Page 12 - Modern Analytical Chemistry

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. . . End of Chapter

y y
List of Key Terms
5E KEY TERMS
The key terms introduced within the chapter are
aliquot (p. 111) multiple-point standardization (p. 109) secondary reagent (p. 107)
external standard (p. 109) normal calibration curve (p. 109) single-point standardization (p. 108) listed at the end of each chapter. Page references
internal standard (p. 116) primary reagent (p. 106) standard deviation about the
linear regression (p. 118) reagent grade (p. 107) regression (p. 121) direct the student to the definitions in the text.
matrix matching (p. 110) residual error (p. 118) total Youden blank (p. 129)
method of standard additions (p. 110)
Summary
5F SUMMARY
The summary provides the student with a brief
In a quantitative analysis, we measure a signal and calculate the and the use of an internal standard. The most desirable standard-
amount of analyte using one of the following equations. ization strategy is an external standardization. The method of review of the important concepts within the chapter.
standard additions, in which known amounts of analyte are added
S meas = kn A + S reag
to the sample, is used when the sample’s matrix complicates the
analysis. An internal standard, which is a species (not analyte)
S meas = kC A + S reag
added to all samples and standards, is used when the procedure Suggested Experiments
To obtain accurate results we must eliminate determinate errors does not allow for the reproducible handling of samples and
affecting the measured signal, S meas , the method’s sensitivity, k, standards.
and any signal due to the reagents, S reag . Standardizations using a single standard are common, but also An annotated list of representative experiments is
To ensure that S meas is determined accurately, we calibrate are subject to greater uncertainty. Whenever possible, a multiple-
the equipment or instrument used to obtain the signal. Balances point standardization is preferred. The results of a multiple-point provided from the Journal of Chemical Education.
are calibrated using standard weights. When necessary, we can standardization are graphed as a calibration curve. A linear regres-
also correct for the buoyancy of air. Volumetric glassware can sion analysis can provide an equation for the standardization.
be calibrated by measuring the mass of water contained or de- A reagent blank corrects the measured signal for signals due to
livered and using the density of water to calculate the true vol- reagents other than the sample that are used in an analysis. The
ume. Most instruments have calibration standards suggested by most common reagent blank is prepared by omitting the sample.
the manufacturer. When a simple reagent blank does not compensate for all constant
An analytical method is standardized by determining its sensi- sources of determinate error, other types of blanks, such as the
tivity. There are several approaches to standardization, including total Youden blank, can be used.
the use of external standards, the method of standard addition,
Suggested Readings
5G Suggested EXPERIMENTS
Suggested readings give the student
Experiments Calibration—Volumetric glassware (burets, pipets, and and internal standards are a common feature of many discussion of the topics introduced
The following exercises and experiments help connect the material in this chapter to the analytical laboratory.
access to more comprehensive
Standardization—External standards, standard additions,
volumetric flasks) can be calibrated in the manner described
quantitative analyses. Suggested experiments using these
in Example 5.1. Most instruments have a calibration sample
within the chapter.
that can be prepared to verify the instrument’s accuracy and
standardization methods are found in later chapters. A good
precision. For example, as described in this chapter, a
project experiment for introducing external standardization,
solution of 60.06 ppm K 2 Cr 2 O 7 in 0.0050 M H 2 SO 4 should
matrix is to explore the effect of pH on the quantitative
give an absorbance of 0.640 ± 0.010 at a wavelength of
350.0 nm when using 0.0050 M H 2 SO 4 as a reagent
analysis of an acid–base indicator. Using bromothymol blue
blank. These exercises also provide practice with using standard additions, and the importance of the sample’s y y
as an example, external standards can be prepared in a pH 9 SUGGESTED READINGS
1G
volumetric glassware, weighing samples, and preparing buffer and used to analyze samples buffered to different pHs
solutions. in the range of 6–10. Results can be compared with those
The role of analytical chemistry within the broader discipline of Laitinen, H. A. “Analytical Chemistry in a Changing World,”
obtained using a standard addition.
chemistry has been discussed by many prominent analytical Anal. Chem. 1980, 52, 605A–609A.
chemists. Several notable examples follow. Laitinen, H. A. “History of Analytical Chemistry in the U.S.A.,”
Baiulescu, G. E.; Patroescu, C.; Chalmers, R. A. Education and Talanta 1989, 36, 1–9.
Teaching in Analytical Chemistry. Ellis Horwood: Chichester, Laitinen, H. A.; Ewing, G. (eds). A History of Analytical
1982. Chemistry. The Division of Analytical Chemistry of
References Hieftje, G. M. “The Two Sides of Analytical Chemistry,” Anal. the American Chemical Society: Washington, D.C.,
1972.
Chem. 1985, 57, 256A–267A.
Kissinger, P. T. “Analytical Chemistry—What is It? Who Needs It? McLafferty, F. W. “Analytical Chemistry: Historic and Modern,”
The references cited in the Why Teach It?” Trends Anal. Chem. 1992, 11, 54–57. Acc. Chem. Res. 1990, 23, 63–64.
chapter are provided so the
student can access them for 1H REFERENCES
further information. 1. Ravey, M. Spectroscopy 1990, 5(7), 11. 113–119; (c) Atkinson, G. F. J. Chem. Educ. 1982, 59, 201–202;
2. de Haseth, J. Spectroscopy 1990, 5(7), 11. (d) Pardue, H. L.; Woo, J. J. Chem. Educ. 1984, 61, 409–412;
(e) Guarnieri, M. J. Chem. Educ. 1988, 65, 201–203; (f) de Haseth, J.
3. Fresenius, C. R. A System of Instruction in Quantitative Chemical
Analysis. John Wiley and Sons: New York, 1881. Spectroscopy 1990, 5, 20–21; (g) Strobel, H. A. Am. Lab. 1990,
October, 17–24.
4. Hillebrand, W. F.; Lundell, G. E. F. Applied Inorganic Analysis, John
Wiley and Sons: New York, 1953. 8. Hieftje, G. M. Am. Lab. 1993, October, 53–61.
9. See, for example, the following laboratory texts: (a) Sorum, C. H.;
5. Van Loon, J. C. Analytical Atomic Absorption Spectroscopy. Academic
Press: New York, 1980. Lagowski, J. J. Introduction to Semimicro Qualitative Analysis, 5th ed.
Prentice-Hall: Englewood Cliffs, NJ, 1977.; (b) Shriner, R. L.; Fuson,
6. Murray, R. W. Anal. Chem. 1991, 63, 271A.
3J PROBLEMS R. C.; Curtin, D. Y. The Systematic Identification of Organic
7. For several different viewpoints see (a) Beilby, A. L. J. Chem. Educ. Compounds, 5th ed. John Wiley and Sons: New York, 1964.
1970, 47, 237–238; (b) Lucchesi, C. A. Am. Lab. 1980, October,
1. When working with a solid sample, it often is necessary to 4. A sample was analyzed to determine the concentration of an
bring the analyte into solution by dissolving the sample in a analyte. Under the conditions of the analysis, the sensitivity is
suitable solvent. Any solid impurities that remain are 17.2 ppm –1 . What is the analyte’s concentration if S meas is 35.2
removed by filtration before continuing with the analysis. and S reag is 0.6? Problems
In a typical total analysis method, the procedure might
5. A method for the analysis of Ca 2+ in water suffers from an
read interference in the presence of Zn 2+ . When the concentration
After dissolving the sample in a beaker, remove any of Ca 2+ is 50 times greater than that of Zn 2+ , an analysis for A variety of problems, many based
solid impurities by passing the solution containing Ca 2+ gives a relative error of –2.0%. What is the value of the
the analyte through filter paper, collecting the selectivity coefficient for this method? on data from the analytical literature,
solution in a clean Erlenmeyer flask. Rinse the beaker 6. The quantitative analysis for reduced glutathione in blood is
with several small portions of solvent, passing these complicated by the presence of many potential interferents. provide the student with practical
rinsings through the filter paper, and collecting them In one study, when analyzing a solution of 10-ppb
in the same Erlenmeyer flask. Finally, rinse the filter glutathione and 1.5-ppb ascorbic acid, the signal was 5.43 examples of current research.
paper with several portions of solvent, collecting the times greater than that obtained for the analysis of 10-ppb
rinsings in the same Erlenmeyer flask. glutathione. 12 What is the selectivity coefficient for this
analysis? The same study found that when analyzing a
For a typical concentration method, however, the procedure
might state solution of 350-ppb methionine and 10-ppb glutathione the
signal was 0 906 times less than that obtained for the analysis
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