Student Comparisons of Analytical Chemical Methods in Undergraduate Chemistry Courses A l k M. Harrison and Kenh E. Peterman York College of Pennsylvania, Country Club Road, York, PA 17403 Chemistry curricula do not always include a "wet analytical" course. Arguments for its inclusion often rest on the nremise that an additional treatment is needed to anchor analytical concepts learned at the freshman level. Opposing views contend that this should not he necessary. Our juniorlevel "wet analytical" course provides the opportunky not only to anchor but also to extend the freshman analytical exposure. In addition, it teaches the relative merit of different analytical methods by requiring the student to compare wet with instrumental procedures. To do this, the students must use statistics to determine whether two methods really give comparable results, and they must also evaluate the advantages and disadvantages of the analytical methods as well as obtain data. Some nrocedures must also be compared in terms of economics. do often we present students with a nrocedure (more or less well outlined) with no opportunity consider when it might be applicable, what it ;or them costs in time or equipment, or how these costs and applications relate to needs for routine quality control versus sporadic use in research. Two sets of experiments comparing chemical analyses are a part of our "wet" course, and two more sets have been added to the instrumental course that followsit. In the latter course, different instrumental methods are used and compared for the same determination. This illustrates to the student that more than one instrumental method may be applicable and that each has inherent advantages and disadvantages in the resolution of a scientific question.
.
Experlment #l-Comparlwn of the Gravimetrlc Determlnation of Sulfate wlth an lndlrect Method by Atomlc Absorption Following the traditional precipitation and filtration of barium sulfate using a known excess quantity of standard barium chloride solution ( I ) , the filtrate and washings are quantitatively transferred to a l-L volumetric flask and diluted to the mark. A set of atomic absorption standards are prepared from a commercially supplied barium solution (1000 ppm) by appropriate dilution. The absorbances of the standards are read on the atomic absorption spectrometer, and a plot of absorbance vs. concentration is obtained from the instrument. The concentration of barium remaining in the unknown sample is determined, additional sample dilutions being performed as needed to bring the unknown solution into the concentration range of the standards. Percent sulfate is calculated from the difference between the original quantity of barium added (also verified by use of AA analysis) and that remaining in solution after the precipitation. The result from the atomic absor~tionanalvsis is com~ared -~~~ with that from thegravimetricanalysis tosee whether sratisticallv comoarable results have been obtained. In addition, the iethods are compared in terms of cost and technician time. ~~~~
~
~
Presented in palt at the 195th American Chemical Society Meeting. Los Angeles. Sept. 1988. 772
Journal of Chemical Education
Experlment#2-Comparing Determlnatlons of Iron Ore by Cerlc Ion Tltratlon Using an lndlcator, Potentlometrlc Titration, and Atomic Absorption Spectroscopy Determination of iron in ore by ceric ion titration is a wellknown analytical procedure (2)as is the potentiometric titration (3).We have the student prepare nine samples: three for each titrimetric analysis and three for atomic absorption analysis. For the AA analysis, the samples are dissolved, but the iron(II1) is not reduced to iron(I1). The iron(II1) . . sam~le solutionsarediluted to theappropriateconcentration ranhe, and the analytical results obtained as in the analysis for barium. The re~ultsfrom each method are compared statistically as well as from rhe standpoint of cost and technician time. A similar experiment can he performed comparing the well-known iodometric titration of copper in brass (4) to an atomic absorption procedure for copper.
Experlment #3-Enzyme Analysls by CW NMR and by UV Instrumentation Several years ago, we developed a method of analysis for a biologically important enzyme using a 60 MHz continuous wave NMR (5).The reaction involved is
A stock solution of awlase I is prepared in 0.1 M ~otassiurn dihydrogen phosphate buffer.~ahplesof this stock solution are diluted with buffer to produce a ranee of enz-me concentrations. The resulting sohtions containing diffe;iug quantities of enzyme are respectively added to a set of vials containing fixed quantities of substrate. Following incubation of the samples for 30 min a t 25 T, the enzyme is heatdenatured by placing the vials in a boiling water bath for 10 min. The proton NMR of each sample is observed on a 0.5-ppm sweep in the region 61.5-2.0 downfield from TMS. In the spectra shown in the figure, the acetyl protons (B) and SCH3 protons (A) of N-acetyl-L-methionine are observed to decrease in intensity with concomitant appearance of peaks for the acetyl protons of acetic acid (D)and SCH8of methionine
(C). The extent of hydrolysis is determined by comparing the integrated area of peak D with the sum of peaks B and D. A linear plot is obtained for percent hydrolysis versus enzyme concentration. The slope of this plot, micromoles of substrate hydrolyzed per milligram enzyme per 0.5 h permits direct calculation of the unit activity of the enzyme (a unit is defied as micromoles of substrate hydrolyzed per milligram enzyme per hour). In the UV method (6). a sinele solution of enzvme and a single solution of substriiteare separately prepared in buffer and added to a cuvette such that the substrate is in large excess. The cuvette is gently inverted several times and the decrease in absorbance over 4 min is read at 238 nm. The instrument can be used to plot the change in absorbance with time. The slope of this plot represents a dynamic reaction and can be used to calculate the unit activity of the enzyme. Comnarable results are obtained from the two methods. ~ o w e k rthe , procedures require good laboratory technique and instrumental skill, especially for the NMR. From these experiments, students can see the analytical canabilities of NMR in biochemical studies in addition to thk traditional UV-visible technique. Experiment #4-Comparison of the Slmunaneous Analysis of Two Components by Vlsible Spectrophotornetry with Their separate Analysis by Atomic Absorption
The concentrations of dichromate and permanganate in a two-component mixture are determined by simultaneous analysis using visible spectrophotometry (7).For atomic absomtion. a series of at least three standard solutions is nrep k e d frhm commercially available stock solutions (i000 -D- D ~for) both chromium and manaanese. Standard nlots are obtained, and the sample is dilute2 until the conce&rations of dichromate and permanganate are in the range of the
standards. The unknown concentrations of chromium and manganese are then determined. To compare the results of these two procedures, students must convert the parts per million of chromium and manganese obtained via atomic absorption to molarity of dichromate and permanganate obtained via visible spectrophotometry. This exercise provides valuable experience in crossreferencing concentration units. Conclusion
We believe that comparative analytical determinations challenge students to consider the advantages and disadvantages that different analytical methods have to offer. Such considerations lead to a better understanding of analytical procedures and help students to integrate more fully the laboratory activities into the theoretical material of the course. Acknowledgment
This material is based upon work supported by the NSF College Instrumentation Project under Grant No. CSI8650418. The authors are grateful for this support, which enabled the purchase of a Varian Model DMS 200 computercontrolled UV-visible spectrophotometer and a Varian Model AAJOB Atomic Absorption Spectrometer, the instruments used for the experiments outlined above. 1. S k w , D.A,: West,
D. M.Fundonor.tilob of AwlyLicd Chemistry, 4th ed.; Saundem: Philadelphia, 1982:pp 724736. 2. Day, R. A,. JI; Underaaad, k L. Qwmtitdiue A m h i s Lnborotary Monrrol. 5th ed.: PrenfirrHall:E n g l a d Cliffs. 1986: p 80. a. Rsf2.0 1M
Volume 66
Number 9
September 1989
773
