August 2004 Chem. Pharm. Bull. 52(8) 929—934 (2004) 929
Potentiometric and Spectroscopic Studies on Aluminium(III) Complexes
of Some Catechol Derivatives
Naciye TÜRKEL, Melek BERKER, and Ulviye ÖZER
Department of Chemistry, Faculty of Arts and Sciences, Uludağ University; Bursa 16059, Turkey.
Received September 24, 2003; accepted April 26, 2004
The interactions of aluminium(III) ion with the triprotic catechol derivatives (H3L), 2,3-dihydroxybenzoic
acid (2,3-DHBA), 3,4-dihydroxyphenylacetic acid (3,4-DHPA), 3,4-dihydroxybenzoic acid (3,4-DHBA), and 
3,4-dihydroxyhydrocinnamic acid (3,4-DHHCA) were investigated in aqueous solution at 25.0 °C. The
Calvin–Bjerrum titration method was adopted for the determination of formation constants of proton–ligand
and aluminium(III)–ligand complexes. Potentiometric and spectroscopic results indicated that these catechol de-
rivatives exhibit a true bidentate character. The chelation occurs via their catecholate sites, with the exception of 
2,3-DHBA. In the case of 2,3-DHBA complexes, the dominant species are either the salicylate type (COO
, O
)
or catecholate type (O
, O
) complex. The protonation constants of ligands and their formation constants of
Al(III) complexes were also correlated. The order of decreasing stabilities of complexes is: 3,4-DHPA3,4-
DHBA3,4-DHHCA2,3-DHBA.
Key words aluminium; benzoic acid; complex; coordination compound
The aqua Al(III) ion is the “hardest” of the trivalent ions are coordinated with one mole of these metal ions in their
commonly found in the environment and in biological sys- complexes. In particular, the constants of formation equilib-
tems. Its effective ionic radius is 54 Å, which is smaller than ria of Al(III) complexes of catechol derivatives have been the
other commonly encountered trivalent metal ions. Therefore subject of a number of investigations.1)
the aqua Al(III) ion has the highest charge/size ratio and high Some of the hydroxy aromatic ligands are interesting
affinity for hard anions. As a result, it has a strong tendency three-protic ligands (H3L), 2,3-dihydroxybenzoic acid (2,3-
to hydrolyze in aqueous solution and its coordination equilib- DHBA), 3,4-dihydroxybenzoic acid (3,4-DHBA), 3,4-dihy-
ria contain rather complicated hydroxo complexes.1,2) The droxyhydrocinnamic acid (3,4-DHHCA), and 3,4-dihydroxy-
equilibria of aqua Al(III) ions were postulated by Baes and phenylacetic acid (3,4-DHPA) are typical examples and they
Mesmer3) including various aluminium-hydroxo species have affinities for metal ions to form stable com-
(log b values calculated for I
0.1 mol · l in parentheses): plexes.19,20,21,26)n Some of their complexes do not hydrolyze
Al(OH)2 (5.461), Al(OH)2 (10.036), Al2(OH)
4
3 and precipitate in aqueous solution, since 3,4-DHBA, 3,4-
(7.7), Al(OH)4 , (23.491), Al13(OH)
7
32 (103.149), DHHCA, and 3,4-DHPA have one carboxylate binding site
Al(OH)3 (13.694), and Al (OH)
5
3 4 (15.737). The forma- besides the catecholate sites that are in chelatable positions.
tion of Al(III) hydroxo complexes and polynuclear species 2,3-DHBA contains three potential binding sites on three ad-
was also examined and defined by Venturini and Berthon.4) jacent ring carbons. It is a good model of the competitive sal-
They showed the existence of Al(OH)2, Al(OH)2 , Al(OH)3, icylate (COO
, O) and catecholate type (O, O) chelation
Al(OH) 44 , Al2(OH)2 , and Al3(OH)
5
4 hydroxo ions. in the same molecule.
5) Kennedy and Powell7) investigated
Alcohols, carboxylic acids, aliphatic monohydroxyacids, Al(III): catechol and 3,4-DHBA complexes using potentiom-
aromatic hydroxy acids, and catechol derivatives are typical etry in aqueous solution in I
0.1 mol · l KCl ionic medium
and effective organic ligands with negative oxygen donors to at 25 °C. They reported the stability constants for the
bind aqua Al(III) ion. Martell et al.2,5) investigated the rela- mononuclear diphenolate complexes of Al(III) with catechol
tions between the basicities of these oxygen donors and the such as AlL, AlL2, and AlL3. They also indicated that mono-
stabilities of their Al(III) complexes. They concluded that the and bis- complexes of catechol become hydrolyzed and then
logarithm of protonation constants of monodentate donors AlL(OH), and AlL2(OH)
4 hydroxo complexes formed. In
and pK sum of bi- or terdentate donors provide a quantative the case of 3,4-DHBA, the existence of Al(HL), Al(HL)2 ,
measure for the hardness of each donor. The order of de- Al(HL)33 , Al(OH)(HL), and Al(OH)(HL)
2
2 complexes was
creasing basicities along with the summation of their approx- shown by potentiometric titration. The carboxylate-coordi-
imate logarithms of protonation constants are proposed to be: nated Al(HL)2-type complex and the carboxyl-protonated
(catecholate ca. 22)(aliphatic monohydroxy acid anion ca. Al(HL)-type complex were also postulated. To assess the
18) (aromatic hydroxy acid anion ca. 16)(alkoxide ca. binding ability of Al(III) ion of humic and fulvic acids,
14)(phenoxide ca. 10)(carboxylate ca. 4). Therefore the which are important organic soil constituents, Kiss et al.14)
complexes of Al(III) ion that are sufficiently stable to hy- performed potentiometric studies. The proton and Al(III)
drolytic reactions and consequent precipitation in aqueous complexes of various bi- and terdentate catechol and salicylic
solution should contain ligands with the highest pK sum. acid derivatives (including 3,4-DHBA and 2,3-DHBA) were
The complexes of catechol derivatives with Al(III) ion in- analyzed and defined at 25 °C at an ionic strength of
cluding metal ions such as Fe(II), Fe(III), Mg(II), Ca(II), 0.2 mol · l KCl, and the formation constants of AlL- and
Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Pb(II), Cd(II), UO 2(II), Al(HL) -type complexes were determined. In the case of
and VO(II) were studied by various researchers.1,2,5—26) They AlL-type chelate, catecholate-type coordination was as-
determined that two or three moles of catechol derivatives sumed. Because of the presence of a separate carboxylic
* To whom correspondence should be addressed. e-mail: nturkel@uludag.edu.tr © 2004 Pharmaceutical Society of Japan
930 Vol. 52, No. 8
group in 3,4-DHBA, a mono protonated (1 : 1) Al(HL) tion to Al(III) as a background electrolyte. Purified nitrogen was circulated
complex forms in which catecholate-type (O, O) coordina- through the jacketed titration cell under slight pressure to exclude carbon
dioxide. All potentiometric titrations were carried out in triplicate. Complex-
tion occurs and the carboxylic group remains protonated. ations were investigated within pH intervals, and at least 1—3-min time in-
Al(HL)-type coordination ion has a log K
4.62 value tervals, were necessary between additions of successive aliquots of sodium
similar to that of the protonation constant of H 2L ion. The hydroxide to reach constant pH.
same behavior toward 3,4-DHBA was also observed in the Spectroscopic Measurements The absorption spectra were recorded on
corresponding VO(IV) complexes.26) In addition, Desroches a Shimadzu UV-2100 spectrophotometer to measure absorbance and to con-
15) firm the existence of deprotonated ligands and complexes at different pHet al. reported the Al(III) complex equilibria of five values as a function of wavelength. Job’s method was used whenever possi-
sequestering ligands, including 2,3-DHBA, Al(HL), and ble.32)
Al2L complexes, were characterized for 2,3-DHBA in Calculations The potentiometric titration curves of four ligands that are
addition to the species mentioned by Kiss et al.14) catechol derivatives and also are carboxylic acids were compared with their
The stability constants of several complexes of substituted Al(III) : ligand systems (Figs. 1, 2). They were investigated by mathematical
analysis of each system. The best complex that accounts for the experimen-
catechol ligands including 2,3-DHBA and 3,4-DHPA were tal data was accepted. Nonlinear least-squares analysis of the data in terms
examined in acidic solution for Fe(III); evidence were found of assumed reactions gave a satisfactory fit in the buffer regions of com-
for the coordination of 3,4-DHPA via the carboxylate group plexes which include Al(III) and one of these ligands in different molar ra-
which presumably does not involve chelation, while the tios. Then the stochiometries of assumed complexes were deduced from the
Fe(III) : 2,3-DHBA system is complicated by mixed-mode shapes of titration curves of ligands and Al(III) : ligand systems. The numberof potentiometric data for each defined pH region is given in Tables 1 and 2.
coordination of both the catecholate and salicylate type. In For computing the protonation constants of ligands33—35)(log K ) and the for-
the investigation of Cu(II), Zn(II), Ni(II), Cd(II), and Mg(II) mation constants of Al(III) complexes (log b) data were utilized in the
complexes with catechol, 3,4-DHBA, and 3,4-DHHCA, the pH
1.5—3.88 range. The hydrolysis of Al(III) ion was completely ne-
introduction of alkyl groups into the phenol ring decreased glected in this pH range, but above pH 4.0 the existence of hydroxo com-
plexes was considered. log K, log b , and standard derivations of these values
their acid strength in the order 3,4-DHBA, 3,4-DHHCA and that are given in Tables 1 and 2 were obtained from related equations using,
3,4-DHPA19); the order of the stability constants was in the RANA computer program described previously.32) To determine the stoi-
agreement with that of Irving & Williams order (Mn(II) chiometries of possible complexes, the degree of formation of Al(III)
Fe(II)Co(II)Ni(II)Cu(II)Zn(II)). The interaction of chelates n was calculated from the formation curves employing the method
21) of Irving and Rossotti.35—38)3,4-DHPA with Cu(II) was also investigated by Kiss et al. The species distribution diagrams
39) of the
Al(III) complexes formed with the above-mentioned ligands at metal-to-lig-
and the ambidentate character of this ligand was observed. In and molar ratios of 1 : 1, 1 : 2 and 1 : 3 as a function of pH are presented in
addition to the chelation via the catecholate sites, coordina- Figs. 3 and 4. 
tion also occurred from a separate carboxylate binding site.
The formation of various Cu(II) oligomeric species was also Results and Discussion
defined. Proton Complexes of Catecholate Derivatives In this
The first aim of this research was the determination of the research, only two protonation constants of each ligand could
formation constants of proton and Al(III) complexes of four be determined. Although 2,3-DHBA, 3,4-DHBA, 3,4-
different hydroxyaromatic ligands in potentiometric and DHHCA, and 3,4-DHPA contain three acidic groups, but
spectrophotometric studies. Although the complexes of 2,3- only two of these acidic groups can lose protons in the mea-
DHBA and 3,4-DHBA were investigated by several au- surable pH range due to the formation of intramolecular hy-
thors,14,15) we have included these two ligands in our ligand drogen bonds between the –COO and –OH groups. There-
series to compare the formation constants of their Al(III) fore the protonation constant of the second phenolate (L3),
complexes with the two other two catechol derivative ligands which is in a meta position to COOH in 2,3-DHBA or in an
in the same ionic medium and temperature. Our second aim ortho position to –OH in 3,4-DHBA, 3,4-DHHCA, and 3,4-
was the comparison of the basicities of these oxygen donor DHPA, can be determined only with uncertainty.14) Therefore
ligands with the stabilities of their complexes. The third aim we introduced only these values that were defined in previous
was the assignment of the species distribution diagrams for investigations.11,19,25,33) The protonation constant of H 2L and
each Al(III) : ligand system to define the conditions to pre- HL2 ions were defined by potentiometric titrations that were
vent the hydrolysis of aqua Al(III) ion and Al(III) complexes. carried out at three different ligand concentrations
(1.0103, 2.02103, 4.04103 M) up to pH 11.5 by in-
Experimental troducing approximately 220 experimental data for each lig-
Materials All the chemicals were of analytical reagent grade. Sodium and. The protonation constant of HL2 ion was also mea-
hydroxide free from carbonate was prepared and standardized with potas-
sium hydrogen phthalate. The stock solutions of Al(III) were prepared by sured spectrophotometrically. To compare our results with
dissolving the appropriate amounts of aluminum chloride (Merck, 99%) in a values reported in the literature the protonation constants of
small excess of HCl,27) and on indirect titration method was used to stan- four ligands are listed in Table 1. It was confirmed that log K
dardize this solution.28) The equivalent points were evaluated using Akalın values of three ligands that have the –COOH group at posi-
and Özer’s29) Gran’s extrapolation method.30) 2,3-DHBA was purchased from tion 4 decrease in the order 3,4-DHPA3,4-DHBA3,4-
Sigma, and 3,4-DHBA, 3,4-DHHCA, and 3,4-DHPA were purchased from
Aldrich. These ligands were used without further purification. Their molecu- DHHCA. 
lar weights were periodically checked by Gran titrations.30) All solutions Aluminium(III) Complexes of Some Catechol Deriva-
were prepared in carbon dioxide–free double–distilled water. tives Due to the differences between the donor group
Potentiometric Measurements A Schott pH-meter (Hofheim, Ger- arrangements of the four aromatic hydroxy ligands, first the
many, accuracy 0.05) fitted with a combined electrode was calibrated with coordination of 2,3-DHBA to Al(III) is explained, and then
acetic acid buffer as well as with standard HCl and NaOH to give hydrogen
ion concentrations directly.31) The Calvin–Bjerrum titration technique was the interaction equilibria of Al(III) ion with the other three
adopted. Measurements were made at 25.00.1 °C and the ionic strength ligands is discussed. 
was maintained at approximately 0.1 M by the addition of standard KCl solu- Al(III) : 2,3-DHBA System. Potentiometric Results
August 2004 931
Table 1. Protonation Constants (log K) for 2,3-DHBA,3,4-DHPA, 3,4-DHBA and 3,4-DHHCA
Equilibria of proton complexes (log K) 2,3-DHBA 3,4-HHPA 3,4-DHBA 3,4-DHHCA
1 OH (L3HÆ̈ HL2) 
OH at 2.position in 2,3-DHBA 13.119) 13.719) 13.111) 12.1525)
OH at 3.position in 3,4-DHBA
2 OH (HL2HÆ̈ H 2L ) 9.910.0133) 9.520.0133) 8.660.0133) 9.300.0133)
OH at 3.position in 2,3-DHBA 10.160.0133a) 9.570.0133a) 9.060.0133a) 9.620.0133a)
OH at 4.position in 3,4-DHPA, 10.110.02a) 9.490.02a) 8.820.04a) 9.640.02a)
3,4-DHBA and 3,4-DHHCA
3 COOH (H LHÆ̈ H L) 2.740.0133) 4.240.0133) 4.260.0133) 4.360.0133)2 3
2.680.01 4.200.01 4.340.01 4.400.01
4 S log K 25.89 27.39 26.26 26.19
a) Spectroscopic data.
The potentiometric titrations of the Al(III) : 2,3-DHBA sys-
tem in 1 : 1 and 1 : 2 molar ratios were performed. The curves
exhibit inflection points at m
2.00 and m
4.00, respec-
tively, where m represents the number of moles of base added
per mole of Al(III) ion (Fig. 1, curves II, III, IV). The forma-
tion of HL2– ion by dissociation of two protons forms 2,3-
DHBA in the m
0.00—2.00 range and the formation of
Al(HL)- and Al(HL)–2-type complex ions occurred as pro-
posed by Kiss et al.14) according to equilibria (1) and (2)
(Table 2). Remarkable drops in pH values were observed es-
pecially in the first buffer regions of the titration curves of
1 : 1 and 1 : 2 Al(III) : 2,3-DHBA systems when compared
with the titration curve of 2,3-DHBA alone. Then log b val-
ues of Al(HL) and Al(HL)–2 complexes calculated by con-
sidering equilibria (3) and (4), respectively. The potentiomet-
ric data indicate the existence of Al(HL)- and Al(HL)–2-type Fig. 1. Potentiometric Titration Curves of Al(III) Chelates of 2,3-DHBA
complexes and they are supported by calculations. The in 0.1 M KCl at 25 °C
agreement of our values with those in the literature14,15) is re- I. 2,3-DHBA alone (TL
2.02103 M). II. (1 : 1) Al(III) : 2,3-DHBA (TAl

flected in Table 2. The observed differences between our val- 2.02103 M, TL
2.02103 M). III. (1 : 2) Al(III) : 2,3-DHBA (TAl
2.02103 M,
TL
4.0410
3
M). IV. (1 : 3) Al(III) : 2,3-DHBA (T 
2.02103 M, T
ues and cited values arise from ionic medium and tempera- Al L

6.06
103 M). 
ture. Thus the assumption of salicylate-type monomeric com-
plexes was sufficient for a fit to the titration curves especially species were calculated from related equations and then
at a lower ligand excess. Kiss et al.14) also assumed the for- drawn as a function of log[H] (Fig. 3). The speciation
mation of catecholate-type (O–, O–) complexes or monohy- curves show that the major species in the defined pH range
droxo-salicylate-type (COO–, O–, OH–) complexes. The sec- are Al(III) ion and Al(HL), Al(HL)2 , Al(HL)(OH), and
ond inflections were noticed at m
3.00 for the 1 : 1 molar Al(HL) 22(OH) complexes in which 2,3-DHBA acts as a
ratio and at m
4.00 for the 1 : 2 molar ratio. Moreover, con- bidentate ligand. The occurrence of the salicylate (COO,
tinued decreases in pH readings after m
2.00 for the 1 : 1 O) bonding mode was confirmed by the speciation curves. 
and m
4.00 for the 1 : 2 molar ratios (pH 3.88) were ob- Spectroscopic Results The stochiometries of complexes
served. Therefore the titration data of either the 1 : 1 or 1 : 2 formed were examined based on the spectroscopic results.
Al(III) : 2,3-DHBA systems above pH 3.88 could be fitted Job’s method was applied by taking the UV/V is spectrum of
only by assuming the formation of a mixed hydroxo com- 2,3-DHBA alone. The pH-dependent change in optical ab-
plex. The shifts in pH values were noted when we waited for sorption for 2,3-DHBA alone was defined from the potentio-
30 min after addition of base (Fig. 1, curves II, III). In addi- metric titration curve. It was noted that the HL type ion of
tion to the formation of Al(HL)- and Al(HL)–2-type com- 2,3-DHBA occurred at pH 3.08. The occurrence of the
plexes, their hydrolysis according to equilibrium (5) between Al(HL)-type complex ion in the pH 2.58—3.83 range was
m
2.00—3.00 and by equilibrium (6) between m
4.00— confirmed by taking the spectra of either 2,3-DHBA solution
5.00 was assumed, respectively. Hence these assumptions or Al(III) : 2,3-DHBA in a 1 : 1 molar ratio at pH 3.05 (Fig. 6,
were supported by calculations (Table 2). curves I, II). The maximum absorbance was observed at
Formation Curve The formation curve, log ligand l
335 nm for the 1 : 1 molar ratio; but 2,3-DHBA alone was
concentration versus number of ligands bound, n, was drawn not absorbed. Job’s plots were drawn at l
335nm that show
for the Al(III) : 2,3-DHBA system. The n values maximum absorbance at the molar fraction of
(1.0n2.0) indicate the occurrence of 1 : 1 and 1 : 2 com- Al(III) XM
0.5 for solution in which the pH is 3.05; in other
plexes, (Al(HL) and Al(HL)2 ), respectively (Fig. 5). words, the stochiometry of the complex confirms the
Speciation Diagram The concentrations of all assumed Al(HL)-type structure. 
932 Vol. 52, No. 8
Table 2. Equilibrium Constants (log K) and formation constants (log b) of Aluminium(III): Catechol Derivatives Chelates at 25 °C, I=0.1 M KCl
Equilibrium 2,3-DHBA 3,4-DHPA 3,4-DHBA 3,4-DHHCA
1) Al3H LÆ̈3 Al(HL)2 H 2.8714)
2.320.01
2) Al32H LÆ̈ Al(HL)24 H3 8.1214) 10.040.01 10.250.01 11.060.01
7.370.01
3) Al3HL2Æ̈ Al(HL) 10.3214), 10.50515)
10.50.05
4) Al32HL2Æ̈ Al(HL)2 18.2614), 18.24415)
18.80.05 
5) Al(HL)OHÆ̈ Al(HL)(OH) 9.740.002
6) Al(HL) Æ̈ 2 14)2 OH Al(HL)2OH 9.82
7.530.02
7) Al3H LÆ̈3 Al(H2L)2H 4.250.01 4.30.01 4.390.01
8) Al (H L)2Æ̈ AlL2 H2 13.260.01 13.040.01 14.550.01
9) Al3L3Æ̈ AlL 17.40.05 16.70.05, 16.877), 16.4714), 15.0334) 15.90.05
10) AlLOHÆ̈ AlL(OH) 8.870.02 7.910.02 8.160.01
11) Al(HL)Æ̈ AlL32 2 2H 13.260.01 13.040.01 14.550.01
12) Al32L3Æ̈ AlL3 28.40.05 26.90.04, 29.887) 14) 34)2 , 29.38 , 27.6 25.80.05
13) AlL3 Æ̈ 42 OH AlL2(OH) 5.380.02 5.30.02 5.20.012
14) Al33H LÆ̈3 Al(HL)33 6 H 17.340.01 16.920.01 19.410.01
15) Al(HL)3Æ̈3 AlL63 3H 22.600.01
16) Al3L3Æ̈ AlL6 38.60.05 35.70.05, 38.647), 38.3514)3 , 37.5534) 34.60.05
Fig. 2. Potentiometric Titration Curves of Al(III) Chelates of 3,4-DHHCA
in 0.1 M KCl at 25 °C
Fig. 3. Species Distribution Curves of the 2,3-DHBA System and the
I. 3,4-DHHCA alone (T 
2.02103L M). II. (1 : 1) Al(III) : 3,4-DHHCA (TAl

3 Metal Ion Al(III) as a Function of log [H
] 
2.0210 M, TL
2.0210
3
M). III. (1 : 2) Al(III) : 3,4-DHHCA (TAl
2.0210
3
M,
TL
4.0410
3 ). IV. (1 : 3) Al(III) : 3,4-DHHCA (T 
2.02103M Al M, TL
6.06
103 M).
Al(III) : 3,4-DHPA, Al(III) : 3,4-DHBA and Al(III) : 3,4-
DHHCA Systems. Potentiometric Results The poten-
tiometric titrations were performed in 1 : 1, 1 : 2 and 1 : 3
molar ratios of Al(III) : ligand systems in 0.1 mol · l KCl
ionic medium at 25 °C. The series of titration curves are
given only for the Al(III) : 3,4-DHHCA system since they are
similar in each system. In 1 : 1 molar ratios of Al(III) : ligand
systems, the first inflection points on potentiometric titration
curves were observed at m
4.00 The potentiometric titration
curves of 3,4-DHHCA alone and the Al(III) : 3,4-DHHCA
system clearly indicate that in m
0.00—1.00 range equilib- Fig. 4. Species Distribution Curves of the 3,4-DHHCA System and the

rium (7) (in Table 2) occurs (Fig. 2, curve II). It was verified Metal Ion Al(III) as a Function of log [H ] 
that in the m
0.00—1.00 range a proton from the uncoordi-
nated carboxyl was titrated; the protonation constants of (8)]. Thus deprotonations of two phenolic hydroxyl groups
H L2 ion for these three ligands are equal to the assumed and the formation of an AlL-type complex according to equi-
formation constants of Al(H 22L) -type ions (Table 1). Then librium (9) were assumed. log K values for equilibria (7) and
the coordination of ligand to Al(III) ion from two phenolate (8) were determined, and then the formation constants (log b)
oxygens that are in the 3 and 4 positions to the carboxyl was for AlL in the pH range for the Al(III) : 3,4-DHPA system, in
considered in m
1.00—3.00 range [Table 2, equilibrium the pH range of 3.77—4.90 for the Al(III) : 3,4-DHBA sys-
August 2004 933
tem, and in the pH range of 4.29—4.86 for the Al(III) : 3,4-
DHHCA system (Fig. 2, curve II) were calculated by intro-
ducing approximately 76 experimental data for each
Al(III) : ligand system (Table 2). Thus the occurrences of
AlL-type complexes that have a catecholate mode of coordi-
nation were supported by potentiometry. In addition to the
log b value for 3,4-DHBA that was determined by Hancock
and Orszulik,34) other log b values7,14) are in a comparable
range with our value. The shifts in pH values beyond pH ca.
4.80 in all Al(III) : ligand systems might be attributed to the
hydrolytic equilibrium (10) that was proposed in the
m
3.00—4.00 range. The occurrence of hydroxo complex
formation was confirmed by calculation of the equilibrium
constant (Table 2, row 10). In the case of the 1 : 2 molar ratio
Fig. 5. Degree of Formation for Al(III) : 2,3-DHBA System, n, as a Func-
of Al(III) : ligand systems, the inflection points of potentio- tion of log L
metric titration curves were observed at m
5.00 and I. (1 : 1) Al(III) : 2,3-DHBA. II. (1 : 2) Al(III) : 2,3-DHBA. 
m
7.00 (Fig. 2, curve III). The stepwise coordination of two
moles of ligands to Al(III) were assumed according equilib-
ria (2) and (11) in the m
0.00—4.00 range (pH 3.48—5.27).
Thus the equilibrium constant of (2) was confirmed; as a re-
sult, the constant of equilibrium (4) was calculated by intro-
ducing 118 experimental points. Hence the occurrence of sal-
icylate (type coordination (COO, O) and the formation of
an Al(HL)2 (type complex were confirmed in the m
0.00—
4.00 range. However, after that the coordination sites change
within the 3,4-DHHCA molecule, and probably the coordina-
tion of adjacent phenolate sites can take place since the for-
mation of AlL32 complex ion by equilibrium (11) was sup-
ported by the potentiometric results. Then its formation con-
stant was calculated by considering the occurrence of equi-
librium (12). When log b values that were reported by several
researchers7,14,32) in the Al(III)3,4-DHBA system are com-
pared for the AlL32 -type complex ion, our value is close to
that of Kennedy and Powell.7) but other log b values are
higher since they were determined at higher temperature and
in a different ionic medium. They also did not take into ac-
count the formation of Al(HL)-type ion. Fig. 6. Absorption Spectra of Al(III) Complexes of 2,3-DHBA in2 0.1 M KCl at 25 °C (pH 3.05)
The hydrolysis of AlL32 chelate was also considered in the I. 2,3-DHBA alone (T 
6.0103 M). II. (1 : 1) Al(III) : 2,3-DHBA (T 
6.0
m
6.00—7.00 range; due to the hydroxo complex formation L Al103 , T 
6.0103M L M).
in this pH range (6.11—6.98), readings were taken every 30
min. The related equilibrium constant for the assumed hy-
drolysis equilibrium (13) was also defined (Table 2). In the confirmed that the formation constants of Al(III) complexes
case of the 1 : 3 molar ratio of the Al(III) : 3,4-DHHCA sys- of these three ligands decrease in the order 3,4-DHPA3,4-
tem, the inflections were shifted to m
6.00. According to DHBA3,4-DHHCA, which is the order of SLog K values
equilibrium (14) (in Table 2), the formation of Al(HL)33 of these ligands. Over the low pH range the salicylic acid-
chelate ion was assumed and its formation constant in the pH type mode of coordination predominates, while the catechol
range 3.45—6.72 was calculated by introducing 224 experi- type is preferred in basic media.26)
mental data. The results supported the formation of Formation Curves The formation curves for
Al(HL)33 between m
0.00 and m
6.00. According to equi- Al(III) : 3,4-DHPA, Al(III) : 3,4-DHBA, and Al(III) : 3,4-
librium (14) the carboxyl and phenolic proton that is in the DHHCA systems in different molar ratios were drawn (Fig.
meta position to the carboxyl proton were titrated and then 7). They have one plateau at n
1.0 for the 1 : 1 at n
2.0 for
salicylate-type coordination (COO, O) occurred in the the 1 : 2 and at n
3.0 in for the 1 : 3 molar ratios of Al(III) to
Al(HL)33 -type chelate (m
0.00—6.00 in acidic media). Due ligand. They indicate that the binding of one, two, and three
to the arrangement of donor groups within these ligands, cat- moles of ligands were occured.
echolate sites act more efficiently and catecholate (O, O) Speciation Diagrams To verify the existences of formed
type coordination occurs in m
6.00—9.00 range. As a result AlL, AlL32 , and AlL
6
3 species in the Al(III) : 3,4-DHPA,
of change in binding mode, the formation of AlL63 type co- Al(III) : 3,4-DHBA and Al(III) : 3,4-DHHCA systems, the
ordination was proposed, and then the constant of equilib- concentration distribution curves of the complexes versus pH
rium (16) was calculated for the Al(III) : ligand system (Table were drawn. The major species in defined pH ranges are AlL,
2). Several authors7,14,34) reported AlL63 -type coordination AlL
3
2 , and AlL
6
3 complexes. The species distribution of the
ions in the 1 : 3 Al(III) : 3,4-DHBA system. Our results also aluminium(III) complexes formed with 3,4-DHHCA at lig-
934 Vol. 52, No. 8
type complexes were confirmed in the pH 4.88—7.96 range
for 3,4-DHPA, pH 4.07—5.96 range for 3,4-DHBA, and pH
5.52—8.41 range for 3,4-DHHCA by potentiometry. But due
to the occurrence of AlL (OH)42 -type complexes and their
precipitation, the validities of AlL32 complexes were not
confirmed spectrophotometrically.
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6
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