Comparison of Analytical Methods

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Alvin

L. Beilby

Pomona College Claremont, California 9171 I

Comparison of Analytical Methods

for the presentation of a unified, cohesive picture of the O n e important aspect of the education tools available to solve analytical problems. One part in analytical chemistry of chemists and others using analytical methods should be the continual emphasis of the structure which can be built upon the framework on the comparison of analytical methods so that these is the continual comparison of methods. students can learn to chose intelligently the proper A technique that can be used to keep students conanalytical methods for the solution of chemical probtinually comparing methods as they are introduced lems. with methods previously discussed is to have them preIn a recent "Provocative Opinion" in THIS JOURNALpare an extensive chart that lists the analytical methods supparting analytical chemistry Wharton ( 1 ) stated and the factors that one considers when comparing that immersed in the two broad phaxes of analytical methods. Each time a new method is introducgd the chemistry-the determination of what is in something students fill in the information for that method and and how much-is the need for an analytical chemist compare this information with the previously entered to be concerned with how to best answer these quesinformation onother methods. tions. A similar concern was voiced by Robinson (g) In the first column of the chart are listed most of the who stated that the relationship between each field currently available methods. One possible source for should be emphasized so that the student is taught such a list is the table of contents of the Fundamental how best to get the data he needs. Review issue of Analytical Chemistry. The headings With the current availability of sophisticated anfor the rest of the columns are the factors that one conalytical instrumentation including minicomputers stusiders when comparing methods. These headings can dents can lose sight of the fact that sometimes purely include such items as the type and size of sample; chemical methods or simple instrumental techniques whether the sample is destroyed; the kind of species are more suitable for the solution of some chemical determined; how good the method is for qualitative, problems unless they have learned to critically evaluquantitative, and structural analysis; what additional ate all available methods. With the continually ininformation can be obtained; the suitability of the creasing number of analytical tools available the ability method in various concentration ranges, i.e., for trace, to critically compare methods is a necessity for anyminor, and major component analysis and for high one faced with the solution of a problem which repurity analysis; the precision and accuracy; the conquires chemical analysis. venience of the method and the time required per analyMost conventional textbooks on analytical chemistry sis both when only one analysis is made and when many or instrumental analysis are organized by methods. analyses are made; and the initial cost to set-up the Usually each sectlon or chapter on a particular method method including the cost of the instrumentation. Also is written to stand by itself so that the instructor does a column can be included for a brief description of the not need to follow the order of presentation given in method in terms of what physical quantities are meathe textbook. For general teaching purposes this sured to obtain the desired information. arrangement is probably the best solution; however, A good way to acquaint students initially with the the weakness of this textbook philosophy is that the use of the chart is to have them complete rows for comparison of methods cannot be discussed systematigravimetry, titrimetry, and any other method with cally throughout the textbook. To overcome this which they may be familiar from an introductory course problem some textbooks have a brief discussion on in analytical chemistry. The inclusion of gravimetry comparison of methods or choice of analytical techand titrimetry gives the instructor the opportunity to niques at the end of the textbook. discuss the modern applications of these methods which In the author's opinion the topic of comparison of are not usually discussed in instrumental analysis textmethods is too important to be relegated to the end. books. It should be emphasized throughout the entire course For a part of the final examination the author has in analytical chemistry or instrumental analysis and given an open-book test consisting of a series of analytishould be included in both the lecture-discussion and cal problems for which the students are to suggest the laboratory portions of the course. Since textsuitable methods for their solutions and to discuss why books do not adequately cover the topic, the instructor they chose each particular method. The comparison must supply the coverage. In the sections which charts prepared by the students can serve as excellent follow the author will describe how he has supplied summaries to aid them in solving the problems. this coverage. The idea for a comparison chart was obtained origLecture-Discussion inally from an article by Melnechuk (4) which contained a chart similar to the one described. Several In a previous paper (5) the author discussed the use of the chemical system concept to provide a framework instrument and apparatus companies have also made Volume 49, Number 10, October 1972

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use of this type of chart in their literature (5-7). Siggia in his book, "Survey of Analytical Chemistry" (8) makes extensive use of comparison charts to summarize the methods applicable to different areas of analysis such as elemental analysis, functional group analysis, identification and structure determination, physical properties, separations, etc. Instructors using Siggia's textbook (8) obviously do not need to employ additional techniques to emphasize the comparison of methods; however, instructors using other textbooks should find the continual preparation of charts by students to be a useful way of stressing the importance of comparing methods throughout the lecturc-discussion portion of an advanced analytical chemistry course.

Although students will gain some feeling for choosing the best met,hod for the solution of a chemical problem from the lecture-discussion, only in the laboratory wherqthcy can actually compare the results obtained by several methods for the analysis of a single sample will they truly appreciate the factors that must be considered in the choice of a method. Again, because of the aim to provide the instructor with flexibility, the conventional compilations of laboratory experiments are usually written as a series of independent experiments. To the author's knowledge only the experiments in "Advanced Analytical Chemistry" by Meites and Thomas (9)are organized around thc analysis of one type of sample by several different methods. I n these experiments the properties of four different inorganic salts are studied by several different techniques. Unfortunately in an area t,hat is changing rapidly, this textbook is somewhat out-of-date since it was published in 1958. Hanrahan (10) discussed a series of experiments using the same unknown sample involving infrared spectroscopy, gas chromatography, and ultraviolet spectroscopy for the analysis of xylcnes. He stated that a continuing problem using the same unknown sample in several experiments tends to generate more student interest than experiments which are treated as completely separate and unrelated problems. Steinfeld (11) in a discussion of an elective projects laboratory which replaced the customary f i s t year laboratory at the Massachusetts Institute of Technology mentioned that one project in quantitative analysis was the comparison of accuracies. The suggestion was given that comparisons he carried out on a number of different methods of analyzing for a particular species under varying conditions such as the presence of interfering species. An example given was the determination of Ag+ by gravimetric, volumetric, and electrochemical methods. The author previously discussed an experiment on the simultaneous potentiometric and photometric endpoint detection in a coulometric titration (12). This experiment allowed students to compare and to choose readily between the two methods of endpoint detection. Obviously there is no one sample that can be used to compare all methods examined in the laboratory. For several years the author has been using copper-based alloys to develop this comparison approach. For 680

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small classes standard reference material samples from the National Bureau of Standards provide easily issued unknowns. Cooper-base alloys do represent actual commercial products, i.e., real samples, although perhaps not as well as others that have been mentioned (13). ' The certified values of the constituents oresent in the National Bureau of Standards samples provide both the instructor and the students with reliable checks on the experimental procedures. The author agrees with Bruckenstein (14) who stated that his experience led him to the conclusion completely contrary to a number of analytical chemists who maintain that it is necessary to make analytical chemistry more palatable to the student by disguising it as one or more minor research problems tailored to fit the time available for the laboratory course while avoiding unknowns as much as possible. Unknowns combined with thc comparison approach in the laboratory can also make analytical chemistry more palatable. Hopefully this paper will encourage other instructors to report samples which they have used and which can be utilized for the comparison of methods. Analysis of Copper-Base Alloys

In this section several different methods which have been used or are proposed for use by the author for the determination of copper, lead, tin, nickel, and zinc in copper-base alloys will be discussed briefly. The list of methods is by no means exhaustive of the available methods but is presented merely as an example of an attempt to use one type of sample for several experiments involving methods commonly included in an instrumental analysis or advanced analytical chemistry laboratory. Constant Potential Electrodeposition and Polarogrophy

One of the common experiments on constant potential electrodeposition is the determination of copper in brass. The determination of copper can be followed by the determination of tin and lead (16). Lead, tin, nickel, and zinc can be determined by polaragraphie procedures using solutions remaining from the electrodeposition analysis (16). In the instructions for the in one textbook i t is stated that soluuol~roerauhic urocedure . . . tions from a previous electrodeposition experiment can he used (16). However, no mention is made in the electrodeposition experiment that the solutions should he saved (16). The author has found it preferable to write-up one single experiment for the analysis of copper-base alloys that combines the several procedures in order to save the students from potential problems. Although lead and tin can be determined by both electrw deposition and polarography, students have been expected to report only the polamgraphic results because of the extreme care required to obtain satisfactory results for electrodeposition of lead and tin. Atomic Absorption Spectroscopy

Lead, tin, nickel, and zinc can be readily determined by atomic absorption spectroscopy (17). In some cases dilution of sample solutions are necessary in order to obtain optimum concentrabions. The need to prepare dilutions provides a. useful exercise for the student,^ to gain a het,ter feeling for the problem of establishing optimum concentrations for analyses. Students can he given approximate percentage ranges of the four constituents in their samples as an aid for the preparation of the appropriate solutions. A t the present time polrtrogrilphy and atomic absorption spectroscopy are the only two methods that the author has used in the laboratory for the determination of lead, tin, nickel, and zinc. The use of only these two methods still gives the student a

good comparison between a "clarsical" electroanalytical method and a modern optical method. The student should he able to observe readily the factors which he must consider if he were required t o choose between these two methods for the analysis of low concentrations of metallic elements in samples. Precision Colorimetry

A technique which is sometimes included in an instrumental analysis experiment is. precisian colorimetry. Generally solutions of a pure compound are employed t o show the use of this technique. Bastian (18) has reported on the determination of high percentages of copper in copper-base alloys by precision colorimetry. Although the author has not yet used this determination in the laboratory, the use of a copper-base alloy sample for a. precision colorimetric experiment would allow the comperison of two high precision methods-constant potential electrodeposition and precision colorimetry. Neutron Activation Analysis

Daly, Hofstetter, and SmidbBIeek (19) have reported on the determination of copper in alloys by fast neutron activation analysis as a student experiment. An adaption of this expzriment is included in a general chemistry laboratory manual emphasizing quantitative measurements by the author and his coworkers ($0). The experiment was performed with moderate success by students.

Conclusion

At the 24th Annual Summer Symposium on Analytical Chemistry held at the National Bureau of Standards in June 1971, the role of the analytical chemist in helping to solve important national problems in solid-state research and electronics,biomedical research, clinical chemistry, agricultural science, air and water pollution control, and oceanography was featured ($8). The continual emphasis on the comparison of methods throughout both the lecture-discussion and laboratory portions of analytical chemistry courses should help t o better prepare students t o be able t o solve problems in these areas and iunew areas as the problems arise. The aurhor ~ v o u l dlike t o w k n o \ v l e d g e rhe arsivtnnee of I'aul- 1,:. \Vhirson in t l ~ ~ : d r v t ! l o n m r n of t thc eornbincd .experiment on the determination of copper, lead, tin, nickel, and zinc in copper-base alloys by constant potential electrodeposition and polarography. Literature Cited (1) (2) (3) (4) (5)

WHARTON. H. W.. J.CREX.EDUO.,47, 58 (1970). ROBINSON, J . W.. Anal. Chem.. 10 (111.33A (19681. BEIGBT,A. L., J. C ~ B I . EDUC.,47,237 (1970). MELNEOHUK, T.,I*t. SCI. Technol., Aug. 1962,14. "Microohemical and Instrumental Anslyais." Applioatioo Data Manual ADM-70, 1963 ed.. Millipore Filter Corp., Bedford, Mssa.,

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Ion-Selective Electrodes

Although one would prohably not choose s. copper ion-selective electrode to determine the copper content of a copper-base alloy, an alloy sample can be employed to prepare an unknown solution for analysis with an electrode that has been standardized with pure copper samples. The presence of other metallic ions in solution of the alloy sample requires the students to consider their effeots on the results. lodometry

Although many students may have determined copper in brass by the classied iodine-thiosulfate titration procedure in an introductory quantitative analysis experiment (Sf), performing this titration in an advanced analytical chemistry laboratory with the aim of comparing a purely chemical method with instrumental methods can provide a. useful experience. As stated previously students can lose sight of the fact that sometimes a classical chemical method will provide the most eonvenient method for the solution of 8. chemical problem.

(6) "Microwave Spectrometer." Tracerlab. Walthsm, Maas.. pp: 6-7. (7) "Anslytical Auger Speotrometer." Varinn Vacuum Divlalon. Palo Alto. Calif.. p. 6. (8) Sroom. 6.. "Survey of Analytical Chemistry." McGraw-Hill Book Co., Ino.. New Ymk. 1968. d Chemistry." (9) MEITES.L., A N D T ~ O X A SH., C., " A d ~ ~ n c eAnalytioal MoGraw-Hi11 Book Co.. Ino., New York. 1958, pp.414608. (10) HANRAXAN. E. S.. J. CHEM.EDOO.,43,321 (19661. J. I., J. C~EI.EDUO..46,232 (1969). (11) STEINFBLD, (121 B e r ~ s rA. , L.. AND LA~oowslrr.C. A,. J. CREM.EDUC.,47.238 (1970). (131 Leer*. S., J . CREM.Eonc.,44,545 (1967). (14) BRUOHENBTE~N. S.. J. A m OBic. A n d . Chem., 50, 1046 (1967). (15) WILLARD.H. H., MERRITT.L. L.,IR..AND D B ~. I ,. A.. "Instrumental Msthods of Analysis" (4th ed.). D. Vsn Nostrand Co.. Ino.. P r i n e ton, N. J., pp. 6423. A N D DEAN,op. cit. pp. 699-700. (16) WVLARD,MERR~TT, (171 "Analytical Methods for Atomic Absorption Spectrophotometry," Perkin-Elmer Corp.. Normdk. Conn., Maroh 1971. p. MT-6. (18) BABTIAN, R.. A n d . Cham., 21,972 (1949). K. J., A N D SXIDRBLEEP,F.. J. CXEX. (19) DAT.~, P. J., HOPBTBTTER. Enm.. 44,412 (1967). (20) Sxmn, R. N.. QUINLAN. J . E., A N D B E I ~ B TA., L.. "Laboratow Manual ior Chemistry: A Quantitative Approaoh," The Ronald Press co., New York. 1969, pp. 145-9. AND BEILBI. OD. cil., PP. 99-105. (21) S r r ~ xQUINLAN, . (221 Chem. E W . Naua, 49 [271, 40 (1971).

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