Leon N. Klattl ond James C. Sheofer2 University of Georgio Athens, 30602
Changing Student Attitudes about Quantitative Analysis Laboratory
All advances in the sciences ultimately rely upon the acauisition of reliable exoerimental data and the ahility to Lse this data to achikve understanding through cesses involving auantitative reasoning. Analytical chemistry, by its v&-nature, offers a unique opportunity to convey this concept, and its corollary of actions based n w n understandine. not onlv to chemistrv students. hut aiso to those in related discipknes. ~ o w e v & the , laboratorv nortion of ouantitative analvsis. . . where this idea should be most clearly demonstrated, entails risks of two extreme types. At one extreme, students are often intimidated by the high level of performance expected, and are often hewildered by the mass of experimental detail. At the opposite extreme the idea of "reliahle data" disappears and the lahoratory degenerates into an undisciplined hodgepodge of "relevant" exercises lacking any objective criteria for judging student performance. The classical inorganic analyses are susceptible to criticisms of the former type, while many attempts to make the laboratory interesting via "relevancy" suffer due to the latter failing. This paper describes results of continuing efforts a t the University of Georgia to design a quantitative analysis laboratory which is relevant to the student's career ohjectives, displays a sense of reality in terms of current analvtical oractice. and orovides reliable and ohiective criteria for jndgini student performance. In accordance with these guidelines a carefullv selected set of inorganic, organic,'and hioanalytical experiments have been developed. The general design of this lahoratory is presented, along with our experience in its operation, and student response to this effect. It should he pointed out that this was an across the board Dromam aimed a t reaching all students taking quantitative analysis. Although our-aporoach has been reasonably successful others should also work, and a t least one such variation has been reported
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,.,
(1).
The makeup of the class is probably typical of that found at most universities of comparable size without a School of Eneineerine that offer a auantitative analvsis course. The &stribution with respect' to major and ciass standing is summarized in Table 1. The large proportion of graduate students are primarily those from the School of Pharmacy. Organization of the Laboratory The lahoratory is divided into two parts as outlined in Table 2. The first segment includes a preset series of core experiments which all students complete. The second part consists of a series of sets of experiments in which the student in each case has the option of selecting from two possible experiments, each demonstrating the same technique, employing the same general method, and of approximately equal difficulty. The core experiments serve to instruct the student in proper equipment usage, the principles of operation, and the limits of accuracy to he Presented in part at the 165 National Meeting of the American Chemical Society, Dallas, Texas, April 1973. To whom correspondence should be addressed. ?Present address: Mohawk Valley Community College, Utica, New York 13501.
Table 1. Distribution of Students Taking Quantitative Analysis According to Major and Year in School D i a t r i b ~ t i ?b~y Major.
Freshman Sophomore Junior Senior Graduate Date for four academic quarters. " 280 rrspnspa. 1231 responses.
Table 2. General Organization of the Laboratory The Core Elprrimrnls The Analytical Balance ( 2 ) Report: Diaerence between W e i g h i " b y Differen-" end "by Addition" Volumtric Calibration ( 2 . 3 ) Report: Reading of Practice Burets Glavimetric Chloride (3,4 ) Report: "?, CI in Unknown Sam le Aeid-Rase Preparation and ~tanda&tion ( 2 , s ) Report: Compare Independently Standardized NaOH and HCI Titration of Soda Ash (3) Report: % N ~ s C Oin I Unknown Sample The E*porimenfol rrptrons Potentiometric Titrations: Weak Acid or Total Cation Content Repan:pKo and Equivalent Weight or meq/g Complexiometric titration: EDTA Milk Powder or Calcium Carbonate Report: % CaO or 7 . C a Redox Titrimetry: Aacorble Acid or Iron ore Report: % Ascorbic Acid or % FeOa Visible Absoqtion Sp~trometry:Blood Glucose of Mn in Steel Report: Glucose Content as mg/lOOm!or % Mn Trace Analysis: Fluorimetry or Hg by Dlthlzone Report: mg Thiamin or mg H g w )
expected in gravimetric and titrimetric analyses. During these initial exercises considerable emphasis is placed upon the importance of systematic preparation, performance, and record keeping so that the mood of the course and student habits are quickly established. In experiments one, two, and four an opportunity is available to develop manipulative ability. These simple procedures may he repeated without penalty until the techniques are mastered so that the student can develop self-confidence in working to the desired level of competency. Gravimetric chloride is included in the core experiments not only for technique, hut also because meticulous attention to detail guarantees good results. Further, we have found that when the student understands the relations hi^ hetween gravimetry and colloid chemistry the response to the gravimetric chloride experiment is greatly improved. ~ F t e rthe students have mastered-the techniques and basic principles illustrated in the core experiments they proceed to the section of the course involving the experimental options. We have recognized that students come to quantitative analysis with different career objectives so that different types of samples might be chosen for analysis. Furthermore, the idea of giving students some part in selecting their program definitely helps to improve interest and student response to the lahoratory by making their effort more meaningful. The experimental options are listed in Table 2. It should be noted that the basic procedures are available in the literature for all the experiments except the ascorbic acid determination which is Volume 51. Number 4, April 1974
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presently in preparation (5). Specific copies of handouts used at the University of Georgia are available upon request. For each set of experiments in the optional group, care has been taken to ensure that the difficulty and the time required for execution is the same in each case. Further, the exnerimental write-uos differ in leneth hv " no more than one page out of an average of four pages per experiment in each set. This minimizes bias in student choices on the basis of length or expected difficulty. The laboratory consists of 18 3-hr periods plus one period devoted to lahoratory cleaning and check out. This is sufficient time for the average student to complete all of the work, but does not allow for either wasted time or repeating any work after the core techniques are mastered. This is a closed lahoratory,.meeting at specified times, the philosophy being that an excess of time tends to condone poor preparation and sloppy results. The experimental results are usually due approximately one week after completion of the experiment and late reports are penalized a t the rate of 5% per day late. These guidelines tend to encourage good work habits, which we feel is an important aspect of the course.
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Experiments
The core set of experiments are well known procedures, and the reader is referred to the indicated references for details. Solid samples for the core set of experiments and for the classical inorganic options are available commercially (Thorn Smith, Analytical Materials, Smith and Underwood Laboratories, 1023 Troy Court, Troy, Michigan 48084). However, for the other experiments the problem of obtaining stablesamples of known and homogenous composition requires further comment. This as well as a hrief descriptionof each optional experiment is given helow. Potentiometric Titrations Determination of a Weak Acid (6-8). This is a potentiometric titration using a p H meter with glass indicator and calomel reference electlodes. Unknown solid pure weak acids are dispensed, and the student is to determine the pK. and the formula weight from a plotted titration curve. The samples are currently under revision and will consist of weak acid mixtures such that all samples will he of approximately equal difficulty, and the "guessing game" inherent in the use of pure compounds will be eliminated. Total Cation Content (9). This is an ion-exchange experiment employing potentiometric titration to determine the equivalence point. The p H meter is required since some of the unknowns may he salts of polyprotic acids. Soluble salts of weak acids or inorganic salts which possess a stable weighable form should he dispensed as samples. Complexiometric Titrations Calcium in Milk Powder or in CaCOa by EDTA (2, 1011). These experiments are carried out almost exactly like the classical titration of Ca in CaC03 except that the milk samples contain less total calcium. Cyanide is added to complex imn. Magnesium is added to the EDTA solution so that either calmagite or eriochrome black-T may he used as indicator. For a parallel procedure hut for calcium in liquid milk consult the experiment reported by McCormick (12). The nonclassical option of calcium in milk powder requires a stable, uniform sample of known composition which dissolves completely during the titration with EDTA. For this purpose a special non-fat milk powder such as that sold in health-food stores is far superior to ordinary milk powder, preferably a product that has been "spray dried." The percentage of calcium in this product varies hut is approximately 1.5%. A series of samples of 240
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varying composition is prepared by adding known quantities of calcium lactate to the milk powder. For a 300-g sample of milk powder about 0.82 g of calcium lactate increases the percentage of Ca by about 0.05%. These samples must then he thoroughly mixed by continuous tumblingfor a t least 24 hr to ensure uniformity. Redox Titrations Determination of Ascorbic Acid (5). This is basically an iodometric procedure (3, 9) in which a measured aliquot of standard KI03 solution is added to an acid solution containing excess KI and a weighed amount of the unknown sample. The liberated iodine oxidizes the carhoncarbon double bond in ascorbic acid. and the excess iodine is titrated using standard sodium thidsulfate. Since ascorbic acid is susceptible to air oxidation, mecial care must be taken in preparing a stable solid sample. Current work indicates that an intimate mixture of boric acid and reagent grade ascorbic acid stored in brown glass bottles does not suffer any significant decomposition over a period of at least eight months. The present series of unknowns vary from 46.48 f 0.93-57.24 f 1.26%; the confidence limits being reported a t the 95% level. Samples should be tumbled continuously for a minimum of 24 hr with occasional vigorous shaking during the first few hours to ensure homogeneity. Determination of Iron in Iron Ore (3, 9). This is a standard classical exneriment involvine dieestion.. nrereduc. tion, and titration with dichmmate. However, the dichromate is standardized directlv using Mohr's salt which greatly reduces the amount of aork in&ved.
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Visible Absorption Spectrophotometry Blood Glucose, Colorimetric. The analysis of glucose is done calorimetrically using a Spectmnic 20, and is essentially the Somogyi-Nelson Method (I3), hut with the modifications that the standard glucose stock solution contains 500 mg glucose/100 ml and 1.25, 2.50, 3.75, 5.00, and 7.50 ml portions of this solution are taken and diluted to 25.0 ml with water to give standards of 25, 50, 75, 100, and 150 mg glucose/ml of solution, respectively, for use in preparing the standard curve. The dilutions can be performed in 18 x 150 mm Pyrex test tuhes calibrated by the student during the experiment. If a piece of Scotch magic tape is wrapped around the top portion of the test tuhe, the 25.0 ml level is easily marked. In our laboratory we use standard size centrifuges available in qualitative analysis laboratories which accept tubes no larger than 10 x 100 mm; so in order to avoid a second transfer of solution we have increased the concentration of sodium hydroxide from 0.08 M to 0.40 M and use 2 ml instead of 7 ml called for in the standard procedure (13). The total volume of each tuhe is therefore 5.0 ml instead of 10.0 ml. Otherwise the procedure is as referenced. This procedure gives a good calibration curve with extreme points at about 80% T and 18% T so that samples in the normal physiological range of 50-125 mg/100 ml glucose will fall in the region of maximum accuracy of the calibration curve. An alternate hut closely related procedure is given in reference (14). This latter procedure may also need revision to suit the needs of a particular situation. Many people who perform a blood glucose analysis ohtain samples from hospitals. In this case neither the student nor the instructor has an objective means of evalnating the results. Even if samples have been previously analyzed by a clinical lahoratory, by the time the student ohtains the sample the glucose content will have changed due to the ~ersistenceof natural enzymatic processes even in stabilized samples a t reduced temperatires. More reliably known samples of artifical stabilized serum can he prepared using reagent grade glucose, egg albumin as a protein fraction, and adding color with food coloring.
In experiments performed a t the University of Georgia a standard is prepared by dissolving 2.500 g of reagent grade glucose in a&ious saturated benzoic acid and diluting to 1 I with saturated henzoic acid solution. The protein fraction is prepared by dissolving albumin in a 3% aqueous NaCl solution. About 3 g albumin/100 ml saline solution is swirled for an hour and then filtered to obtain a clear pmtein fraction. This solution foams rather badly so that a few drops of a silicone antifoaming agent may be pmfitahly added. Individual samples ranging from 50 mg/100 ml-100 mg/100 ml glucose are then prepared by pipetting ~ o r t i o n sof the standard into 100-ml volumetric flasks. T o each volumetric flask 2.00 ml of the albumin solution and a ~ ~ r o x i m a t e lone-half v ml of colorine aeent (ratio of vell& to red fo-od coloring 15:l) is addid a n d the solut~ons are then diluted to volume with saturated henzoic acid solution. Individual samples should be about 3 ml dispensed in sterilized test tubes. Care should be taken to keep everything sterile. Mouth pipetting can be a disaster in this regard. The samples, if kept frozen until used, will remain stable and of known glucose content for a t least three weeks. However, no study has been made of long-term stability. The use of an antifoaming agent not only facilitates preparation of the samples but also makes them easier for the students to handle so that more satisfying results are obtained. I t should also be noted that the saturated benzoic acid should be saturated a t about 0-5°C; otherwise a great deal of excess benzoic acid will be present in the student samples causine difficultv in ~ i ~ e t t i n e . ~ a n ~ a n e in s eSteel, dolor%etrie (3, lo). This is a classical determination presented in many textbooks. One should take care to obtain powdered metal samples which tend to be readily soluble, rather than hearing balls or turnings which are difficult to dissolve. Trace Analysis
Trace Analysis by Fluorimetry. T o date no completely satisfactory fluorimetric analysis has been found which is hoth highly interesting and gives reliable results in the hands of the average student. We have attempted to perform the analysis of thiamin (Vitamin BI) by a fluorimetric procedure which involves the reaction of thiamine with Hg(I1) (15). Although this experiment requires the use of a constant temperature bath a t 40°C. this is not the critical point and should not deter any one from trying it. Rather, the problem seems to arise from the extreme dilution required in obtaining solutions of the proper concentration range when one begins with a solid standard and solid unknown. Cument work centers around the determination of salicyclic acid and/or aspirin in alkaline solution (16) but no student results are presently available. This work is being
Table 3. Student Selections on the Experimental Options (96) Weak Acid
85.5
Milk Powder Calcium Carbonate
82.4
A s m r b i ~Acid Imn Oxide
94.4 5.6 92.1
Total Cation Con-t
14.5
17.6
Blood G l u m Mn in Steel
7.3
conducted using a Coleman model 12C filter fluorimeter. This instrument with necessary accessories cost a h a t $700. The problems experienced have not been traceable to an instrumental source. Trace Amlysis for Mercury. This is,a colorimetric determination employing in chloroform to extract . ~dithizone . and concentrate mercury as the mercury dithizonate. In order to eliminate some of the problems inherent in carrying out trace determinations, e.g., surface adsorption and contamination we have prepared a single mercury solution (10 pg/ml Hg(JI) in 0.5 M H2SOr) to be used hoth by students to prepare calibration curves and by instructors in dispensing specified volumes as unknowns. The procedure is further modified from that given by Kennedy, et al., (17), such that only one 250-ml separatory funnel is required per student and the total experiment can be performed in one laboratory period. Student Results and Response
Although the expected accuracy of the usual inorganic experiments has been studied in some detail and typical results published, it remains to he established whether the organic determinations compare in terms of accuracy and precision. A comparison of student data taken over two successive quarters (Fig. 1) shows that in fact the ascorbic acid determination is comparable to the classical carbonate determination. The results for the analysis of milk powder, (Fig. 2) while slightly biased toward positive deviations have a smaller relative standard deviation than the ascorbic acid determinations. It must be noted that since the soda ash samples are significantly different in percent purity than are the milk powder samples, the percent relative standard deviations should be compared in Figure 2. These curves and statistics are obtained by comparing individual student reports to the average of the class for each unknown sample. The plots were derived from histograms of the deviation of individual student's reports from the student average on all the samples. Statistical values are averages of calculations performed on SOD1 1%
M l L I POWDER
77 Rsporls
102 11.W.lS
5 . 1.21 % N a p , R c l Sld D l " .
5 .0034
%Eo
RcI 5.0 O w .L.55%
206%
DEVIATIONS
DEVIATIONS Figure 1. Student results obtained with Na2C03 and ascorbic acid sampies. Both series of unknowns have an assay of a b u t 50% with essentially equal ranges. The dotted lines represent the +ls and +2s values. respectively.
Figure 2. Student results obtained with Na2C03 and calcium in milk powder. The Na2C03 unknowns have an assay of about 50% while the milk powder samples have an assay of about 1.7%. Data reported as absolute deviations in terms NasCOa and %Ca, respectively. Dotted lines represent the +ls and +2s values, respectively.
Volume 51. Number 4, April 1974
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each set of data for reports by students on a particular unknown sample. The composite average of the student's reports presented for the ascorbic acids and milk powder samples, after dropping outlines a t the 90% level, are within the 95% confidence limits based upon quadruplicate analysis of each sample by instructors in the course. In the case of milk powder samples the correlation between student mean and the accepted value is best for the samples to which a larger amount of calcium lactate was added. Generally, student response has been excellent as indicated by improvements in attitude and student evaluations of the course. However, the most interesting indication is derived from the choice of experiments as shown in Table 3. Although this may not be surprising, it certainly lends support to the idea that students do, in fact, prefer to conduct experiments using organic and bioorganic samples which appear to them more intimately related to real life.
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Journal of Chemical Edocafion
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Brsms*dt, W. R., Korfmaehu. ,,m,s \Lo,",.
W. A,,
Lwofl, T., 3. CHEM. EDUC.. 50. 252
(2) Harris, W . E.. and Kratochvil, B., "Chemical Analpb: An Intensive lntmduction foModern Andpis," Barnes aod Noble, I"".. New Ymk, 1970. (31 Bladel. W. J.. and Mcloeha. V. W.. "Elsmcntw Quantitative Analysis.'' 2nd Ed. Hamor 8"dRo.. NovYork. 196. (4) Shcaffer. J. C., Department of Chemistry, Univemity of Gangia, Athens, Georgia 30602. (unpublbhedl. (5) Bailey, David. Department of Chemistry, LDbanon Valley Collage, Annvilla, Pa.. 17W3. locmnalcommunicathn). (61 P e e s o k , ~ .L., "ExprimcnU i n ~ o d e r nMethods of Chemical Analmi8.l. John Wiley and Sons,Inc.. New York. 1968. (7) Stunoek, P. E.. J. CHEM. EDUC., 45,258 119681. (8)MaiM, L., 3. CHEM. EDUC., 49,682 (19721. (91 Fischer. R. B., and Peten. D. G.. "Quantitative Chemical *ns1,mis: 3rd Ed., W. B. Snunden Co.. Philadelphia, Pa.. 1%8. (101 Fleschka. H.A,, Barnard Jr.. A. 3.. and S f u m k . P. E., "Quantitative Analytical Chemlatry, Short lntrduction fa Practice... vo1. n. Barnes and Noble. I"
