Determination of complex formation constants for Cu(II)–Alizarin complexone with amines by capillary zone electrophoresis

Analytica Chimica Acta 398 (1999) 75–82

Determination of complex formation constants for Cu(II)–Alizarin complexone with amines by capillary zone electrophoresis Takashi Yokoyama a,∗ , Kimiko Tashiro a , Tomoaki Murao a , Ayumu Yanase a , Jun Nishimoto b , Michio Zenki a a

Department of Chemistry, Faculty of Science, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan b Department of Chemistry, Faculty of Science and Engineering, Saga University, Honjo-machi, Saga 840-8502, Japan Received 9 February 1999; received in revised form 9 April 1999; accepted 11 May 1999

Abstract Complex formation constants between Cu(II)–Alizarin complexone (Cu(II)–ALC) and amines were investigated by capillary zone electrophoresis. The electrophoretic mobility (µep ) of Cu(II)–ALC was obtained from the migration time of 3 ␮g ml−1 Cu(II) in a borate buffer in the presence of 1 × 10−3 mol dm−3 ALC of pH 9.1 at 20 kV. The µep value decreased with increasing amine concentration as additive in the buffer. This indicates coordination of the amines with Cu(II)–ALC. The first and second coordination equilibrium constants (dm3 mol−1 ) were 1 ± 1 and (3.33 ± 0.01) × 106 , 100 ± 1 and (4.73 ± 0.01) × 106 , 69 ± 4 and (1.21 ± 0.07) × 105 , 2 ± 1 and (2.74 ± 0.06) × 105 , 150 ± 20 and 74 ± 7, 5 ± 1 and 74 ± 3, 42 ± 1 and 12 ± 3, 57 ± 1 and 9 ± 2, (2.9 ± 0.2) × 108 and 990 ± 90, and 7900 ± 200 and 780 ± 70 for ammonia, methylamine, ethylamine, n-propylamine, pyridine, 2-methylpyridine, 3-methylpyridine, 4-methylpyridine, imidazole, and 1-methylimidazole, respectively. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Capillary electrophoresis; Alizarin complexone; Amine; Copper; Equilibrium constant

1. Introduction Many separation systems of metal ions with a chelating reagent by capillary zone electrophoresis (CZE) have been reported [1–6]. In several cases, separations of metal ions with the chelating reagent being an aminopolycarboxylic acid, such as ethylenediamine-N,N,N0 ,N0 -tetraacetic acid (EDTA) and trans-1,2-diaminocyclohexane-N,N,N0 ,N0 -tetraacetic acid (CDTA) have been investigated [7–19]. However, the separation of metal ions with Alizarin ∗ Corresponding author. Tel.: +81-86-256-9490; fax: +81-86-254-2891 E-mail address: [email protected] (T. Yokoyama)

complexone (ALC: 3-[N,N-bis(carboxymethyl)aminomethyl]-1,2-dihydroxyanthraquinone) as a class of aminopolycarboxylic acids has not been reported. Since ALC forms complex with many metal ions [20,21], it is expected that ALC is a good chelating reagent for the analysis of metal ions by CZE. For EDTA and CDTA, the coordination sites of metal ions are saturated with two nitrogen and four oxygen atoms of EDTA and CDTA, which form 1 : 1 complexes with metal ions. In the case of ALC, since ALC has four coordinating sites of a single nitrogen atom and three oxygen atoms, it can form a ternary complex which is coordinated water molecules at vacant coordination sites of metal–ALC complexes [22]. In cases of complexes capable of fast formation and

0003-2670/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 9 9 ) 0 0 4 1 6 - X

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capillary (GL Science, Tokyo) was used. Sample was introduced in the hydrodynamic mode, keeping the capillary end at a height of 25 mm for 60 s. The applied voltage was 20 kV. Solutes were monitored at 220 nm by direct UV absorption. UV–Vis absorption spectra were measured by a UV-2400PC spectrophotometer (Shimadzu, Tokyo).

dissociation, the separation of metal ions is accomplished mainly due to the difference in the formation or acid dissociation constants of complexes [23]. In cases of complexes with multidentate ligands including metallochromic species, the complex formation and dissociation equilibria are not generally fast compared to partially complexed metal ions using ligands such as 2–hydroxyisobutyric acid. Therefore, it is expected that the separation of metal ions by ALC is due to the fast ligand exchange reactions on the remaining coordination sites of the central metal ion saturated in a dynamic equilibrium with solvent molecules, buffer, or additives. Furthermore, a binuclear complex in which the mole ratio of the metal ion, such as Al(III) and Fe(III), to ALC is 2 : 1, can form in 20% aqueous dioxan of pH 4–5 [21]. It is also well-known that fluoride ion coordinates with La(III)–ALC and Ce(III)–ALC [22,24–28]. The Ce(III) ion with ALC forms [Ce(ALC)(H2 O)2 ] in an aqueous solution of pH 5.0–5.2, and [Ce(ALC)(H2 O)F]− is formed in the presence of the fluoride ion [22]. Also, Langmyhr et al. have reported that the ternary complex La(III)–ALC–F− forms a dimer [La2 (ALC)2 F2 ]2− [25]. Anfalt et al. have reported that the composition of the La(III)–ALC–F− complex is [La(ALC)4 F2 ] [26]. Furthermore, Yuchi et al. have reported the species La2 (ALC)2 F [27]. These results show that water molecules coordinated with the metal–ALC complexes could be exchanged with the other ligand, such as another ALC and the fluoride ion. Therefore, the separation behavior of the metal–ALC complexes by CZE could be influenced on some coordinating additives in the buffer. The separation behavior of the metal–ALC complexes by CZE is interesting. Accordingly, the separation behavior of the metal–ALC complexes and the effect of the additives in the buffer are investigated by CZE. Coordination equilibrium constants of Cu(II)–ALC complex with amines as the additives are determined by CZE.

The capillary was first conditioned with the buffer (10 min) immediately prior to injection. The column was washed systematically with 0.1 mol dm−3 KOH (2 min), and then rinsed for 2 min between runs.

2. Experimental

3. Results and discussion

2.1. Apparatus

3.1. Complex formation between B(III), Fe(II), Co(II), Ni(II), Cu(II), and Zn(II) and ALC

Capillary electrophoresis was carried out with CAPI 3000 (Otsuka, Osaka, Japan) at 30◦ C. A 500 mm long (378 mm to the detector cell), 75 ␮m i.d. fused silica

2.2. Chemicals ALC was purchased from Dojindo (Kumamoto, Japan). Ammonium chloride, methylammonium chloride, ethylammonium chloride, n-propylammonium chloride, i-propylammonium chloride, dimethylammonium chloride, trimethylammonium chloride, diethylammonium chloride, triethylamine, di(n-propyl)ammonium chloride, di(i-propyl)ammonium chloride, pyridine (Py), 2-methylpyridine (2-MePy), 3-methylpyridine (3-MePy), 4-methylpyridine (4-MePy), imidazole (Im), dioxan (DO), methanol (MeOH), N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), and acetonitrile (AN) were purchased from Nacalai Tesque (Kyoto, Japan). 1-Methylimidazole (1-MeIm) was purchase from Aldrich (Milwaukee, WI). All reagents were used as received. The buffer was prepared from sodium tetraborate, ALC, and additives such as amines, ethers, and amides. The Fe(II), Fe(III), Co(II), Ni(II), Cu(II), and Zn(II) solutions were prepared from nitrate salts (Wako, Osaka, Japan). All solutions were filtered through a 0.45 ␮m PTFE filter (Advantec, Tokyo). 2.3. Procedure

Visible absorption spectra for ALC at various pH values have been described by Leonard et al. [28].

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Fig. 1. Acid dissociation equilibrium of ALC in an aqueous solution of pH 9.1.

Fig. 2. UV–Vis absorption spectra of 6 × 10−5 mol dm−3 ALC in 1 × 10−2 mol dm−3 Na2 B4 O7 (a) and 5 × 10−2 mol dm−3 Na2 HPO4 (b) buffer of pH 9.1.

These spectra corresponded with those in our work. Also, pKa1 = 2.40 (–COOH), pKa2 = 5.54 (2–OH), pKa3 = 10.07 (≡NH+ ), and pKa4 = 11.98 (1–OH) of ALC have been determined by Ingman [29,30]. In an aqueous solution of pH 9.1, the acid dissociation equilibrium of ALC is as illustrated in Fig. 1. In a phosphate buffer of pH 9.1, the maximum visible absorption wavelength of ALC was ca. 520 nm. However, it was at 455 nm in a borate buffer of pH 9.1, as shown in Fig. 2. It is well-known that boric acid forms a complex with phenolic acids [31–33]. Therefore, this indicates complex formation of the borate ion with ALC, as illustrated in Fig. 3. The mole ratio of metal ion to ALC for metal–ALC complexes was determined spectrophotometrically. The results obtained by a continuous variation method for the Cu(II)–ALC system in a 1 × 10−2 mol dm−3 Na2 B4 O7 buffer of pH 9.1 are shown in Fig. 4. The absorbance at 520 nm increased with increasing mole fraction of Cu(II) from

0 to 0.571. Also, the absorbance at 455 nm decreased with increasing mole fraction. Therefore, this shows that borate was dissociated from the ALC–borate complex due to coordination of the ALC–borate complex with Cu(II). Also, the variation of absorbance at 520 nm showed that the mole ratio of Cu(II) to ALC in the Cu(II)–ALC complex was 1 : 1. However, when the mole fraction of Cu(II) was above 0.714,

Fig. 3. Structure of the ALC–borate complex.

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Fig. 4. Variation of visible absorption spectra for the Cu(II)–ALC system in 1 × 10−2 mol dm−3 Na2 B4 O7 buffer of pH 9.1. Mole fraction of Cu(II): (a) 0, (b) 0.143, (c) 0.285, (d) 0.428, (e) 0.571, (f) 0.714, (g) 0.857, (h) 1.

ALC might coordinate with Fe(III), as with the borate ion. 3.2. Separation of Fe(III), Co(II), Ni(II), Cu(II), and Zn(II) with ALC by CZE

Fig. 5. Structure of the metal–ALC complex in a borate buffer of pH 9.1.

there was no isosbestic point at 505 nm. This shows an existence of another species of the Cu(II)–ALC complex above the 0.714 mole fraction. Also, the variations of visible absorption spectra for Co(II)–, Ni(II)–, and Zn(II)–ALC were approximately similar to those of Cu(II)–ALC. Each of the mole ratios of Co(II), Ni(II), and Zn(II) to ALC was also 1 : 1. Therefore, the structure of these metal–ALC complexes in the borate buffer of pH 9.1 was determined as illustrated in Fig. 5. However, the mole ratio of Fe(II) to ALC could not be determined spectrophotometrically in the borate buffer because the maximum visible absorption wavelengths for Fe(II)–ALC and for the ALC–borate complex were too close to be distinguished from each other. Since Fe(II) was oxidized by dissolved oxygen, the 1-hydroxyl group of

Fig. 6 shows an electropherogram of Fe(III), Co(II), Ni(II), Cu(II), and Zn(II) in the borate buffer of pH 9.1 in the presence of 1 × 10−3 mol dm−3 ALC. Separa-

Fig. 6. Electropherogram of Fe(III), Co(II), Ni(II), Cu(II), and Zn(II) in 1 × 10−2 mol dm−3 Na2 B4 O7 buffer with 1 × 10−3 mol dm−3 ALC of pH 9.1; each metal ion, 1 ␮g ml−1 ; applied voltage, 20 kV.

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Fig. 7. Electropherogram of Fe(III), Co(II), Ni(II), Cu(II), and Zn(II) in 1 × 10−2 mol dm−3 Na2 B4 O7 buffer with 1 × 10−3 mol dm−3 ALC in the presence of 5 v/v% Py of pH 9.1; each metal ion, 1 ␮g ml−1 ; applied voltage, 20 kV.

tion of these complexes for five transition metal ions with ALC could be accomplished due to the pKa of H2 O coordinated with these complexes. However, the separation of these metal ions could not be improved by change of pH because, at pH > 9.1, precipitation of these metal hydroxides occurred in the capillary, and at pH < 9.1, ALC was precipitated. Furthermore, in a phosphate buffer of pH 9.1, the separation of these metal ions was poor because precipitation of the metal phosphates occurred. A peak of Fe(II) had a migration time similar to that of Fe(III). Also, the peak height of 5 ␮g ml−1 Fe(II) was less than that of 5 ␮g ml−1 Fe(III). This means that only Fe(III), after oxidation by dissolved oxygen, appeared as a single peak. Accordingly, the peak of Fe(II) did not appear. 3.3. Complex formation constants of Cu(II)–ALC with amines Fig. 7 shows an electropherogram of Fe(III), Co(II), Ni(II), Cu(II), and Zn(II) in a borate buffer of pH 9.1, with ALC in the presence of Py. The migration time of Cu(II) was shorter than that in the absence of Py.

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Furthermore, the migration time of Cu(II) was shorter than that of Fe(III). This means that the net charge of Cu(II)–ALC decreased and its molecular size increased after the addition of Py. Also, the visible absorption spectrum of Cu(II)–ALC in the presence of Py was similar to that in absence of Py. This means that a single nitrogen and three oxygen atoms of ALC coordinating with the Cu(II)–ALC complex were not dissociated from Cu(II) due to the coordination of Py. In other words, an exchange reaction of water molecules or hydroxide ions coordinating with Cu(II)–ALC with Py would have less influence on the visible absorption spectra of Cu(II)–ALC complex. Therefore, the coordination equilibria illustrated in Fig. 8 were taken to occur. This behavior of Cu(II)–ALC in CZE was also observed in the presence of 2-MePy, 3-MePy, 4-MePy, Im, 1-MeIm, NH3 , and monoalkylamines except for i-PrNH2 . However, in cases of i-PrNH2 , dialkylamines, trialkylamines, AN, DO, MeOH, DMF, and DMSO, it was not observed. This is due to a weak basicity for AN, DO, MeOH, DMF, and DMSO, and due to the steric hindrance for i-PrNH2 , dialkylamines and trialkylamines. For Fe(III)–, Co(II)–, Ni(II)–, and Zn(II)–ALC, the same behavior as for Cu(II)–ALC was not observed. Fig. 9 shows the dependencies of apparent electrophoretic mobility (µep ) for Fe(III)–, Co(II)–, Ni(II)–, Cu(II)–, and Zn(II)–ALC on Py concentration. The µep value of Cu(II)–ALC decreased appreciably with increasing Py concentrations compared to those Fe(III)–, Co(II)–, Ni(II)–, and Zn(II)–ALC. Therefore, the coordination with Py to Fe(III)–, Co(II)–, Ni(II)–, and Zn(II)–ALC would be less than that with Cu(II)–ALC. The µep value of Cu(II)–ALC was obtained from Eq. (1): µep = µep0 XCuS0 + µep1 XCuS1 + µep2 XCuS2

(1)

where µep0 and XCuS0 , µep1 and XCuS1 , and µep2 and XCuS2 are electrophoretic mobility and mole fraction of H[Cu(ALC)(H2 O)(OH)]− , H[Cu(ALC)(OH)S]− , and [Cu(ALC)S2 ]− , respectively, and S is an amine. The KCuS1 and KCuS2 values were calculated from Eq. (2) by fitting µep1 , KCuS1 , and KCuS2 : µep =

µep0 +µep1 KCuS1 [S] + µep2 KCuS1 KCuS2 [S]2 1 + KCuS1 [S] + KCuS1 KCuS2 [S]2 (2)

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Fig. 8. Coordination equilibria between Cu(II)–ALC and Py.

Fig. 9. Dependencies of electrophoretic mobilities for Fe(III)–, Co(II)–, Ni(II)– Cu(II)–, and Zn(II)–ALC on Py concentration; 4, Fe(III); 䊉, Co(II); 䊏, Ni(II); 䊊, Cu(II); N, Zn(II).

The µep0 and µep2 values were obtained from the electrophoretic mobility of Cu(II)–ALC in the borate buffer without amine and that in the borate buffer with excess amine, respectively. Also, the amine concentration was corrected by the pKa value [34]. The ionic strength was kept at 0.03 during the measurement. The dependencies of the µep values for Cu(II)–ALC on the amine concentration and the fitted curves are shown in Fig. 10 (a) and (b). The KCuS1 , KCuS2 and log β CuS2 values with the pKa of these amines are

summarized in Table 1, where β CuS2 = KCuS1 KCuS2 . The log β CuS2 value of n-PrNH2 was smaller than those of MeNH2 and EtNH2 , in spite of the almost similar pKa . Also, i-PrNH2 was not coordinated with Cu(II)–ALC. These mean a steric hindrance between alkyl groups of amines and functional groups around the coordination sites of Cu(II)–ALC. The log β CuS2 values of 2-, 3-, and 4-MePy were smaller than that of Py, although the pKa values of 2-, 3-, and 4-MePy were larger than that of Py. This is also due to steric

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Table 1 Coordination equilibrium constants of amines with Cu(II)–ALC Amine

pKa

log β CuS2

KCuS1 (dm3 mol−1 )

KCuS2 (dm3 mol−1 )

NH3 MeNH2 EtNH2 n-PrNH2 Py 2-MePy 3-MePy 4-MePy Im 1-MeIm

9.25a 10.657a 10.807b 10.708b 5.25a 5.97b 5.68b 6.02b 6.953a 6.95a

6.52 8.67 6.92 5.74 4.05 2.57 2.70 2.71 11.46 6.79

1±1 100 ± 1 69 ± 4 2±1 150 ± 20 5±1 42 ± 1 57 ± 1 (2.9 ± 0.2) × 108 7900 ± 200

(3.33 ± 0.01) × 106 (4.73 ± 0.01) × 106 (1.21 ± 0.07) × 105 (2.74 ± 0.06) × 105 74 ± 7 74 ± 3 12 ± 3 9±2 990 ± 90 780 ± 70

a b

25◦ C. 20◦ C.

Fig. 10. Dependencies of electrophoretic mobilities for Cu(II)–ALC on concentrations of aromatic amines (a) and of alkyl amines (b) and their fitting curves; (a) 䊊, Py; 䊉, 2-MePy; 䊏, 3-MePy; N, 4-MePy; 䊐, Im; 4, 1-MeIm; (b) 䊐, NH3 ; 䊉, MeNH2 ; 䊏, EtNH2 ; 䊊, n-PrNH2 .

hindrance, like alkylamines. Since the coordination sites of Cu(II)–ALC were relatively tight, the steric structure factor of amines was important for the coor-

dination of amines with Cu(II)–ALC. A similar effect has been reported in the coordination equilibrium constants of solvent molecules to [Ni(tmc)]2+ (tmc: 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) [35–37]. A tendency for the coordination of alkylamines was different from that of aromatic amines. In the case of alkylamines, the KCuS2 values were larger than the KCuS1 values. This would mean that a hydrogen-bond formation between the oxygen atoms of the carbonyl and hydroxyl groups for Cu(II)–ALC and hydrogen atoms of the alkylamines interfered with the first coordination of alkylamines. In other words, the hydrogen atoms of the alkylamines behave as strong acceptors. The protons of –NH2 for the single coordinating alkylamine would lead the coordinating water in Cu(II)–ALC into the outer sphere. Consequently, the KCuS2 value was larger than the KCuS1 value. In the case of aromatic amines, since there is no hydrogen atom on the coordinating nitrogen atom, the KCuS1 values were larger than the KCuS2 value except for 2-MePy. Since the 2-Me group of 2-MePy was largely the steric hindrance in the coordination with transition metal ions [38–40], its first coordination equilibrium constant was small. The 2-Me group of 2-MePy coordinating with Cu(II)–ALC could also lead the coordinating water to Cu(II)–ALC to its outer sphere, due to the steric hindrance. The log β CuS2 of Im was the largest value among the observed amines. This would be due to the lesser steric hindrance, no hydrogen atom of the coordinating nitrogen atom (=N–) and the increased basicity of the nitrogen atom in =N– due to internal hydrogen-bonding between the hydrogen

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atom in =N–H and the nitrogen atom in the =N–CH= groups of Im [41]. Therefore, the log β CuS2 of 1-MeIm was smaller than that of Im in spite of the similar pKa values.

4. Conclusions The species of Cu(II)–ALC in an aqueous borate buffer of pH 9.1 was characterized. Also, the coordination equilibrium constants of several amines with Cu(II)–ALC were determined. It was found that the amines of generally large pKa and less steric hindrance efficiently shortened the migration time of Cu(II)–ALC. Also, oxygen-coordinating ligands such as alcohols, ketones, and amides could not be exchanged with water molecules coordinated with Cu(II)–ALC. The best ligand for Cu(II)–ALC was Im. In this method, the detection limit (S/N = 3) for Cu(II) was 200 ng ml−1 .

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