In the Laboratory
Coulometric Titration of Ethylenediaminetetraacetate (EDTA) with Spectrophotometric Endpoint Detection: An Experiment for the Instrumental Analysis Laboratory Kathryn R. Williams,* Vaneica Y. Young, and Benjamin J. Killian Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, United States *
[email protected] In the analytical chemistry curriculum, it is essential for students to have hands-on experience with a wide variety of methods—a challenging task in the time constraints of the usual two-course sequence. To optimize usage of laboratory time, we have designed several experiments which combine two or more analytical methods: TGA and FTIR (1);UV spectrophotometry, HPLC, and capillary electrophoresis (2); and solid-phase microextraction and GC-MS (3). This article describes a new instrumental analysis experiment that combines constantcurrent coulometry with spectrophotometric endpoint detection for the determination of ethylenediaminetetraacetate (EDTA or Y4). This compound occurs widely in consumer products and is recognized as a persistent pollutant in wastewater (4). Coulometric titrations have been the focus of a number of experiments described previously in this Journal (4-24). Only one of these utilized endpoint detection by absorbance measurements, but with a colored indicator (12). There have been relatively few reports of spectrophotometric endpoints involving the analyte or titrant as the chromophore (25, 26). The samples, blood-collection tubes containing a solution of K3EDTA, increase the instructional value by demonstrating a practical application of complexation chemistry. To prevent the blood sample from clotting, an anticoagulant—either EDTA, citrate, or heparin—must be added to the collection tube. Citrate and EDTA both act by sequestering calcium ion, whereas heparin deactivates the thrombin blood-clotting factor (27). Because the type and concentration of the coagulant can affect subsequent blood analyses (27), knowledge and control of the quantity of EDTA present in blood-sampling tubes can be very important in clinical work. Tubes already treated with EDTA are commercially available at very low cost. Overview of the Experiment The EDTA is titrated with Cu2þ electrolytically generated at 10 mA from a heavy-gauge copper wire anode with H2(g) generation as the auxiliary reduction reaction. Because the Cu2þ is sequestered as soon as it is formed, there is no need to separate the electrode compartments. The reaction is performed in an acetate buffer at pH 4.0, where the EDTA exists predominantly as H2Y2-. The reaction is monitored by measuring the increase in absorbance of the CuY2- product using a Spectral Instruments model 440 fiberoptic UV-Vis spectrophotometer in the time-based mode. Although the CuY2- complex can be detected
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at 745 nm (28), measurements are taken in the UV region, where the molar absorptivity is about 200 times larger. Prior to analyzing the collection tube contents, students determine the optimum wavelength(s) by comparing UV spectra of ca. 10-4 M solutions (all in acetate buffer) of uncomplexed H2Y2-, uncomplexed Cu2þ, CuY2-, and the diluted contents of a collection tube. The students use two different wavelengths for the analysis: 240 nm, where there is significant background absorbance from the acetate buffer and another wavelength of their own choosing (usually around 270 nm). Students are asked to comment in their reports on the effect of background noise at 240 nm. For the coulometric titration, students quantitatively transfer the contents of a blood-collection tube to a beaker and add acetate buffer and sufficient water to immerse the electrodes and the fiber optic probe. The spectrophotometer is programmed to obtain readings at the two chosen wavelengths at 5 s intervals. The UV data acquisition and the constant-current source are activated simultaneously. At 10 mA, the titration is complete in about 3 min, and three collection tubes are analyzed by each student group. Hazards This experiment utilizes common chemicals (acetate buffer, EDTA, and Cu2þ) of low toxicity. The very small generating current presents no shock hazard. Results The absorbance versus time data obtained at 270 nm in a typical student titration are shown in Figure 1. The absorbance increases steeply as the CuY2- complex forms. After the EDTA is consumed, the slope is only slightly greater than zero, owing to the very small absorbance of the excess Cu(H2O)62þ. Although the endpoint could be read directly from the plot, students are instructed to construct two least-squares lines, which are equated to calculate the time at the intersection. Results from several student groups are shown in Table 1. According to the manufacturer, the allowed range for the mass of K3EDTA 3 2H2O is 3.0-5.5 mg. For the lot represented in Table 1, the manufacturer provided an analysis of 4.44 mg with a standard deviation of 0.09 mg. The student results show excellent agreement with an average of 4.46 ( 0.12 mg (N = 18). The
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r 2011 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 88 No. 3 March 2011 10.1021/ed100627q Published on Web 01/11/2011
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In the Laboratory
a consumer product having clinical applications. The experiment also reinforces concepts of complexation chemistry and electrolysis. Literature Cited
Figure 1. Plot of absorbance (270 nm) vs coulometric generation time (9.992 mA). The endpoint time is 186.62 s. The sharp change above 280 s is due to copper deposition on the Pt cathode (observed visually). Table 1. Student Results for Coulometric Titration of EDTA Wavelength/nm Current/mA End point/s K3EDTA 3 2H2O/mg 240 270 240 270 240 270 240 270 240 270 240 270 240 270 240 270 240 270
9.974 10.118 10.123 9.964 9.992 9.996 9.998 10.012 9.988
189.33
4.330
189.79
4.341
192.62
4.469
191.02
4.432
196.20
4.555
195.59
4.541
193.95
4.432
194.33
4.440
186.58
4.275
186.62
4.276
192.61
4.415
191.56
4.391
200.98
4.608
200.27
4.592
193.76
4.449
191.80
4.404
204.19
4.677
202.45
4.637
results also show that the background absorption at 240 nm has little effect on the accuracy. However, students should find that the absorbance of the buffer can result in a less defined transition at the equivalence point thus, emphasizing matrix effects in spectroscopic analysis. The experiment is easily completed in a single 3-h laboratory period. Students have been very enthusiastic about the analysis of
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Supporting Information Available Student handout and notes for the instructor. This material is available via the Internet at http://pubs.acs.org.
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