Complexation and intramolecular redox decomposition of cerium(IV) hydroxo complexes with oxycarboxylic acids

Article abstract of DOI:10.1023/A:1020303822320

Spectrophotometry, pH-metry, and kinetic method were applied to study the complexation and redox decomposition of cerium hydroxo complexes formed in the systems Ce⁴⁺-SO₄^2--H_mL, where H _mL (m = 2, 3, 4

Full text of DOI:10.1023/A:1020303822320

Russian Journal of Applied Chemistry, Vol. 75, No. 6, 2002, pp. 866 874. Translated from Zhurnal Prikladnoi Khimii, Vol. 75, No. 6, 2002,
pp. 886 894.
Original Russian Text Copyright
2002 by Voskresenskaya, Skorik.
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
Complexation and Intramolecular Redox Decomposition
of Cerium(IV) Hydroxo Complexes with Oxycarboxylic Acids
O. O. Voskresenskaya and N. A. Skorik
Joint Institute for Nuclear Research, Dubna, Moscow oblast, Russia
Tomsk State University, Tomsk, Russia
Received January 15, 2001; in final form, March 2003
Abstract Spectrophotometry, pH-metry, and kinetic method were applied to study the complexation and
redox decomposition of cerium hydroxo complexes formed in the systems Ce4+ SO42 HmL, where HmL
(m = 2, 3, 4) are, respectively lactic, malic, and tartaric acids.
A study of thermodynamic and kinetic character-
istics of complexation and redox decomposition of
complexes formed in the first stage of oxidation by
cerium(IV) of polar organic compounds is of theoret-
ical and practical interest. It is rather important in
connection with the problem of stabilization of un-
stable oxidation states of metals [1 4] and use of
these systems as a source of radicals in radical poly-
merization [5, 6], and also in view of their other ap-
plications analytical [7], in a theoretical study of
the electron transfer rates in intermediate complexes
[8], and in separation of a mixture of rare-earth
elements [1 3].
could be revealed between the stability of the com-
plexes and the rate and apparent activation energy of
the redox interaction in cerium(IV) complexation with
oxyacids [3]. It has been shown that intramolecular
redox interaction cannot be completely eliminated in
systems of this kind, with only its partial suppression
possible [3, 4].
As characteristics of the redox interaction were
employed the conditional rate of the redox process,
its apparent activation energy, and reducing power
of an organic ligand [3]. Such a direct characteristic
of the kinetic activity of the complexes as rate con-
stant of their intracomplex redox decomposition was
For example, separation of a mixture of rare-earth not determined in the previous studies mentioned
elements is based on the ability of cerium to form here. It is also evident that such a property of an or-
compounds in which it is in oxidation state +4. This ganic ligand as its reducing capacity may play an im-
leads to a pronounced difference between the proper-
ties of cerium compounds and those with rare-earth
elements in oxidation state +3 and improves the ef-
ficiency of methods for separation of rare-earth el-
ements. However, the problem of cerium stabilization
in this anomalous oxidation state, especially in
complexation with oxyacids, has been studied insuf-
ficiently.
portant part in the case of a bimolecular (free-radical),
but not intermediate, mechanism occurring in oxida-
tion of oxy acids by cerium(IV) [9]. A study of the
problem of stabilization in the given context is in-
separable from a study of the mechanism and kinetics
of the redox process occurring in the systems.
The kinetics of oxidation of oxycarboxylic acids
by cerium(IV) was studied in [5, 9 13]. In [10 12],
a mechanism of the redox process was postulated, in
which an intermediate complex is formed and the de-
composition of this complex is the rate-determining
Previously, the influence exerted by the stability
constants of complexes, reducing ability of ligands,
their denticity, and other factors on the rate of the
redox interaction in systems containing cerium(IV) stage of the process. However, it has been noticed that
and organic ligands (carboxyl-containing complexons,
oxycarboxylic acids, etc.) [3]. It has been established
kinetic data are insufficient for confirming the forma-
tion of such an intermediate complex [11]. A syner-
that there is a correlation between the rate of the red- gistic effect of the carboxylic and hydroxy groups of
ox interaction in the systems and the instability con-
stant of cerium(IV) complexes. No simple relationship
oxyacids, and also intermediate and free-radical
mechanisms of oxidation of organic ligand by ceri-
1070-4272/02/7506-0866 $27.00 2002 MAIK Nauka/Interperiodica

COMPLEXATION
OF CERIUM(IV) HYDROXO COMPLEXES WITH OXYCARBOXYLIC ACIDS 867
um(IV) for, respectively, oxy acids and succinic di-
carboxylic acids have been noted [9]. A bridge mech-
anism of electron transfer in such an intermediate
has been suggested and necessity for revealing the
states in which the metal and ligand are present in
this complex has been indicated [13].
It is noteworthy that the complexation pre-equilib-
rium in the redox process has not been studied spe-
cially, being either postulated or studied using meth-
ods yielding only effective complexation constants
related to a fixed pH value and indeterminate form of
the ligand and metal in the complex. At the same
time, the kinetic dependences themselves were inter-
preted on the basis of assumptions about the com-
position, structure, and reactivity of the complexes
and characterized by effective rate constants. Data on
the rate constants of intramolecular decomposition of
cerium(IV) complexes with organic ligands are un-
available in the literature. Of interest in this regard are
generalized methods for investigation of complexa-
tion, proposed in this study, and their application to
analysis of the kinetics and thermodynamics of the
initial stages of the processes mentioned, the mech-
anism of these processes, and the reactivity of inter-
mediate complexes involved.
A comparative evaluation of the effective forma-
tion constants of intermediate cerium(IV) complexes
with lactic
oxyacid and other hydroxyl-contain-
ing compounds in a perchlorate medium was made
in [5]. The composition and stability of complexes
have been studied in a nitrate medium for the malate
.
0
0
Fig. 1. (a, b, d)
D
composition and (c, e)
D
com-
z
complex [Ce(NO3)zMalt]2 [3, 14] and binuclear tar-
position diagrams for isomolar series of solutions. (y) Con-
tent of ligand. System: (a) Ce(IV) SO H Lact (c =
tot
2.46 10 M, pH 2.87, I = 2, T = 25.0 C, = 400 nm),
(b e) Ce(IV) SO H Tart [(b, c) c = 1.17 10 M,
pH 2.24, I = 2, T = 24.0 C, = 400 nm; (d, e) c = 1.03
2
z
trate complex of cerium(IV) [Ce2(NO3)zTart]6 [3,
4
2
2
15]. Other researchers [3, 14, 15] could not study the
complexation in a sulfate medium in the systems
we investigated.
2
2
4
4
tot
tot
3
10 M, pH 2.3, I = 2, T = 25.0 C,
= 340 nm].
In the present study, the methods commonly used
to investigate complexation and generalized to the
case of kinetically active complexes were applied to
determine in a sulfate medium the composition, stab-
ility constants 1, rate constants k2, and activation
energy of intramolecular redox decomposition of
lactate, malate, and tartrate hydroxo complexes of
cerium(IV) [CeOH Hm x L]3 (m = 2, 3, 4) formed
in the first stage of oxidation by cerium(IV) of the
above-mentioned organic ligands:
x
.
0
0
Fig. 2. (1) D c /c and (2)
D
c /c diagrams for the
L
M
L M
4+
2
3
system Ce SO H Malt. c = 1.88 10 , c = 3.00
4
3
M
L
4
2
10 2.11 10
M; pH 2.70; I = 2; T = 18.0 C;
1
x
0
CeOH3+ + H m _ Lx
[CeOHH m x L ]3
_
= 400 nm; D = 0.376; D = 0.09.
M
x
k2
The number x of protons displaced from the ligand
Hm L in complexation was determined. The mech-
anism of the redox process observed in the systems
Ce3+ + LHmx = 1
+ H2O,
(1)
x
= k1 / k , k1 >> k 1 >> k2.
1
1
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 6 2002

868
VOSKRESENSKAYA, SKORIK
The composition of intermediate complexes formed
in the systems Ce4+ SO42 Hm L at the instant of mix-
ing of the reaction mixture components was studied
by the methods of isomolar series (Fig. 1) and molar
ratios (Fig. 2). The results obtained indicate that 1 : 1
complexes are predominant in all the systems in the
pH range studied. The formation of complexes with
1 : 1 composition in the systems studied was ad-
ditionally confirmed by using kinetic analogues of
the above methods [Figs. 1c and 1e; Fig. 2, curve 2;
Eq. (16)], which are particularly appropriate in the
cases of low yield or low light absorption by the com-
plexes.
The data on the composition of the complexes, ob-
tained in the present study, is in agreement with the
assumption that an intermediate cerium tartrate com-
plex of composition 1 : 1 is formed in oxidation of
tartaric acid by cerium(IV) [10, 12] and also with
the conclusions of [5] that the 1 : 1 composition of
intermediate organocerium complexes is independent
of the denticity of the organic ligand and can be re-
garded as their proof. The composition of the malate
complex of cerium(IV) in a sulfate medium coincides
with that in a nitrate medium, established in [3. 14].
4+
2
Fig. 3. log
pH diagrams. System: (I) Ce SO
4
1eff
3
2
H Lact (c = 6.0 10 , c = 3.0 10 M; I = 2; T =
18.0 C; = 400 nm); (II) Ce SO H Malt [(1) c
2.14 10 , c = 2.00 10 M; T = 17.7 C; (2) c
2.04 10 , c = 2.00 10 M; T = 28.0 C; and (3) c
3.21 10 , c = 3.00 10 M; T = 27.0 C; I = 2,
400 nm]; (III) Ce SO H Tart [(1) c = 1.22 10
c =1.20 10 M; T = 28.0 C; (2) c = c = 2.14
2
M
L
4+
2
=
=
=
=
4
3
M
M
3
3
L
3
3
L
M
3
3
The form in which a coordinated ligand is present
in the complexes and their stability constants were
determined using the D pH method [16]. As the
dominant form of cerium(IV) in the pH range studied
was taken the monohydroxo form CeOH3+ [17].
L
4+
2
3
,
4
4
M
3
L
M
L
3
3
10 M; T = 28.0 C; (3) c = 2.04 10 , c = 2.00
M
L
3
3
10 M; T = 28.0 C; and (4) c = 3.02 10 , c
=
M
L
3
1.65 10 M; T = 24.0 C; I = 2,
= 400 nm].
The number x of protons displaced from the ligand
molecule HmL by metal ion in attainment of equilib-
rium
was studied, and the law governing its rate was estab-
lished.
.
Table 1. Stability constant and rate constant of redox decomposition of lactate complex with cerium(IV) by the D0 pH*
3
2
method (cM = 6 10 , cL = 3 10 M; I = 2; T = 18.0 C;
= 400 nm; D0 = 0.610)
.
1
D0
DM
D0, s
pH
cK0 103, M
log
log 1eff f
log kapp
log k2
1eff
5
5
5
5
5
5
4
4
4
4
4
4
4
4
4
0.307
0.304
0.304
0.309
0.311
0.311
0.322
0.332
0.328
0.327
0.331
0.339
0.347
0.357
0.342
0.300
0.300
0.299
0.300
0.301
0.297
0.294
0.294
0.277
0.284
0.298
0.274
0.215
0.217
0.175
1.0 10
1.5 10
3.0 10
4.0 10
4.5 10
8.5 10
1.6 10
1.5 10
1.8 10
2.7 10
3.2 10
4.3 10
4.2 10
4.4 10
4.5 10
1.24
1.35
1.50
1.61
1.70
1.83
1.94
1.94
2.03
2.06
2.12
2.20
2.27
2.33
2.43
0.059
0.077
0.097
0.174
0.194
0.268
0.532
0.720
0.864
0.792
0.900
1.302
1.896
1.956
2.184
0.48
15.35
15.25
15.05
15.09
14.96
14.85
14.95
15.12
15.00
14.91
14.89
14.90
15.00
15.05
14.86
1.23
1.28
1.49
1.36
1.37
1.50
1.48
1.32
1.32
1.53
1.56
1.52
1.35
1.35
1.31
3.23
3.28
3.49
3.36
3.37
3.50
3.48
3.32
3.32
3.53
3.56
3.52
3.35
3.35
3.31
0.36
0.26
0.00
0.05
0.20
0.52
0.68
0.74
0.72
0.81
0.98
1.22
1.38
1.33
*
log
1
= 14.97 0.18, log k = 3.40 0.10 (18 C).
2
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 6 2002

COMPLEXATION
OF CERIUM(IV) HYDROXO COMPLEXES WITH OXYCARBOXYLIC ACIDS 869
Table 2. Stability constants, rate constant, and activation energy of malate complex with cerium(IV), determined by
.
3
3
the D0, D0 pH* method [cM = 2.14 10 , cL = 2.00 10 M; I = 2; T = 17.7 C; = 400 nm; D0 = 0.310 (run no. 1);
3
3
cM = 3.21 10 , cL = 3.00 10 M; I = 2; T = 27.0 C;
= 400 nm; D0 = 0.246 (run no. 2)]
.
1
D0
DM
D0 104, s
pH
cK0 103, M
log kapp
log k2
log
log
1
1eff
Run no. 1
0.228
0.231
0.229
0.230
0.244
0.250
0.262
0.309
0.310
0.305
0.315
0.306
0.227
0.228
0.220
0.215
0.210
0.212
0.208
0.128
0.111
0.094
0.085
0.066
0.4
0.3
0.9
1.1
2.1
2.7
3.2
5.0
4.9
5.0
5.0
5.1
1.67
1.79
1.90
2.05
2.27
2.42
2.38
2.69
2.75
2.85
2.96
3.07
0.024
0.073
0.200
0.316
0.680
0.775
1.060
1.988
2.000
1.954
2.000
1.967
1.23
1.61
1.65
1.54
1.49
1.54
1.48
1.40
1.39
1.40
1.40
1.41
3.23
3.61
3.65
3.55
3.49
3.55
3.48
3.40
3.39
3.40
3.40
3.41
0.76
18.31
18.45
18.61
18.64
18.34
18.01
18.48
1.26
1.76
2.01
2.55
2.67
3.02
Run no. 2
0.158
0.160
0.162
0.162
0.167
0.185
0.155
0.155
0.150
0.148
0.148
0.140
1.0
2.0
3.7
5.0
5.1
7.8
1.68
1.85
1.96
2.06
2.06
2.20
0.095
0.158
0.360
0.412
0.559
1.227
0.02
0.10
0.01
0.08
1.96
1.80
2.20
2.28
2.19
2.26
2.14
2.00
1.02
1.26
1.68
1.75
1.94
2.85
18.54
18.28
18.38
18.15
18.34
18.83
*
log
= 18.39 0.18, log k = 3.43 0.10 (17.7 C) (run no. 1); log
= 18.42 0.24, log k = 2.17 0.10 (27 C),
1 2
E = 1133.0 1.3 kJ mol 2(run no. 2).
1
Table 3. Stability constant and rate constant of redox decomposition of tartrate complex with cerium(IV), determined
.
3
3
by the D0, D0 pH* method (cM = 1.22 10 , cL = 1.20 10 M; I = 2; T = 28.0 C; = 400 nm; D0 = 0.317)
.
1
D0
DM
D0 103, s
pH
cK0 103, M
log
log kapp
log k2
1eff
5
5
5
4
4
4
4
0.170
0.163
0.167
0.173
0.171
0.172
0.217
0.238
0.273
0.279
0.292
0.155
0.150
0.154
0.156
0.151
0.153
0.155
0.146
0.152
0.150
0.121
0.20
0.30
0.35
0.40
0.45
0.45
1.70
1.50
1.60
1.25
1.30
1.74
1.79
1.80
1.81
1.90
1.98
2.22
2.35
2.38
2.44
2.50
5.18 10
28.25
3.86
3.21
3.66
3.12
3.10
3.36
3.70
3.98
3.90
3.96
3.89
3.88
3.92
3.96
9.34 10
27.97
27.95
28.05
27.76
27.44
27.34
27.24
27.72
27.67
27.98
9.57 10
1.27 10
1.45 10
1.33 10
4.59 10
4
6.46 10
8.78 10
9.27 10
1.05 10
4
4
3
*
log
= 27.76 0.32, log k = 3.93 0.04 (28 C).
1
2
R1
x
CeOH3+ + HmL
[CeOHHm xL]3
+ xH+, (2)
log
= log R1 + xpH
(4)
1eff
by comparing the D0 pH data of series nos. 2 4 (D0
is light absorption by the reaction mixture at instant
[CeOHHm xL][H]x
1eff [H]x,
(3)
R1
=
=
[CeOH][HmL]
of time
As follows from analysis of the dependence of
log on pH (Fig. 3, Tables 1 3), the high-charge
= 0).
was evaluated graphically as the slope of the plot of
log against pH
1eff
1eff
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 6 2002

870
VOSKRESENSKAYA, SKORIK
Table 4. Stability constant and rate constant of redox decomposition of malate complex with cerium(IV), determined
.
by the D0, D0 cL/cM method (cM = 1.88 10 , cL = 3.00 10 2.11 10 M; pH 2.70; I = 2; T = 18.0 C;
= 400 nm; D0 = 0.376; DM = 0.090)
*
3
4
2
.
1
D0
D0 103, s
cL/cM
cK0 103, M
log kapp
log k2
log
1
0.132
0.180
0.215
0.240
0.260
0.279
0.306
0.327
0.350
0.364
0.370
0.374
0.374
0.375
0.379
0.375
0.377
1.5
2.5
3.1
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.0
7.4
7.3
7.5
7.6
7.5
7.5
0.16
0.32
0.48
0.64
0.80
0.96
1.27
1.60
2.23
3.19
4.15
5.10
6.70
8.45
9.55
10.40
11.20
0.296
0.592
0.822
0.986
1.117
1.243
1.420
1.558
1.709
1.801
1.841
1.866
1.866
1.873
1.880
1.873
1.880
1.70
1.63
1.58
1.61
1.60
1.60
1.59
1.59
1.58
1.59
1.58
1.60
1.59
1.60
1.61
1.60
1.60
3.52
3.45
3.40
3.43
3.42
3.42
3.41
3.41
3.40
3.41
3.40
3.42
3.41
3.42
3.43
3.42
3.42
19.08
19.17
18.50
18.22
18.09
18.05
18.01
18.03
18.11
18.24
18.41
18.75
18.60
18.76
18.73
*
log
= 18.45 0.34, log k = 3.42 0.03 (18.0 C).
2
1
ion CeOH3+ displaces 2, 3, or 4 protons from the
ligand molecule in complexation with, respectively,
lactic, malic, and tartaric acids. In this case, similarly
to that of interaction of oxyacids with the cations
Sc3+, Ga3+, In3+, and Th4+ [18], protons of the oxy
groups of oxyacids are displaced because of the high
coordination numbers of rare-earth elements and the
effective negative charge on electron-donor oxygen
atoms.
y and z can be estimated by the inequalities y 0,
3 [19].
0
z
For the complexation equilibrium (5), the equilib-
rium constant
:
1
[CeOHL]
=
(7)
1
[CeOH][L]
(charges are omitted) was calculated from the data of
the series D0 pH and the ascending portion of the
saturation curve by the formula
Consequently, the complexation pre-equilibrium
in oxidation of the acids under consideration by ce-
rium(IV) can be represented as
cK0
=
f = 1eff f,
(8)
1
(cM cK0 )(cL cK0 )
1
x
CeOH3+ + Lx
[(CeOH)L]3
(5)
m
f = 1 + Bi [H]i
i = 1
or, which is equivalent in terms of the theory of ionic
equilibria,
for each point of the dependence
log = log + log f
1
x
CeOH3+ + Lx
[(CeO)HL]3
,
(6)
(9)
1
1eff
where x = 2, 3, and 4 for, respectively, H2Lact,
H3Malt, and H4Tart.
with the subsequent averaging of the log
obtained (Tables 1 4).
values
1
It is noteworthy that, in a sulfate medium, the
x
formula [CeOHL]3
should be only regarded as
The agreement between the results obtained by dif-
ferent methods (D pH, molar ratios) in determining
the stability constants for, e.g., the malate complex of
an abbreviated designation of all the possible sub-
stances with variable number of ions of the ionic
x + y
background
z[(NH4)yCeOHL(SO4)z]3
z, where
cerium(IV) (log
= 18.39, log
= 18.45, Tables 2
y
1
1
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 6 2002

COMPLEXATION
OF CERIUM(IV) HYDROXO COMPLEXES WITH OXYCARBOXYLIC ACIDS 871
and 4) indicates that the basic equilibria in solution
are taken into account correctly. The log
value
1
x
for the complex [CeOHL]3 (x = 2, 3, 4), found
in a sulfate medium by averaging data of two to
four series for each of the systems, grows in order
H2Lact < H3Malt < H4Tart with increasing ligand den-
ticity from log
= 14.97 0.18 for H2Lact to
1
log
= 27.83 0.23 for H4Tart.
1
The kinetics and mechanism of the redox process
observed in the system was studied by analyzing the
.
dependence of its initial rate D0 on the equilibrium
concentration of hydrogen ions at the initial con-
centration of the organic ligand in terms of the ge-
neralizations of the D0 pH and D0 cL/cM methods to
.
the case D0
0 (see below), which can be named
.
.
the D0, D0 pH and D0, D0 cL/cM methods, respec-
tively.
The rate constants of the redox decomposition of
x
[CeOHL]3 complexes were calculated from the data
of series D0 pH, D0 cL/cM (supplemented with the de-
.
.
pendences D0 pH, D0 cL/cM) with the use of the equi-
librium concentrations of the complex, calculated from
the series D0 pH, D0 cL/cM (Tables 1 4):
.
0
0
Fig. 4. (1) D pH, (2) D pH, and (3)
D
pH diagrams.
3
M
_
D0 DM
4+
2
System: (a) Ce SO
H Tart (c = c = 2.14 10 M,
4
4 M L
cM, cM < cL;
4+
2
_
D0 DM
I = 2, T = 28.0 C, = 400 nm), (b) Ce SO H Malt
4
3
cK0
=
3
3
(10)
(c = 2.14 10 , c = 2.00 10 M; I = 2; T = 17.7 C;
M
L
_
D0 DM
4+
2
3
= 400 nm), (c) Ce SO
H Lact (c = 6.0 10 ,
4
2 M
cL, cL < cM,
_
2
D0 DM
c
= 3.0 10 M; I = 2; T = 18.0 C; = 400 nm).
L
where cM and cL are the initial concentrations of
the metal ion and ligand, DM the optical density of
the metal ion solution, and D0 the light absorption
by the reaction mixture at the instant of time = 0,
corresponding to a yield of the complex equal to unity.
.
The analysis of nonlinear dependences D0 pH
.
(Fig. 4) and D0 cL/cM (Fig. 2) within the series D0,
.
.
D0 pH, D0, D0 cL/cM was supplemented with anal-
.
ysis of linear dependences D0 cK0 (Fig. 5):
.
D0 = A + kapp cK0,
(11)
.
0
0
4+
2
Fig. 5.
D
c
diagrams. System: (1) Ce SO H Lact
K
4
2
3
2
(c = 6.0 10 , c = 3.0 10 M; I = 2; T = 18.0 C;
= 400 nm); (2) Ce SO H Malt (c = 1.88 10
allowing, in particular, graphical evaluation of the rate
constants of the intramolecular redox decomposition.
M
L
4+
2
3
,
4
3
M
3
2
c
= 3.00 10 2.11 10
M; pH 2.70; I = 2; T =
L
An analytical calculation of the logarithm of the
apparent rate constant of intramolecular redox de-
composition was carried out for each point of the
4+
2
18.0 C; = 400 nm); (3) Ce SO H Tart (c = c
=
4
4
M
L
3
2.14 10 M; I = 2, T = 28.0 C,
= 400 nm).
dependence
Since the apparent rate constants of redox decom-
.
1
position of the complexes, kapp (l mol 1 s ) depend
log kapp = log D0
log c0K
(12)
on their extinction coefficients:
with the subsequent averaging of the log kapp values
obtained.
kapp
=
K l/k2
(13)
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 6 2002

872
VOSKRESENSKAYA, SKORIK
1
(l is the thickness of the absorbing layer of the solu-
tion), logarithms of the true rate constants k2 (s )
E = 132.71 1.40 kJ mol for H2Lact, H3Malt, and
H4Tart.
1
of the intramolecular redox decomposition were also
calculated (Tables 1 4):
The rate constants and activation energy of intra-
molecular redox decomposition, obtained in the pres-
ent study, are in agreement with the corresponding
rate constants and activation energy (log k2 = 3.36
0.06, recalculated to 18 C) and activation energy
log k2 = log kapp
log Kl.
(14)
The calculated k2 values were used to evaluate, in
accordance with the formula
1
(E = 131.8 1.2 kJ mol ) for cerium tartrate
[(CeOH)2Tart]2+ [20] and quinate complexes of ce-
rium(IV) CeOHL]+ [21].
log k2 T log k2 T
2
(15)
E = 2.303R
1 T1T2
T
The rise in the rate pseudoconstants, observed in
going from H2Lact to H3Malt [12] and from H2Malt
to H4Tart [3, 19], is, thus, only apparent and reflects
the increase in the extinction coefficients of the com-
plexes in the series in question. Not only the com-
position of the intermediate complex [5], but also its
reactivity are found to be independent, in the given
series, of the denticity of the organic ligand. The rise
1
[R = 8.314 J K 1 mol is the gas constant, T the ab-
solute temperature (K); T = T2 T1, T2 > T1], the ac-
tivation energy of redox decomposition of the com-
plexes (Table 2).
.
The fact that the dependence D0 = f(cK0 ) re-
mains linear in the concentration ranges under study
x
(Fig. 5) and the constancy of the calculated log kapp
in the stability of the complexes [CeOHL]3
.
.
value within the series D0, D0 pH, D0, D0 cL/cM
(Tables 1 4) indicate that a single kinetically ac-
tive complex is predominant in the systems; and the
equality to zero of its free term A (Fig. 5) points
to the absence of any contribution from the bimolec-
ular pathway to the process within the concentration
ranges studied. Thus, we have an intermediate mech-
anism of oxidation of the organic ligand by ceri-
um(IV).
(x = 2, 3, 4) with increasing ligand denticity is not
accompanied by stabilization of cerium(IV) in this
series of oxyacids.
The independence of the true rate constants k2 and
the activation energy E of intramolecular redox de-
composition from the denticity of oxyacids can be
accounted for (as is done in [20]) by the common
(single-electron) mechanism of electron transfer via
one and the same bridge group in the complexes
x
In this case the initial rate of the redox process,
found using generalized methods of isomolar series
and molar ratios, depends on the yield of the complex
[CeOHL]3 (x = 2 4). According to the assumption
made in [13], this group is the oxo group of the ceryl
ion, which can form a hydrogen bond with the alcohol
hydroxyl of tartaric acid. In terms of the theory of
ionic equilibria this is equivalent to the formation of
a hydrogen bond between a deprotonated alcohol hy-
droxyl of anions of oxyacids and the hydroxy group
of monohydroxo ion of cerium(IV) [Eqs. (5) and (6)].
0
kapp KcM , cM < cL;
.
D0 = kapp cK0
=
(16)
0
kapp KcL, cL < cM;
0
K
[
= (D0 DM)/(D0 DM) is the yield of the com-
plex at instant of time = 0] and reaches a maximum
simultaneously with its maximum yield at the stoi-
chiometric ratio of the components. In the case in
question, this indicates that an intermediate complex
of composition 1 : 1 is formed in the system (Figs. 1c
and 1e; Fig. 2, curve 2).
Thus, the following structure of the intermediate
complexes formed in oxidation by cerium(IV) of the
ligand series under consideration H2Lact (I), H3Malt
(II), and H4Tart (III), can be suggested:
O
O
O
O
C
C
The logarithm of the apparent rate constant of
redox decomposition of the complexes, log kapp (12),
grows in the order lactic < malic < tartaric acid
from 1.39 0.06 to 0.11 0.04 (18 C). However,
the logarithms of true rate constants, log k2, calculated
using expression (14), are constant and equal to
3.40 0.05 for all the complexes (18 C). The activa-
tion energy of intramolecular decomposition also
takes a constant, within experimental error, value
HC O e
HC O e
e
e
HO
Ce4+
HO
Ce4+
O
O
H3C
H2C
C O
C
I
II
HC O e
O
e
HO
Ce4+
.
HC
.
.
O
C O
O
III
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 6 2002

COMPLEXATION
OF CERIUM(IV) HYDROXO COMPLEXES WITH OXYCARBOXYLIC ACIDS 873
The equation for the initial rate of the redox pro-
cess occurring in the systems can be represented, with
account of expressions (3), (7), and (12), as
The pH value was measured in a mixture of com-
ponents, after the recording of the optical density
(
1 min and also
10 min), and in cerium(IV)
sulfate solutions with a pH-673 precision pH meter.
dcM
dt
x
= k2R1[CeOH3+][HmL][H+]
The step constant of protonation of the carboxy
group in the lactic acid anion was also determined
at an ionic strength I = 2
= k2 1[CeOH3+][Hm xLx ] (x = m = 2, 3, 4). (17)
This equation establishes the type of the functional
dependence of the rate on the equilibrium concentra-
tion of hydrogen ions in solution and indicates that
oxyacids are involved in the redox process not as
molecules, contrary to the previous assumption, but as
Lx anions, where x = 2, 3, 4.
[H2L]
=
.
(18)
2
[HL][H]
The value log
= 3.72 0.02, obtained for
2
H2Lact, is in agreement with log
= 3.69 (I = 2)
[7]. The logarithms of step constants2of protonation of
carboxy groups in the malic acid anion (log 1 = 3.22,
To conclude, it should be mentioned that the redox
reactions analyzed in the present study can be, ap-
parently, regarded as indicator reactions [22] for the
corresponding complexation pre-equilibria and used
not only to solve the inverse problem of determining
the rate constants of intramolecular redox decomposi-
tion, but also to tackle with the direct problem of
finding the equilibrium concentrations of the com-
plexes (and, as a consequence, of their stability con-
stants) by kinetic methods.
log
= 4.71) were taken from [24], and those for
the ta2rtaric acid anion (log
from [25].
= 2.95, log
= 4.11),
1
2
Since, according to the dissociation constants of
oxy groups, oxyacids are weak acids, the following
value of the logarithm of the dissociation constant of
the oxy group was used in calculations for all oxy-
acids: log B1 = 14.6 (obtained by averaging the data
of [16, 25]; in [25], the value of log B1 was deter-
mined by mathematical modeling). The logarithms of
the total protonation constants Bi = [HiL] / [L][H]i,
i > 1, were taken to be as follows: log B2 = 18.31 for
H2Lact; log B2 = 19.31, log B3 = 22.53 for H3Malt;
and log B2 = 25.90, log B3 = 30.01, and log B4 =
32.96 for H4Tart.
EXPERIMENTAL
Twice recrystallized lactic H2Lact, malic H3Malt,
and tartaric H4Tart acids, oxycarboxylic acid of chem-
ically pure grade, and cerium(IV) sulfate Ce(SO4)2
4H2O of analytically pure grade were used in the
study. The required ionic strength of solution, I = 2,
was created with ammonium sulfate of analytically
pure grade. The initial solutions wee prepared from
precisely weighed portions. The concentration of
organic acid solutions was refined by pH-metric titra-
tion with a NaOH solution containing no CO23 ions.
The content of cerium (IV) in the salt and a freshly
prepared solution were determined by back titration
with Mohr’s salt in the presence of ferroin [23] before
and after an experiment.
CONCLUSIONS
(1) A generalization of the methods used to study
complexation (D pH method and methods of molar
ratios and isomolar series) to the case on nonzero rate
of redox decomposition of complexes was proposed.
(2) The above methods were used to determine
the composition, stability constants, true rate con-
stants, and activation energies of intramolecular redox
decomposition of hydroxo complexes of cerium(IV)
with a number of organic ligands; the law governing
the rate of the redox process occurring in the systems
and its intermediate mechanism was established.
The optical density of the solutions was recorded
with a Specord UV VIS recording spectrophotometer
and a KF-5 photoelectrocolorimeter with MEA-4 re-
cording unit, mainly at a wavelength of 400 nm. As
the time of reaction onset was taken the instant at
which the mixing vessel, in which the starting com-
ponents were placed, was turned upside-down. The
initial optical density of the reaction mixture of the
metal and the ligand D0 was found by linear extrapo-
lation of the kinetic curves plotted in semilogarithmic
coordinates to the instant of time = 0. The solutions
were thermostated using a Wobser thermostat.
(3) The absence of stabilization of cerium(IV), with
the stability of its complexes increasing in the series
of the oxycarboxylic acids considered, and the in-
dependence of the reactivity of these complexes from
the denticity of an organic ligand were demonstrated.
(4) The independence of the rate constants and ac-
tivation energy of redox decomposition of complexes
from the denticity of oxyacids was interpreted in
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 6 2002

874
VOSKRESENSKAYA, SKORIK
terms of the bridge mechanism of redox decomposi-
11. Mehrotra, R.N. and Chosh, S., Z. phys. Chem. (DDR),
1965, vol. 230, no. 3/4, pp. 231 239.
12. Lahid, A. and Alexander, A., J. Chem. Soc. Dalton
Trans., 1974, no. 23, pp. 2521 2530.
tion as transfer of an electron via one and the same
m
bridge group in the complexes [CeOHL]3
3, 4).
(m = 2,
13. Luzan, A.A. and Yatsimirskii, K.B., Zh. Neorg.
Khim., 1968, vol. 13, no. 12, pp. 3216 3221.
14. Vakhramova, G.P., Pechurova, N.I., and Spitsyn, V.I.,
Izv. Akad. Nauk SSSR, Ser. Khim., 1974, no. 10,
pp. 2361 2366.
(5) A conclusion was made that the redox reactions
considered in the present study can be used as in-
dicator reactions in investigating the corresponding
complexation equilibria by kinetic methods.
15. Vakhramova, G.P., Pechurova, N.I., and Spitsyn, V.I.,
Zh. Neorg. Khim., 1974, vol. 19, no. 10,
pp. 2722 2725.
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