J. CHEM. RESEARCH (S), 1998 395
Table 2 Dependence of kobs on [HSO4 ] for the IrIII catalysed
oxidation of formaldehyde by CeIV. [HCHO]T 0.3, [CeIV]T
Use of eqn. (10) in (9) aords eqn. (11) after rearrangement
4.4Â10 3, [H ] 1.75, [Ir]T 2Â10 6 mol dm 3, 25 8C
1=ꢀk0HSO4 ꢀQ2=m ꢀQ2Q3HSO4 =m
ꢀ11
3
[HSO4 ]/mol dm
104 kobs/s
0.50
0.75 1.0
11.0 7.3
1.2
5.4
1.4
4.3
1.75
3.1
1
where
18.5
2kK2Ir H
T
m
1 K2Ir
T
where
f
is the fraction of [CeIV
]
kinetically active.
T
Under the conditions K1[HCHO]T (1 K2[Ir]T)w1, eqn. (8)
and n Q2, p Q2Q3 [cf. eqn. (2)]. From
a
1/k0[HSO4 ] vs. [HSO4 ] at ®xed [H ], [HCHO]T and [Ir]T
plot of
reduces to (9) which is in the form of eqn. (1).
1
the estimated Q3=0.88 dm3 mol at 25 8C conforms well
0
to the reported value (=0.60 mol 1 dm3).4b McAuley,12a
Hanna and Sarac12b and Hintz and Johnson9 have also
identi®ed Ce(SO4)2 as the kinetically active species in
aqueous H2SO4 in cerium(IV) oxidation reactions.
2f kK2Ir HCHO
T
T
kobs
ꢀ9
1 K2Ir
T
The equilibrium constant for complex formation1 between
Ce(SO4)2 and HCHO is 8.2 dm3 mol at 30 8C in aqueous
1
H2SO4 media. In the present system, Ce(SO4)2 is kinetically
active and its thermodynamic avidity for complex formation
is much greater4a,b,9 than that of Ce(SO4)2. Hence, it is
reasonable to consider K1 [=formation constant for the
Ce(SO4)2±HCHO complex]w8.2. It justi®es the approxi-
mation K1[HCHO]T (1 K2[Ir]T)w1. From the hydrogen-
sulfate dependence, Ce(SO4)2 has been identi®ed as
Experimental
Standard stock solutions of CeIV and catalyst were prepared as
reported earlier.2b Formaldehyde (BDH, AR) free from methanol
was used for preparing stock formalin solution and standardised
against alkaline iodine as usual. All other chemicals used were of
analytical grade. The procedure for kinetic measurements has been
discussed earlier2b±d and kobs were computed from the linear plot of
log[CeIV] vs. time.
kinetically active and under the experimental conditions its
concentration9 is ca. 10 7 mol dm
3
by considering the
successive equilibrium constants4 for formation of dierent
sulfatocerium(IV) species. Hence, under these conditions,
We thank CSIR (New Delhi), UGC (New Delhi) and
Visva-Bharati for ®nancial support.
the concentrations of species
I and II are less than
10 7 mol dm 3. Owing to these very small concentrations,
the kinetically involved species I and II could not be spec-
trally identi®ed. Their negligible concentrations also justify
Received, 4th February 1998; Accepted, 7th April 1998
Paper E/8/00995C
the approximations [HCHO]T1[HCHO] and [Ir]T1[IrIII
used in obtaining eqn. (8).
]
In the given scheme, K2 probably involves the outer-
sphere association of species I and catalyst followed by
electron transfer leading to species II which may be
CeIII(S)IrIV. In fact, such reaction mechanisms are well
documented5±7 in dierent metal ion catalysed electron
References
1 P. N. Sankhla and R. N. Mehrotra, J. Inorg. Nucl. Chem., 1972,
34, 3781.
2 (a) A. K. Das and M. Das, J. Chem. Soc., Dalton Trans., 1994,
589; (b) A. K. Das and M. Das, Indian J. Chem., Sect. A., 1995,
34, 866; (c) A. K. Das and M. Das, Int. J. Chem. Kinet.,
1995, 27, 7; (d) P. N. Saha, S. K. Mondal, D. Kar, M. Das,
A. K. Das and R. K. Mohanty, J. Chem. Res., 1997, (S) 364;
(e) R. K. Mohanty, M. Das and A. K. Das, Transition Met.
Chem., 1997, 22, 487.
transfer reactions. Participation of the catalytic cycle IrIII
/
IrIV has been also argued by previous workers10 in other
cerium(IV) oxidations.
By using eqn. (9), a linear plot (re0.99) of 1/kobs vs.
1/[Ir]T leads to the relationship: 1/slope 2fkK2=kb (say)
1
and 1/intercept 2fk ka. The values are: 103ka/s
=
1.2320.10 (20 8C), 1.8020.15 (25 8C), 2.3020.15 (30 8C)
3 G. Arcoleo, G. Calvaruso, F. P. Cavasino and C. Sbriziolo,
Inorg. Chim. Acta, 1977, 23, 227.
and 3.0320.2 (35 8C) with activation DH%=4323 kJ mol
,
1
4 (a) T. J. Hardwick and E. Robertson, Can. J. Chem., 1951,
29, 828; (b) L. T. Bugaenko and H. Kuan-Lin, Russ. J. Inorg.
Chem., 1963, 8, 1299; (c) S. K. Misra and Y. K. Gupta, J. Chem.
Soc. A, 1970, 2918; (d) S. Lakshmi and R. Renganathan, Int. J.
Chem. Kinet., 1996, 28, 713.
5 Y. A. El Tantawy, A. I. Abu-Shady and I. I. Ezzat, J. Inorg.
Nucl. Chem., 1978, 40, 168.
6 R. N. Banerjee, A. Das and S. Dasgupta, J. Chem. Soc., Dalton
Trans., 1989, 1645.
7 M. I. Sambrani and J. R. Raju, Indian J. Chem., Sect. A, 1991,
30, 369.
8 R. P. Bell, Adv. Phys. Org. Chem., 1966, 4, 1; P. Valenta,
Collect. Czech. Chem. Commun., 1960, 25, 853.
9 H. L. Hintz and D. C. Johnson, J. Org. Chem., 1967, 32, 556.
10 Y. B. Yatsimirskii, J. Indian Chem. Soc., 1974, 51, 32; Y. R.
Sarma and P. K. Saiprakash, Indian J. Chem., Sect. A, 1980, 19,
1175.
DS%= 156210 J K 1 mol 1; 10 3 kb/dm3 mol 1 s 1=0.522
0.08 (20 8C), 1.0620.05 (25 8C), 2.1620.10 (30 8C),
3.6720.20 (35 8C) with the activation parameters
1
.
DH%=9426 kJ mol 1, DS%=126216 J K 1 mol
Iridium(III) is an inert centre while CeIV is a relatively
more labile one.11 Consequently, the equilibria leading to
dierent sulfato species of CeIV are only important in
the present kinetics to explain the [HSO4 ] dependence. In
aqueous sulfuric acid media the important cerium(IV) species
are4b,c Ce(SO4)2, Ce(SO4)2 and HCe(SO4)3 . By considering
the relative values of Q1, Q2 and Q3 which are the successive
formation equilibrium constants for the species Ce(SO4)2
,
Ce(SO4)2 and HCe(SO4)3 respectively, [Ce(SO4)2] can be
reasonably given by eqn. (10).
11 cf. R. G. Wilkins, in The Study of Kinetics and Mechanism of
& Bacon,
CeIV H
Reactions of Transition Metal Complexes, Allyn
Boston, 1974.
2
T
CeꢀSO4
f CeIV
T
HSO4 ꢀQ2 Q2Q3HSO4
12 (a) A. McAuley, J. Chem. Soc., 1965, 4054; (b) S. B. Hanna and
S. A. Sarac, J. Org. Chem., 1977, 42, 2063.
ꢀ10