Download Titanium(IV) citrates in aqueous

Titanium(IV) citrates in aqueous-chloride solutions
Bezryadin Sergei Gennadevich1*+, Chevela Vladimir Vsevolodovich2*,
Ajsuvakova Olga Pavlovna1, Ivanova Valentina Yurevna2
1
Department of chemistry, Orenburg State Agrarian University,
Chelyuskintsev st., 18, Orenburg, 460014. Orenburgskaya region. Russia.
Tel.: (3532) 779506 E-mail: [email protected]
2
Department of Inorganic Chemistry, AM Butlerov Institute of Chemistry of Kazan (Volga Region)
Federal University, Kremlevskaya st., 18, Kazan, 420008. The Republic of Tatarstan. Russia.
Tel.: (843) 2315416
_______________________________________________
* Leader of the research area; + Corresponding author
Keywords: titanium (IV) citrate, complex formation, potentiometric titration, mathematical
simulation
Abstract
The titanium (IV) - citric acid system was studied by the method of potentiometric titration in
conjunction with mathematical simulation for the ratios of metal-to-ligand is 1:1, 2:3, 1:2 and 1:3. The
composition, stability and the quantity of accumulation of citrate titanium (IV) in aqueous solution were
calculated. It was found that for the equimolar ratio of the reactants di-, tri- and tetranuclear particles is
formed, while for excess of ligand mononuclear complex forms [Ti(H4-nCit)3]4-3n (n = 2-4) at pH ≤ 8 and
[Ti(OH)2(Cit)2]6- at pH ≥ 8 are dominated.
Introduction
The titanium (IV) complexes play an important role in practice of analytical and inorganic
chemistry, biochemistry and geochemistry. Although traditionally titanium is regarded as an
immobile element of Earth’s crust, now there is an information [1], suggesting that the solubility of
its compounds may be higher than previously thought. Apparently, a considerable role is played by
the interaction of titanium with polyfunctional chelating ligands.
It follows that formation complexes processes of the titanium(IV) with α-hydroxyacids,
particularly citric acid, require a thorough study. In modern literature the question about that titanium
- a biologically active element whose influence on people increases is widely discussed. [1-4].
Therefore, due to the prevalence of titanium in the environment and its nonunique biological
activity the value of information on the structure and stability of its coordination compounds, as well
as equilibria in aqueous solutions containing ions of titanium has increased to date. (IV).
Although the importance of the study of the complexation of titanium (IV) with citric acid
emphasized repeatedly, the above complexes remain poorly understood-governmental. Despite the
considerable amount of work done in this area, information about the hydroxyacid complexes of
titanium (IV), cited in the literature are sketchy and mainly concerning the synthesis of citrate
crystals of titanium (IV) from solutions [2-10].
Experimental Section
Complexation in the titanium (IV) - citric acid was studied by potentiometric titration method in
combination with the mathematical modeling. We investigated the following reagents ratios - 1:1, 1:2, 1:3 and
2:3. To identify complex forms concentration the range 0.0013-0.02 mole/l of metal was studied. We used
"pure for analysis" qualification titanium(IV) chloride and citric acid. All working solutions are prepared with
twice distilled water. An aliquot of the pure titanium chloride(IV) was added dropwise with stirring to a
solution of citric acid. This technique allows us to do it without additional acidification of the system.
Potentiometric measurements were carried out at a pH-meter pH-150M to 0.01 units. pH. The
temperature of the working solution was maintained at 25 ± 0.1 ° C. Because of the fact that for the
equilibration in the system- Ti(IV)-α-hydroxyacid takes time the titration of the prepared solutions was carried
out every other day.
Results and Discussion
Fig. 1-2 shows the experimentally obtained curves describing the Berrum’s function
dependence of pH for titanium(IV) - citric acid system in a ratios of metal to ligand 1:1, 1:2, 1:3, 2:3.
For all four ratios there is a general pattern in the curves.
First, there is a sharp increase in Berrum’s function to the inflection point at pH 6.5 and the
curve changes after bending.
In the case of 1:2 and 1:3 ratios the curve flattens out. On these curves we can highlight a range
in which the function of formation varies slightly (pH 7-9). The presence of ñ constant arias can be
explained by the formation of stable complex forms.
The curve on the plateau is not observed for the ratios of 1:1 and 2:3 after the inversion point.
The Berrum’s function with pH continues to increase, but the rise curve becomes flatter.
Since these dependencies experimental points do not lie on a one curve, it must be assumed
that in the Ti4+-citric acid system at a ratio of metal: ligand close to equimolar, the formation of
polynuclear complex may be formed.
Some authors interpret the experimental curves from the standpoint of existence of a simple
model that includes only the mononuclear complexes, both for systems with an equimolar ratio of Ti
(IV) - citrate, and for cases where the ligand is taken with an excess of 2-4 fold. [11].
Therefore, they offered the same schemes of equilibrium in citric acid solution containing ions
of titanium(IV) [2, 3, 6]. It should be emphasized that the information about the constants of
formation titanium(IV) citrate complexes does not provide in the literature, with the exception of
work. [11].
Data given in the literature describe the behavior Ti4+-H4Cit at a sufficiently high concentration
of the components (0.01-0.04 mol/l), the same data for more dilute solutions are absent.
We have studied the system TiCl4-H4Cit by potentiometric titration over a wide pH range
1.10-10.5. The complexation of titanium (IV) with citric acid was studied in essentially zero ionic
strength. It minimized the effect of the supporting electrolyte on the complex particles formation in
the studied system.
For the simulation of the equilibrium in the titanium(IV) - citric acid the program «Chemical
Parameters of equilibria in solutions with solid phases» (CPESSP) was used [12]. In drawing up the
stoichiometry matrix we used the values of the titanium(IV) hydrolysis constants according to P.
Comba [13] and L. Ciavatta data [14].
If the complexation reactions in solutions containing particles of H+, Ti4+, H4Cit are presented
in the form of formal equations

[Ti p H 4 q  r Citq ]4 p  r  rH 
pTi 4   qH 4Cit 

then complexation equilibrium constants can be calculated by the equation:
 pqr 
[Ti p H 4q -r Citq4p -r ][ H  ]r
[Ti 4  ] p [ H 4Cit ]q
8
8
7
7
6
6
5
5
n 4
n 4
BTi=0,02M, BTi=0,0211M
BTi=0,0013M, BTi=0,00137M
3
BTi=0,01M, BTi=0,01055M
BTi=0,0013M, BTi=0,0042M
BTi=0,0026M, BTi=0,0027M
3
BTi=0,02M, BTi=0,0628M
BTi=0,01M, BTi=0,0314M
BTi=0,01M, BTi=0,02105M
2
BTi=0,0013M, BTi=0,0027M
BTi=0,0026M, BTi=0,0082M
2
BTi=0,0026M, BTi=0,0055M
BTi=0,02M, BTi=0,0421M
BTi=0,01M, BTi=0,0157M
BTi=0,0013M, BTi=0,0020M
1
1
2
3
4
5
6
7
8
9
BTi=0,02M, BTi=0,0314M
1
BTi=0,0026M, BTi=0,0041M
10
1
2
3
4
5
6
7
8
9
10
pH
pH
Fig. 1. Dependence of Berrum function of pH
in TiCl4 – citric acid system
in a ratio of reagents 1:1-1:3
(ВTi – 0.0013М и 0.01М)
Fig. 2. Dependence of Berrum function of pH
in TiCl4 – citric acid system
in a ratio of reagents 1:1-1:3
(ВTi – 0.0026М и 0.02М)
Calculation Berrum’s function was performed with using the following equation:
10
n~ 
pH
 (Valiq  V NaOH )  (c NaOH V NaOH )  10 pH 14  (Valiq  V NaOH )
c H 4Cit  Valiq
The models of complex formation at equimolar ratio cited in the literature do not give an
adequate description of the experiment. It is assumed that the 1:1 ratio is typical only for acidic pH
range [11, 15], whereas the broad region of pH values are undescribed (pH> 6).
However, based on data on the interaction of the ions of elements of groups III-IV with
hydroxy-acids, the formation of polynuclear complex forms also may be assumed in this case [1620].
To improve the convergence of the experimental data with the results of mathematical
modeling in the matrix of stoichiometric we were introduced the complexes with a higher degree of
nucleation. The mathematical modeling of the equilibrium composition at an equimolar ratio showed
that the titanium (IV) - citric acid is formed a series of complexes of the 1:1, 2:2, 3:3, 4:4 Ti: citrate
ratio.
Complex speciation in the system starts with form [TiHCit]+, dominated in the highly acidic
region (pH <1.5):

[TiHCit ]  3H 
H 4Cit  Ti 4 

Further interconversions of pH 1.5-2.5 can be represented as follows:
dimerization

 2[TiCit ]0 

[Ti2OHCit2 ]
2[TiHCit ] 

2 H 
H 
The hydroxyform [Ti2OH(Cit)2]- is polymerized while the pH is growth, and in an acidic aria
(pH=2.5-6.5) trimers describe equilibrium better then monomers:
trimerization


 2[Ti3 (OH ) 4 Cit3 ]4 


3[Ti2OHCit2 ] 
2[Ti3 (OH )3 Cit3 ]3 


3 H 
2 H 
2 H 

 2[Ti3 (OH )5 Cit3 ]5 

 2[Ti3 (OH )6 Cit3 ]6



2 H 
2 H 
Complex species nuclearity increases in the alkaline range of pH. According to the simulation
the accumulation proportion of complex 4:4:26 ([Ti4(OH)10(Cit)4]10-) began to increase at pH 6.5.
Then this complex becomes in more deprotonated species, and alkaline region at a pH of 7-9.5
dominance of tetramer of complexes is observed:
tetramerization

 3[Ti4 (OH )10 Cit4 ]10 

 3[Ti4 (OH )11 Cit4 ]11 


4[Ti3 (OH )6 Cit3 ]6 




6 H
3 H
3 H

 3[Ti4 (OH )12 Cit4 ]12


3 H 
Thus, our scheme of complex formation is consistent with the titanium(IV) ions marked a
tendency to exist in solution in the form of particles of different degree of nuclearity [21]. The
decreasing nuclearity in the strongly alkaline range can be attributed to the destruction of complex
shapes and forming titanium(IV) hydroxoсomplex [Ti(OH)4(Cit)]4-:

 4[Ti3 (OH )10 Cit3 ]10 

12[Ti(OH )4 Cit ]4
3[Ti4 (OH )12 Cit4 ]12 


6 H
3 H
Modeling of equilibria in the presence of excess ligand we started with a simple scheme,
compiled from the literature data [2, 3, 5, 8, 12]. It should be noted that such complex formation
scheme is generally true, however, contained information in the literature mainly deals with acidic
pH, while the alkaline region of pH> 8.0 remains poorly understood.
According to [11] the 1:3 complexes - [Ti(H2Cit)3]2-, [Ti(H2Cit)(HCit)2]4-, [Ti(HCit)2(Cit)]6-,
[Ti(Cit)3]8- are the most common for the titanium (IV) - citric acid system in excess of the ligand.
At the same time, in the literature there is no consensus on the question of the existence of
forms with different metal: ligand ratios. The hydroxoform of 1:2 [Ti(Cit)2(OH)2]6 was found in the
alkaline - range [11]. This fact agrees with the data of the authors [2, 3] that the complexed ligands
fraction gradually decreases with the growth of the pH.
Deng et al. [3] based on the NMR data suggest that there might be different titanium(IV)
citrates in solutions even in the case where the pH of the system is much higher than 7.0.
Complexation Ti4+ with citric acid in aqueous solution with an excess of ligand can be
described a relatively simple model:

[TiCit ]0 

[Ti ( H 2Cit )3 ]2 

[Ti ( H 2Cit ) 2 ( HCit )]3 


[TiHCit ] 




H 
2 H 4Cit ,  H 
H 
H 

[Ti ( H 2Cit )( HCit ) 2 ]4 

[Ti ( HCit )3 ]5 

[Ti ( HCit ) 2 Cit ]6 







H 
H 
H 
H 

[TiHCit (Cit ) 2 ]7  

[Ti (Cit )3 ]8 
[Ti(OH ) 2 (Cit ) 2 ]6



H 
H 
 H  ,  Cit 4
The fact of the formation of the 1:1 complex but not 1:3 at first attracts attention. The inclusion
complexes of 1:3:4 and 1:3:5 aggravate the consistency of the simulation and leads to a discrepancy
between theoretically calculated and experimental curves, so 1:3:4 and 1:3:5 forms were removed
from the stoichiometric matrix.
The main stages of mathematical modeling are presented in Table. 1-3.
We proposed scheme including the 1:1, 2:2, 3:3, 4:4, and 1:3 complexes for the ratios of metal:
ligand 1:2 and 2:3. Mathematical treatment of the results of potentiometric titration showed that the
shape of 1:2 except hydroxocomplexes [Ti(Cit)2(OH)2]6-) have no effect on the value of the criterion
of Fmin. It is significant that the 2:3 ratio for the low concentration of metal and the ligand the
equimolar forms are dominated in solution, whereas the proportion of particles [Ti(HCit)2(Cit)]6-,
[Ti(Cit)3]8-, [Ti(Cit)2(OH)2]6- begins to increase with the concentrations growths of the reactants in
the system.
The model №7 best of all corresponds the experimental data. The deviation of the experimental
values of the formation function of the theoretically calculated values of this parameter does not
exceed 2%.
Comparison of experimental and theoretical curves for the metal: ligand 1:1, 1:2 and 1:3 ratios
is shown in Fig. 3 (BTi = 0.0026 mol/l).
Logarithms of the formation constants βpqr of the titanium(IV) citrates is shown in the table 4.
Тable 1. The results of mathematical modeling of equilibrium
in the Ti(IV) - citric acid system to an equimolar ratio of the reactants
(N = 172, non-significant forms marked braces)
Model’s №
1
2
3
Model
[Ti(HCit)]+ + [Ti(Cit)]0 + {[TiOH(Cit)]-} + {[Ti(OH)2(Cit)]2-} +
{[Ti(OH)3(Cit)]3-} + [Ti(OH)4(Cit)]4- (model 1)
Моdel 1 (-non-significant forms) + [Ti2OH(Cit)2]- + {[Ti2(OH)2(Cit)2]2-}
+ {[Ti2(OH)3(Cit)2]3-} + {[Ti2(OH)4(Cit)2]4-} + {[Ti2(OH)5(Cit)2]5-} +
{[Ti2(OH)6(Cit)2]6-} + {[Ti2(OH)7(Cit)2]7-} (-non-significant forms)
Моdel 2 (-non-significant forms)+ [Ti3(OH)3(Cit)3]3- + [Ti3(OH)4(Cit)3]4+ [Ti3(OH)5(Cit)3]5- + [Ti3(OH)6(Cit)3]6- + [Ti3(OH)10(Cit)3]10- +
[Ti4(OH)10(Cit)4]10- + [Ti4(OH)11(Cit)4]11- + [Ti4(OH)12(Cit)4]12- (model 3)
Fmin
Fpr
R, %
45.8
0.274
2.58
17.8
0.110
1.61
13.1
0.082
1.38
Fmin
Fpr
R, %
41.4
0.140
1.85
41.0
0.139
1.84
Тable 2. The results of mathematical modeling of equilibrium
in the Ti(IV) - citric acid system for 1:1-1:2 reactant ratios
(N = 303, non-significant forms marked braces)
Model’s №
4
5
Model
Mоdel 3 + {[Ti(H3Cit)2]2+} + {[Ti(H3Cit)(H2Cit)]+} + {[Ti(H2Cit)2]0} +
[Ti(H2Cit)(HCit)]- + [Ti(HCit)2]2- + [Ti(HCit)(Cit)]3- + [Ti(Cit)2]4- +
{[TiOH(Cit)2]5-}+ [Ti(OH)2(Cit)2]6-(Моdel 4)
Моdel 4 (-non-significant forms) +
{[Ti2(OH)(Cit)4]9-} +
10[Ti2(OH)2(Cit)4] (Model 5)
Тable 3. The results of mathematical modeling of equilibrium
in the Ti(IV) - citric acid system for 1:1-1:3 reactant ratios
(N = 567, non-significant forms marked braces).
Model’s №
6
7
Моdel
Моdel 5 + [Ti(H2Cit)3]2- + [Ti(H2Cit)2(HCit)]3- + [Ti(H2Cit)(HCit)2]4- +
[Ti(HCit)3]5- + [Ti(HCit)2(Cit)]6- + [Ti(HCit)(Cit)2]7- + [Ti(Cit)3]8- (Моdel
6)
[Ti(HCit)]+ + [Ti(Cit)]0 + [Ti(OH)4(Cit)]4- + [Ti2OH(Cit)2]- +
[Ti2OH(Cit)2]- + [Ti3(OH)3(Cit)3]3- + [Ti3(OH)4(Cit)3]4+ [Ti3(OH)5(Cit)3]5- + [Ti3(OH)6(Cit)3]6- + [Ti3(OH)10(Cit)3]10+ [Ti4(OH)10(Cit)4]10- + [Ti4(OH)11(Cit)4]11- + [Ti4(OH)12(Cit)4]12+ [Ti(OH)2(Cit)2]6- + [Ti(H2Cit)3]2- + [Ti(H2Cit)2(HCit)]3+ [Ti(H2Cit)(HCit)2]4- + [Ti(HCit)3]5- + [Ti(HCit)2(Cit)]6+ [Ti(HCit)(Cit)2]7- + [Ti(Cit)3]8- (Моdel 7)
Fmin
Fpr
R. %
98.97
0.178
2.09
97.7
0.178
2.08
7
6
5
n
4
1 - BTi = 0.0026M, CH4Cit = 0.0027M
3
2 - BTi = 0.0026M, CH4Cit = 0.0027M
3 - BTi = 0.0026M, CH4Cit = 0.0055M
4 - BTi = 0.0026M, CH4Cit = 0.0055M
2
5 - BTi = 0.0026M, CH4Cit = 0.0082M
6 - BTi = 0.0026M, CH4Cit = 0.0082M
1
2
3
4
5
6
pH
7
8
9
10
Fig. 3. Comparison of experimental and theoretical curves (curves 1, 3, 5
were created by potentiometric results; curves 2, 4, 6 were
obtained as result of the mathematical treatment of the experimental data).
Тable 4. Titanium(IV) citrates composition and stability.
Complex
[Ti(HCit)]+
[Ti(Cit)]0
[Ti(OH)4(Cit)]4[Ti2OH(Cit)2][Ti3(OH)3(Cit)3]3[Ti3(OH)4(Cit)3]4[Ti3(OH)5(Cit)3]5[Ti3(OH)6(Cit)3]6[Ti3(OH)10(Cit)3]10[Ti4(OH)10(Cit)4]10[Ti4(OH)11(Cit)4]11[Ti4(OH)12(Cit)4]12[Ti(OH)2(Cit)2]6[Ti(H2Cit)3]2[Ti(H2Cit)2(HCit)]3[Ti(H2Cit)(HCit)2]4[Ti(HCit)3]5[Ti(HCit)2(Cit)]6[Ti(HCit)(Cit)2]7[Ti(Cit)3]8-
Stoichiometry matrix
Ti (p)
H4Cit (q)
H+ (r)
1
1
3
1
1
4
1
1
8
2
2
9
3
3
15
3
3
16
3
3
17
3
3
18
3
3
22
4
4
26
4
4
27
4
4
28
1
2
10
1
3
6
1
3
7
1
3
8
1
3
9
1
3
10
1
3
11
1
3
12
4+
lgβpqr
2.85±0.06
1.56±0.05
-23.26±0.15
2.96±0.13
2.10±0.14
-2.05±0.27
-7.01±0.27
-12.47±0.19
-43.42±0.39
-28.47±0.27
-36.22±0.54
-44.42±0.41
-23.88±0.21
1.19±0.11
-2.03±0.17
-5.95±0.11
-10.89±0.43
-15.54±0.18
-21.07±0.52
-27.35±0.20
Thus, we have proposed schemes reflecting equilibrium solution of citric acid containing ions
of titanium(IV) (Fig. 4-5).
Fig. 4. Scheme of interconversation in the Ti(IV) – citric acid system by the 1:1
ratio (comparison of the complexes is commensurated by table 4)
Fig. 5. Scheme of interconversation in the Ti(IV) – citric acid system by the 1:1,
1:2, 1:3, 2:3 ratios (comparison of the complexes is commensurated by table 4)
We calculated the share of accumulation of citrate titanium (IV) using the data obtained from
the formation constants. Distribution of the complex forms depending on the pH of the selected
metal and the ligand concentration is shown in Figure 6-7.
0,8
0,8
0,7
H4Cit
H4Cit
0,7
H3Cit
0,6
-
H3Cit
2-
H2Cit
2-
H2Cit
3-
HCit
2+
Ti(OH)2
0,6
0,5
TiHCit
0
TiCit
4Ti(OH)4Cit
0,4
Ti2OHCit2
3-
0,5
HCit
+
TiHCit
0
TiCit
6Ti(OH)2(Cit)2
0,4
Ti(H2Cit)3
-

-
3-

Ti3(OH)3Cit3
2-
4-
Ti3(OH)4Cit3
0,3
3-
Ti(H2Cit)2(HCit)
5-
Ti3(OH)5Cit3
4-
0,3
6-
Ti3(OH)6Cit3
Ti(H2Cit)(HCit)2
5-
Ti(HCit)3
10-
Ti3(OH)10Cit3
0,2
10-
Ti4(OH)10Cit4
0,2
6-
Ti(HCit)2(Cit)
11-
Ti4(OH)11Cit4
7-
Ti(HCit)(Cit)2
12-
Ti4(OH)12Cit4
0,1
0,1
8-
Ti(Cit)3
0,0
0,0
1
2
3
4
5
6
7
8
9
10
pH
1
2
3
4
5
pH
6
7
8
9
10
Fig. 6. Distribution of the complex forms in
Fig. 7. Distribution of the complex forms in
the system of TiCl4-citric acid depending
on the pH (BTi= 0.01 М : CH4Cit= 0.01055 M).
the system of TiCl4-citric acid depending
on the pH (BTi= 0.02 М : CH4Cit= 0.0628 M)
Conclusions
It is shown that in the TiCl4 - citric acid exist in a broad pH range the 1:1, 2:2, 3:3, 4:4, 1:2, 1:3
varying deprotonation. For citrate titanium (IV) formation constants and the proportion of complex
forms of data storage were calculated. The forms of citrate titanium (IV) of 2:2, 3:3, 4:4 varying
degrees of protonation were discovered for the first time.
References
[1]
K. Buettner, A. Valentine. Bioinorganic chemistry of titanium. Chem. Rev. 2012. Vol.112. No.3.
P.1863-1881.
[2]
Y.-F. Deng, Z.-H. Zhou, H.-L. Wan. pH-dependent isolations and spectroscopic, structural, and
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