edlted by DUANE SELL
Palatine lilmo8s 60067
A Spectrophotometry Unit for Advanced Chemistry Students Susan M. Diehl-Jones Brother Rice High School, 7101 Lahser Road, Birmingham, MI 48010 As a four-year, college preparatory school for young men, Brother Rice High School fosters an environment dedicated to academic excellence and extensive extracurricular activities. Over 95% of the students continue their education beyond high school. The chemistry curriculum includes a 12-week st;dy of chemistry which is incorporated into the required freshman-level integrated science program and a full-year elective course, which couples rigorous, traditionally theoretical presentations with a Chem-Study laboratory approach. There is, unfortunately, little time or laboratory space left to accommodate advanced courses since our science department faculty (4112 members: Z112 biology, 1 chemistry, 1 physics) provides analogous programs in biology and physics. Yet, we have a significant number of students with solid scientific backgrounds who are both capable of and enthusiastically interested in advanced science study. Directed-study courses have evolved as an opportunity to satisfy the needs of several students who have a special interest in extending the scope of their study beyond that of a full-year science course. Our directed-study course differs from a course of independent study in that the activities, readings, experiments, and general content are specified by a faculty member who also agrees to teachlmoderate the study. This approach has allowed us to develop and revise
materials for each study, thereby providing for some flexibility in content while assuring that the students are meeting similar criteria for the credit awarded. The areas studied each year depend on student interest, faculty availability, and budgetary limitations. Topics have included qualitative analysis, organic chemistrv. electronics. and anatomv-nhvsioloev. The dikkted-study chemistry courses were Ganged so that onlv those students who had at least one vear of hieh school chemistry and had maintained an above-average per?ormance record were ~ e r m i t t e dto register for a course. T o receive one-half of an academic credit in a science laboratory course, students a t Brother Rice are recluired to have 4000 min of .
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986
Journal of Chemical Education
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a week after &boo1 for the 20-wk period. These meetings ~rovidedfor instructor lecture, Deer interaction, cooperative experimentation, discussion, and easily-accessible instructor consultation. Additional laboratory time was arranged flexibly throughout the day on an individual basis so that students could work on their own projects. A Spectrophotometry Directed-Study Course
The impetus for the choice of this topic for directed study came from the donation of a Beckman DU spectrophotometer to the Brother Rice High School Science Department. Initially, the objective was to design, develop, and test an introductory secondary level spectrophotometry program for use as an independent study. However, as I began the task of searching for a variety of spectrophotometric experiments that were applicable to the advanced secondary student, I became nninfullv > ~ ~ ~aware - of ~ the ~ lack ~ ~ of information available on the uses and applications of this important chemical analysis method at this level. The flexibility, time, and money necessary to develop such a unit for individual use were impractical, and conseouentlv the Droeram described herein is a directedstud& uniitic [ h e .o w a r i h e d i o r m . i t ('l'al)lv : { I , s r u d r n t i s i i h mitted their most "publishable" work: a comprehensive, typewritten, research report. The educational philosophy behind the student research projects could he simply stated as follows: "Unless a student can use or apply what he knows, he has not learned." I t is this author's belief that the best way to use all the knowledge accumulated over the months of lahoratory work is to apply this knowledge to a new (or unknown) situation. This application of individual might have involved a re~r~anizationlintegration skills, techniques and ideas, and/or a proposal/prediction of results. ~~
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~
Experiment I:
lo3-.The spectrophotorneter was used to determine the concentration of Cu2+ as [Cu(NH3)412+which remains in equilibrium with excess copper(I1) iodate. Only two solutions are required for this experiment. This author used 0.150 M CuS04 and 0.320 M HI03. Students prepared the solid Cu(I03)2, filtered the precipitate, and determined the Cu2+concentration in the filtrate by complexation with NH3. T o determine the C U ( N H ~ ) ~ students ~ + , had of their unknown solution. This experiment offers the following advantages: 1) It allows students to recognize the small but important steps often omitted in textbook procedures. 2) It reinforces the introductory laboratory technique of scanning. 3) It requires a minimum amount of solution preparation and laboratory time. 4) It can be done over an extended time period. 5) It allows students to gain experience in the use of a buret and in ~erforminefiltrations.
The disadvantages of this experiment seem to he (1)the apparent inability to obtain accurate results, and (2) the rather large numbers of calculations required to obtain the K , value. Table 2.
Experimentation
The following three experiments were carried out by students as a reqnired part of the course. They proved especially useful in the program for the reasons cited below. Table 1.
List of Speclfic Objectlves of the Spectrophotometry Unit
I. Data collection A. Precise mechanical manipulation of the spectrophotometer B. lndentification by scanning of the maximum absorbance band C. Measurement of any % T data D. Calculation of absorbance from % T data E. Use of an Mganized laboratory notebaak for recording of data and calculations 11. Preparation of standards from a stack solution A. Safe and precise use of pipets and volumetric glassware B. Recognition of the differences in precision of various laboratory equipment 111. Preparationof a stack solution IV. illustrationof and correlation of the principles of Beer's Law A. Proof of "absorbance o. concentration'' by comparing soi~tionsof differing concentration B. Graph absorbance data to obtain a calibration curve C. identificationof areas on a calibration curve where Beer's Law -~ mav ~,not aodv .. . Camo.na!ion 01 aala oola ned from rpeclroscopy wth 0 otnerltheoreloa. m w to oola n experimental res.11~ Awareness 01 tne .SRF 01 Beer r -a* in m e variobs lle ds of E chemistry V. Presentation of experimental work in written form which integrates science with mathematics and English communication skills VI. Extensionof the principles of spectrophotometryto new or unknown situations ~
Solubillty Product of Coppeflll) Iodate (1)
Serles of Experiments for the Directed Study Spectrophotometry Unit
De~cription Introductory Reading Spectrophotometry: Mechanics and Measurementa K., of Cu(l0dz Kinetics or Reduction of Methyl Orange by SnCI, P O P Determination(I) PO,-' Determination(11) POP- Determination Biological Fluids Chlorophyll Separationa Metallic Trace Analysis (Pb. Hg) pk, of AcidIBase Indicator Individual Project Area Legend: A-AMlytical. Physical 'Author's experiments.
Area
Initial
Modified
9 10
7 8
Final
A P P E E B OIE AIE A
B-Biological,
E-Environmental,
... 7 (for 7ilab oeriods)
O-Organic.
P-
Table 3. Format for Presentation of Each Student's Most "Publlshable" Work from the Individual Project I. Disc~ssionof the objectives of the experiment 11. Background information-including reference to literature studies Ill. Procedure, including any adjustments, adaptations, andlor substitutionS performedafterdiscovering how to make the project "work" by using the suggested literature guidelines IV. Display of the data V. Calculations (only the set-ups) and results VI. Discussion of the possible sources of error VII. Conclusion Vlll. BibliqraphyIFootnotes
Volume 60 Number 11 November 1983
987
Experiment 11: Phosphate Determination in Detergents. Ascorbic Acid Method (2) T h e objective of this experiment was to determine the amount of PO& in several detergents by comparing the relative intensities of the blue-colored molyhdate complex ion of Pod3- with a series of standard phosphate solutions. Samples were treated with ascorbic acid, ammonium molyhdate, and sulfuric acid. This experiment offers the following advantages: 1) Students are exposed to partsper million terminology and calculations, the importance of temperature and timing in color develooment. and contamination orohlems that must he avoided in trace analvsis 2) Students develop th; patience necessary for laboratory work because detergents dissolve slowly. 3) Solution preparation is minimal: only an ammonium molybdate-sulfuricacid solution and a phosphate stock solution must be made. 4) Although perhaps not the best experiment to use for trace analysis exposure, students are presented with the problem of how to make standards from a stock solution. The disadvantages of this experiment are: 1) The lengthy procedure that must be completed in one laboratory
session. 2) 'The need to wash glasswnrr with phosphate-freedpwrpent prior I U cnch lah,,r.rtury sesawn ill order I < prevent phuiphate rtjn-
tamination. 3) Thecolor sensitivity of the molyhdate complex as both boiling and cwling times affect the color intensity. 4) The inaccuracy of the results. Experiment ill: Phosphate Determination in Detergents: Tin(//) Chloride Method (3) In this experinwnt, phosphnte solutions were treated with amnit~~rium mol\.hdate. tin1111chloride. h\.drochloric acid, and sulfuric acid soLtions to generate the hlue-colored molybdate complex ion. As before, the intensity of this complex was compared to the intensity of the complex generated from standard samples. While the tin(I1) chloride method required the preparation of four solutions, the precision obtained was greatly improved over that of the ascorbic acid method. Perhaps this was due, in part, to the students'familiarity with the experiment and perhaps, in part, to the specific temperature and time directions during the color development stage given by the tin(I1) chloride procedure. T h e accuracy of this method, although improved, still left a lot to be desired. Student Research Projects Each student was required to choose an individual project. Below are brief descriptions of four of the individual projects which were completed. Project I: The Determination of Iron in Blood Serum (4) The purpose of this experiment was to determine the amount of iron in blood serum. Three solutions were prepared: a protein precipitant (trichloroacetic acid, hydrochloric acid, thioglycolic acid), a chromogen solution (sodium acetate and Ferrozine), and an iron standard. Theoreticallv, the protein .precipitant releases the iron from . serum protein- and prec~pitatrjthe pr~,teins.'l'h~chromugvo reacts with the irontll) ions to produce a sulurion whI Craphw the absorlwnce versus 1111for each uf the tenahtions: one ~rdphper us\~clcngth:twgraphs rornl. 1, Dercnniuariun of In wncentrlrmn ior +.id1solution H I each wavelength. 5) Determination of the HIn concentrationfor each solution at each wavelength. 6) Determining the In- to HIn concentration ratio for each solution.
7) Graphii the logarithm of I n to HIn ratio versus pH for all solutions and determine the pK. value for the two indicators.
In order to understand the chemical concepts and calculations reauired bv this experiment, the student was reauired to review pH, buffers, equilihri&n constant, and general acid-base equilibria. This experiment offers several advantages, namely exposure
to buffers, acid-base equilibria, and the use of pH meters. From an instructor's s t a n d ~ o i n tseveral , areas needed to be closely considered:
all measurements within a short time. 3) The numerous solution preparations may make this experiment
better suited to partners than to individual students. Project IV: Simultaneous Determination of a TwoComponent Mixture (8) This experiment was designed to verify that, in a twocomwonent mixture (for examwle, chromium(II1) and cob&(11) or nickel(I1) and permanganate), the total ahsorbance is eaual to the sum of the individual absorbances and also to determine the concentration of each component in the mixture. Several solutions, includinrr a set of standards for each component and a t least one "&known" mixture containing the ions; were prepared. Although not a difficult preparation, it does require about 2 hr for solution preparation. The advantages of this experiment are: 1) The straightforward procedure as referenced and outstanding
accuracy of the experiment. 2) This experiment can be easily completed in six lab periods. The disadvantages of this experiment are minimal: 1) Some "trial and error" is involved in determiningaconcentration for each component which will give a transmittance greater than 10%but not more than 90%.
2) The student is also required to do an extensive amount of
encounter." In addition, several indicated that having experience in collecting data and organizing it into a clear but brief report was an asset in every technicallahoratory course they encountered. The research nroiect had the ereatest long-ranee imnact. Students noted [hat the project iiqued theirUindi
