV)]2l .. . (21)

which is the same as the experimental rate law (5) when pH is fixed.

Regarding the [H+] dependence of Mn(II) catalysed reaction, there are conflicting reports in the literature. Mn(II) catalysed reaction has been considered to be [H+] ion independent by several workers9,41-43 . On the other hand, many others 1 0,44-46 found the rate to increase with decrease in [H+]. Connick and Zhang8

found the overall reaction to have an order of -1 in [H+], but ascribed entire hydrogen ion dependence to uncatalysed pathway while catalysed pathway was considered to be [H+] independent. On making the same assumptions as in the case of CdS-catalysed reaction that the steps (8, 9) are inversely dependent and that the steps (17-19) are independent of [H+], the observed nature of [H+] dependence can be explained.

In their proposed mechanism, Berglund et al?,9 have assumed step (~2) to be the initiation reaction.

... (22)

The replacement of step (6) by (22) as the initiation reaction and keeping the remaining steps (7-12) and (17-] 9) the same, leads to the rate law (23) .

Rubs = A [S(lV») 15 + B [Mn(lI)] [S(lY») IS + C [Mn(II») [S(IV)]uS

. .. (23)

A, B and C are the composite rate constants and include half-order dependence in the concentration of trace metal ion impurity M(IIT). This rate law is neither similar to the experimental rate law (5), nor contains cross term [Mn(II)] [S(IV)] which has been repeatedly noticed by several workers 7-10,21. We, therefore, consider this mechanism unlikely.

For Mn(II) catalysed reaction Connick and Zhang8

determined the rate law (24) at pH 4.5 under the experimental conditions which match our reaction conditions.

Rate = 'Iea [HS03]2 + k~ [HS03] [Mn(II)] + ky [Mn(II)]2 . . . (24)

518 INDIAN J CHEM, SEC. A, MAY 2000

A comparison of Eq. (24) with rate law (3) for CdS catalysed and rate law (S) for Mn(n) catalysed reaction

shows ka. = k3 [H+r2 = Ro and kp = ke [H+rl . These values are collected in Table 6. Under similar reaction conditions, the Ro term is slightly lower under our conditions, but of the same order of magnitude. This could be due to the acetate ion comRlexing of trace metal ions which may have some role 6. The values of Ro determined from CdS and Mn(n) catalysed reactions are in good agreement despite the complex nature of systems under investigation. Allowing for the temperature difference the value of kp appears to be slightly high in our case. This could be due to the fact that in the presence of acetate ion, Mn(III) could be present as acetate complex. Since acetate ions are known to stabilise Mn(III) , the former may act as a

catalyst for the oxidation of Mn(ID by SOs '. A similar situation prevails in the sulphite induced autoxidation of manganese(n), where the presence of azide ion catalyses the autoxidation of Mn(II)(ref.3\).

At pH 4.5 and 2SoC, the value of ky (Eq. 24) is reported8 to be 98.6 L mol- I s-I. Under our reaction conditions at pH 4.S and [Mn(n)] = 2

x to-5 - 2 x to-6 mol L- I this term will range from 4

x to-8 to 4 x to-10 mol L -I s-I which will be

negligible compared to the observed rate values of 2.4

x 10-7 to 11.6 x 10-6 mol L -I s-I determined in our

conditions and possibly this is the reason why we did not observe quadratic term in Mn(II) in our rate law.

Berglund et al?,9 at pH 4.0 found the experimental rate law (2S) for Mn(n) catalysis,

Rate = l(k [Mn(lI)j + k [Mn(II)]2) I (A + [Mn(I1)]) j [HS031 ... (25)

which at low [Mn(II)] reduces to Eq. (26) and at high [Mn(n)] reduces to (27).

Rate = (k I A) [Mn(II)] [S(lV») ... (26)

Rate = k [S(lV)] + k' [Mn(II)j [S(lV)] ... (27)

Both rate laws agree on the existence of a cross term [Mn(II)] [S(IV)] as found by us and others7- IO,21,41-43.

The values kl A and k' have been reported 7,9 to be

1.2 x t03 L mol-I s-I and .68 L mol- I s-I respectively at 2SoC and pH 4. Our value for cross term is 6.3

L mo\-I s-I at pH 4.S and 30°C and that of Connick

and Zhang8 is 1.23 L mo\-I s -I at 2SoC and pH 4.S.

An explanation for high rates and rate constants reported for Mn(II) catalysed reaction by Berglund and some other workers is not obvious. Despite the difference in reported rates, most studies, agree under appropriate reaction conditions, on the existence of the cross term, Iq, [Mn(II)] [S(IV)].

The [Mn(lI)i term has not been noted by us. This could be due to several reasons. First, the occurence of [Mn2+]2 term requries the use of high [Mn(II)] which we could not do as the reaction is likely to become oxygen mass transfer controlled owing to high reaction rate and consequently higher rate of depletion of solution phase 02 than the rate of its diffusion from atmosphere into the reaction mixture. Second, the quadratic term has been ascribed to the formation of a

bridged complex 7,9 MnS03-Mn2+, the formation of which in the presence of complexing acetate ions,

which themselves form complex CH3COOMn +, would be hindered and hence less likely. The absence of uncatalysed rate term, Ro , in several previous studies on Mn(n) system at low pH is as expected7,9,IO,45,46 because at low pH this term makes a very small contribution.

A comparison of CdS and Mn(n) catalysed reactions shows them to differ in respect of cross term kdMn(II)][S(IV)] only, which is absent in the former.

Rightly so, because a reaction of SOs ' with Cd(II) to yield Cd(III) is not possible. The presence of the term [CdS] [S(IV)]2 in Eq. (3) and [Mn(II)] [S(IV)]2 in Eq. (S) is noteworthy. In several oxidation and autoxidation studies, similar rate terms have been noted previously 14,15,47,48.

Ethanol inhibition has been explained through the

scavenging of SOs . (ref. 49) and S04' . (ref. 8) radicals.

Recent studies suggest that the scavenging of SOs .

S05 ' + C2H50H ) products .. . (28)

k29

S04 ' + C2H50H ) SO~-+ CH3CHOH + H+ ... (29)

radica\s8,28 to be unimportant. Indeed, the upper limit of the rate constant, k28 (~ 103 at pH 9)38 reflects the nature of the experiment in which the radicals were being lost to self reaction and the actual rate constant is very low50. In view of this ~e have considered steps (12) and (29) as termination steps in presence of ethanol.

MANOJ et. al.: KINETICS OF S(lY) BY D10XYGEN 519

The mechanistic steps (6-14) for CdS catalysed reaction and steps (6-12) and (17-19) for Mn(II) catalysed reaction together with the step (29) lead to the same general rate law, (30) for both reactions in the presence of ethanol.

... (30)

where Ra\c and Robs represent the rates In presence and absence of ethanol respectively, B = kIO [S(lV)] and C = k29 . For CdS catalysis,

and for Mn(II) catalysis,

A = 1(k29k9 / 2k I 2)1 1(k~+k9) [S(IV)]

+ kl7 K [S(lV)] [Mn(H)] + klR [Mn(II)]1 [S(lV)]

... (31)

.. . (32)

These rate laws require the plot between

1/(Robs - Ra\c) and 1/ [C2H50H] to be linear. Such plots for CdS and manganese(II)-catalysed reactions are shown in Figs. (6) and (7). The agreement with Eq. (30) is only qualitative and that too in the region of relatively high ethanol concentration. Thus, the real situation may be more co'mplex than visualised here. The same concentration of ethanol is seen to inhibit manganese(II) catalysed reaction more severely than CdS-catalysed reaction.

Considering the same two reactions (12 and 29) as termination steps in presence of alcohol, Connick and Zhang obtained the rate law (33).

Rate = Ro - b [alcohol] . . : (33)

where Ro is the rate in the absence of alcohol and b is a composite rate constant. This rate law requires that at high enough [alcohol], such that Ro = b [alcohol] , the rate should reduce to zero. Further, at still higher [alcohol], such that b [alcohol] > Ro, the rate law (33) predicts a negative rate which is meaningless. In deriving rate law (33), while calculating the steady state concentration of S04' radicals, Connick and Zhang ignored, perhaps inadvertently, step (29). If this is not done, the same rate law as (30) is obtained for both the uncatalysed 17 and Mn(II) catalysed8 reactions.

The effect of alcohol in the case of several metal oxides (Table 3) shows the oxides to fall in two categories, whereas the transition metal oxide catalysed

reactions are only slightly affected, the .reactions catalysed by non-transition metal 'oxides completely siezed. This can be interpretated to mean that in the case of transition metal oxides, the catalysed reaction probably occurs ' largely through anon-radical mechanism as postulated earlier6 whereas in the case of the non- transition metal oxides, the catalysis is due to radical mechanism.

Application to atmospheric chemistry

In atmospheric situations, the aqueous phase concentrations of S(lV) and Mn(II) are less than I

x 10-5 mol L -I. The pH region of importance is between 4~6. Under these conditions, [Mn(II)12 terms

.' 8 al observed by Connick and Zhang, Martin and

Huss et al. IO and [Mn(II)][HS03']2 term observed by us will be unimportant. Therefore, for calculation of atmospheric conversion rates due to aqueous phase Mn(II)-catalysed S02, autoxidation, the applied rate law (34), at constant pH, must contain only cross term.

R .. hs = k c [Mn(II)] [S(lV)] .. . (34)

In the pH region of significance, kc is inversely dependent on pH as found by us . From the rate and pH data of Connick et al.8 also, it is observed that the overall reaction has roughly inverse dependence on [H+]. We suggest the use of Eq. (35) for calculation of rate due to Mn(II) ion catalysis, in the pH region 4-6.

. . . (35)

The value of kef[H+] is equal to k~ in terms of rate law (24). From k~ value of 1.23 L mol-I s-I reported

by Connick and Zhang8 at pH 4.5 and 25°C, ke can be

calculated to be 3.9 x 10-5 L mol-I s-I . We recommend that this value of ke should be used in atmospheric calculations. However, if acetic acid is present in atmospheric a~ueous systems, as has been reported by many workers , use of our values would be more appropriate. For calculating the atmosphere conversion rates due to CdS catalysis, Eq. (36) should be used.

, . , (36)

520 INDIAN J,CHEM, SEC. A, MAY 2000

Acknowledgement

This work was supported by Earth System Science Division, Department of Science and Technology, New Delhi. Partial support from the CSIR, New Delhi is also acknowledged. Authors are thankful to Dr. Robert E Huie and anonymous referee for useful suggestions.

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