An Individualized Approach to QualitativeAnalysis B l a n c a L. Haendlerl, Robert Cook, a n d G e o r g e S i e m i e n c o w Lafayette College, Easton, PA 18042 T h e r e has been considerable debate in recent years about t h e value of teachine analvsis . (.1.. 2). . Although - oualitative . rarely taught now as a separate course, i t is frequently part of t h e freshman chemistry laboratory, a n d new methods of teaching i t a r e still being sought. Several articles have appeared recently which suggest using computer simulation i n addition t o or i n place of actual laboratory experience ( 3 , 4 ) . I n this paper, we would like t o describe a new approach t o qualitative analysis which we have successfully introduced i n t o our freshman chemistry laboratory. Qualitative analysis ha; significant potential for t h e teaching of descriptive chemistry, but this potential frequently gors unrealized when standard separation schemes are used. Hs\,inp. specific directions t o follow fur every step almost insures &at many students will n o t really learn t h e chemistry involved, a n d we have found t h a t laboratory quizzes are a n ineffective remedy for this problem. This "cookbook" approach leaves little room for student initiative. Our approach involves selecting a unique set of ions for each student a n d requiring t h e student t o work out a separation scheme based on t h e results of exoerimentation with certain reagents a n d with t h e help of some reference materials. This idea has been employed by others (5-7), b u t our scheme is considerably more extensive i n scope and difficulty. W e d o feel t h a t t h e "hands-on" approach is necessary t o achieve a real understanding of t h e chemistry, a n d hence have n o t adooted t h e computer simulation approach. W e have made use bf a compute; as a n aid t o devisi& t h e various combinations of ions, however. ~~
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The Experimental Approach The set of ions chosen for possible analysis is given in the table. The ions are divided into groups according to their behavior with four primary reagents: 6 M HCI, 3 M H2S01, 6 M NH40H, and 3 M NaOH. Group I contains ions which form insoluble chlorides (plus ammonium ion), group I1 those which form insoluble sulfates, group I11 those which form complexes with ammonia, group IV those which form insoluble hydroxides, and group V those which form amphoteric hydroxides (plus arsenic). This list is neither inclusive nor exclusive, but it is simply one which we have foundconvenient. Sodium and potassium ions were not included because we felt that the flame tests did A computer program2 was written which selects and prints out as many different combinations of ten of these twenty-two ions as there are students in the class. The program selects an average of two ions from each group.3 However, it is written so that certain combinations may contain only one ion from any of thegroups and three ions from either group 1V or group V. The program also avoids selecting CaZt and SrZ+together, as they have proven very difficult to separate, and
Cations Chosen for Analysis Group I
NH~+.~ g + H&+, , pb2+ Ba2+.Ca2+. Pbzt, Sr2+ Cd2+, Co2+, CuZt, Ni2+ Bi3+. Fe3+. Hg2+, Mg2+, Mn2+ AP+. AsM, Cr3+, SbSC, Sn4+. Zn2+
Group I I Group ill Group IV Group V
it selects only one ion which forms an insoluble sulfate. At the same time, between three and seven of the ten ions chosen are selected randomly and printed out as an unknown. The experiment requires eight to nine weeks at 3 hr per week depending on class size and student aptitude. Our course has approximately 140-150 students divided into seven sections of 20-25. Although each student is assigned a specific laboratory section, we have found it essential to provide times when students can put in additional work if they wish. During the first week, the students are told which ten ions they have been assigned and are given reference sheets4 which describe the "expected" behavior of each ion with the four primary reagents as well as other important reactions. (When more than one oxidation state is important, we include the chemistry of both, but they are not reauired to distineuish between different states exceot for mercurv.) '?'he" m--. e then s&oosed to test the behavior of each of their ions with ....J ~~~-~~ each of the four primary reagents. During the second week, they perform confirmatory tests on each of their ions. The procedures for these are taken from standard texts and are also handed out. This completes the preliminary part of the experiment and the only part in which the students are given specific procedural instructions. Students are cautioned to be alert for results which do not match the reference sheets and to rely on their awn observations. The students are then given the next three weeks to devise and perform experiments which allow them to work out a separation scheme with which they can successfully analyze a mixture containing equal amounts of all ten of their ions. These schemes are handed in and checked by the instructor as they are developed. After successful analysis of thpir known mixtures, the studrnt, are ciwn their u11knownsand rhrw to i w r weeks toanalyze thern.Thq then hand i n thrlr rrwlts alunp: w~rha renort whlrh d e w l h t s in drtai. haw thrv developed their schemes. The experiment can only he successful if the students are truly working on their own. It is important, therefore, not to answer students' questions directly, hut to encourage them to find their own answers hy reading and direct experimentation. Their reference material includes the sulfide chemistry of their ions, but they are not
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Author to whom communications should be addressed. Present address: Chlorox Technical Center, P.O. Box 493. Pleasanton, CA 94566. A copy of the program is available upon request. For the purposes of the program, Ca2+ is put into group I, and Mg2+ into group II, and PbZ+ is listed in group II only. Copies of these are available upon request.
Volume 59
Number 4
A ~ r i l1982
333
allowed use thiudcetamide in their srhemm mleus they rannot *para& certain iunv without it. Thev t'req~~~nrl? d~scoverthat many confirmatory tests can be performed without prior separation. We allow this as long as they have experimental verification. It is importent to follow the progress of individual students closely toavoid tm much wasted time. We have found our approach to he significantly more demanding of the instructors than the ~cookhwkmschemes,but it is camoletelv manaeeable and the laboratorvsessions are not cha" otic.
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Preparation of Solutions and Unknowns I t is nrcwsiiry to haw solutions of all the cations available. We usrd 0.2 .A1 sulutions uf the nitrate salw (or therhloride iithr nitrate was nor awilnhlr). Studen- were csutrmed ru note if they were using a chlor~desolutiun in order t u anticipate poasible prrcipilarion reactions. It is necessary to use sulutim~sof 5n4r, As5', and Sb'- to prevent an oxidation-reduction reaction if Ha2" is also prrsrnr. Smre rh? final known mixture contains ten ions, students were also told to keep aeonstant dilution factor of 1/10 if they usedmixtures of fewer than ten ions in various experiments and tests. Since each unknown was different, they had to be prepared individually. This was accomplished by filling unknown bottles from a series of burets, each of which contained a solution of one of the ions. Since our bottles contained 30 ml, 3 mlof each required ion solution was dispensed and the remaining volume filled with distilled water to give the proper dilution factor. Six milliliters of certain ions which were expected to be particularly difficult (Al,Ca, Sr, and Zn) was used. The preparation of unknowns was done by the individual laboratory instructors. Grading We assigned 50 points to the unknown results and 50 to the report. T h e reports were graded by individual instructors as they saw fit. Generally, the quality of thereports was excellent. The unknowns were graded according to the frequency of the errors made. All mistakes from the class were tabulated and a table of the frequency of each error constructed, A certain of points related to this frequency was then taken off for each mistake. One difficulty with grading this experiment is that the students have turned in only one unknown analysis and areport for %9 weeks of what for most of them has heenvery hard work. Because of this we felt it advisable to establish a minimum grade of 70. The average grade for the experiment was 89. T h e students seemed to feel that this method was fair. Results and Student Response T h e average number of errors (both as ions missed and as
334
Journal of Chemical Education
ions reported in error) was 1.9 per student. In the past, we have done qualitative analysis in this cuursr by the standard sulfide results on a general unmeth& and have ty&cally known of 1.5 errors per student, considering the fact that students using the new method had to work out their own schemes and did not have the benefit of doing four individual group unknowns before the general, we are by our results. T h e response of the students to this approach has been overwhelmingly enthusiastic. They were asked to comment on 1he value uf the experimrnt in their reporti, and many of them did so at length. Although many expressed frustration at the difticultips uf workinp.out their own schemes,. thev .alro expressed a great sense of'a~corn~lishment a t finally overcoming these obstacles. Many students described strong positive feelings about being ahle to attack and solve a problem independently. It was particularly gratifying that some students were ahle to work out improvements in standard methods which were quite successful. It was also obvious that the discrepancies between what the references said should happen and what actually did happen gave them a much more realistic impression of scientific experimentation than most standard laboratory experiments. It was very clear that many more students had genuinely mastered the chemistry of their ions by the end of the experiment than was the case when we used the traditional method. Summary We feel that our approach to qualitative analysis has sigOne of the standard nificant advantages Over schemes. It provides the students with a genuine experience in individual and independent problem solving, hut one which is not beyond their abilities and which produces good results. We also feel that this method produces a real understallding of the descriptive chemistry of some of the common cations and their reactions' Literature Cited Kr,n,,,,, (21 ~,,k,,
D.
(19751.
EDUC,,13,419 i19 66), .I?., H. F., d..
w.. H.M.V,
J, D..
J. cmM. EDUC, sz, 581
,,J, CHBM, EDUC,,50,556,19731, ( I ) B ~ ~ ~B., F J.I C , HEM. ~ouc., 56.108 (19791. ( 5 ) Thompmn,M. L..and Bix1er.J. W., J.CHEM. EDUC..48.113 (1971). (6) O'cnnnor, R., and woelfd, W. c., reem em an ~ i b r a r yof Laboratory Separates in Chemistry: W. H. Freemen Company, San Francisco. 1371. Vol. 11, No. 1121. (7, 0 ~ h ~ d t . E., c . J.CHEM.EDUC..S~,~~~ (19741.
