In the Laboratory
Determination of Anionic Surfactants Using Atomic Absorption Spectrometry and Anodic Stripping Voltammetry
W
Richard John and Daniel Lord School of Applied Science, Griffith University, Gold Coast Campus, Parklands Drive Southport Gold Coast, QLD, 4214, Australia; *
[email protected] Atomic absorption spectroscopy (AAS) and anodic stripping voltammetry (ASV) are techniques typically restricted to metal analysis. An experiment developed for our undergraduate teaching program demonstrates the utility of these techniques for the determination of organic species, in this case, the indirect analysis of anionic surfactants. The method involves formation of an extractable complex between the synthetic surfactant anion and the bis(ethylenediamine)diaquacopper(II) cation. This complex is extracted into chloroform and then back-extracted into dilute acid. The resulting Cu(II) ions are determined by AAS and ASV. Students are required to determine the concentration of a pre-prepared “unknown” anionic surfactant solution and to collect and analyze a real sample of their choice. The experiment was designed as a final-year undergraduate laboratory for the subject Analytical Chemistry within our Environmental Science degree. It demontrates the following analytical principles: indirect analysis compleximetric analysis liquid–liquid (solvent) extraction back extraction (into dilute acid) analytical recovery analysis using flame-AAS and ASV, two of the most popular methods for trace metal analysis
In their write-up, students are expected to discuss each of the above principles and make a critical comparison of metal analysis by AAS and ASV. The experiment expands on a previous publication in this Journal for the determination of anionic surfactants, which employed a colorimetric endpoint for the analysis (1). Background Anionic surfactants are widely used for industrial and domestic purposes, including laundry powders, dish-washing liquids, shampoos, emulsifiers for industrial solvent cleaners, and agricultural and horticultural chemical delivery systems. Although most anionic surfactants in use today are biodegradable and essentially nontoxic to humans (2), a need still exists for effective monitoring of these compounds. Aside from the visual pollution (in the form of foaming) associated with surfactant use, the environmental impact of surfactant release in natural water systems can be much more serious. For example, several authors have reported impaired respiratory function in fish caused by damage to the gill epithelium at concentrations below 3 mg L{1 (3–6 ). In addition, damage to chemoreceptors of fish at concentrations as low as 0.5 mg L{1
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has been shown to impair feeding behavior (7 ). While the more active marine adults (such as fish) are more susceptible to the presence of anionic surfactants than less active adults (such as crustaceans), the developmental stages of all forms of marine life are extremely susceptible to even trace levels of these compounds. In particular, invertebrate and fish larvae show susceptibility to anionic surfactant concentrations of 0.05 mg L{1 (8) and 0.16 mg L{1 (9), respectively.
Analysis of Anionic Surfactants The APHA standard method for the determination of anionic surfactants is the Methylene Blue Active Substances (MBAS) method (10). This method employs the methylene blue cation, which forms a 1:1 ion pair with the surfactant anion. The ion pair is extracted into chloroform and determined spectrophotometrically. Unfortunately, the MBAS method lacks specificity because many sulfonates and simple inorganic anions such as cyanate, nitrate, thiocyanate, sulfide, and chloride also form extractable compounds with methylene blue, resulting in positive interferences. For this reason, the MBAS method for anionic surfactants was replaced as the Australian standard method by a method that uses the bis(ethylenediamine)diaquacopper(II) cation as the extracting agent (11). This ion permits selective quantitative extraction of surfactant anions into an organic solvent (1). The surfactant anion reacts with the aquated bis(ethylenediamine)copper(II) ion (12) and is extracted into chloroform as a neutral inner sphere complex (eq 1). The [Cu(en)2(RSO2O)2](org) complex is back-extracted with dilute acid, releasing free Cu(II) into the aqueous phase. The copper concentration is then determined using AAS and ASV. [Cu(en)2(H2O)2]2+(aq) + 2RSO2O{(aq) → [Cu(en)2(RSO2O)2](org) + 2H2O(aq)
(1)
en = ethylenediamine (H2NCH2CH2NH2) R = hydrocarbon group (C12H25C6H4) In contrast to the Australian standard method for anionic surfactant analysis, which utilizes flame AAS, the instrumentation required by ASV is simpler and more robust, without the need for specialized gases and extraction systems. The simple instrumentation means that very little operator expertise is needed, making it ideal for undergraduate students. Unlike ASV, flame-AAS cannot be used to measure copper concentrations below 0.02 ppm, and the calibration curve for ASV generally extends over more orders of magnitude than AAS. Of its spectroscopic competitors, only flameless AAS is able to achieve the same sensitivity as ASV, but at a much higher cost.
Journal of Chemical Education • Vol. 76 No. 9 September 1999 • JChemEd.chem.wisc.edu
In the Laboratory
Experimental Procedure
A = 0.0349 x concn - 5 x 10-4)
0.018
0.010
The preparation of reagents and full experimental procedures are based on the Australian standard method for anionic surfactant analysis, AS 3506 (11). A brief outline of the procedure and required equipment is given below. Full details can be found in the online lab documentation.W
0.008
General Procedure
0.016
A324.8nm
0.014 0.012
0.006 0.004 0.002 0 0
0.1
0.2
0.3
0.4
0.5
Concentration (ppm) Figure 1a. AAS calibration curve for Cu(II) standards.
0.014
A = 0.0271 x concn - 2 x 10-5)
0.012
A324.8nm
0.010 0.008 0.006 0.004 0.002 0 0
0.1
0.2
0.3
0.4
0.5
Concentration (ppm) Figure 1b. AAS calibration curve for dodecylbenzene sulfonic acid standards.
Students are required to prepare anionic surfactant calibration standards of 0.05, 0.1, 0.2 and 0.5 mg L{1 by serial dilution of a pre-prepared 1000 mg L{1 stock solution. The anionic surfactant standard used in this experiment is dodecylbenzene sulfonic acid. Foam formation should be avoided because the concentration of surfactant in the foam phase is significantly greater than in the aqueous phase. Compressed air can be used to collapse any foam that forms. Two hundred milliliters of each standard is pipetted into 250-mL separating funnels. The copper ethylenediamine reagent (pre-prepared) and chloroform are then added. After liquid–liquid extraction the chloroform layer is run off into a centrifuge tube and centrifuged for 2 min at 2500 rpm to achieve complete phase separation. (Alternatively, phase separation can be achieved by allowing the chloroform extracts to stand for 20 min.) Ten milliliters of the clarified extract is then pipetted into a 25-mL stoppered measuring cylinder and 10.00 mL of 0.1 M HCl is added. This is shaken and allowed to stand for 20 min, after which the aqueous layer is removed and analyzed for Cu(II) by AAS and ASV. A blank solution is also analyzed by using 200 mL of deionized water in place of the surfactant standards. Students are required to determine the concentration of a pre-prepared “unknown” (the concentration is known to the lab demonstrators) dodecylbenzene sulfonate solution by comparison with standards. In addition they are required to collect and analyze a real sample of their choice (see below).
Sampling
35
Ip = 68.031 x concn + 0.1964
30
Ip / µA
25 20 15 10 5 0 0
0.1
0.2
0.3
0.4
0.5
Concentration (ppm) Figure 2. ASV calibration curve for dodecylbenzene sulfonic acid standards.
Samples should be collected in glass bottles and, as soon as practicable, filtered through a 0.45 ± 0.05-µm cellulose acetate membrane filter. The filtered sample should be stored at 4 °C and analyzed within 48 hours of collection. With concentrations below 1 ppm, adsorption onto the wall of the sample container will lead to reduced surfactant levels in the bulk aqueous phase. Addition of alkali phosphate will minimize adsorption errors (12). Three readily available sources of anionic surfactant samples on our campus were used in this experiment: the water from (i) the sink of a cafeteria kitchen, (ii) the dishwasher from the same kitchen, and (iii) a domestic washing machine at the end of its wash cycle. These samples, while not typical environmental samples, were convenient examples to demonstrate the principles of sampling and analysis required in this experiment. It should be noted, however, that the anionic surfactant concentrations in these samples exceeded the calibration standards, necessitating suitable dilution of the samples by the students.
JChemEd.chem.wisc.edu • Vol. 76 No. 9 September 1999 • Journal of Chemical Education
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In the Laboratory
Results and Discussion The ratio of copper to surfactant in the extracted complex is 1:2. Given the experimental procedure used and assuming 100% extraction efficiency, a sample containing 0.1 ppm of surfactant should yield a Cu(II) concentration of 0.13 ppm after back-extraction into acid. Figure 1a shows a typical calibration curve obtained by students for the determination of Cu(II) standards by AAS. The students use this curve for comparison with the calibration curve obtained for analysis of the dodecylbenzene sulfonic acid standards (Fig. 1b). The ratio of the slopes for these two curves (Cu/surfactant) is (0.0349/0.0270), or 1.29. This equates with the ratio expected for 100% extraction efficiency (0.13 ppm/0.10 ppm = 1.30). The students use this procedure as a check to ensure they have 100% (or close to it) recovery of surfactant. Figure 2 shows a typical calibration curve obtained for dodecylbenzene sulfonic acid standards using an ASV finish. A similar check on percent recovery could be performed using ASV analysis of Cu(II) standards, although we do not make this part of our procedure. The results of three typical student analyses of laboratory prepared unknown surfactant samples are shown in Table 1. As the table shows, we find that the AAS and ASV results correlate very well and errors tend to result from students’ technique (dilutions, extractions, preparation of standards, etc.) rather than from the end analyses. Students are also required to collect a sample containing anionic surfactant from one of three sites on campus. Table 2 shows the results obtained from three student analyses. Again, the AAS and ASV results show good correlation. Details of interferences for anionic surfactant analysis of real samples can be found in ref 12. These are also outlined in the lab documentation. Specific details concerning the ASV and AAS parameters are also provided in the lab documentation.W Acknowledgments We wish to thank Mark Imisides and Melinda John for helpful discussions in the preparation of this experiment. Many thanks also to Greg Hope, Paul Duckworth, and AD Instruments for advice and the use of the Maclab/4e interface and potentiostat. Note W Supplementary materials for this article are available on JCE Online at http://jchemed.chem.wisc.edu/Journal/issues/1999/Sep/ abs1256.html.
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Table 1. Concentration of Laborator y-Prepared "Unknown" Samples Student No.
Result/ppm True value/ ppm AAS ASV
1061923
0.10
0.12
0.13
593106
0.20
0.20
0.21
455969
0.50
0.49
0.48
Table 2. Concentration of "Real" Samples Collected by Students Sample Commercial sink Commercial dishwasher Domestic washing machine
Result/ppm AAS 121 5.16 70.0
ASV 122 5.49 71.7
Literature Cited 1. Crisp, P. T.; Eckert, J. M.; Gibson, N. A. J. Chem. Educ. 1983, 60, 236–238. 2. Swisher, R. D. Surfactant Biodegradation, 2nd ed.; Dekker: New York, 1987. 3. Swedmark, M.; Braaten, B.; Emanuelsson, E.; Granmo, A. Marine Biol. 1971, 9, 183. 4. Schmid, J.; Mann, H. Nature 1961, 192, 625. 5. Swisher, R. D.; O’Rourke, J. T.; Tomlinson, H. D. J. Am. Oil Chem. Soc. 1964, 41, 746. 6. Eisler, R. Am. Fisheries Soc. Trans. 1965, 94, 26. 7. Bardach, J. E.; Fujiya, M.; Hall, A. Science 1965, 148, 1605. 8. Renzoni, A. Arch. Oceanogr. Limnol. 1975, 18(2), 99. 9. Lesyuk, I. I.; Kostyuk, A. O.; Lemishko, A. A.; Reshetito, S. G.; Kotsqumbuas, I. Y. Vopr. Ikhtiol (Russia) 1983, 23(b), 993. 10. Standard Methods for the Examination of Water and Waste Water, 16th ed.; American Public Health Association: Washington, DC, 1985. 11. AS 3506. Waters. Determination of Filtrable Anionic Surfactants— Copper–Ethylenediamine Flame Atomic Absorption Spectrometric Method; Standards Association of Australia: N.S.W Australia, 1987. 12. Crisp, P. T.; Eckert, J. M.; Gibson, N. A. Anal. Chim. Acta 1975, 78, 391–396.
Journal of Chemical Education • Vol. 76 No. 9 September 1999 • JChemEd.chem.wisc.edu
