Article pubs.acs.org/jced
Interaction of Phytate with Ag+, CH3Hg+, Mn2+, Fe2+, Co2+, and VO2+: Stability Constants and Sequestering Ability Clemente Bretti, Rosalia Maria Cigala, Concetta De Stefano, Gabriele Lando, and Silvio Sammartano* Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, Università di Messina, Viale Ferdinando Stagno D’Alcontres, 31, I-98166 Messina (Vill. S. Agata), Italy S Supporting Information *
ABSTRACT: The stability of M/Phy species (M = Ag+, CH3Hg+, Mn2+, Fe2+, Co2+, VO2+, and Phy = Phytate) was determined in NaNO3, at I = 0.10 mol·dm−3 and T = 298.15 K, potentiometrically. For the VO2+/Phy system, the measurements were performed at different ionic strengths (to 0.45 mol·dm−3) and ionic media (NaNO3 and Na2SO4). In general, the formation of six MHqPhy (with 0 ≤ q ≤ 5) species was observed. For the CH3Hg+/Phy system, the formation of three polynuclear species, M2HqPhy (with 2 ≤ q ≤ 4), was also evidenced. The general stability trend is: VO2+ ∼ CH3Hg+ > Co2+ > Mn2+ ∼ Fe2+ > Ag+. The sequestering ability of phytate toward the considered metal cations was evaluated at different pH values and increased with increasing pH, except for methylmercury and vanadyl. Literature values were critically compared, where possible, with data reported in this work. The dependence of the formation constants of the MHqPhy species on the number of protons and on the first metal hydrolysis constants was modeled. Furthermore, a relationship between the stability of the mononuclear MHqPhy and the polynuclear MpHqPhy species was found. These three relationships were then combined, for testing and predictive purposes. The last contribution of this research group to the series entitled “Speciation of phytate ion in aqueous solution” is reported in ref 1.
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INTRODUCTION 1,2,3,4,5,6-Hexakis(dihydrogen phosphate)myo-inositol, also known as phytic acid (Phy) and its salts (also known as phytines2) are widely present in nature, mostly in grain, and it is the principal storage form of phosphorus in many plant tissues, especially bran and seeds.3 In the past, the role of phytic acid in human and nonruminants was called into question. In fact, the presence of phytic acid may reduce the bioabsorption of minerals in the intestine, engendering mineral deficiencies in people whose diets rely on these foods for their mineral intake, such as those in developing countries.4 On the other hand, phytic acid may be considered as a nutrient, providing an antioxidant effect,5 and preventing colon cancer, reducing oxidative stress in the lumen of the intestinal tract6,7 and kidney stone formation.7−9 Moreover, it has been demonstrated that phytate may reduce glucose absorption10 and may have a neuro-protective effect in chelating iron.11 In other works, published by Davidsson et al.12 and Lonnerdal,13 it is reported that the presence of phytic acid in soy decreases the manganese absorption. Other properties have been continuously discovered; for example, owing to its low cost and toxicity, phytic acid can be used in the remediation strategies for the removal of different cations14−17 or in electrochemistry for the preparation of modified electrodes.18 Some comprehensive reviews were published on these topics (see refs 19−24 and references therein). Also from a chemico-physical point of view, phytate is a very interesting molecule, because it forms quite strong ion pairs with most of the metal ions and shows an high charge © XXXX American Chemical Society
density, with 12 displaceable protons in the whole pH range; see the structure in Figure 1. In the last years, some reviews
Figure 1. Structural formula of phytic acid.
have been published on the acid−base and thermodynamic properties of phytate and on its interaction with different metals, organo-metal cations, and uncharged molecules. In this work, the stability of various M/Phy systems (with M = Ag+, CH3Hg+, Mn2+, Fe2+, Co2+, VO2+) was studied. The Received: July 6, 2012 Accepted: September 10, 2012
A
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Table 1. Stability Constant Values of the MpHqPhy Speciesa Reported in the Literature M 2+
Co Co2+ Co2+ Mn2+ Mn2+ Mn2+ Fe2+ Fe2+ a
Ib
medium
T/K
log K10
log K11
log K12
log K13
log K14
log K15
log K16
log K17
ref
0.10 0.15 0.17 0.15 0.10 3.00 0.10 0.15
KCl NaClO4 Et4NClO4 NaClO4 KCl NaClO4 KCl NaClO4
309.15 310.15 293.15 310.15 309.15 298.15 309.15 310.15
9.73
8.65
7.96 9.7 12.7
7.04 9.1 9.82
10.46
9.91
9.50
3.27 6.26 5.26 7.20 6.01
12.35
11.99
11.66 11.44
11.22 10.52
9.44 8.99
3.76 6.96 6.48 8.44 6.54 3.85 8.09 7.71
2.97 4.0 2.94
10.85
5.02 7.9 7.85 8.78 7.87
7.43 5.95
6.77
26 28 25 28 26 29 27 28
5.13
Refers to eq 1. bIn mol·dm−3.
Table 2. Details of the Chemicals Used in This Work
a
chemical name
source
initial mole fraction purity
purification method
final mole fraction purity
analysis method
phytic acid methylmercury chloride cobalt chloride manganese chloride silver nitrate vandyl sulfate Mohr’s salta hydrochloric acid nitric acid sodium hydroxide sodium nitrate sodium sulfate water nitrogen
Aldrich Aldrich Aldrich Aldrich Aldrich Aldrich Aldrich Aldrich Aldrich Aldrich Aldrich Aldrich Aldrich Rivoira
95 % 99 % ≥ 98 % > 98 % > 99 % unknown > 99 % 37 % 70 % standard solution > 99 % 99 % for trace analysis > 99.9 %
ion-exchange resin
99 % 99 % ≥ 98 % > 98 % > 99 % 99 % > 99 % 37 % 70 % standard solution > 99 % 99 %
potentiometry complexometric titration EDTA titration EDTA titration Mohr’s method redox + photometric titration spectrophotometric titration volumetric titration volumetric titration volumetric titration
> 99.9 %
As (FeSO4(NH4)2SO4·6H2O).
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EXPERIMENTAL SECTION Chemicals. Phytic acid solutions were prepared weighing Aldrich dipotassium salt (K2H10Phy) and passing it over a strong cationic exchange resin (Dowex 50W X 8) in H+ form. The concentration was checked potentiometrically by alkalimetric titrations, and flame emission spectrometry was used to determine the potassium concentration, which was lower than the LOQ (limit of quantification) ( 1 were performed to investigate the formation of polynuclear species, whose formation has been observed for other monocharged cations.19 For each experiment, independent titrations of strong acid solutions with standard base were carried out under the same medium and ionic strength conditions as the systems to be investigated, to determine the standard electrode potential (E0) and the acidic junction potential (Ej = ja·[H+]). In this way, the pH scale used was the free scale, pH ≡ −log10[H+], where [H+] is the free proton concentration. For each titration, 80−100 data points were collected, and the equilibrium state during titrations was checked by adopting some usual precautions46 (e.g., checking the time required to reach equilibrium and performing back-titrations). For measurements performed at low ionic strengths, the contribution of the ligand to the total ionic strength was considered. All the experimental details are summarized in Table 3. Four potentiometric measurements with a metallic ISE-Ag + electrode (model 6.0331.010, purchased by Metrohm) were also performed at pH = 5.5. The standard electrode potential was determined, with respect to a calomel reference electrode, titrating a solution containing NaNO3 at a given concentration with standard AgNO3 solution. The measurements were performed in the same way, but adding phytic acid to the titrand solutions. Calculations. The calculation programs are reviewed in ref 47. The nonlinear least-squares computer program ESAB2M was used for the refinement of all the parameters of the acid− base titrations (E0, Kw, liquid junction potential coefficient, ja, analytical concentration of reagents). The BSTAC and STACO computer programs were used in the calculation of complex formation constants. The ES4ECI program was used to draw the speciation and sequestration diagrams and to calculate species formation percentages. The LIANA program was used to fit different equations. According to recent works,35,48 the charge (z) of phytate in Na+ aqueous media is z = −7, since the strong interactions between highly charged anions, such as phytate, and the cation of the supporting electrolyte lower the effective charge of the ligand, so that the completely deprotonated phytate form is
p M + HqPhy = M pHqPhy
K pq
(1)
or p M + qH + Phy = M pHqPhy
βpq
(2)
When q < 0, equilibria refer to hydrolytic species, whereas if p = 0, equilibria refer to protonation constants. Throughout the work, uncertainties are given as a 95 % confidence interval (95 % C.I.). The conversion from the molar to the molal concentration scale was obtained using the equation: c/m = d + a1·c + a2·c2 (at T = 298.15 K, d = solvent density = 0.9971 g·cm−3; c = molar concentration; m = molal concentration; a1 = 0.02983151, a2 = −6.4516·10−4 in NaNO3, and a1 = 0.01696676, a2 = −3.23·10−6 in Na2SO4).
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RESULTS AND DISCUSSION Protonation of Ligands and Hydrolysis of Metal Ions. In the equilibrium analysis, the building of a reliable and robust model is fundamental. For this reason the study of the different M/Phy system was preceded by an analysis of literature data concerning the protonation constants of phytic acid.35,48 A similar consideration can be made concerning the hydrolysis of the metal cations. In this case, the data reported in Baes and Mesmer49 were considered reliable. Even if these data refer mainly to perchlorate medium, their accuracy is sufficient for our purposes. In fact, both nitrate and perchlorate anions can be considered as “weakly interacting” with the most of metal cations. Regarding vanadyl hydrolysis and its interaction with sulfate ion we used the data reported in Berto et al.43 For the interaction of Fe2+ with ammonia and with sulfate, and for the ammonia protonation constant, we used the data reported in Martell et al.50 Sulfate protonation data were taken from De Robertis et al.51 The metal hydrolysis constant values are reported as Supporting Information, whereas the phytate protonation constants are collected in Table 4. Formation of Metal−Proton−Phytate Species. The analysis of the potentiometric titration data, obtained in the experimental conditions summarized in Table 3, allowed us to determine different MpHqPhy species. In the investigated M/ Phy systems, the speciation schemes proposed consists of six MHqPhy species (with 0 ≤ q ≤ 5), namely, MPhy, MHPhy, MH2Phy, MH3Phy, MH4Phy, and MH5Phy. In the case of the CH3Hg+/Phy system, owing to the excess of methylmercury in some measurements, the M2H2Phy, M2H3Phy, and M2H4Phy C
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47.39
log β24d
40.88 32.80 46.67 48.16 47.84 46.62 47.18
13.43 ± 0.30 ± ± ± ± ±
40.74 42.36 41.99 41.30 41.59
2.99 4.59 4.16 2.94 3.50 0.07 0.06 0.08 0.08 0.12 log β14d
± ± ± ± ± 33.67 35.45 35.33 34.21 34.37
3.55 5.30 4.80 4.11 4.40 0.08 0.05 0.07 0.09 0.12 log β13d
± ± ± ± ± 25.04 27.21 26.80 25.30 25.56
4.68 6.61 6.34 5.22 5.38 0.08 0.07 0.07 0.08 0.14 log β12d
± ± ± ± ± ± ± ± ± ±
0.10 0.17 0.08 0.15b 0.11 log β11d
5.58 7.84 7.34 5.84 6.10
log K12b
c
log K13b
c
log K14b
c
log K15b
c
0.08 0.05 0.08 0.10 0.18 log β15d
log K22b D
0.10 0.13 0.08 0.20 0.19 ± ± ± ± ± 0.0962 0.103 0.101 0.100 0.106
6.40 8.52 8.10 6.74 7.14
a
0.0962 0.103 0.101 0.100 0.106 Ag+ CH3Hg+ Co2+ Fe2+ Mn2+
In mol·dm−3. bRefers to eq 1. c± 95 C.I. dRefers to eq 2.
log β10d
6.33 8.16 8.05 6.39 6.66
c
Small discrepancies can be noted for the stability of the CoH3Phy species, that is, slightly higher than the NiH3Phy species (0.3 log K units), and for that of the MnH3Phy species, also slightly higher than the FeH3Phy species (0.15 log K units). Concerning the difference between the stability of the CoH3Phy and the NiH3Phy species it should be remarked that the error associated to their values is 0.07 (at 95 % C.I.) for the CoH3Phy and 0.3 (at 95 % C.I.) for NiH3Phy.35 Similar considerations can be done for the difference in the stability of the MnH3Phy and FeH3Phy species. Concerning the formation constants of the VO2+/Phy system we can observe that, in general, in both the considered ionic media, NaNO3 and Na2SO4, the formation constants slightly decrease with increasing ionic strength. For example, in Na2SO4 the stability of the VOH4Phy species is log K14 = 4.97 and 4.90, at I = 0.0981 and 0.4509 mol·dm−3, respectively. Similarly, at comparable ionic strength (e.g., I ∼ 0.1 mol·dm−3), the formation constants obtained in NaNO3 are lower than those in Na2SO4, because at the same ionic strength the sodium concentration in Na2SO4 is lower than in NaNO3, in particular cNa(Na2SO4) = 0.66·cNa(NaNO3). It should be noted that the stability constants of the VO2+/Phy system were obtained considering the VO2+/SO42− complexes. To show the complexing ability of phytate toward cobalt, the distribution diagram of the CoHqPhy species is shown in Figure
log K11b
Mn 2 + < Fe 2 + < Co2 + < Ni 2 + < Cu 2 + > Zn 2 +
c
If we consider the stability values of the MH3Phy species, with respect the literature values for the Zn2+, Cu2+, and Ni2+ cations,35 the Irving-Williams affinity series is respected.
log K10b
VO2 + ∼ CH3Hg + > Co2 + > Mn 2 + ∼ Fe 2 + > Ag +
Ia
Table 5. Experimental Stability Constant Values of the MpHqPhy Species in NaNO3 at T = 298.15 K Reported in This Work
species were also found. For the VO2+/Phy system we could not determine the stability of the MPhy and the MHPhy species. All of the stability constant values reported in this work are listed in Tables 5 and 6. In the case of the Ag+/Phy system, it was possible to compare the formation constant values for the AgH5Phy− species, mainly present at pH = 5.5, obtained both by ISEAg+ (log K15 = 3.04 ± 0.05) and ISE-H+ measurements (log K15 = 2.99 ± 0.08). The difference between the two values is lower than the experimental errors (considering ± 95 % C.I.); therefore they cannot be considered significantly different. Comparing the stability of the different MHqPhy species, it is possible to observe that the stability of the VOHqPhy and the (CH3Hg)HqPhy species are higher with respect the other MHqPhy species. For example, the stability trend observed for the MH3Phy species was:
log β22d
In mol·dm−3.
15.81 17.64 17.53 15.87 16.14
8.78 18.19 27.07 34.70 40.66 45.35 48.04
M
a
9.48 19.46 28.99 37.19 43.68 48.85 51.87
6.40 8.52 8.10 6.74 7.14
H + Phy = HPhy 2H+ + Phy7− = H2Phy5− 3H+ + Phy7− = H3Phy4− 4H+ + Phy7− = H4Phy3− 5H+ + Phy7− = H5Phy2− 6H+ + Phy7− = H6Phy− 7H+ + Phy7− = H7Phy
I = 0.45a
Ag CH3Hg+ Co2+ Fe2+ Mn2+
6−
log β23d
7−
log K23b
I = 0.1
equilibrium +
+
log Kq a
12.02 ± 0.11
log K24b
Table 4. Phytate Protonation Constants Used in Calculations from Cigala et al.35
10.31 ± 0.12
Article
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Table 6. Experimental Stability Constant Values for the (VO)HqPhy Species in NaNO3 and Na2SO4 at Different Ionic Strengths and at T = 298.15 K log K1qa NaNO3 q
I = 0.102b
2 3 4 5
± ± ± ±
8.10 6.90 4.64 3.68 2 3 4 5
a
0.10 0.08 0.08 0.09
Na2SO4 I = 0.442b
c
27.61 35.91 41.80 47.24
8.07 6.71 4.91 3.72
± ± ± ±
0.07 0.04 0.03 0.05
26.24 33.76 39.60 44.38
I = 0.0981b 8.49 7.02 4.97 4.01 log β1qd
± ± ± ±
0.15 0.10 0.11 0.13
I = 0.451b 8.09 6.75 4.90 3.94
28.01 36.06 42.17 47.64
± ± ± ±
0.12 0.10 0.08 0.09
26.15 33.69 39.49 44.53
Figure 3. Distribution diagrams of the CH3Hg+/Phy species vs pH. Experimental conditions: cCH3Hg = cCl = 1 mol·m−3, cPhy = 4 mol·m−3, in NaNO3 at I = 0.103 mol·dm−3 and T = 298.15 K. Species: MPhy (1), MHPhy (2), MH2Phy (3), MH3Phy (4), MH4Phy (5), MH5Phy (6), M2H3Phy (7), M2H4Phy (8), M + MCl + M(OH) (9) (charges omitted for simplicity).
Refers to eq 1. bIn mol·dm−3. c± 95 C.I. dRefers to eq 2.
2 . In the reported experimental conditions (cCo = 1 mol·m−3, cPhy = 4 mol·m−3, in NaNO3 at I = 0.101 mol·dm−3 and T =
0.5. In these experimental conditions, the formation of the polynuclear (CH3Hg)2HqPhy species occurs to a very low extent. In the concentration condition of equimolarity, cM = cPhy = cCl = 4 mol·m−3, the molar fraction of the three polynuclear species is quite similar, and it is x ∼ 0.2. Distribution diagrams for the other M/Phy systems are reported as Supporting Information. Sequestering Ability of Phytate. In the last years, we proposed an empirical parameter (called pL0.5 or pL50) that, once the experimental conditions (ionic strength, ionic medium, temperature, pH, and metal concentration) are fixed, can give an objective representation of the sequestering ability of a ligand (L) toward a metal ion (M). A detailed description of the method is given, for example, in ref 52. Briefly, the mole fraction (x) of a generic metal ion M (present in trace) complexed by a generic ligand L may be expressed as a function of pL, where pL ≡ −log cL with cL = total ligand concentration. This function is represented by a sigmoid curve (similar to a dose−response curve), with asymptotes of 1 for pL → −∞ and 0 for pL → +∞:
Figure 2. Distribution diagram of Co2+/Phy species vs pH. Experimental conditions: cCo = 1 mol·m−3 ; cPhy = 4 mol·m−3, in NaNO3 at I = 0.101 mol·dm−3 and T = 298.15 K. Species: MPhy (1), MHPhy (2), MH2Phy (3), MH3Phy (4), MH4Phy (5), MH5Phy (6), and M (7) (charges omitted for simplicity).
⎡ ⎤ 1 x=⎢ ⎥ (pL − pL ) 0.5 ⎦ ⎣ 1 + 10
298.15 K) phytate totally binds Co2+ at pH > 5. The formation percentages of the various protonated species are high, for example, the mole fraction (x) of the CoH5Phy is 0.8 at pH = 4.5, and 0.5 for the CoH2Phy species at pH = 8.5. Despite the fact that the stability constant values of the CH3Hg+/Phy system are higher than those of the Co2+/Phy, in the same experimental conditions the formation percentages of the various (CH3Hg)HqPhy species are lower than the corresponding CoHqPhy species; see Figure 3. The relatively low formation percentages of the (CH3Hg)pHqPhy species are due to the high tendency of the methylmercury cation to undergo hydrolysis (log βMOH = −4.54) and to form complexes with chloride (log βMCl = 5.25). For simplicity, in Figure 3 the molar fractions of the M(OH), MCl, and M species are reported as summation of the single contribute and are shown as a dotted line. Even if the phytate concentration is higher than both methylmercury and chloride, the sum of the molar fractions of M, M(OH), and MCl is higher than 0.25 thorough the entire pH range, whereas, in the same range, the single molar fraction of the protonated complex species is lower than
(3)
where the parameter pL0.5 represents the total ligand concentration necessary to sequester 50 % of the metal ion. Therefore, the higher the pL0.5 is, the stronger is the binding ability toward a given cation. In this work, we calculated the sequestering ability of phytate toward the considered metal cations at different pH values, and the corresponding pL0.5 values are reported in Table 7. In the case of the VO2+/Phy system, the pL0.5 values were also determined at different ionic strengths and ionic media and are reported in Table 8. The dependence of the pL0.5 values on pH in the case of the Co2+/Phy, Mn2+/Phy, Fe2+/Phy, and Ag+/ Phy systems is linear and was modeled by means of an empirical relationship (pL0.5 = a + b·pH), which can be used as a predictive tool. The four fits (see Figure 4) are characterized by the same slope, b = 0.70 ± 0.01, and different intercepts (a). In particular, a = 0.32 ± 0.05, −0.45 ± 0.07, −0.82 ± 0.07, and −1.05 ± 0.07 for Co2+, Mn2+, Fe2+, and Ag+, respectively. The correlation coefficients (r) of the fits are always higher than E
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Table 7. pL0.5 Valuesa for M/Phy Systems Calculated at I = 0.10 mol·dm−3 in NaNO3 and T = 298.15 K
reported by these authors with data found in the literature for the same systems, owing to different experimental conditions and different speciation models. In any case, the most important literature findings concerning the binding ability of phytate toward the considered metal cations are reported in refs 25−29 and summarized in Table 1. To our knowledge no works have been published on the interaction of phytate with methylmercury, whereas very few data are reported on the Ag+/ Phy and VO2+/Phy systems by Anderson53 and Williams,54 respectively. The former reported a study on the silver/phytate interaction, precipitating the solid octasilverphytate salt, whereas the latter performed a spectrophotometric study on the VO2+/Phy system. They reported that, for a solution containing both vanadyl and phytate, with molar ratio 6:1, the complex is characterized by a stoichiometry of 1:1 (at pH ∼ 1), whereas the formation of sparingly soluble species occurred at pH > 1 with the formation of successive complexes, up to 4:1 (VO2+:Phy) species. In this work we performed measurements in the excess of ligand; therefore we observed the formation of scarcely soluble species only at pH ∼ 9.0. Regarding the other M/Phy systems, the stability constant value reported by Vasca et al.29 for the MnH5Phy species (log K15 = 3.85 in NaClO4 at I = 3 mol·dm−3) is in a fairly good agreement to the value reported in this work (log K15 = 3.50 in NaNO3 at I = 0.10 mol·dm−3), as well as the data reported by De Carli et al.26 for various CoHqPhy species (in KCl at I = 0.1 mol·dm−3 and T = 309.15 K), whose differences with analogous species here reported are lower than 1 log K units. In the same work, the authors reported data for the Mn2+/Phy system, which, in this case, are significantly higher than our values (difference >2 log K units). The results reported by BebotBrigaud et al.25 for the Co2+/Phy system are not comparable with our values, since they were obtained in a noninteractive supporting electrolyte (Et4NClO4). In any case, the fact that values reported in Et4NClO4 are higher than values here reported in NaNO3 is consistent. The data reported by Torres et al.28 for the Co2+/Phy, Mn2+/Phy, and Fe2+/Phy systems are higher than our values by several order of magnitude, as well as those reported by Quirrembach et al.27 for the Fe2+/Phy system. The explanation of these discrepancies is not easy, but the different set of phytate protonation constants used in the calculation plays a key role. Lonnerdal13 and Davidsson et al.12 reported that the affinity of phytate toward manganese is lower than that for copper and zinc; this is in agreement with results reported in this work. Some investigations were performed by Evans and Pierce,55 but they did not report data on the interaction of phytate with manganese cations. Estimation of Formation Constants by Empirical Relationships. The analysis of the data reported in this work together with the available literature data allowed us to perform general considerations on the binding ability of phytate toward the different metal cations. For example, in previous sections we observed that the Irving−Williams affinity series is respected, except for some small discrepancies. In addition to this qualitative aspect, we performed some quantitative analysis, which can be used for predictive, testing, or other purposes. Before the description of these correlations, it is necessary to note that the data used in this paragraph (in addition to the experimental results of this work) were critically selected, and those that populated the training set are reported in refs 1, 30−40, for the interaction of phytate with Li+, Na+, K+, Cs+, Mg2+, Ca2+, Ni2+, Cu2+, Pb2+, Zn2+, Cd2+, Sn2+, (CH3)2Sn2+,
pL0.5
a
+
pH
Ag
4.0 5.0 6.0 7.4 8.1 9.0
1.76 2.64 3.18 3.96 4.46 5.23
CH3Hg
+
3.15 3.59 3.37 2.97 2.91 2.91
Co2+
Fe2+
Mn2+
2.93 3.81 4.41 5.52 6.11 6.77
1.72 2.71 3.55 4.40 4.83 5.36
2.27 3.20 3.90 4.70 5.14 5.77
± 0.1 (95 % C.I.) for all of the systems.
Table 8. pL0.5 Valuesa for the VO2+/Phy System at Different pH, Ionic Media, and Ionic Strengths at T = 298.15 K pL0.5
a
NaNO3
Na2SO4
NaNO3
Na2SO4
pH
I = 0.102b
I = 0.098b
I = 0.442b
I = 0.451b
4.0 5.0 6.0 7.4 8.1 9.0
2.44 3.31 3.87 4.30 4.28 4.13
2.56 3.47 3.90 4.22 4.28 4.29
3.04 3.93 4.54 4.77 4.76 4.29
3.11 3.90 4.42 4.65 4.64 4.32
± 0.1 (95 % C.I.) for all of the systems. bIn mol·dm−3.
Figure 4. Dependence of pL0.5 on pH in NaNO3 at I = 0.10 mol·dm−3 and T = 298.15 K. Systems: ○, Co2+/Phy; △, Mn2+/Phy; ◇, Fe2+/ Phy; □, Ag+/Phy.
0.99, and the mean deviation (m.d.) is always lower than the error on the pL0.5 values, m.d. < 0.10. This relationship can be useful to predict the pL0.5 values at a given pH, without the knowledge of the stability, protonation, and hydrolysis constants. For the VO2+/Phy and CH3Hg+/Phy systems, the dependence of the pL0.5 on pH is not linear; therefore it cannot be modeled with a simple relationship. Comparisons. As pointed out for other systems, when dealing with a so complex molecule, with high charge and many protonation constants, the determination of the stability constants is very challenging. In fact, both the ligand protonation and the metal hydrolysis constants are key factors, which may significantly affect the reliability of the data of a given system. As an example, the number of phytate protonation constants considered by various authors may vary from six to twelve. It is very difficult to compare the results F
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Pd2+, La3+, Al3+, Fe3+, and Cr3+ cations. The data on the metal hydrolysis constants are reported in refs 49 and 56. A linear correlation between the equilibrium constant of the first hydrolysis step (as −log βMOH) and the stability of the MH3Phy species (as log K13) was found. This correlation, valid
Figure 6. Correlation between the stability of the different protonated MHqPhy species with respect to the number of protons (q). For simplicity we report as (Y) the function Y = (log K1q − log K13)·z−2. Correlation coefficient r = 0.88. The equation is: Y = (−0.39 ± 0.01)·(q − 3).
Figure 5. Log K13 vs −log βMOH. Correlation between the stability of the MH3Phy species and the MOH species for different charged metal cations at I = 0.10 mol·dm−3 in Na+ media and T = 298.15 K. Cations: △, M+; ○, M2+; □, M3+. Correlation coefficient, r = 0.92, intercept, a = 16.0 ± 0.9, slope, b = −1.00 ± 0.10.
This equation can be useful to estimate the stability of an generic MHqPhy species, knowing the first hydrolysis constant of the cation and, of course, its charge. From previous data performed on polynuclear species of various M/Phy systems it was possible to establish another correlation, on the dependence of the stability of the mononuclear species with respect to the polynuclear species (unpublished data from this laboratory). In particular, the formation constant values of the log K2q species (expressed as in eq 1) vs log K1q show a linear relationship, with correlation coefficient r = 0.997, slope b = 1.80 ± 0.01, and intercept a = 0. The t test performed in this set confirmed that a = 0 at 99 % of the confidence interval. For example, considering the (CH3Hg)2H3Phy species, the difference between the calculated (log K23 = 11.90 by eq 7) and the experimental (log K23 = 12.02, Table 5) values is 0.12 log units. Furthermore, the stability of the log K3q species is plotted with respect to the log K2q species (expressed as in eq 1). The correlation coefficient is r = 0.999, the slope is b = 1.34 ± 0.01, and the intercept is a = 0.
for mono-, bi-, and trivalent metal cations, is shown in Figure 5 at I = 0.10 mol·dm−3 in NaNO3 and is (log K13 ± 0.5) = 16.0 + log βMOH
(4)
with a correlation coefficient r = 0.92. Considering 18 metal cations, only Hg2+, Pd2+, and La3+ are outside the confidence bands (95 % C.I.), whereas the stability constants determined in this work are in a good agreement with the calculated values. For example, the calculated value for the MnH3Phy species is log K13 = 5.28, and the experimental value is log K13 = 5.38. For this correlation, the “not strictly metal” cations (CH3Hg+, VO2+, and (CH3)2Sn2+) were not considered. The second empirical relationship regards the dependence of the formation constants on the number of protons of the complex. This property, observed in the past for other systems, is also valid for those reported in this work. Considering the stability of all the MHqPhy species, at a given ionic strength values of I = 0.10 mol·dm−3, we report in Figure 6 the plot of a function (Y) vs the number of protons (q). The function Y is:
where z stands for the charge of the cation under investigation (e.g., z = 1 for CH3Hg+ and z = 3 for Fe3+). The fit, based on 92 complex species, is: (5)
(log K3q ± 0.4) = 1.34·log K 2q
(8)
log K = log K 0 − 0.51·z*·√I /(1 + 1.5·√I ) + C·I
with a correlation coefficient, r = 0.88. This correlation, more general than the others, may be applied to all of the mononuclear MHqPhy complex species with 1 ≤ z ≤ 3. Combining eq 5 with eq 4, we obtain: log K1q ± 0.5 = 16.0 + log βMOH − 0.39·(q − 3) ·z 2
(7)
Combining eq 7 with eq 8, the slope of the plot log K3q vs log K1q (not reported) can be easily calculated, multiplying the two slopes as: b (eq 7)·b (eq 8) = 2.41 ± 0.02. Dependence on Ionic Strength and Ionic Medium. In this work we reported formation constant values at I = 0.10 mol·dm−3, therefore we are not able to perform ionic strength dependence calculations. We overcome this problem considering literature formation constant values of MHqPhy species reported at different ionic strengths in ref 35, for M = Cu2+, Zn2+, Pb2+, and Ni2+.
Y = (log K1q − log K13) ·z −2
log K1q ± 0.3 = log K13 − 0.39·(q − 3) ·z 2
(log K 2q ± 0.5) = 1.80·log K1q
z* =
(9)
∑ (charges)2 reactants − ∑ (charges)2 products
(K = formation constant; K0 = formation constant at infinite dilution). Equation 9 is also valid for the overall equilibrium constants, β. C is a linear function of ionic strength, and usually,
(6) G
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ing more data (e.g., data for M+/Phy and M3+/Phy systems) that in the future will be available.
this simple choice is sufficient to explain the experimental data trend in a wide ionic strength range, generally Co2+ > Mn2+ ∼ Fe2+ > Ag+. In the case of the CH3Hg+/Phy system three polynuclear species were determined, namely, M2H2Phy, M2H3Phy, and M2H4Phy species. The stability constants reported in this work were used to determine the sequestering ability of phytate toward the various cations studied. Except for CH3Hg+ and VO2+, the sequestering ability linearly increases with increasing pH. Therefore, at pH = 4.0 phytate preferentially binds CH3Hg+ among the six cations, whereas at pH = 9.0 the situation is very different and phytate selectively sequester Co2+ with respect to the other cations. Considering the available stability data of various M/Phy systems, it was possible to analyze the general behavior of phytate. Three useful relationships were found, which can be used for testing and predictive purposes. The first correlation regards the stability of the MH3Phy species (for 18 mono-, di-, and trivalent metal cations) with respect to the first hydrolysis step. The correlation is linear with a correlation coefficient r = 0.92. The second refers to the dependence of the stability of the MHqPhy species with respect to the number of protons of the complex, and also in this case, using 92 complex species, we found a good accordance. The third correlation shows a linear dependence of the stability of the mononuclear MHqPhy species with respect to the polynuclear M2HqPhy and M3HqPhy species. The three correlations can also be combined and used for the estimation of the stability of unknown MHqPhy species with an uncertainty of 0.5 log K units.
(10)
This equation can be used to calculate rough values of the parameter C at a given z* value, which may be applied to the formation constants in Table 5 to estimate the data at different ionic strength by means of eq 9. The stability constant values at I = (0, 0.5, and 1.0) mol·dm−3 for the M/Phy systems studied in this work are listed in Table 9. Table 9. Estimated Stability Constants at Different Ionic Strengths Using the Debye−Hückel Equation and the Calculated Ionic Strength Dependence Parameter at T = 298.15 K log Kpqa cation Ag+
CH3Hg+
Co2+
Fe2+
Mn2+
a
p
q
I = 0b
I = 0.5b
I = 1.0b
1 1 1 1 1 1 1 1 1 1 1 2 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
0 1 3 4 5 0 1 2 3 4 5 2 3 4 0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 3 4 5
8.0 7.7 6.7 5.6 4.2 3.5 10.1 9.5 9.0 7.5 6.0 5.1 15.4 13.6 11.4 11.2 10.7 9.6 8.1 6.2 5.1 9.8 9.0 8.1 7.0 5.5 3.9 10.2 9.3 8.3 7.2 5.8
5.3 5.4 4.8 4.0 3.0 2.6 7.4 7.2 7.0 5.9 4.7 4.2 12.1 10.9 9.5 6.1 6.3 5.9 5.1 3.9 3.5 4.8 4.7 4.4 4.0 3.2 2.3 5.2 5.0 4.6 4.2 3.5
4.7 4.8 4.3 3.6 2.6 2.2 6.9 6.7 6.5 5.5 4.4 3.8 11.4 10.4 9.0 5.2 5.5 5.1 4.5 3.3 3.0 3.8 3.8 3.6 3.4 2.6 1.8 4.2 4.1 3.9 3.5 2.9
■
ASSOCIATED CONTENT
* Supporting Information S
Metal hydrolysis constants, distribution diagrams, and dependence of the stability constant values. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: +39-090-6765749. Fax: +39-090-392827. Funding
We thank the University of Messina for partial financial support. Notes
The authors declare no competing financial interest.
■
Refers to eq 1. bIn mol·dm−3.
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From a general point of view this procedure can be extended to eq 6, to enable the calculation of the formation constants of various MHqPhy species at different ionic strengths. However, we suggest to use this procedure only for the calculation of tentative formation constant values. To improve the robustness of the model, the determination of general values for the ionic strength dependence parameter, must be performed considerH
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