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230 Modern Analytical Chemistry

32. The relevant equilibria for extracting a neutral metal–ligand Distribution Ratio for
complex from an aqueous solution into an organic phase are pH Hg 2+ Pb 2+ Zn 2+
shown in the following diagram.
1 3.3 0.0 0.0
ML 2
2 10.0 0.43 0.0
Organic 3 32.3 999 0.0
K D
Aqueous 4 32.3 9999 0.0
b 2 5 19.0 9999 0.18
M 2+ + 2L – ML 2 6 4.0 9999 0.33
(a) Derive an expression for the distribution ratio for this 7 1.0 9999 0.82
extraction. (b) Calculate the extraction efficiency when 8 0.54 9999 1.50
50.0 mL of an aqueous solution that is 0.15 mM in M 2+ 9 0.15 9999 2.57
–
and 0.12 M in L is extracted with 25.0 mL of the organic 10 0.05 9999 2.57
phase. Assume that K D is 10.3 and b 2 is 560.
(a) Suppose that you have 50.0 mL of an aqueous solution
2+
2+
33. Derive equation 7.30. containing Hg , Pb , and Zn . Describe how you
2+
34. The following information is available for the extraction of would go about effecting a separation in which these
22
2+
4
Cu into CCl 4 with dithizone: K D,c =7 ´10 ; b=5 ´10 ; metal ions are extracted into separate portions of the
4
–5
K a,HL =3 ´10 ; K D,HL = 1.1 ´10 ; n = 2. What is the organic solvent. (b) Under the conditions you have
2+
extraction efficiency if 100 mL of an aqueous solution of selected for extracting Hg , what percent of the Hg 2+
2+
–7
1.0 ´10 M Cu that is 1 M in HCl is extracted with 10 mL remains in the aqueous phase after three extractions with
–4
of CCl 4 containing 4.0 ´10 M HL? 50.0 mL each of the organic solvent? (c) Under the
2+
conditions you have chosen for extracting Pb , what is
35. Cupferron is a ligand whose strong affinity for metal ions
the minimum volume of organic solvent needed to extract
makes it useful as a chelating agent in liquid–liquid
2+
99.5% of the Pb in a single extraction? (d) Under the
extractions. The following distribution ratios are known for 2+
2+
2+
2+
the extraction of Hg , Pb , and Zn from aqueous conditions you have chosen for extracting Zn , how
many extractions are needed to remove 99.5% of the Zn 2+
solutions to an organic solvent.
if each extraction is to use 25.0 mL of organic solvent?
7 M SUGGESTED READINGS
The following paper provides a general introduction to the Benoit, G.; Hunter, K. S.; Rozan, T. F. “Sources of Trace Metal
terminology used in describing sampling. Contamination Artifacts During Collection, Handling, and
Majors, R. E. “Nomenclature for Sampling in Analytical Analysis of Freshwaters,” Anal. Chem. 1997, 69, 1006–1011.
Chemistry.” LC•GC 1992, 10, 500–506. Keith, L. H., ed. Principles of Environmental Sampling, American
Further information on the statistics of sampling is covered in the Chemical Society: Washington, DC, 1988.
following papers. Keith, L. H. Environmental Sampling and Analysis—A Practical
Kratochvil, B.; Goewie, C. E.; Taylor, J. K. “Sampling Theory for Guide, Lewis Publishers, Boca Raton, FL, 1991.
Environmental Analysis,” Trends Anal. Chem. 1986, 5, The following sources provide additional information on
253–256. preparing samples for analysis, including the separation of
Kratochvil, B.; Taylor, J. K. “Sampling for Chemical Analysis,” analytes and interferents.
Anal. Chem. 1981, 53, 924A–938A. Anderson, R. Sample Pretreatment and Separation, Wiley:
The following sources may be consulted for further details Chichester, 1987.
regarding the collection of environmental samples. The paper Baiulescu, G. E.; Dumitrescu, P.; Zuaravescu, P. G. Sampling. Ellis
by Benoit and colleagues provides a good discussion of how Horwood: New York, 1991.
easily samples can be contaminated during collection and Compton, T. R. Direct Preconcentration Techniques. Oxford
preservation. Science Publications: Oxford, 1993.
Barceló, D.; Hennion, M. C. “Sampling of Polar Pesticides from Compton, T. R. Complex-Formation Preconcentration Techniques.
Water Matrices,” Anal. Chim. Acta 1997, 338, 3–18. Oxford Science Publications: Oxford, 1993.
Batley, G. E.; Gardner, D. “Sampling and Storage of Natural Gy, P. M. Sampling of Particulate Materials: Theory and Practice;
Waters for Trace Metal Analysis”, Wat. Res. 1977, 11, 745-756. Elsevier: Amsterdam, 1979.