Indian Journal of Chemistry
Vol. 38A, January I 999, pp.61-64
Oxidation of chromium (III) by periodate in aqueous alkaline medium
-
A kinetic study
S C Hiremath, S M Tuwar & S T Nandibewoor*
P.G.Department of Studies in Chemistry,
Kamatak University, Dharwad 580 000, India
Received 30 July /998; revised 26 November /998
The kinetics of oxidation of chromium(I1I) by periodate in aqueous alkaline medium at 27°C obeys the rate law
d[Cr(ITI)]
dt
=
[Cr(ITI)][J(VII)]
{
K
kl + b W
Kl OH-
where K i s di ssociation constant of the equilibrium H)O/"
I
[
""
]
}
"" H2 10/+H+,
kl and k2 are the rate constants for
the slow step s of the reaction s H2 10/ - Cr(OH)4' and H310t- Cr(OH)4 ' respectively. The con stants k and k2 are
l
calculated and u sed to regenerate the experimental rate constants at various condition s.
The periodate ion has been exten sively u sed in oxidation
of organic compounds, particularly in the oxidation of gly­
col sI. The mechan i sm of oxidation of 1 ,2-diol s by periodate
2
has been shown to proceed by the way of covalent link i n­
volving a cyclic intermediate. Thi s correspond s to an inner­
3
sphere mechan i sm . However, the literature on the mechani sm
of oxidation reaction s of inorganic material s by periodate i s
scanty.
The kinetics of oxidation of chromium(III) in alkaline me­
dium has been studied4 with a few oxidants. The results of
studies, in particular the nature of reactive chromium(III) ion,
has kindled a further interest in alkaline chromium(III) chem­
i stry. As the reduction potential sS of coupes Cr(VI) / Cr(lll)
and 1° ' / 1°3 ' are - 0.1 3 V and + 0.70 V respectively at pH >
4
1 2, the title reaction i s feasible. Oxidation of chromium(III)
6
by periodate has been studied in acidic media, where the sec­
ond order dependence on chromium(III) and first order on
periodate was observed. The pre sent investigation of the same
reaction in alkaline media i s different and hence the title study
to understand the alkaline chemistry of chromium (III) and
periodate.
Materials and Methods
Reagent grade chemical s and doubly distilled Hp were
u sed throughout. A stock solution of periodate was prepared
by di ssolving a known mass of KIO iRiedel) in Hp and stored
in a vessel wrapped with aluminium foil to avoid photochemi ­
cal reaction. The solution was standardi sed iodometrically7 at
neutral pH. The chromium (III) solution for every kinetic run
was prepared freshly from recry stall i sed �Cr2(SOJ4 ' 24Hp
(BDH) i n Hp and w a s standard i sedK by oxi dation to
chromium(VI) [heating with �SP (BDH) in the presence of
R
silver(I)] followed by titration of chromium(VI) against iron
(II) in acidic medium. The chromium(III) solution at pH > 1 2
was clear when freshly prepared, and turbidity meter indicat­
i ng the homogeneitl of solution up to ca. 40 h. Chromium(VI)
and iodate solution s were prepared by di ssolving �Cr20 and
7
KI03 in water re spe ctively. NaCl04 and NaOH were used to
adj u st the ionic strength and alkalinity respectively, to the re­
quired values.
Kinetic studies
Kinetic runs were followed u sing equivalent concentra­
tion s of reactants at 27 ± 0.1 °C unless otherw i se stated. The
reaction w a s i n i tiated by m i x i n g the fre sh l y prepared
chromium(lII) solution to the thermally equilibrated periodate
solution which al so contained the required quantity of sodium
hydroxide and sodium perchlorate to maintain [alkali] and ionic
strength con stant respectively. The progress of the reaction
was monitored by measuring the absorbance of one of the prod­
ucts, chromium(VI) in the reaction solution i n a l cm cell with
a thermo stated compartment of a Hitachi 1 50-20 spec tropho­
tometer coupled with HI-Tech (SFA- 1 2) Rapid Kinetic Ac­
3
cessory at 372 nm (E = 4670 ± 2 % dm mol" cm' l ) where the
other species in the mixture do not ab sorb significantly. The
concentration of chromium(III) at any time was obtained by
INDIAN J CHEM, SEC. A, JANUARY 1 999
62
accounting for the chromi um(VI) concentration . The kinetic
runs were reprod ucible within ± 5%.
Second order rate constant (ks) were obtained at eq uivalent
concentrations of chromi um(III) and periodate from a plot of
1/ [Cr(III)] vers us time. S uch p lots were linear up to two half
l ives completion of the reaction and ks val ues were reprod uc­
ible within ± 5 % . Whi le varying the concentrations of
chromi um(III) and periodate,initial rates were also obtained
[chromium(VI)] vers us time plots ,by plane mirror method.
R uns under conditions of pH greater than 12 lead to satisfactory results. However,at lower
[OH-]
of around less than
0.05 mol dm·3 ,the reaction mixture tended to become turbid
presumably d ue to the precipitation of chromi um(III) hydrox­
ide , Cr(OH) . Hence, sol ubility of chromium(I1I) had been
3
tested with a Systronic Nephelo Turbidity Meter-I 3 I and pH
meter. And also in view of the tendency of chromi um(III)
sol ution to undergo precipitation after standing for 36 hours
as tested by Nephelo Turbidity meter,the runs were restricted
to freshly prepared solutions. It may be noted that the onset of
slow precipitation starts after about 40 hours. However,the
kinetics was a lso studied for aged chromium(lII) sol ution i .e.
before getting the precipitation. It was observed that the or­
ders in oxidant, red uctant and alkali concentrations remain the
same,except that the marginal decrease in second order rate
constant,ks with ageing of chromi um(III) sol ution.
Use of polythene / acrylic ware and q uartz or polyacrylate
cells gave the same res ults as with the glass vessels and cells,
indicating that the s urfaces play no important role on the reac­
tion rate. The effect of dissolved oxygen on the rate of reac­
tion was checked by preparing reaction mixture and following
the reaction in the atmosphere of nitrogen . No significant dif­
ference between the results obtained under nitrogen and in pres­
ence of air was observed. In view of the ubiquitous contami­
nation of carbonate in basic sol ution the effect of carbonate on
the reaction was also studied. Added carbonate up to 1.0 x
10.3 mol dm-3 showed no effect on the reaction rate. However,
fresh sol utions were used while performing the experiments.
Results
Stoichiometry
Different sets of the reaction mixtures containing different
amounts of chromi um(III) and periodate in 0.50 mol dm-3 a l­
kali and 1.0 mol dm-3 ionic strength were allowed to react for
over 8 h at 27°C in an inert atmosphere and then analysed.
Chromi um(VI) was analysed spectrophotometrically. When
chromium(I1I) was in excess,the unreacted chromi um(III) was
analysed by oxidising it to chromi um(VI) as disc ussed else­
where. The total chrom i um(VI) in the solution was analysed
by spectrophotometry. S ubtracting the chromi um(VI) initially
present from it, the remaining concentration corresponds to
chromi um(III). Periodate was determined iodometrical ly at
ne utral pH. Iodate was determined iodometrically in 1.0 mol
dm-3 acid accounting for chromi um(VI) and periodate in the
total oxidant. The results showed 2 :3 stoichiometry for the
reaction as in Eq . (I ).
2Cr(III) + 3I(VII)
OH-
)
2Cr(VI) + 3I(V)
. .. ( 1)
Reaction order
Oxidation of chrom i um (III) by periodate was shown to be
second order,one each in [oxidant] and [reductant] . The sec­
ond order rate constants,k, val ues were l inear when they are
eq uivalent concentrations . However,under simi lar conditions
when [Cr(III)] is not eq uivalent to [I(VII)] ,it was found that
there was a lack of constancy in ks (Table I) val ues. T h e
order in chromi um(III) was unity between 0.25 x 10-4 and 2.5
X 10-4 mol dm-3 of [Cr(III)] at fixed [ I(VII)] of 1 .5 X 1 0.4 mol
dm-3. Order in periodate was a lso unity between 0. 30 x 1 0 4
and 4.5 x 10-4 mol dm-3 of [I (VII)] at constant [Cr(III)] of 1 .0
-
x
10-4 mol dm- 3. The effect of [ 0 H -] on the reaction was
studied from 0.15 to 1.0 mol dm-3 at fixed [Cr(III)] and [I(VII)]
and 1= 1.0 mol dm- 3; the order in [ O H - ] was found to be 0.5 (Table l ).
Effect 0/added products
Initially added products, chromi um(VI) and iodate in the
concentration range of2.0 x 10-5 to 1.0 x 10-4 mol dm-3 al l other
concentrations being constant did not have significant effect.
on the reaction rate.
Effect o/varying ionic strength and dielectric constant
The ionic strength of the reaction medi um was varied from
0.5 to 2.0 mol dm-3 with NaC I0 at fixed [reactants] and [al­
4
kali] . The results showed that ionic strength did not have any
significant e ffect on the reaction. Likewise the dielectric con­
stant (D) of the reaction medi um was varied by varying the
content of t-butanol with water which is inert towards oxidant.
The second order rate constant,k decreased with the decrease
in the dielectric constant of the edi um. A plot of log ks ver­
s us liD was linear with negative slope.
�
Effect a/temperature
I t has been reported 4 i n the c ase of oxi dation of
chromi um(III) by hexacyanoferrate(III) in alkal i ,the rate of
reaction decreased with increase in temperature. However, for
periodate oxidation ,rate increased with increase in tempera­
t ure. The temperature effect was studied at 27 ,32 ,37 ,42 and
47°C at fixed [reactants] ,[alkali] and ionic strength and from
the plot of log ks vers us liT, the activation parameters of the
reaction were obtained as Ea = 89 ± 2 kJ mol- I , log A = 16.5 ±
0.5 , !lS' = - 6 4 ± 4 JK- I mol- I , !1H' = 8 4 ± 2 kJ mol · 1 and
flG' = 103 ± 2 kJ mol·l.
Discussion
In alkaline medium ,the results obtained are different
from what were obtained in aq ueous acidic medi um. In acidic
media,order in [chromi um(III)] was two,[periodate] ,one and
HIREMATH et at. : KINETICS OF PERIODATE OXIDATION OF CHROMIUM (III)
reaction was inverse dependent on [H+]. The inverse [H+] de­
pendence has been explained by considering the deprotonated
periodate as active species which would be present in acidic
medi um. W hereas i n the prese nt s t udy the order i n
[chromi um(III)] and [periodate] is unity each and i t is inverse
fractional in [alkal i ] .
In aq ueo us alkaline medi um, the periodate exists9 in fo ur
2
4
forms, 1 ° . . H}IO� . , H210/ and di meric form, H 2120w ..
4
Among these,H }IO/· and H }O/ are hydroxyspecies, which
are in equil ibri um as in Eq . (2).
(2)
The unprotonated periodate,104· ex ists mostly at ne utral pH
and dimeric form,H2120w 4· is known to be present at very
h igh [alkal i]. Hence,it is possible to conc l ude that under the
cond it ions of pH e mployed viz, [ 0 H
-
]
=
0.50 mol d m· "
the periodate may exist as H)O/· and H 2 IO/ which can be
considered as pri me active species. As for the red uctan t,
chromi �m(III),the species Cr(OH) 2+,Cr(OH)2+, hydrolytic oli­
gomers, Cr(OH) , and Cr(OH)4· in addition to higher poly mers
·
w
are known
to exist in aqueous sol ution. In ac id ic media,
Cr(OH) 2+ ,Cr(OH)2 + and hydroly tic ol igo mers existll and in
the pH range 7 to 10, Cr(OH)} is known to exist in solid
s tate. Chromi um(III) exhibits greater sol ub ility at higher pH
by its amphoteris m d ue to the predominance of species
---"'-
H J 102.
(,
�
I-h lOr,' + Cr(OHk
H}
10/
leVI) +
}
H 2 10fl · + H
KI
--. H2 lOt + Cr(OH)4
slow
I(VI)
fast
I(VI) + Cr(IlI)
+
Cr(IV)
k2
Cr(OH)4- --. I-b 10/ + Cr(OH)4
slow
I(VI)
Cr(IV)
fast
Cr(IV) -----. 1(V) + Cr(V)
+
I(VII) + Cr(V) -----. I(VI) + Cr(VI)
I(VIJ)
(2a)
+
fast
+
(4)
(5)
(6)
Cr(IV)
(7)
Cr(IV) -----. I(VI) + Cr(V)
(8)
-----. I(V) + Cr(Vl)
(9)
I(VI) + Cr(V)
-----. leV)
(3)
fast
Table 1 - Effe ct of [Cr(lll)], [I(VII)] and [ OH-] on
periodate oxidation of chromium(III) in aqueous alkali at
27°C and I 1 .0 mol dm·}
=
[ OH-]
[Cr(lll)]
[I (VII)]
mol dm·}
1 04
mol dm·} mol dm·}
0.25
0.50
0.75
1 .0
1 .5
2.0
2.5
1 .5
1 .5
1 .5
1 .5
1 .5
1 .5
1 .5
0.5
0.5
0.5
0.5
0.5
0.5
0. 5
24
26
27
27
26
24
22
28
28
-28
28
28
28
28
1 .0
1 .0
1 .0
1 .0
1 .0
1 .0
0.30
0.50
1 .0
1 .5
2.25
3.0
0.5
0.5
0.5
0.5
0.5
0.5
30
30
28
27
26
24
28
28
28
28
28
28
0. 1 5
0.25
0.35
0.50
0.70
0.85
1 .0 1
58
48
35
27
23
22
21
59
42
34
28
24
22
21
x 1 04
X
1 .5
1 .0
1 .0
1 .5
1 .5
1 .0
1 .5
1 .0
1 .0
1 .5
1 .5
1 .0
1 .5
1 .0
Error ± 5%
k, (dm) mol ·1
Exptl.
Cal cd
Cr(OH)4·. The polymeric species are formed only on long
s tanding. Hence the active species of Cr(lll) in sol uble form
at alkaline med ium is considered to be as Cr(OH)4· and
also the pH employed in this study, reveals the existence of
Cr(lll) completely as Cr(OH\ .
A mechanism w ith ox idant species, H }O/ and H }IOt
and red uc tant,Cr(OH)4· can accoun t for the experi mental re ­
sul ts as in Sche me I where H 210/ and H }IOh 2· exist in eq ui­
l ibrium s tep (2). These two interact w ith Cr(OH) · in two
4
d ifferent slow steps (3) and (4) to yield Cr(lV) and I(V I)
spe cies and followed by o ther fast steps to g ive the prod ucts
of the reaction.
The rate law of the Sche me I can be derived fro m the
co mb ined slow s teps (3) and (4).
d[Cr(IIl)]
dt
From Eq. (2) the rate law ( 1 0) becomes (I I )
fast
- d[Cr(III)]
dt
Scheme I
63
=
LCr(TII)l[I(VIl)l
f1
k 1+
k
2K
I
K OH-
[
w
]
)
... ( II)
INDIAN J CHEM, SEC. A, JANUARY 1 999
64
concentration of alkali. Hence the study is restricted to the
second order conditions of low concentrations. The increase
in rate with increase in dielectric constant of reaction medium
is qualitatively expected from the above mechanism where,
ions of like charges interact in the slow steps. Since both the
2
Cr(OH)4' and H3106 . [(OH)310t] or H2IOt [(OH)2IO/J are
reactive species, the electron transfer may take place via in­
ner-spehere mechanism, through the common bridging ligand
60
50
7..
l...
0
E
'i:
"0
..
...
'
�
40
30
o H group or through the hydroxo-chromium to oxygen­
iodine. Hence there may be negligible effect of. ionic
strength; abnormally large value of observed second order
rate constants and relatively high value of apparent entropy of
activation. This has also been supported by earlier work 1 6.
-
20
10
Inverse dependence of reaction rate on [OH - ] may be due
o
·2.0
10
1/[OH-] d,.,jl
40
moll
5.0
6·0
Fig 1 . - Venfication of rate law ( 1 1 ) in the form of ( 1 2)
(conditions as in Table 1 ).
where k, and k2 represent the rate constants for the slow
steps (3) and (4) respectively, K, represents the dissociation
constant ofEq. (2) and Kw is ionic product of water. It may
be noted that I(VII) oxidation of Cr(lll) takes place with single
electron transfer in rate determining steps (3) and (4) which is
2
in accordance with well-accepted principle' . It explains the
order of unity �ach in oxidant and reductant. Besides these,
some other reactions in th e fast steps are taking place involv­
ing Cr(IV), Cr(V) and I(VI) intermediates. In acidic media,
chromium(V) is relatively stable and its ESR spectrum has
3
been recorded 1 ; chromium(IV) is the one of the most potent
4
oxidants known i n the acidic medial and can oxidise
manganese(lI) to manganese(III). Chromium(VI) itself loses
much of its oxidising capacity in alkaline media as evidenced
by reduction potentiaP and this trend appears to apply to its
lower oxidation states as well. The results of this study reveal
that these chromium species are much weaker oxidants in al­
kaline medium. The mechanism shown in Scheme 1 and rate
law (II ) may be verified i n the form of equation ( 1 2) by ploting
"
ks versus 1 / [ O H ] . The plot sho �ld be linear as shown in
Fig. l .
-
... ( 1 2)
The slope and intercept of such plot lead to the values of
kI = 1 4 dm 3 mol · l s·1 and k2 = 1 . 8 x 1 0 3dm 3 mol · l s . 1 . Using
2
these constants and the K = 2.5 x 1 0 · 1 from the
I
reported 9 value, the rate constants are regenerated (Table I).
It has been reported I S that the periodate exists in dimeric
form in its higher concentration which is also true for the higher
to the involvement of oxidant species, period ate (Eq. 2) in two
forms viz. H210t and H3IO/', which are involved in the slow
steps of reaction as shown in Scheme l in which the nega­
tively charged chromium(III) species and periodate species as
well as the oxygen -iodine bond being weakened which facili­
tates the transfer of electron or transfer of oxygen atom in op­
posite direction.
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2
3
4
5
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8
9
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