In the Laboratory edited by
Advanced Chemistry Classroom and Laboratory
Joseph J. BelBruno
Infrared Spectroscopy Determination of Lead Binding to Ethylenediaminetetraacetic Acid
Dartmouth College Hanover, NH 03755
Simona Dragan and Alanah Fitch Loyola University Chicago, Department of Chemistry, 6525 N. Sheridan, Chicago, IL 60626
We have developed a thematic instrumental lab sequence based on lead analysis of community-derived samples (1). After a few introductory labs, students begin a series of spectroscopic measurements of lead, such as UV–vis analysis via dithizone extraction (2) and FTNMR built upon the chelation with ethylenediaminetetraacetic acid (EDTA) (3). A Fourier transform infrared experiment based on the dithizone–lead chelation was also included and the students were asked to identify the Pb–S vibrational peak. Since the chelate stability is low, the signal dependance on pH is quite high, and the location of the Pb–S band lies within a highly populated area, the lab usually is fraught with failure. Therefore, we have considered the well-known approach of metal complexation to EDTA studied extensively in the 1950s and 1960s (see below) by infrared spectroscopy. In this lab students get acquainted with infrared spectroscopy by interpreting the spectrum of EDTA, by comparing it to that of disodium ethylenediaminetetraacetate (Na 2EDTA) and lead(II) ethylenediaminetetraacetate (PbEDTA), and by noting the changes that complexation of EDTA to sodium and lead produce. Based on the spectral differences between PbEDTA and EDTA, we have developed a method to determine the amount of lead in a mixture of PbEDTA and EDTA by quantitating the vibrational changes produced when the ratio of these compounds is varied. Experiment Ethylenediaminetetraacetic acid and disodium ethylenediaminetetraacetate used in this study were purchased from Aldrich and used without further purification. PbEDTA was prepared according to a procedure adapted from literature (4 ). Solutions with equal number of moles of Na2EDTA and Pb(NO3)2 were mixed together and warmed almost to the boiling point, at pH 6.0. The white crystalline PbEDTA was obtained by slowly evaporating the solution at room temperature. The mixtures of PbEDTA and EDTA were obtained by mixing together solutions with equal concentrations of Pb(NO3)2 and EDTA, at pH 6.0, in the volumetric ratios of 0:1, 0.25:1, 0.5:1, 0.75:1 and 1:1, followed by slow evaporation. Infrared spectra of EDTA and EDTA–metal complexes were determined by having the above compounds (or mixtures) in solid state, dispersed in mineral oil (Nujol). A small amount of compound, equivalent to 10 µmol of EDTA ligand, was finely ground (particle size < 2 µ m) on an agate mortar and uniformly spread in a drop of mineral oil. The fine paste was then pressed between two plates of NaCl to create a thin film that is almost translucent. All spectra were recorded on a Matson FTIR spetrophotometer in the middle IR region (4,000 to 400 cm ᎑1) at a 2 cm᎑1 resolution. 1018
Results and Discussion
IR Spectrum of Nujol The IR spectrum of Nujol (one drop smeared out between two plates of NaCl) is shown in Figure 1a. It is a typical spectrum of straight-chain hydrocarbons. The wavenumber region between 3000 and 2840 cm᎑1 consists of bands due to absorption from C–H stretching (ν ) of methyl groups (symmetrical at 2872 cm᎑1 and asymmetrical at 2962 cm᎑1). Symmetrical and asymmetrical stretchings of the methylene group occur near 2926 and 2853 cm᎑1, respectively. The latter are well distinguished in the spectrum of Figure 1, whereas the methyl stretching vibrations are hidden. The bending vibrations (δ) of the C–H bonds in the methylene group occur in the region 1550–1350 cm᎑1, of which the methylene symmetric scissoring band is visible at 1460 cm ᎑1 . The absorption band at 1377 cm᎑1, arising from the symmetrical bending of the methyl C–H bonds, is very stable when the methyl group is attached to another carbon atom. The weak band from the methylene rocking vibration (ρ), in which all of the methylene groups rock in phase, appears between 775 and 645 cm᎑1 and is characteristic for straight-chain alkanes of seven or more carbons (5). The origin of the band at 2230– 2375 cm᎑1 is unknown. EDTA Spectrum The EDTA spectrum (represented as H4Y structure drawn in Fig. 2a) in Figure 1b shows a quite large absorption band centered around 3000 cm᎑1, which is usually assigned to O–H stretching due to internal dimerization of the carboxyl groups that are separated by the tertiary nitrogen and methylene groups, as shown in Figure 2b. The C–H stretching bands from methylene groups of acid and Nujol are superimposed on this large O–H stretching band. The C=O stretching bands in acids are considerably more intense than ketonic C=O stretching bands. In dimerized saturated aliphatic acids the C=O group absorbs in the region of 1720– 1706 cm᎑1. EDTA exhibits the carboxylic C=O stretching band at 1697 cm᎑1, in a very good agreement with literature data (6 ). The slight shift could be due to the presence of the zwitterionic EDTA that results from the proton exchange between the COOH and the aminic nitrogen with possible hydrogen bonding, as shown in Figure 2c (7 ). This form may be obtained upon crystallization of an aqueous solution of EDTA, since the equilibrium free EDTA
zwitterionic EDTA
very likely occurs in solution. Therefore, N–H “free” asymmetrical and symmetrical stretching modes usually present in the 3500–3300 cm᎑1 region are weak, being visible only in the Na2EDTA spectrum. The N–H scissoring vibration
Journal of Chemical Education • Vol. 75 No. 8 August 1998 • JChemEd.chem.wisc.edu
In the Laboratory
which is usually observed in the 1650-1580 cm᎑1 region of the spectrum may be overlapped by the strong absorption of the carboxylic C=O. Two bands arising from C–O stretching and O–H bending usually appear in the spectra of carboxylic acids near 1320–1210 and 1440–1395 cm᎑1, respectively. In EDTA, the latter is of moderate intensity and occurs in the same region as the CH2 scissoring vibration of the CH2 group adjacent to carbonyl. The weak absorption bands that can be seen in the region of 1250–1020 cm᎑1 are assigned to the C–N stretching vibration.
Figure 1. Infrared spectra of (a) mineral oil, (b) ethylenediaminetetraacetic acid, (c) disodium ethylenediaminetetraacetate (Na2H2Yⴢ2H2O), and (d) lead ethylenediaminetetraacetate (PbH2Y). 10 µmol of each EDTA compound was uniformly spread in a drop of mineral oil.
(a) HOOC HOOC
CH2 CH2
N CH2 CH2 N
CH2 COOH CH2 COOH
(b) O
C
C
O O
O N CH2 CH2 N H H H H O C C O O O (c) CH2 COOH
HOOC CH2 +
+
N CH2 CH2 N CH2 CH2 H H C C O– O – O O Figure 2. Structure of (a) ethylenediaminetetraacetic acid (EDTA); (b) EDTA, with internal dimerization of the carboxyl groups separated by methylenes and tertiar y aminic nitrogen showing bifurcated hydrogen bonding; (c) EDTA zwitterion resulting from the proton exchange between COOH and aminic nitrogen. Hydrogen bondings are very likely to be present.
IR Spectrum of Na2EDTA The IR spectrum of Na2EDTA (represented as Na2H2Y), shown in Figure 1c, has a relatively large band with minimum at 1633 cm᎑1. This band can be considered the result of the overlapping of the un-ionized carboxyl band at 1697 cm᎑1, as in EDTA, and the free ionized COO absorption band, which would have arisen at 1605 cm᎑1 had all the hydrogens been substituted with sodium (4 ). Only the asymmetrical C=O stretching band in carboxylate, usually near 1650–1550 cm᎑1, is observed in the IR spectrum. So far, Nujol has proven to be a good nonpolar dispersing medium for the IR study of EDTA and Na 2EDTA complexes because it does not exhibit any absorption bands around 1700 cm᎑1, the region characteristic of stretching vibration of the C=O carboxylic bond. This vibration is present in the spectrum of the carboxyl group regardless of the manner in which it may be bonded. For all spectra recorded, much attention is given to the bands in the region between 1800 and 1500 cm᎑1 where shifts in absorption bands are expected to correlate with changes in the structure of coordinated compound and the ratio of free EDTA to metal-bound EDTA. Although the structures of metal chelate compounds of EDTA derivatives are too complex to allow any theoretical approach, infrared studies are still useful for distinguishing the un-ionized, free ionized, and coordinated carboxyl groups in these compounds. The method is based on the simple rule that the un-ionized and uncoordinated COO stretching bands occur at 1750–1700 cm᎑1, whereas the free-ionized and coordinated COO stretching bands are present at 1650–1550 cm᎑1; the exact frequency depends on the nature of metal (8). Spectrum of PbEDTA The spectrum of PbEDTA (represented as PbH2Y), shown in Figure 1d, has two distinctive absorption peaks at 1697 and 1558 cm᎑1, the latter due to the coordinated carboxylate. The coordinated COO absorption peak is shifted to lower frequencies compared to the coordinated COO from the Na2EDTA spectrum and to the frequency reported for Na4EDTA (Na4Y) solid salt, variation that can be accounted for in terms of the nature of the oxygen–metal bond. If the bond is primarily ionic the possibility for carboxylate resonance is at maximum. Since an increase in carboxylate resonance imparts enhanced single-bond character to the carbonyl group, it also lowers the frequency of the C=O stretching vibration. In consequence, the C=O stretching frequency provides some information related to the ionic character of the bond. The positions of the carbonyl peak maxima arising from the COOM group in EDTA are influenced by several factors, but two are particularly apparent: the charge and the size of the metal ion. High charge and small size appear to enhance the covalency of the metal–oxygen bond. An interesting correlation was observed when the charge-to-size ratio q/r of the metal ion is compared with the position of the carbonyl peak maximum for each complex (9). When q/r is larger than 3.6, carbonyl absorption maxima occur in the 1625–1650 cm᎑1 region, indicating covalency in the metal– carboxylate bond; but when q/r is less than 3.6, the maxima occur between 1590 and 1615 cm᎑1. Pb II has q/r = 1.5 and r = 1.32 Å, values that may explain the position of the coordinated carboxylate band at even lower frequency. McConnell and Nuttall (10), in their Raman spectroscopy study of Na2PbEDTA, assigned the intense bands at 355
JChemEd.chem.wisc.edu • Vol. 75 No. 8 August 1998 • Journal of Chemical Education
1019
In the Laboratory C O
O
–
2+
Pb HOOC CH2 N
N CH2 COOH CH2 – O
CH2 O CH2 C
Figure 3. Structure of lead ethylenediaminetetraacetate as a tetrahedral tetracoordinate complex with covalent Pb–N bonds and ionic Pb–carboxylate bonds.
and 435 cm᎑1 as ν M–N, since for lead the metal–carboxylate bonds are predominantly ionic, whereas the metal–nitrogen bonds are covalent. They also found two types of M–O stretching bonds in the 300–550 cm᎑1 region in the spectra of both solid Na2PbEDTA and its solution, with an increase in intensity for the solid. This change is compatible with the change in the nature of bonding because in the solid, carboxylate–metal interactions are frozen. The overall similarity of the Raman spectra of the solid and solution species justifies the assumption that the stereochemistry in the two phases is similar. Sawyer and Paulsen (4 ) found a shoulder in the PbIIEDTA IR spectrum at 1645 cm᎑1, which could possibly be attributed to some covalent character for one or two metal– carboxylate bonds. An alternate conclusion is that one or more of the carboxylate groups of the EDTA molecule are not bonded to the lead ion. This is close to Langer’s (11) conclusion that lead is four-coordinated in the solid state and six-coordinated in solution, and that two arrangements are possible for the tetradentate tetracoordinated EDTA. Langer also found the major band lower by 26 cm᎑1 for the Pb2EDTA complex than for the completely ionized carboxylate group of K4EDTA (1595 cm᎑1), indicating a bond character less than 1.5 for both carbon-to-oxygen bonds of the carboxylate groups. Based on the spectrum of PbEDTA shown in Figure 1d and the conclusions of McConnell (10), Sawyer (4), and Langer (11), the structure of PbEDTA is presented in Figure 3, with divalent Pb tetracoordinated, two covalent metal–nitrogen bonds, and two ionic metal–carboxylate bonds. An alternate structure could have the lead ion pentacoordinated, the fifth position being occupied by the lone pair of s2 electrons. The two carboxylic groups not coordinated to the metal ion may be engaged in intramolecular or intermolecular hydrogen bonding.
IR Spectra for PbEDTA and EDTA Mixtures Infrared spectra were run for mixtures of PbEDTA and EDTA (equivalent to 10 µ mol EDTA ligand) finely ground and uniformly spread in a drop of mineral oil. The spectra are shown in Figure 4a–4e. As the concentration of PbEDTA increases, the peak at 1558 cm ᎑1 becomes larger and larger while the peak at 1697 cm᎑1 is being overlapped by the newly formed absorption peak. The magnitude of each of these peaks is given by the distance from the minimum to the baseline between the shoulder in the 1860–1780 cm ᎑1 region and the shoulder in the 1565–1500 cm᎑1 region, the exact coordinates of each baseline being different from one spectrum to another. Although the optical path length varies with the pressure with which the sodium chloride plates are compressed, 1020
it is possible to quantitate the amount of lead present in a mixture of PbEDTA and EDTA by plotting the ratio of the 1558 cm᎑1 peak to the 1697 cm᎑1 peak vs the molar ratio of PbEDTA (xPbEDTA). As shown in Figure 5, a straight line starting from origin with a high correlation coefficient is obtained. This may suggest that infrared spectroscopy can be used in quantitative analysis to determine the ratio of lead-bound EDTA to unbound EDTA by comparing the peak heights in the fingerprint 1850–1500 cm᎑1 region. Conclusion The general trend of decreasing the stretching frequency of the carboxylate C=O with increasing ionic character of the metal–carboxylate bond was observed for both disodium and lead(II) ethylenediaminetetraacetate salts, the shift being more pronounced in the case of the lead compound. The band at 1558 cm᎑1 in the spectrum of PbEDTA is even lower than the band for coordinated carboxylate in disodium lead(II) ethylenediaminotetraacetate at 1600 cm᎑1 as reported (4), but very close to the carboxylate band in Pb2EDTA complex (10). Therefore, the observed spectroscopic features lead to the conclusion that the four carboxylate groups are not equivalent: two groups exist as COOH not coordinated to the metal (the peak at 1697 cm᎑1 as in EDTA); the other two are coor-
Figure 4. Infrared spectra of PbEDTA and EDTA mixtures, the molar ratio of PbEDTA being: (a) 0; (b) 0.25; (c) 0.5; (d) 0.75; (e) 1.0. Each of these mixtures has a total of 10 µmol of ligand.
1558 to 1697 cm–1 peak height ratio
CH2
0.6
y = 0.6081x 2 R = 0.9902
0.4
0.2
0
0
0.25
0.5
0.75
1
xPbEDTA
Figure 5. 1558 to 1697 cm᎑1 peak height ratio vs molar ratio of PbEDTA.
Journal of Chemical Education • Vol. 75 No. 8 August 1998 • JChemEd.chem.wisc.edu
In the Laboratory
dinated to the metal. The bond character for both carbonto-oxygen bonds of these groups was reported as being less than 1.5, thus explaining the position of the carboxylate band at 1558 cm᎑1. The literature data from the 1960s and 1970s reported only description of structures of lead–EDTA compounds, with the coordination number varying from six to less than six or four. Based on the infrared spectrum acquired, we formulated the PbEDTA (PbH2Y) structure as a tetracoordinate complex, eventually pentacoordinate with the pair of lone s2 electrons in the fifth position. The method that we chose to record the infrared spectra proved to be adequate, since mineral oil used for dispersing the solid compounds does not exhibit any absorption bands between 1800 and 1550 cm᎑1, the region characteristic of stretching vibration of the C=O carboxylic bond. This method is especially useful when quick preparation of samples is desired, such as giving hands-on opportunity to each student in a large class. In addition, this experiment produces a very stable lead complex that can be measured in a relatively background-free region of the IR spectrum. To our knowledge this is the first time that a method to quantitate the amount of lead bound to EDTA by simply comparing the peak height of the most prominent peaks in the 1800–1550cm᎑1 region has been reported. A potential application of this method could be determination of lead
extracted by binding it to ethylenediaminetetraacetic acid, excess EDTA being added. Acknowledgment This research was supported with Loyola University Chicago capital budget funds. Literature Cited 1. Fitch, A.; Wang, Y.; Mellican, S.; Macha, S. Anal. Chem. 1996, 68, 727A–731A. 2. Sandell, E. B. Colorimetric Determinations of Traces of Metals; Interscience: New York, 1950. Murcia, N. S.; Lundquist, E. G.; Russo, S. O.; Peters, D. G. J. Chem. Educ. 1990, 67, 608–611. 3. Fujiwara, S.; Ogimura, Y.; Nagashima, K. Chem. Instr. 1968, 1(1), 21–32. 4. Sawyer, D. T.; Paulsen, P. J. J. Am. Chem. Soc. 1959, 81, 816–820. 5. Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds; Wiley: New York, 1991. 6. Nakamoto, K. Infrared Spectra of Inorganic and Coordination Compounds; Wiley: New York, 1963. 7. Chapman, D.; Lloyd, D. R.; Prince, R. H. J. Chem. Soc. 1963, 3645–3658. 8. Sawyer, D. T.; Paulsen, P. J. J. Am. Chem. Soc. 1959, 80, 1597– 1603; 1960, 82, 4191–4197. 9. Sievers, R. E.; Bailar, J. C., Jr. Inorg. Chem. 1962, 1, 174–182. 10. McConnell, A. A.; Nuttall, R. H. Spectrochim. Acta 1977, 33A, 459–462. 11. Langer, H. G. J. Inorg. Nucl. Chem. 1964, 26, 767–772.
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