Chelation behaviour of CE(III), Th(IV), and UO2(VI) with 5,7-dihydroxy-6-formyl-2-methylbenzopyran-4-one Schiff bases

Article abstract of DOI:10.1007/PL00010120

The stability constants of the stepwise chelate formation between Ce³⁺, Th⁴⁺, and UO,²⁺₂ ions and some Schiff bases derived from 5,7-dihydroxy-6-formyl-2-methylbenzopyran-4-one were determined conductometrically as well as potentiometrically applying the Irving and Rossotti techniques. The results indicate the formation of 1:1 and 1:2 (M:L) chelates. The stability constants increase with decreasing ionic strength, decreasing temperature, and decreasing percentage of ethanol. In different organic solvents (70% v/v solvent-water media), the stability constant values follow the order MeOH < EtOH < 1-propanol < 2-propanol < acetone < DMSO depending on both the dielectric constant and the hydrogen bonding structure of the medium. The thermodynamic parameters (ΔG, ΔH, and ΔS) were calculated; they indicate that the chelate formation is an exothermic process and that the degree of disorder of the solution increases upon chelation.

Full text of DOI:10.1007/PL00010120

Monatshefte fuÈr Chemie 129, 985±998 (1998)
Chelation Behaviour of Ce(III), Th(IV),
and UO2(VI) with 5,7-Dihydroxy-6-formyl-2-
methylbenzopyran-4-one Schiff Bases
Yousry M. Issa1;Ã, Omyma E. Sherif1, and Safaa M. Abbas2
1
Chemistry Department, Faculty of Science, Cairo University, Giza, Egypt
2
Chemistry Department, Faculty of Science, Cairo University, Beni Suef, Egypt
Summary. The stability constants of the stepwise chelate formation between Ce3‡, Th4‡, and UO22‡
ions and some Schiff bases derived from 5,7-dihydroxy-6-formyl-2-methylbenzopyran-4-one were
determined conductometrically as well as potentiometrically applying the Irving and Rossotti
techniques. The results indicate the formation of 1:1 and 1:2 (M:L) chelates. The stability constants
increase with decreasing ionic strength, decreasing temperature, and decreasing percentage of
ethanol. In different organic solvents (70% v/v solvent-water media), the stability constant values
follow the order MeOH < EtOH < 1-propanol < 2-propanol < acetone < DMSO depending on both the
dielectric constant and the hydrogen bonding structure of the medium. The thermodynamic
parameters (ÁG, ÁH, and ÁS) were calculated; they indicate that the chelate formation is an
exothermic process and that the degree of disorder of the solution increases upon chelation.
Keywords. Ce3‡; Th4‡; UO22‡; Schiff bases; Chromone.
Das Chelierungsverhalten von Ce3‡, Th4‡ und UO22‡ mit Schiffschen Basen vom Typ
5,7-Dihydroxy-6-formyl-2-methylbenzopyran-4-on
3‡
Zusammenfassung. Die Stabilitatskonstanten fur die stufenweise Chelatbildung von Ce -, Th4‡
È
-
È
und UO22‡-Ionen mit von 5,7-Dihydroxy-6-formyl-2-methylbenzopyran-4-onen abgeleiteten Schiff-
schen Basen wurden konduktometrisch und potentiometrisch mit Hilfe der Irving-Rossotti-Technik
bestimmt. Die Ergebnisse zeigen die Bildung von 1:1- und 1:2-Chelaten (M:L) an. Die
È È
Stabilitakonstanten steigen mit steigender Ionenstarke, abnehmender Temperatur und abnehmendem
È È
Anteil von Ethanol. In verschiedenen organischen Losungsmitteln (70% v/v Losungsmittel-Wasser-
È
Gemische) folgen die Stabilitatskonstanten der Reihe MeOH < EtOH < 1-Propanol < 2-Propanol
È È
< Aceton < DMSO, hangen also sowohl von der Dielektrizitatskonstante als auch von der Wasser-
stoffbruÈckenbindungsstruktur des Mediums ab. Die thermodynamischen Parameter (ÁG, ÁH und
È
ÁS) wurden bestimmt; sie weisen darauf hin, daû die Bildung solcher Chelate exotherm verlauft und
È
daû und der Unordnungsgrad der Losung bei der Chelierung steigt.
Introduction
The condensation of primary amines with carbonyl compounds yields Schiff bases
[1, 2]. The complexation of Schiff bases with lanthanides has been the subject of
Ã
Corresponding author

986
Y. M. Issa et al.
1
X ˆ H (a), o-COOH (b), o-OH (c), o-OH, m-Cl (d), o-OH, p-SO3H (e)
Scheme 1
many investigations [3±11] including determination of stepwise and overall
formation constants and evaluation of the effects of ionic strength and temperature
of the medium on the stability of the chelates.
The present paper deals with conductometric and potentiometric studies of the
chelates of Ce3‡, Th4‡, and UO22‡ with Schiff bases prepared from 5,7-dihydroxy-
6-formyl-2-methylbenzopyran-4-one and aniline, anthranilic acid, o-aminophenol,
2-amino-5-chlorophenol, and 4-amino-3-hydroxybenzenesulfonic acid (Scheme 1).
The conductometric titrations were carried out in 70% (v/v) ethanol-water whereas
the potentiometric titrations were performed according to Irving and Rossotti [12]
at different ionic strengths (0.14, 0.10, 0.06, and 0.02 M NaCl) and different
temperatures (55, 45, 35, and 25ꢀC). The effects of varying alcohol content (90,
80, 70, and 60% (v/v)) and different solvents (methanol, ethanol, 1-propanol,
2-propanol, acetone, and DMSO) on the stability constants of the metal chelates
were also studied. The thermodynamic parameters (ÁG, ÁH, and ÁS) of the
formed chelates were calculated.
Results and Discussion
Conductometric titration curves indicate that the increase of conductance upon
‡
addition of metal ion solution is mainly due to the release of H ions. Taking the
chelation behaviour of Ce3‡ with 1a as an example, one can formulate
‡
‡
2H2L ‡ Ce3‡ „ 2H ‡ ‰CeꢁHL† Š
2
Further addition of metal ions beyond complete conversion of HL to an 1:2
species will produce a new 1:1 species according to
‡
‡
‡
‰CeꢁHL† Š ‡ Ce3‡ „ 2‰CeLŠ ‡ 2H
2
The Schiff bases possess two centers of chelation. The ®rst one stems from the
azomethine group and the OH group in its ortho position, and the second one
involves the chromone carbonyl group in position 4 and the deprotonated hydroxy
group in position 5. Accordingly, addition of metal ions to the 1:1 ML species
results in a new 2:1 M2L species:
‡
4‡
‰CeLŠ ‡ Ce3‡ „ ‰Ce2LŠ
The conductometric titration curves for the complexes show three breaks
(Fig. 1) at molar ratios of 2:1, 1:1, and 1:2, indicating the formation of three types
of complexes with stoichiometric ratios 2:1, 1:1, and 1:2 (M:L). Higher molar

Chelation of Ce3‡, Th4‡, and UO22‡ with Schiff Bases
987
Fig. 1. Conductometric titration curves for 4 Â 10ꢂ5M solutions of Schiff bases 1a±e with 10ꢂ3M
Th4‡ ions
ratios can be excluded due to steric hindrance generated upon accommodating
more than two Schiff base molecules around the metal ion. The stability constants
of the 1:1 and 1:2 chelates (logꢀ1 and logꢀ2) were calculated from pH-metric
titration results.
From the pH titration curves of the mixtures containing Schiff bases and metal
ions, the average number of ligand molecules attached to a metal ion (ꢀn) and the
free ligand exponent ( pL) were calculated and plotted against each other, thus
affording the formation curves of the metal complexes (Fig. 2). The ꢀn values never
exceed 2 for all complexes, indicating the formation of 1:1 and 1:2 (M:L)
complexes only. The possibility of formation of binuclear chelates was considered
negligible due to the very low concentrations of the metal ions used (10ꢂ4 M) [13].
The stability constants of the complexes were studied with respect to the effect of
ionic strength, temperature, ethanol percentage, and solvent type. Under all
conditions, the stability constant values exhibit the following decreasing order [14]:
Th4‡> Ce3‡> UO22‡. This can be attributed to the decrease of charge on the central
metal ion [15] which decreases the electrostatic attraction between the cation and
the Schiff base.

988
Y. M. Issa et al.
Fig. 2. Formation curves of UO22‡ chelates with Schiff bases 1a±e (0.106 M NaCl, 298 K) in 70% v/v
ethanol-water
logꢀ1 Ce3‡
logꢀ2 Ce3‡
logꢀ2 Th4‡
logꢀ2 Th4‡
logꢀ2 UO22‡
logꢀ1 UO22‡
p
Fig. 3. Plots of logꢀ1 and logꢀ2 (mean values) vs ꢁ for chelates of 1a with Ce3‡, Th4‡, and UO22‡
at 298 K (70% v/v ethanol-water)
In case of Ce3‡ and Th4‡ chelates, the stability constants follow the order 1c >1d
>1a >1b >1e, whereas in the case of UO22‡ chelates the order is 1c > 1a > 1d >1b
>1e. This behaviour correlates with the effect of substitution on the phenyl ring as
follows:
1) The high stability of Schiff base 1c complexes can be attributed to the presence
of the OH group in the o0-position relative to the azomethine group, forming a

Chelation of Ce3‡, Th4‡, and UO22‡ with Schiff Bases
989
Table 1. logꢀ1 and logꢀ2 values for Ce3‡, Th4‡, and UO22‡ chelates with Schiff bases 1a±e at different
ionic strengths
ꢁ
(NaCl)
Ce3‡
Th4‡
UO22‡
logꢀ1
logꢀ2
logꢀ1
logꢀ2
logꢀ1
logꢀ2
mean SD
value
mean SD
value
mean SD
value
mean SD
value
mean SD
value
mean SD
value
1a
1b
1c
1d
1e
0.146 6.73 0.02 12.08 0.02 6.84 0.03 13.58 0.03 6.30 0.03 11.02 0.01
5.37 0.03 9.98 0.02 5.52 0.02 11.01 0.02 5.13 0.02 9.82 0.02
7.02 0.03 13.21 0.04 7.63 0.03 14.84 0.03 6.57 0.03 11.98 0.03
6.79 0.01 12.84 0.03 7.54 0.04 14.69 0.02 6.12 0.03 11.12 0.02
4.42 0.03
8.38 0.02 5.35 0.03 10.03 0.02 4.05 0.01
7.80 0.02
1a
1b
1c
1d
1e
0.106 7.04 0.03 12.64 0.01 7.30 0.02 14.06 0.02 6.68 0.02 12.55 0.03
5.63 0.03 10.45 0.03 5.85 0.01 11.42 0.02 5.54 0.05 10.17 0.03
7.34 0.05 13.85 0.01 7.98 0.02 15.74 0.02 6.76 0.02 12.66 0.02
7.10 0.04 13.23 0.03 7.88 0.02 15.50 0.03 6.33 0.04 12.07 0.03
4.71 0.02
8.86 0.04 5.64 0.03 10.44 0.02 4.37 0.04
7.84 0.02
1a
1b
1c
1d
1e
0.066 7.47 0.03 12.99 0.02 7.81 0.02 14.39 0.02 7.00 0.03 12.90 0.02
6.00 0.02 11.25 0.02 6.22 0.03 11.81 0.03 5.81 0.02 10.98 0.03
7.66 0.01 14.66 0.03 8.51 0.02 16.71 0.50 7.01 0.02 12.92 0.02
7.29 0.04 13.94 4.99 8.37 0.03 15.52 0.51 6.79 0.03 12.02 0.03
5.02 0.03
9.26 0.03 6.20 0.02 10.72 0.02 4.77 0.02
8.62 0.03
1a
1b
1c
1d
1e
0.026 7.86 0.03 14.68 0.03 8.33 0.02 15.11 0.01 7.23 0.02 13.33 0.02
6.40 0.04 12.08 0.04 6.76 0.02 12.94 0.04 6.17 0.03 11.84 0.02
7.98 0.03 15.23 0.02 8.83 0.03 17.22 0.02 7.40 0.03 14.20 0.02
7.59 0.02 14.29 0.04 8.74 0.03 16.92 0.02 7.11 0.02 13.29 0.03
5.35 0.03 10.15 0.04 6.80 0.02 10.89 0.03 5.22 0.03 10.00 0.03
1a
1b
1c
1d
1e
0.00a 8.52 0.02 15.51 0.03 9.29 0.03 17.40 0.02 7.86 0.12 15.17 0.02
7.12 0.04 13.48 0.03 7.51 1.16 14.01 0.02 6.84 0.03 13.07 0.04
8.64 0.02 16.55 0.04 9.68 0.02 18.48 0.02 7.78 0.01 14.91 0.02
8.11 0.03 15.21 0.04 9.50 0.03 18.12 0.03 7.67 0.03 14.63 0.04
5.94 0.04 11.19 0.05 7.69 0.04 11.53 0.02 5.91 0.02 11.16 0.04
SD: standard deviation; mean values obtained by interpolation at half ꢀn, successive approximation, and
a
mid point methods; logꢀ values obtained by linear regression analysis
new reactive center for chelation through NO0; the double chelation through
NOO0 contributes to the complex stability.
2) The lower stability of 1d chelates is caused by the -I effect of the Cl atom para
to the OH group in the anilino ring which decreases the electron density around
the OH group, thus decreasing its ability for chelation and therefore the stability
of the complexes. In case of Ce3‡ and Th4‡, the small size of these ions renders
them unaffected by the steric effect of the Cl atom, and their complexes with 1d

990
Y. M. Issa et al.
Table 2. logꢀ1 and logꢀ2 values for Ce3‡, Th4‡, and UO22‡ chelates with Schiff bases 1a±e at different
temperatures
T
Ce3‡
Th4‡
UO22‡
(K)
logꢀ1
logꢀ2
logꢀ1
logꢀ2
logꢀ1
logꢀ2
mean SD
value
mean SD
value
mean SD
value
mean SD
value
mean SD
value
mean SD
value
1a
1b
1c
1d
1e
328
318
308
298
273a
6.11 0.01 11.10 0.04 6.44 0.02 12.70 0.02 5.99 0.02 11.11 0.01
4.91 0.04 9.23 0.02 5.12 0.02 9.88 0.02 5.00 0.01 8.77 0.04
6.37 0.06 11.96 0.01 7.18 0.01 13.71 0.01 6.04 0.06 11.33 0.01
6.24 0.03 11.83 0.02 6.95 1.01 13.54 1.01 5.55 0.02 10.75 0.02
4.11 0.05
6.31 0.03 11.48 0.02 6.74 0.02 13.15 0.02 6.19 0.02 11.55 0.02
5.12 0.03 9.62 0.01 5.35 0.01 10.29 0.01 5.17 0.03 9.28 0.26
7.95 0.03 4.92 0.02
9.38 0.02 3.92 0.02
6.67 0.03
1a
1b
1c
1d
1e
6.63 0.03 12.82 0.03 7.43 0.05 14.25 0.05 6.26 0.03 11.81 0.02
6.43 0.05 12.19 0.02 7.25 0.02 14.10 0.02 5.75 0.02 11.12 0.02
4.27 0.04
6.64 0.03 11.90 0.02 7.02 0.02 13.60 0.02 6.42 0.02 11.95 0.03
5.39 0.02 10.03 0.04 5.60 0.02 10.80 0.02 5.35 0.01 9.78 0.03
8.23 0.05 5.09 0.03
9.70 0.03 4.06 0.04
6.99 0.03
1a
1b
1c
1d
1e
7.10 0.02 13.30 0.02 7.71 0.04 14.99 0.04 6.48 0.03 12.22 0.04
6.75 0.05 12.65 0.03 7.50 0.01 14.99 0.01 6.04 0.03 11.50 0.02
4.52 0.04
8.52 0.03 5.27 0.03 10.04 0.03 4.22 0.02
7.23 0.03
1a
1b
1c
1d
1e
7.04 0.01 12.54 0.01 7.30 0.02 14.06 0.02 6.68 0.02 12.55 0.02
5.63 0.03 10.45 0.02 5.85 0.01 11.42 0.01 5.54 0.04 10.17 0.03
7.34 0.02 13.85 0.01 7.98 0.02 15.74 0.02 6.76 0.02 12.66 0.03
7.10 0.04 13.23 0.02 7.88 0.02 15.50 0.02 6.33 0.03 12.07 0.03
4.71 0.02
8.86 0.03 5.64 0.02 10.44 0.02 4.37 0.03
7.84 0.07
1a
1b
1c
1d
1e
7.86 0.04 13.18 0.03 7.74 0.01 15.74 0.01 7.25 0.01 13.64 0.02
6.21 0.02 11.47 0.02 6.35 0.03 12.59 0.03 5.96 0.01 11.47 0.01
8.22 0.04 15.75 0.03 8.39 0.02 17.48 0.02 7.18 0.03 13.78 0.02
7.83 0.03 14.67 0.01 8.41 0.02 17.09 0.02 6.81 0.02 12.91 0.02
5.22 0.03
9.51 0.03 6.10 0.03 11.20 0.03 4.80 0.03
9.05 0.04
SD: standard deviation; mean values obtained by interpolation at half ꢀn, successive approximation, and
a
mid point methods; logꢀ values obtained by linear regression analysis
are more stable than those with 1a. In case of the UO22‡ ion, its large diameter
[16] in combination with the bulky Cl atom decreases the stability of UO22‡ -1d
chelates due to steric effects; hence, the stability of UO22‡ complexes with 1d is
lower than that of its complexes with 1a.
3) The lower stability of 1b chelates relative to those of 1c can be attributed to
their lower pKa values (1b: 6.42, 7.30, and 9.44; 1c: 6.70, 8.26, and 11.22),
indicating an increase of the ionic character of the 1b chelates and therefore
point to facilitated hydrolysis.

Chelation of Ce3‡, Th4‡, and UO22‡ with Schiff Bases
991
logꢀ1 Ce3‡
logꢀ2 Ce3‡
logꢀ1 Th4‡
logꢀ2 Th4‡
logꢀ1 UO22‡
logꢀ2 UO22‡
Fig. 4. Plots of logꢀ1 and logꢀ2 mean values vs I/T for chelates of 1a with Ce3‡, Th4‡ and UO22‡
0.106 M NaCl 70% v/v ethanol-water
4) The electron withdrawing effect of the bulky SO3H group in p-position of the
azomethine group (in addition to the OH group in o-position to the -CH ˆ N-
group) decreases the stability of 1e chelates.
The ionic strength of solutions is related to the activity coef®cient [17]
according to log f ˆ A2 Á Zi2 Á ꢁ1=2 where f is the activity coef®cient, A is a constant,
and Zi is the ionic charge. Therefore, an increase in ionic strength will decrease
the activity coef®cient and, consequently, the activity of the ions. The tendency for
chelation thus decreases as re¯ected by the decreasing stability constants of the
È
metal chelates. This is in full agreement with the Debye-Huckel equation ([18];
p
p
logꢀ ˆ logꢀꢀ ꢂ ꢁ0:509 Z2 ꢁ=ꢁ1 ‡ ꢂ ꢁ††; ꢁ: ionic strength of the medium, ꢀ:
stability constant at a given ionic strength, ꢀꢀ: thermodynamic formation constant
at zero ionic strength, ꢂ: activity coef®cient).
Thermodynamic formation constants (at ꢁ ˆ 0) were obtained by applying
p
linear regression analysis. The plot of logꢀ1 and logꢀ2 vs. ꢁ (Fig. 3) is linear with
a variation coef®cient ꢃ 0.98 [16]. The obtained results are listed in Table 1.
The values of logꢀ1 and logꢀ2 at different temperatures (Table 2) show that the
stability constants of the complexes decrease with increasing temperature. This can
be attributed to the thermal hydrolysis of the metal complexes [18]. The values of
ÁG for the ionization process can be calculated from Eq. (1) [20]:
ꢂÁG ˆ 2:303RT logꢀ
Substituting in ÁG ˆ ÁH -TÁS and resolving for logꢀ affords Eq. (2) [19]:
logꢀ ˆ ÁS=2:303R ꢂ ÁH=2:303R Á 1=T
ꢁ1†
ꢁ2†
A plot of logꢀ vs. 1/T therefore gives a straight line (Fig. 4; variation coef®cient
ꢃ 0.97) with a slope of ÁH/2.303R and an intercept of ÁS/2.303R.
The negative values of both ÁG and ÁH indicate that the complexation
reaction is exothermic. All ÁS values are positive due to displacement of the

992
Y. M. Issa et al.
Table 3. Thermodynamic parameters of Ce3‡ chelates with Schiff bases 1a±e
mean
value
T
ꢂÁG
ꢂÁH
ÁS
(J Á molꢂ1 Á Kꢂ1
114.69
(K)
(kJ Á molꢂ1
)
(kJ Á molꢂ1
52.42
)
)
1a
1b
1c
1d
1e
logꢀ1
logꢀ2
6.11
6.31
328
318
308
298
328
318
308
298
38.31
38.36
39.09
40.10
69.59
69.78
70.06
71.43
6.64
7.04
11.10
11.48
11.90
12.54
63.78
224.60
logꢀ1
logꢀ2
4.91
5.12
328
318
308
298
328
318
308
298
30.78
31.12
31.73
32.07
57.88
58.48
59.05
59.53
36.89
69.17
91.75
5.39
5.63
9.23
176.81
9.62
10.03
10.45
logꢀ1
logꢀ2
6.37
6.63
328
318
308
298
328
318
308
298
39.94
40.30
41.80
41.81
74.99
77.93
78.30
78.89
51.42
98.10
124.25
236.07
7.10
7.34
11.96
12.82
13.30
13.85
logꢀ1
logꢀ2
6.24
6.53
328
318
308
298
328
318
308
298
39.12
39.69
39.74
40.44
74.17
74.11
74.48
75.36
47.34
78.12
120.42
223.64
6.75
7.10
11.83
12.19
12.65
13.23
logꢀ1
logꢀ2
4.11
4.27
4.52
4.71
7.95
8.23
8.52
8.86
328
318
308
298
328
318
308
298
25.77
25.96
26.61
26.83
49.84
50.03
50.16
50.47
45.87
51.90
79.33
151.01

Chelation of Ce3‡, Th4‡, and UO22‡ with Schiff Bases
993
Table 4. Thermodynamic parameters of Th4‡ chelates with Schiff bases 1a±e
mean
value
T
ꢂÁG
ꢂÁH
ÁS
(J Á molꢂ1 Á Kꢂ1
127.11
(K)
(kJ Á molꢂ1
)
(kJ Á molꢂ1
41.50
)
)
1a
1b
1c
1d
1e
logꢀ1
logꢀ2
6.44
6.74
328
318
308
298
328
318
308
298
40.38
40.97
41.33
41.58
79.63
79.93
80.07
80.19
7.02
7.30
12.70
13.15
13.60
14.08
63.72
242.76
logꢀ1
logꢀ2
5.12
5.35
328
318
308
298
328
318
308
298
32.14
32.56
32.97
33.36
62.02
62.55
63.66
65.13
57.34
69.05
98.44
5.60
5.85
9.88
192.11
10.29
10.80
11.42
logꢀ1
logꢀ2
7.18
7.43
328
318
308
298
328
318
308
298
45.02
45.16
45.39
45.46
86.06
86.62
88.25
89.66
44.11
99.60
135.72
256.14
7.71
7.98
13.71
14.25
14.99
15.74
logꢀ1
logꢀ2
6.95
7.25
328
318
308
298
328
318
308
298
43.57
44.07
44.16
44.89
84.89
85.71
87.62
88.29
51.07
131.89
262.83
7.50
7.88
13.54
14.10
14.88
15.50
121.37
logꢀ1
logꢀ2
4.92
5.09
328
318
308
298
328
318
308
298
30.85
30.94
31.03
32.13
58.81
59.03
59.11
59.47
39.34
56.22
94.62
5.27
5.64
9.38
176.81
9.70
10.04
10.44

994
Y. M. Issa et al.
Table 5. Thermodynamic parameters of UO22‡ chelates with Schiff bases 1a±e
mean
value
T
ꢂÁG
ꢂÁH
ÁS
(J Á molꢂ1 Á Kꢂ1
113.73
(K)
(kJ Á molꢂ1
)
(kJ Á molꢂ1
31.86
)
)
1a
1b
1c
1d
1e
logꢀ1
logꢀ2
5.99
6.19
328
318
308
298
328
318
308
298
37.56
37.63
37.80
38.05
69.84
70.21
70.35
71.49
6.42
6.68
11.14
11.55
11.95
12.55
63.72
212.18
logꢀ1
logꢀ2
5.00
5.17
5.35
5.54
8.77
9.28
9.78
10.17
328
318
308
298
328
318
308
298
31.35
31.43
31.50
31.56
54.99
56.41
57.58
57.93
22.85
50.97
94.62
214.09
logꢀ1
logꢀ2
6.04
6.26
328
318
308
298
328
318
308
298
37.87
38.05
38.15
38.51
71.04
71.79
71.94
72.11
29.41
63.72
115.65
215.04
6.48
6.76
11.33
11.81
12.22
12.66
logꢀ1
logꢀ2
5.55
5.75
328
318
308
298
328
318
308
298
34.80
34.95
35.56
36.06
67.40
67.60
67.71
68.75
33.73
65.71
104.18
194.97
6.04
6.33
10.75
11.12
11.50
12.07
logꢀ1
logꢀ2
3.92
4.06
4.22
4.37
6.67
6.99
7.23
7.84
328
318
308
298
328
318
308
298
24.58
24.68
24.85
24.89
41.82
42.49
42.57
44.66
20.78
38.23
74.55
205.49

Chelation of Ce3‡, Th4‡, and UO22‡ with Schiff Bases
995
logꢀ1 Ce3‡
logꢀ2 Ce3‡
logꢀ1 Th4‡
logꢀ2 Th4‡
logꢀ1 UO22‡
logꢀ2 UO22‡
Fig. 5. Plots of logꢀ1 and logꢀ2 (mean values) vs. percentage of ethanol for chelates of 1a±e with
Ce3‡, Th4‡, and UO22‡ at 0.106 M NaCl and 298 K
highly oriented and ordered water molecules by Schiff base molecules, thus
increasing the disorder in the solution by complexation.
The stability constants at 273 K were obtained applying linear regression
analysis. The plots of logꢀ1 and logꢀ2 vs. T give straight lines with correlation
coef®cients ꢃ0.98. The stability constant temperature coef®cient (rate of change of
logꢀ with temperature) ranged between 0.01 and 0.03.
The investigation of solvent effects on the stability constants of the metal
chelates was carried out in different ethanol-water compositions and in different
solvents. The results, listed in Table 6, show that as the percentage of ethanol
increases the stability constants of the complexes also increase. This can be
attributed to the higher solvation ability of water as compared to ethanol that may
compete with Schiff bases in terms of coordination with the metal cation. In
addition, the lower dielectric constant of ethanol may also increase the electrostatic
contribution of the metal-Schiff base bond formation, thus leading to an increase of
the stability constants with its increasing amount in solvent mixture [21]. The plot
of logꢀ vs. ethanol percentage gives a straight line (Fig. 5) [22]. The stability
constants of the chelates in pure ethanol and pure water are obtained by linear
regression analysis.
The dielectric constant of any solvent-water medium can be calculated using
Eq. (3) where D is the dielectric constant and n is the mole fraction:
Dꢁmedium† ˆ ꢁDꢁsolvent†  nꢁsolvent†† ‡ ꢁDꢁwater†  nꢁwater††
ꢁ3†
The stability constants of metal-Schiff base chelates in different solvents of
varying polarities (dielectric constants: MeOH, 32.62; EtOH, 24.3; 1-propanol,
20.1; 2-propanol, 18.3; acetone, 20.7; DMSO, 4.7) increase following the sequence
MeOH < EtOH < 1-propanol < 2-propanol < acetone < DMSO (Table 7), indicating
that the dielectric constant of the medium is not the predominant factor affecting
the change of logꢀ values since their relationship is not linear. This can be
attributed to the dependence of the stability constants on both the dielectric

996
Y. M. Issa et al.
Table 6. logꢀ1 and logꢀ2 values for Ce3‡, Th4‡, and UO22‡ chelates with Schiff bases 1a±e at different
percentages of ethanol
%EtOH Ce3‡
Th4‡
UO22‡
logꢀ1
logꢀ2
logꢀ1
logꢀ2
logꢀ1
logꢀ2
mean SD
value
mean SD
value
mean SD
value
mean SD
value
mean SD
value
mean SD
value
1a
1b
1c
1d
1e
1a
1b
1c
1d
1e
1a
1b
1c
1d
1e
1a
1b
1c
1d
1e
1a
1b
1c
1d
1e
1a
1b
1c
1d
1e
60
70
6.87 0.02 12.27 0.02 7.17 0.02 13.93 0.03 6.47 0.03 11.79 0.03
5.51 0.02 10.16 0.02 5.66 0.02 10.82 0.03 5.36 0.03 9.79 0.05
7.22 0.02 13.43 0.02 7.78 0.02 15.09 0.02 6.65 0.03 11.99 0.03
6.83 0.01 13.15 0.02 7.63 0.02 14.84 0.03 6.20 0.02 11.54 0.01
4.60 0.02
8.62 0.02 5.51 0.01 10.72 0.02 4.28 0.01
7.43 0.01
7.04 0.02 12.54 0.01 7.30 0.02 14.06 0.02 6.68 0.02 12.55 0.03
5.63 0.02 10.45 0.03 5.85 0.01 11.42 0.02 5.54 0.04 10.17 0.03
7.34 0.02 13.85 0.01 7.98 0.02 15.74 0.02 6.76 0.02 12.66 0.02
7.10 0.02 13.23 0.03 7.88 0.02 15.50 0.03 6.33 0.03 12.07 0.03
4.71 0.02
8.86 0.04 5.64 0.02 10.44 0.02 4.37 0.03
7.84 0.01
80
7.26 0.02 12.97 0.01 7.43 0.02 14.35 0.04 6.84 0.03 12.72 0.02
5.80 0.03 11.20 0.03 6.10 0.02 11.94 0.03 5.82 0.01 10.97 0.02
7.62 0.03 14.85 0.04 8.20 0.03 15.94 0.02 7.12 0.02 13.09 0.01
7.46 0.03 14.17 0.02 8.10 0.02 15.75 0.02 6.52 0.02 12.56 0.03
4.99 0.03
9.34 0.02 5.83 0.01 11.41 0.03 4.69 0.02
8.77 0.02
90
7.53 0.03 13.88 0.03 7.69 0.02 14.88 0.03 7.08 0.02 13.49 0.02
6.36 0.02 11.92 0.03 6.45 0.02 12.13 0.03 6.52 0.02 11.47 0.02
7.98 0.01 15.07 0.02 8.57 0.02 16.82 0.03 7.76 0.21 13.60 0.01
7.86 0.02 14.93 0.01 8.37 0.03 16.20 0.02 6.94 0.02 13.01 0.01
5.32 0.02 10.23 0.02 6.00 0.03 11.97 0.02 5.22 0.02
9.50 0.01
100a
7.74 0.02 15.75 0.04 6.06 0.03 15.10 0.03 7.46 0.00 14.77 0.03
6.50 0.05 12.44 0.03 6.66 0.03 12.67 0.02 6.74 0.01 12.00 0.02
8.18 0.01 15.78 0.03 8.76 0.02 17.27 0.03 8.00 0.02 14.98 0.02
8.17 0.01 15.41 0.03 8.62 0.01 16.66 0.01 7.07 0.02 13.37 0.02
5.52 0.02 10.58 0.02 6.18 0.02 12.26 0.03 5.54 0.01 10.17 0.02
0a
5.53 0.01
3.90 0.01
5.62 0.02
4.73 0.02
3.08 0.02
8.97 0.01 6.13 0.05 11.95 0.03 5.28 0.02
6.41 0.02 4.05 0.06 8.24 0.03 2.99 0.10
8.69 0.06
6.22 0.03
8.89 0.03
8.62 0.04
3.03 0.02
9.86 0.03 6.19 0.01 12.46 0.04 5.31 0.03
9.16 0.02 6.17 0.04 12.33 0.04 4.69 0.02
5.28 0.03 4.50 0.04
7.60 0.02 2.29 0.04
SD: standard deviation; mean values obtained by interpolation at half ꢀn, successive approximation, and
a
mid point methods; logꢀ values obtained by linear regression analysis
constant and the hydrogen bonding structure of the solvent and is illustrated as
follows:
1) Within the alcoholic series (from methanol to 2-propanol), the solvation power
decreases and the stability constants of the complexes increase with decreasing
dielectric constant [23].

Chelation of Ce3‡, Th4‡, and UO22‡ with Schiff Bases
997
Table 7. logꢀ1 and logꢀ2 values for Ce3‡, Th4‡, and UO22‡ chelates with Schiff bases 1a±e in different
solvents (70% (v/v) solvent-water)
Solvent
Ce3‡
Th4‡
UO22‡
(dielectric)
logꢀ1
logꢀ2
logꢀ1
logꢀ2
logꢀ1
logꢀ2
Mean SD
value
Mean SD
value
Mean SD
value
Mean SD
value
Mean SD
value
Mean SD
value
1a
1b
1c
1d
1e
1a
1b
1c
1d
1e
1a
1b
1c
1d
1e
1a
1b
1c
1d
1e
1a
1b
1c
1d
1e
1a
1b
1c
1d
1e
MeOH
(35.34)
6.82 0.02 13.38 0.02 7.12 0.02 13.81 0.02
5.50 0.02 10.22 0.02 5.76 0.03 11.11 0.03
7.22 0.02 13.52 0.02 7.80 0.00 14.93 0.03
6.89 0.01 12.95 0.02 7.73 0.02 14.72 0.02
6.50 0.02 11.89 0.02
5.43 0.03 9.76 0.03
6.64 0.00 12.23 0.03
6.15 0.02 11.53 0.02
4.55 0.02
8.61 0.02 5.35 0.03 10.25 0.03
4.20 0.03
7.66 0.03
EtOH
7.04 0.02 12.54 0.01 7.30 0.02 14.06 0.02
5.63 0.02 10.45 0.03 5.85 0.01 11.42 0.02
7.34 0.02 13.85 0.01 7.98 0.02 15.74 0.02
7.10 0.02 13.23 0.03 7.88 0.02 15.50 0.03
6.68 0.02 12.55 0.02
5.54 0.01 10.17 0.02
6.76 0.02 12.66 0.02
6.33 0.02 12.07 0.03
(29.13)
4.71 0.02
8.86 0.04 5.64 0.02 10.44 0.02
4.37 0.02
7.84 0.02
1-PrOH
(26.47)
7.22 0.02 13.80 0.01 7.41 0.03 14.23 0.02
5.79 0.03 10.84 0.03 5.92 0.03 11.42 0.02
7.54 0.03 14.37 0.04 8.14 0.02 15.80 0.03
7.40 0.03 13.84 0.02 8.00 0.03 15.42 0.02
6.94 0.03 12.40 0.02
5.90 0.03 11.11 0.02
7.02 0.02 13.12 0.03
6.73 0.03 12.63 0.02
4.89 0.03
9.19 0.02 5.74 0.02 10.92 0.03
4.70 0.02
8.46 0.03
2-PrOH
(24.87)
7.31 0.03 14.12 0.03 7.58 0.02 14.65 0.03
5.90 0.02 11.20 0.03 6.02 0.01 11.53 0.02
7.65 0.01 14.79 0.02 8.21 0.02 15.91 0.02
7.12 0.02 13.62 0.03
6.06 0.01 11.50 0.02
7.39 0.02 13.91 0.02
7.49 0.02 14.50 0.01 8.11 0.02 15.62 0.02 16.97 0.02 12.81 0.02
5.05 0.02 9.44 0.02 5.90 0.01 11.08 0.03 5.01 0.01 9.32 0.03
Acetone
(27.00)
7.89 0.02 14.95 0.04 8.08 0.02 15.84 0.03
6.31 0.05 11.59 0.03 6.97 0.02 13.21 0.01
8.16 0.01 15.44 0.03 8.99 0.02 17.22 0.02
8.00 0.01 15.14 0.03 8.73 0.02 16.84 0.02
5.48 0.02 10.09 0.02 6.37 0.02 12.12 0.02
8.33 0.01 15.55 0.01 8.63 0.02 16.72 0.03
6.71 0.01 12.35 0.02 7.42 0.02 14.13 0.03
8.43 0.02 16.11 0.03 9.23 0.03 17.72 0.03
8.32 0.02 15.92 0.02 9.04 0.04 16.90 0.02
5.92 0.02 10.77 0.03 6.88 0.22 13.22 0.02
7.90 0.02 14.11 0.03
6.77 0.02 12.43 0.01
8.16 0.02 15.12 0.02
7.73 0.02 13.24 0.02
6.00 0.02 11.20 0.02
8.36 0.02 14.83 0.03
7.10 0.02 13.22 0.03
8.62 0.03 16.12 0.03
8.02 0.04 14.19 0.02
6.30 0.02 11.93 0.02
DMSO
(12.76)
SD: standard deviation; mean values obtained by interpolation at half ꢀn, successive approximation, and
mid point methods
2) Although acetone (20.7) has a dielectric constant higher than 1- and 2-propanol
(20.1 and 18.3, respectively), the complexes have higher stability constants in
acetone than in the latter solvents. This can be attributed to a decrease in
hydrogen bonding structure which assists the complex formation in acetone
rather than in 1- and 2-propanol [24] or, in other words, to the smaller solvation
capacity of acetone.

998
Y. M. Issa et al.: Chelation of Ce3‡, Th4‡, and UO22‡ with Schiff Bases
Experimental
All chemicals used were of the highest purity available (BDH or Merck products). The solvents were
puri®ed according to Ref. [25]; H2O was distilled twice.
The Schiff bases were prepared by condensation of 5,7-dihydroxy-6-formyl-2-methylbenzopyran-
4-one [26] with substituted anilines in absolute alcohol for at least one hour except for 4-amino-3-
hydroxybenzenesulfonic acid which is insoluble in ethanol. It was dissolved in 4% (w/w) NaOH, and
the resulting Schiff base was precipitated by acidi®cation to pH 2.4 with dilute acetic acid.
10ꢂ3 M CeCl3 Á 6H2O, Th(NO3)4, and UO2(CH3COO)2 Á 2H2O solutions were prepared and
standardized with EDTA using the appropriate indicator [27, 28]. Both CeCl3 Á 6H2O and Th(NO3)4
were dissolved in water, whereas Schiff bases and UO2(CH3COO)2 Á 2H2O were dissolved in ethanol
due to their insolubility in H2O.
Conductometric titrations of 50 ml (4 Â 10ꢂ5 M) Schiff base solutions with standard metal ion
solutions were carried out using a CM-1K conductivity meter (cell constant: 0.975). Potentiometric
titrations were performed applying the Calvin-Bjerrum pH-metric titration technique as adopted by
Irving and Rossotti [12, 17] using a digital pH-meter (Seibold G103).
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Received January 30, 1998. Accepted (revised) March 19, 1998