Article pubs.acs.org/IECR
Conductometric Performance of Two-Pole and Five-Ring Conductivity Cell Probes for Lanthanide Determination Using EDTA and DCTA as Potential Sequestering Agents Komal Matharu,† Susheel K Mittal,† and S. K. Ashok Kumar*,‡ †
School of Chemistry & Biochemistry, Thapar University, Patiala, 147004, Punjab, India Materials Chemistry Division, School of Advanced Sciences, VIT University, Vellore, 632014, Tamil Nadu, India
‡
S Supporting Information *
ABSTRACT: An improved conductometric titration method has been developed for the easy detection of lanthanides in the presence of interfering ions using a five-ring conductivity probe. These measurements were compared with those from a two-pole conductivity cell. Conductometric titrations of lanthanides with ethylenediaminetetraacetic acid (EDTA) and trans-1,2diaminocyclohexanetetraacetic acid (DCTA) as complexing agents were successfully carried out in the presence of alkaline earth metal ions, transition metal ions, and other metal ions, such as Al3+ and Th4+, as interfering ions. The performance of the titrations of binary mixtures of Al(III) and lanthanides further improved and distinct end points corresponding to the stoichiometric ratio could be observed in mixed solvent media containing 1,4-dioxane (10−20%) which otherwise showed interference in aqueous media.
1. INTRODUCTION Electrolytic conductance has been used to investigate the nature and properties of ions in solution. This routine analysis has received considerable attention mainly because of the high accuracy and the experimental simplicity. Direct conductometric analysis has limited applications1 due to the nonselective character of the technique,2,3 but conductometric titrations have been found as a selective technique in the method introduced by Kuster and Gruters4 in 1903. This technique can be used for numerous applications in chemical,5,6 biological,7−9 and environmental laboratories.10−14 For this reason, a variety of commercial conductometers are frequently used in many laboratories. In addition, in-line conductometry is a universal detection strategy for flow injection analysis (FIA),15 ion chromatography (IC),16−22 and capillary electrophoresis (CE).23 Two-pole conductivity cells have been used commonly in laboratories for various analytical purposes. However, due to some demerits of the two-pole cell, advanced cells such as three-pole, four-pole,24 and five-ring conductometric cells25 have been developed to obtain desirable results in qualitative and quantitative determinations of analytes. To compare the performance of a two-pole cell and a fivering conductivity cell, authors used chelates such as ethylenediaminetetraacetic acid (EDTA) and trans-1,2-diaminocyclohexanetetraacetic acid (DCTA) in the form of their alkali metal salts for determination of lanthanide ions in solution. These chelates are known and have been used in volumetric analysis26,27 for a long time. They are being used for the quantitative determination of more than 45 elements by direct and back-titration procedures to obtain a visual end point28,29 or by use of various techniques such as pH metry,30 spectrophotometry,31 capillary electrophoresis,32 high performance liquid chromatography,33 and ion chromatography.34 However, the exact use of these reagents for quantitative determination of © 2012 American Chemical Society
lanthanides using a conductometric technique is still not known. Our recent work filled this research gap using EDTA as a reagent and a five-ring conductivity cell as a probe.35 A detailed literature survey confirmed that there has been no study on the conductometric performance of two-pole and fivering conductivity cell probes for lanthanide determination using DCTA as potential receptor and its comparison with EDTA.
2. EXPERIMENTAL SECTION 2.1. Reagents and Solutions. Solutions of metal ions were prepared by direct weighing of AnalaR grade (99.9%) metal chlorides supplied by Chengdu Beyond Chemicals, China, in double distilled deionized water having a conductivity of less than 3 μS cm−1. The ligands ethylenediaminetetraacetic acid tetrasodium salt (Na4EDTA, L1) and trans-1,2-diaminocyclohexanetetraacetic acid (DCTA) and metal salts of various metal ions were obtained from Aldrich, USA. Na4EDTA was used as received while the tetrasodium salt of trans-1,2-diaminocyclohexanetetraacetic acid (Na4DCTA, L2) was prepared from pure DCTA after its reaction with a calculated amount of sodium hydroxide. The Ln(III) solutions were standardized against EDTA solution using xylenol orange as an indicator. 2.2. Instrumentation. Two instruments were used to perform conductometric titrations: (i) CM 180 conductivity meter (ELICO, India) digital display with accuracy and repeatability of ±1% with two-pole conductivity cells having cell constants 0.1, 0.45, and 1.0 cm−1; (ii) 856-conductivity touch control module and five-ring conductivity sensor (Metrohm, Switzerland) as a standalone system. Received: Revised: Accepted: Published: 11328
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Figure 1. Conductometric titration curves for 50 mL of Ce(III) (1 × 10−3 M) with L1 and L2 (1 × 10−2 M) in the absence of coligand HIBA using two-pole and five-ring conductivity cells.
Figure 2. Conductometric titrations of binary mixtures of lanthanides (25 mL of 1 × 10−3 M Ce(III) + 25 mL of 1 × 10−3 M La(III)) vs 1 × 10−2 M L1 and L2 in the presence of coligand HIBA using five-ring probe and two-pole cell.
The instrument was maintained at 25 ± 1 °C (except for titrations where 18 ± 0.1 °C was kept constant) using a water thermostat (Julabo F12-ED refrigerated/heating circulator) which has a working range of −20 to 100 °C. For pH measurements a digital pH meter Century (India) CP 901 having a range of pH 0−14 and accuracy of ±0.1 pH was used. The instrument is glass electrode based with Ag/ AgCl, KCl(saturated) as reference electrode. 2.3. Procedure for Specific Conductance Measurement. Lanthanide chloride solution (50 mL of 1 × 10−3 M) was placed in a titration vessel, and the conductivity was measured by using the 856-conductivity module and a five-ring conductivity sensor (Metrohm, Switzerland) at 18 °C. Ligand (L1/L2) solution (1 × 10−2 M) was added to the titration vessel in 0.5 mL increments, and the conductivity was measured until the ligand (L1/L2) added was sufficiently larger than the 1:2 metal:ligand (L1/L2) stoichiometry. During each titration the concentration of ligands, L1/L2, was 10 times greater than the concentration of the analytes taken. The values of conductivity versus volume of ligand (L1/L2) added were plotted, where sharp inflections in the conductometric titration curves gave the equivalence points. 2.4. Effect of pH and Effect of Supporting Electrolyte. Conductivity measurements for the titration of lanthanides with ligand (L1/L2) were also carried out in the presence of buffer of pH 4 (sodium acetate−acetic acid buffer solution), but no effect of the buffer was noticed on the equivalence point except for
the greater magnitude of conductivity. This is due to the fact that the initial pH of the lanthanide solution was found to be 4.0, which is suitable for a stable complex formation with ligand (L1/L2), and during titration the pH varied only by 4.0 ± 0.3 up to the equivalence point due to the small release of H+ ions from 2-hydroxyisobutyric acid (HIBA) on Ln(III)−L1/L2 complex formation. Hence, buffer solution was not used in subsequent studies. Supporting electrolytes are required in controlled-potential experiments to decrease the resistance of the solution, eliminate electromigration effects, and maintain a constant ionic strength. The selectivity may be increased by the relative rates of diffusion of the analyte and the interferents through the choice of the supporting electrolyte and given applied potential. For this reason we carried out conductometric titration of different concentrations of Ce(III) ions against L1 and L2 solution in the presence of KCl as supporting electrolyte (100:1 KCl and titration system). Since no difference in the equivalence point was observed in the presence of supporting electrolyte, it was not used in subsequent studies.
3. RESULTS AND DISCUSSION 3.1. Complexing Behavior of EDTA and DCTA. The strong metal−ligand interactions in the metal−EDTA and metal−DCTA complexes originate from two sources: first, six strong donor atoms in EDTA (two nitrogen and four oxygen) 11329
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Figure 3. (a, b) Conductometric titrations of binary mixtures of lanthanides (25 mL of 1 × 10−3 M Ce(III) + 25 mL of 1 × 10−3 M Th(IV)) vs 1 × 10−2 M L1 and L2 in the absence of coligand HIBA using five-ring probe and two-pole cell. (c, d) Conductometric titrations of binary mixtures of lanthanides (25 mL of 1 × 10−3 M Ce(III) + 25 mL of 1 × 10−3 M Th(IV)) vs 1 × 10−2 M L2 in the presence of coligand HIBA using five-ring probe and two-pole cell.
(positively charged) and a cathode (negatively charged); (ii) an electrolyte solution; and (iii) a battery (current reading detection unit). The number of ions determines the amount of current generated which indicates the concentration of electrolytes. The electric field over the cross section of a real cell is never homogeneous due to the polarization effect at the electrode/ electrolyte interface and electrolyte decomposition. Therefore, it is not possible to calculate the actual conductivity of the cell. For this reason, the cell constant is constant only over a limited range of conductivity values.42 The appropriate value for the cell constant for titration depends on the anticipated level of conductivity of the titration system. In general, a cell constant of 1.0 cm−1 is useful for monitoring lanthanide solutions having a concentration range of 10−2 M or greater, whereas cells with a cell constant of 0.45 cm−1 can be used for the titration of 10−3 and 10−4 M Ln(III) solutions against L1 or L2 with high accuracy and a cell constant of 0.1 cm−1 is suitable for 1 × 10−5 M Ln(III) solutions. Results listed in Table 1S in the Supporting Information clearly show that the five-ring conductivity probe is more beneficial than a two-pole conductivity cell as it shows wide linearity and a quick as well as stable response. This is so because a two-pole conductivity cell suffers from some drawbacks, such as polarization effects at higher conductivities, a wall effect caused by stray current, and the effect of varying the position of the cell in the
surround the metal ion in the complex in order to achieve the maximal number of possible donor−acceptor interactions; second, the anionic character of the ligand also contributes to the stabilization of the complex.36 Four carboxylate oxygens, each carrying a negative charge, can establish a strong electrostatic attraction with the captured metal ion and, hence, form complexes of different stabilities with different lanthanides.37 Stability constant values of the lanthanide−ligand (L1/L2) complexes largely increase along the lanthanides series. From the stability constant values of Ln(III)−L1/L2 complexes,38 it can be concluded that DCTA is a stronger complexing agent than EDTA. The log K values increase from 16.26 for the lanthanum complex to 21.51 for the lutetium complex. The only structural difference between EDTA and DCTA is that the nitrogen atoms in the latter are connected by a cyclohexane ring instead of two methylene groups. Consequently, there is no free rotation about the bonds between the nitrogen atoms. In general, it is expected that DCTA chelates would be more stable due to the increased basicity of the nitrogen atoms and more favorable entropy change upon chelation of the metal ions with DCTA due to the preorientation of the nitrogen atoms.39−41 3.2. Criteria for Selection of Cell Constant, Measuring Range, and Detection Limit. A two-pole conductometric cell consists of (i) two electrodes, including an anode 11330
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Figure 5. Conductometric titrations of binary mixtures of lanthanides (25 mL of 1 × 10−3 M Ce(III) + 25 mL of 1 × 10−3 M Ag(I)) vs 1 × 10−2 M L1 and L2 in the absence of coligand HIBA using five-ring probe and two-pole cell.
Figure 4. Conductometric titrations of binary mixtures of lanthanides (25 mL of 1 × 10−3 M Ce(III) + 25 mL of 1 × 10−3 M Ca(II)) vs 1 × 10−2 M L1 and L2 in the absence of coligand HIBA using five-ring probe and two-pole cell.
observed just after the equivalence point when a two-electrode cell is used. This sudden change in conductivity is not observed with a five-ring electrode, which may be due to polarization of the electrodes in the two-electrode cell. 3.3. Analysis of Lanthanide Mixtures. The method adopted for the analysis of lanthanide(III) mixtures remains the same as discussed in our previous paper.35 Conductometric titrations of binary mixtures of Ce(III) and La(III) ions were carried out with both ligands L1 and L2 separately using both conductivity cells (two-pole and five-ring) in the presence of HIBA (as coligand) in purely aqueous media at 18 ± 0.1 °C. Shapes of the conductometric titration curves for formation of the Ce(III)−L2 complex show trends similar to those with L1 up to the first equivalence point corresponding to the formation of a 1:1 complex. After the first equivalence point, the change in slope of titration curve is more prominent with L2 while with L1 a relatively less sharp change is observed. As observed in Figure 2a, the large decrease (30 μS) in the case of L2 compared to relatively less decrease (15 μS) for L1 can be explained on the basis of their respective structures. L2 because of trans-substituted cyclohexane groups is likely to show more rigidity on complexation than the corresponding change in the case of L1, which is linear in structure and less rigid as compared to L2. Further addition of L2 after the first equivalence point [for Ce(III)−L2 complex] results in the formation of La(III)−L2 complex, and during this phase the
measuring vessel, which are absent in a five-electrode cell.24,25 Hence, for different concentrations of solution (1 × 10−1−1 × 10−7 M) cells of different cell constant values are required to obtain the actual conductance of the solution with accuracy. To study the performance of two-pole and five-ring conductivity probes, conductometric titrations of various lanthanides were carried out using EDTA and DCTA as ligands. The study shows that conductivity changes on complexation with L1 and L2 are sufficient for quantitative analysis of lanthanide(III) ions. Conductometric titrations with ligands (L1/L2) exhibited a special property; i.e., during complexation, very small incremental variation in the conductivity of the solution is seen until the equivalence point, after which the value of conductivity of the solution increases sharply, as the ligand (L1/L2) added to the solution remain unreacted, thereby increasing the number of currentcarrying species in the solution. The small incremental variation observed up to the equivalence point is due to the formation of [Ln(III)−L1]− or [Ln(III)−L2]− complex and the replacement of H+ ions with less mobile Na+ ions in the solution. In Figure 1, part a for five-ring electrodes, show that, during the titration of Ln(III) ions with L1/L2, the conductivity trend remains same throughout the titration, however, during the titration of Ln(III) with L1/L2, a dip in the conductivity is 11331
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conductivity first decreases, stays almost constant, and then increases until the second equivalence point corresponding to the formation of 1:1 La(III)−L2 complex. The titration using the two-pole conductivity cell undertakes the curve as shown in Figure 2b, where a composite end point can be observed, unlike two separate end points with the five-ring electrode. However, L1 is a more useful reagent than L2 in conductometric titration for the determination of lanthanides. 3.4. Analysis of Lanthanides with Other Metal Ions Present. Nine metal ions were selected to study possible interference in the analysis of lanthanides by this method. These included alkaline earth metal ions (Mg2+ and Ca2+), transition metal ions (Fe3+, Ni2+, Cu2+, Ag+, and Pb2+), and other metal ions (Al3+ and Th4+). Results obtained for the conductometric titrations of binary mixtures of Ce3+ solution (25 mL, 1 × 10−3 M) with an equal concentration of interfering metal ion (25 mL, 1 × 10−3 M) with L1 and L2 using two-pole and five-ring electrodes are given in Table 2S in the Supporting Information. Metal ions can be divided into four groups on the basis of their stability constants with EDTA: 1. log K > 20; di-, tri-, and tetrapositive cations, including Fe3+, In3+, Bi3+, Ga3+, Tl3+, Th4+, Zr4+, and Hg2+ 2. log K = 12−20; dipositive transition metals, lanthanides, and Al3+ 3. log K = 7.5−12; alkaline earth metals 4. log K < 7.5; alkali metals, Ag+, and Tl+ Conductometric titrations were carried out on both of these electrodes taking some metals from each group. The shapes of the titration curves and the appearance of equivalence points can be correlated to their respective metal groups as cited above. Results obtained show that by using the conductometric titration method we can determine both lanthanides and group 1 metal ions simultaneously. Here, the first and second equivalence points in the conductometric titration curve correspond to group 1 metal ions and the lanthanide ion, respectively (Figure 3a,b), because the stability constant values of these metals are higher than those of lanthanides. In a binary mixture of lanthanides and group 1 metal ions with L2, the presence of coligand (HIBA) is necessary as shown in Figure 3c,d while in the case of L1 this condition is not necessary, indicating better results with L1. Lanthanides in the presence of groups 2 and 3 metal ions can be determined easily by the conductometric technique using L1 as well as L2. In the plots of these conductometric titrations using L1 and L2 as a ligand, two distinct end points were observed: the first end point corresponds to the lanthanide ion and the second end point corresponds to group 2/group 3 metal ion as shown in Figure 4. However, in the case of L2, only one end point is observed, which corresponds to lanthanide ion concentration. To obtain equivalence points for lanthanide and group 2/group 3 metal ions, HIBA needs to be added to the system with L2 as a completing agent (Figure 4a,b). This unique behavior may be due to the slow kinetics of complexation of L2 with groups 2 and 3 metal ions compared to that with L1. In the case of systems containing a mixture of lanthanide and group 4 metal ions only one end point is observed, which corresponds to the lanthanide ion (Figure 5). Our technique does not show any evidence of complexation of group 4 metal ions with ligands L1 and L2 even in the presence of HIBA. 3.5. Determination of Lanthanides in the Presence of Al(III) Ions. Al3+, a group 2 metal ion, showed interference in the determination of lanthanides by conductometry in aqueous
media. This may be due to the fact that the complexation of Al3+ with EDTA is a slow process, especially if aluminum is present in the form of hydroxide. Also, the formation constant for the reaction Al3+ + Y4− → AlY− (K = 1016.1) does not indicate the extent of formation of AlY− in water as hydrogen ions from water compete with aluminum for the coordinating sites on EDTA anion and hydroxyl ions from water compete with EDTA for coordinating with aluminum. A series of conductometric titrations were carried out for the simultaneous determination of Al(III) and lanthanide ion in solution. In pure aqueous media only one end point was observed which corresponds to the total amount of Ce(III) and Al(III) ions present in the system as shown in Figure 6 [the end point at more than total concentration of Ce(III) and Al(III) in Figure 6a may be due to delayed complexation of Al(III) with DCTA in aqueous media]. To avoid these side reactions, we carried out this
Figure 6. Conductometric titrations of binary mixtures of lanthanides (25 mL of 1 × 10−3 M Ce(III) + 25 mL of 1 × 10−3 M Al(III)) vs 1 × 10−2 M L1 and L2 in the presence of coligand HIBA using five-ring probe and two-pole cell.
conductometric titration in the presence of the nonpolar aprotic solvent 1,4-dioxane, taking HIBA as a coligand. This resulted in two different end points corresponding to the stoichiometries of cerium(III) and aluminum(III) ions, respectively, as shown in Figure 7a and Figure 7b using 5-ring electrode. However, with 2-pole probe only single end point corresponding to the total volume of Ce(III) and Al(III) can be achieved as seen from Figure 7c. 11332
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Experiments were conducted to validate the proposed methods based on two-probe and five-ring conductivity cells. Conductometric titrations with two-pole and five-ring electrodes were carried out to determine La(III) and Gd(III) in synthetic water samples by titrating against L1 and L2. Results of the titrations are summarized in Table 3S in the Supporting Information, which clearly indicates a high order of similarity with the analysis done using standard popular methods such as spectrophotometry43 (arsenazo method).
4. CONCLUSIONS Usefulness of the complexing reagents EDTA and DCTA for lanthanide determination has been demonstrated using a twopole conductivity cell and a five-ring conductivity cell. The following conclusions can be drawn from the experimental work: (i) The five-ring conductivity cell is a better electrode for determination of binary mixtures of lanthanides because unlike the two-pole cell it is possible to obtain separate end points for each lanthanide. (ii) The difficulty in determination of lanthanides in the presence of aluminum observed with both types of electrodes (two-pole, five-ring) can be overcome by using a 1,4-dioxane− water (1:4) medium instead of a pure water system. (iii) Using the five-ring electrode, two distinct end points are observed in comparison to only one end point with the twopole electrode for the determination of lanthanides in the presence of aluminum in dioxane−water medium. (iv) For single ion determination, the ligands EDTA and DCTA give equally good titration curves. However, for the determination of binary mixtures of lanthanides, DCTA can be regarded as a better ligand because it gives well-marked changes in the slopes of titration curves using the five-ring electrode. For binary mixtures of lanthanides with other group metal ions such as alkaline earth and transition metal ions, EDTA gives better results.
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ASSOCIATED CONTENT
S Supporting Information *
Four- and five-ring conductivity cells. Tables listing results of the conductometric titration of Ln(III) vs L1 and L2, simultaneous determination of Ln(III) and interfering ions using L1 and L2 in the presence and absence of HIBA, and validation of the proposed method for the determination of Ln(III) in pure and dosage forms. This material is available free of charge via the Internet at http://pubs.acs.org.
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Figure 7. Conductometric titrations of binary mixtures of lanthanides (25 mL of 1 × 10−3 M Ce(III) + 25 mL of 1 × 10−3 M Al(III)) vs 1 × 10−2 M versus L1 and L2 in the presence of coligand HIBA using fivering probe and two-pole cell in 20% dioxane media.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: +91-416-2243092. Notes
The authors declare no competing financial interest.
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3.6. Validation of the Proposed Method for the Determination of Ln(III) in Pure and Dosage Forms. Repeatability and reproducibility are two important characteristics of any analytical method. Both parameters were studied in this work. To evaluate the discrepancies in the response for successive runs using the same conductivity cell, the repeatability was evaluated by performing three determinations with the same Ln(III) standard solution.
ACKNOWLEDGMENTS
The authors are thankful to the Naval Research Board, DRDO (Government of India), for awarding the research project entitled “A Cost-Effective Conductometric Method for Trace Level Determination of Rare Earth Metal Ions” for the period 2009−2011. 11333
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