Investigation of Experimental Parameters A Student Project for Quantitative Analysis Donald L. Campbell University of Wisconsin-Eau Claire, Eau Claire, WI 54702 Experiments typically performed in introductory quantitative analvsis courses are selected to illustrate imoortant analytical methods and to help students develop specific laboratory skills. Usually the experimental parameters for each experiment have been set to optimize response, eliminate interference, and avoid other sources of error. Despite the instructor's efforts to emohasize the many choices that have been madeindesigninga methodofanalysis,studrnts tend to follow the "wokhook" with little aoorecintion of the need for careful experimental design. Individual student projects are-often included in the laboratory work to foster student involvement in method cvaluation and optimization. One type ol' proiect is the assi~mmentof an analysis problem for which no method is pmvidcd. Thc students are asked to find suitable methods in the literature, choose the most appropriate method for the ssmole in auestion. and carrv i t out. ORen students treat this a s a n exercise in finding an alternative "cookbook" and sometimes do not have the expertise to evaluate alternative methods very effectively, particularly if the methods involve unfamiliar instrumental techniques. I t can also be rather difficult for the instructor to orovide the needed wunsel, special reagents, and equipment for a class of 20-30 students, each pursuing some chosen method of analysis. A second approach is to build a prescribed process of method development or evaluation into one or more of the experiments for the course. This approach has been applied to the spectrophotometric determination of acetylsalicylic acid in asoirin in a verv well desiened exoeriment.' In a third approach, we have adapted this in&stigation of exoerimental oarameters of analvtical methods to a studentdesigned project, as described below.
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' Street, K. W., Jr. J. Chem. Educ. 1988, 65,914-915
Experimental Parameters Project The student selects a question concerning the limits of some exoerimental oarameter in one of the oreviouslv completed analyses and then designs a n experiment to answer it. For example, in the determination of calcium in fruit juice by atomic absorption spectroscopy, the question could be "What minimum lanthanum concentration is needed to avoid phosphate interference for the phosphate concentrations typically found in the fruit juice?" The student might then design a n experiment in which the lanthanum concentration is varied in fruit juice samples of known phosphate content. I t is generally possible for students to design effective experiments to answer their questions because they are already familiar with the analytical method to be investigated. The questions the students choose may be their own or generated by the instructor. For example, students may be asked to generate a question or two for each experiment a s they carry them out. Then they choose their project from amone these ouestions. For one recent class I selected sever& questidns related to. one experiment so the class could focus on that exoeriment and comoare findines. The projects may investigate a variety of aspects of the experimental design.
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1.Determine whether specificsample components might interfere with the method of analysis. 2. Determine whether oarticular oracedures in the analvtical method might be eliminated or shdrtened to save time &d re-
agents. 3. Determine whether the method can be extended to different types of samples.
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bv Some auestions that have been successfullvinvestieated " studeits are listed in the table below. After choosing a project question and doing some reading onthe subject, the students designasetofexperiments that
Examples of Questions Successfully Investigated Experiments Determination of acetylsalicylic acid (ASA)in aspirin by acid-base titration
Determination of Vitamin C in breakfast drink powder by iodometric titration
Determination of sodium fluoride in toothpaste by ion-selective electrode
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Journal of Chemical Education
Project Questions Does the variability of measured ASA content result primarily from variability in the tablets or from the precision limits of the analytical method? HOW is the precision of the method affected by the presence of undissolved solid binder? Does its removal affectthe accuracy or precision of the titrations? Is it possible to successfullyanalyze samples with low Vitamin C content bv decreasina the concentrations of the standard reaoents ueo7 DO'S gn f cant errors resL t from leav ng ~odne sot~ton3 in contact w th a r for a per~odof I me oeforetltrat on wttn stanoaro sod d m rn OSL fatesol~ton? How ong s too ong? How isthe precision or accuracy of the procedure affected by omining pH bufferingor ionic strength control? Is it possible to extend the method to toothpastes containing sodium monofluorophosphate instead of sodium fluoride? What sample pretreatment might release the fluoridefor detection by the electrode?
will answer the question. Their question and experimental design are submitted in writing for approval by the instructor before work begins; often the instructor suggests improvements in the design. After carrying out the experiments, the students complete their report that summarizes their experimental results and states their conclusions regarding the project question. If the results are inconclusive, they provide suggestions for additional experiments. Discussion The type of project described has many different advantages. The number of possible projects is very large, and the difficulty of projects varies. Thus, students can all have a unique project, suited to their interests and abilities. Students actually design the experimental protocol to obtain an answer to a specific question, a basic aspect of the scientific process that is much too uncommon in introductory science courses. Students gain some insight into the
design of the particular analytical method they choose to investigate. This tvDe of oroiect is nractical for both laree and smaU " classes because students typically use methods already familiar to them and reagents and equipment that are already available. The instructor can concentrate on counseling students on experimental design, rather than helping them fmd needed reagents and equipment. Finally, the student projects can provide useful information that allows for improvement in the analytical procedure or a better understanding by the instructor, as well as the student, as to the practical limits of the method. In three semesters with projects of this type, it has been apparent that students take a genuine interest in finding experimental answers to their questions. Withappropriate counseling, they can design and carry out successful experiments.
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Volume 68 Number 9 September 1991
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