D. Harvey - Modern Analytical Chemistry (794078), страница 69
Текст из файла (страница 69)
Assume that KD is 10.3 and β2 is 560.33. Derive equation 7.30.34. The following information is available for the extraction ofCu2+ into CCl4 with dithizone: KD,c = 7 × 104; β = 5 × 1022;Ka,HL = 3 × 10–5; KD,HL = 1.1 × 104; n = 2. What is theextraction efficiency if 100 mL of an aqueous solution of1.0 × 10–7 M Cu2+ that is 1 M in HCl is extracted with 10 mLof CCl4 containing 4.0 × 10–4 M HL?35. Cupferron is a ligand whose strong affinity for metal ionsmakes it useful as a chelating agent in liquid–liquidextractions. The following distribution ratios are known forthe extraction of Hg2+, Pb2+, and Zn2+ from aqueoussolutions to an organic solvent.Distribution Ratio forpHHg2+Pb2+Zn2+123456789103.310.032.332.319.04.01.00.540.150.050.00.4399999999999999999999999999999990.00.00.00.00.180.330.821.502.572.57(a) Suppose that you have 50.0 mL of an aqueous solutioncontaining Hg2+, Pb2+, and Zn2+.
Describe how youwould go about effecting a separation in which thesemetal ions are extracted into separate portions of theorganic solvent. (b) Under the conditions you haveselected for extracting Hg2+, what percent of the Hg2+remains in the aqueous phase after three extractions with50.0 mL each of the organic solvent? (c) Under theconditions you have chosen for extracting Pb2+, what isthe minimum volume of organic solvent needed to extract99.5% of the Pb2+ in a single extraction? (d) Under theconditions you have chosen for extracting Zn2+, howmany extractions are needed to remove 99.5% of the Zn2+if each extraction is to use 25.0 mL of organic solvent?7M SUGGESTED READINGSThe following paper provides a general introduction to theterminology used in describing sampling.Majors, R. E.
“Nomenclature for Sampling in AnalyticalChemistry.” LC•GC 1992, 10, 500–506.Further information on the statistics of sampling is covered in thefollowing papers.Kratochvil, B.; Goewie, C. E.; Taylor, J. K. “Sampling Theory forEnvironmental Analysis,” Trends Anal. Chem. 1986, 5,253–256.Kratochvil, B.; Taylor, J. K. “Sampling for Chemical Analysis,”Anal. Chem.
1981, 53, 924A–938A.The following sources may be consulted for further detailsregarding the collection of environmental samples. The paperby Benoit and colleagues provides a good discussion of howeasily samples can be contaminated during collection andpreservation.Barceló, D.; Hennion, M. C. “Sampling of Polar Pesticides fromWater Matrices,” Anal.
Chim. Acta 1997, 338, 3–18.Batley, G. E.; Gardner, D. “Sampling and Storage of NaturalWaters for Trace Metal Analysis”, Wat. Res. 1977, 11, 745-756.Benoit, G.; Hunter, K. S.; Rozan, T. F. “Sources of Trace MetalContamination Artifacts During Collection, Handling, andAnalysis of Freshwaters,” Anal. Chem. 1997, 69, 1006–1011.Keith, L. H., ed. Principles of Environmental Sampling, AmericanChemical Society: Washington, DC, 1988.Keith, L. H. Environmental Sampling and Analysis—A PracticalGuide, Lewis Publishers, Boca Raton, FL, 1991.The following sources provide additional information onpreparing samples for analysis, including the separation ofanalytes and interferents.Anderson, R. Sample Pretreatment and Separation, Wiley:Chichester, 1987.Baiulescu, G.
E.; Dumitrescu, P.; Zuaravescu, P. G. Sampling. EllisHorwood: New York, 1991.Compton, T. R. Direct Preconcentration Techniques. OxfordScience Publications: Oxford, 1993.Compton, T. R. Complex-Formation Preconcentration Techniques.Oxford Science Publications: Oxford, 1993.Gy, P. M. Sampling of Particulate Materials: Theory and Practice;Elsevier: Amsterdam, 1979.1400-CH07 9/8/99 4:04 PM Page 231Chapter 7 Obtaining and Preparing Samples for AnalysisGy, P. M.
Sampling of Heterogeneous and Dynamic Materials:Theories of Heterogeneity, Sampling and Homogenizing. Elsevier:Amsterdam, 1992.Gy, P. ed. Sampling for Analytical Purposes. Wiley: New York, 1998.Karger, B. L.; Snyder, L. R.; Harvath, C. An Introduction toSeparation Science, Wiley-Interscience: New York, 1973.Kingston, H. M.; Haswell, S. J., eds. Microwave-EnhancedChemistry: Fundamentals, Sample Preparation, andApplications.
American Chemical Society: Washington, DC,1997.Majors, R. E.; Raynie, D. E. “Sample Preparation and Solid-PhaseExtraction,” LC•GC 1997, 15, 1106–1117.Miller, J. M. Separation Methods in Chemical Analysis, WileyInterscience: New York, 1975.231Morrison, G. H.; Freiser, H. Solvent Extraction in AnalyticalChemistry, John Wiley and Sons: New York, 1957.Pawliszyn, J. Solid-Phase Microextraction: Theory and Practice,Wiley: New York, 1997.Smith, R.; James, G. V. The Sampling of Bulk Materials. RoyalSociety of Chemistry: London, 1981.Sulcek, Z.; Povondra, P.
Methods of Decomposition in InorganicAnalysis. CRC Press: Boca Raton, FL, 1989.Thurman, E. M.; Mills, M. S. Solid-Phase Extraction: Principles andPractice, Wiley: New York, 1998.A web-site dedicated to sample preparation, which contains usefulinformation about acid digestion and microwave digestion, isfound athttp://www.sampleprep.duq.edu/sampleprep/7N REFERENCES1. Youden, Y.
J. J. Assoc. Off. Anal. Chem. 1981, 50, 1007–1013.2. Fricke, G. H.; Mischler, P. G.; Staffieri, F. P.; et al. Anal. Chem. 1987,59, 1213–1217.3. (a) Cohen, R. D. J. Chem. Educ. 1991, 68, 902–903; (b) Cohen, R. D.J. Chem. Educ. 1992, 69, 200–203.4. Keith, L. H. Environ. Sci. Technol. 1990, 24, 610–617.5. Flatman, G.
T.; Englund, E. J.; Yfantis, A. A. In Keith, L. H., ed.Principles of Environmental Sampling. American Chemical Society:Washington, DC, 1988, 73–84.6. Ingamells; C. O.; Switzer, P. Talanta 1973, 20, 547–568.7. Viseman, J. Mat. Res. Stds. 1969, 9(11), 8–13.8. Blackwood, L. G. Environ. Sci. Technol. 1991, 25, 1366–1367.9. Duce, R. A.; Quinn, J.
G.; Olney, C. E. et al. Science, 1972, 176,161–163.10. Tanner, R. L. In Keith, L. H., ed. Principles of Environmental Sampling.American Chemical Society: Washington, DC, 1988, 275–286.11. (a) Sandell, E. B. Colorimetric Determination of Traces of Metals,Interscience Publishers: New York, 1950, pp.
19–20; (b) Sandell, E. B.Anal. Chem. 1968, 40, 834–835.12. Zubay, G. Biochemistry, 2nd ed. Macmillan: New York, 1988, p. 120.13. Meites, L. Handbook of Analytical Chemistry, McGraw-Hill: New York,1963.14. Glavin, D. P.; Bada, J. L. Anal. Chem. 1998, 70, 3119–3122.15. Fresenius, C. R. A System of Instruction in Quantitative ChemicalAnalysis.
John Wiley and Sons: New York, 1881.16. Jeannot, M. A.; Cantwell, F. F. Anal. Chem. 1997, 69, 235–239.17. Zhang, A.; Yang, M. J.; Pawliszyn, J. Anal. Chem. 1994, 66,844A–853A.18. Renoe, B. W. Am. Lab. August 1994, 34–40.19. McNally, M. E. Anal. Chem. 1995, 67, 308A–315A.20. “TPH Extraction by SFE”, ISCO, Inc., Lincoln, NE, Revised Nov. 1992.21. “The Analysis of Trihalomethanes in Drinking Water by LiquidExtraction”; US Environmental Protection Agency, EnvironmentalMonitoring and Support Laboratory, Cincinnati, OH, 9 Sept. 1977.22. Official Methods of Analysis, 11th ed., Association of Official AnalyticalChemists, Washington, DC, 1970, p.
475.23. Aguilar, C.; Borrull, F.; Marcé, R. M. LC•GC 1996, 14, 1048–1054.24. Corl, W. E. Spectroscopy 1991, 6(8), 40–43.25. Kratochvil, B.; Taylor, J. K. Anal. Chem. 1981, 53, 924A–938A.26. Engels, J. C.; Ingamells, C. O. Geochim. Cosmochim. Acta 1970, 34,1007–1017.27. Guy, R.
D.; Ramaley, L.; Wentzell, P. D. J. Chem. Educ. 1998, 75,1028–1033.28. Maw, R.; Witry, L.; Emond, T. Spectroscopy 1994, 9, 39–41.29. Simpson, S. L.; Apte, S. C.; Batley, G. E. Anal. Chem. 1998, 70,4202–4205.1400-CH08 9/9/99 2:17 PM Page 232Chapter 8Gravimetric Methodsof AnalysisGravimetry encompasses all techniques in which we measure massor a change in mass.
When you step on a scale after exercising you aremaking, in a sense, a gravimetric determination of your mass.Measuring mass is the most fundamental of all analyticalmeasurements, and gravimetry is unquestionably the oldest analyticaltechnique.2321400-CH08 9/9/99 2:17 PM Page 233Chapter 8 Gravimetric Methods of Analysis2338A Overview of GravimetryBefore we look more closely at specific gravimetric methods and their applications,let’s take a moment to develop a broad survey of gravimetry. Later, as you readthrough the sections of this chapter discussing different gravimetric methods, thissurvey will help you focus on their similarities.
It is usually easier to understand anew method of analysis when you can see its relationship to other similar methods.8A.1 Using Mass as a SignalAt the beginning of this chapter we indicated that in gravimetry we measure massor a change in mass. This suggests that there are at least two ways to use mass as ananalytical signal. We can, of course, measure an analyte’s mass directly by placing iton a balance and recording its mass.
For example, suppose you are to determine thetotal suspended solids in water released from a sewage-treatment facility. Suspended solids are just that; solid matter that has yet to settle out of its solution matrix. The analysis is easy. You collect a sample and pass it through a preweighed filter that retains the suspended solids. After drying to remove any residual moisture,you weigh the filter. The difference between the filter’s original mass and final massgives the mass of suspended solids. We call this a direct analysis because the analyteitself is the object being weighed.What if the analyte is an aqueous ion, such as Pb2+? In this case we cannot isolate the analyte by filtration because the Pb2+ is dissolved in the solution’s matrix.We can still measure the analyte’s mass, however, by chemically converting it to asolid form.
If we suspend a pair of Pt electrodes in our solution and apply a sufficiently positive potential between them for a long enough time, we can force thereactionPb2+(aq) + 4H2O(l)t PbO2(s) + H2(g) + 2H3O+(aq)to go to completion. The Pb2+ ion in solution oxidizes to PbO2 and deposits on thePt electrode serving as the anode. If we weigh the Pt anode before and after applyingthe potential, the difference in the two measurements gives the mass of PbO2 and,from the reaction’s stoichiometry, the mass of Pb2+. This also is a direct analysis because the material being weighed contains the analyte.Sometimes it is easier to remove the analyte and use a change in mass as theanalytical signal.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
