ROBERT REEVES Mattborough School 250 S. Rossmore Avenue Los Angeles, CA 90004
QualitativeAnalysis of Some Transition Metals Cary Kilner Exeter Area High School, Exeter, NH 03833 Mv students alwavs seem pleased to be presented with an interesting rpalitative lab after a run of the more usual quantitative experiments: not only do they get good grades, but i t is often possible to embody more dramatic and visual descriptive chemistry in that format. The instructor's task is to find an imaginative way to re-package demonstrations as full-length experiments. It seemed to me that qualitative analysis for iron, nickel, cobalt, and copper using reactions that produce colored complexes or precipitates might be an interesting prohlem for this format. In addition, the wide variety of outcomes from these reactions gives the instructor plenty of ancillary material to discuss if desired. My classroom discussions vary with the interests and talents of the current students, but I do cover a number of related topics including: alternative tests for ferric ion (thiocyanate ion versus ferrocyanide); the chemistry of the iron hydroxides, including the gelatinous nature of the precipitates, their complicated hydrolysis reactions and their reactions in boiling water; the copper-ammonia complex versus the formation of insoluble copper hydroxide when using aqueous ammonia; the pink-to-blue color change when cobalt ion reacts with potassium ferrocyanide; the nickel-ammonia complex versus the hydroxide, the reaction with potassium ferrocvanide and the unique test using dimethyl glyoxime; colloids, gels, and precipitates and separations using a centrifuge versus a filter. I n the laboratory, the students make their initial observations using the test reagents on a set of knowns. I ask them to prepare a table for recording their observations before beginning the lab. A simple grid format is adequate. I remind them to observe the colors carefullv, and to distinguish clear from cloudy and gelatinous from settling precipitates. The quicker and more inquisitive students are directed to a centrifuge; they share their more conclusive data on solution versus ~recioitationwith the class bv making.a labeled display of thei; resilts. Observations finished, each student uses his or her data chart and independently chooses what he or she considers the most conclusive test for each cation, with the option of adding another test for confirmation in the case of similarities. The next dav I prepare a set of unknowns in large test tubes and add the-test reagents. During class, I walk around the room with each test tube in turn and quiz the students on the identity of the cation present. The students use their data tables. An alternative, of course, would be to distribute unknowns for testing. However, my students find it refreshing to apply what they have done in lab to another situation. I lab directions each year, striving for a tend to rewrite
' Weast et al., "Handbook of Chemistry and Physics." section on
Special Solutions and Reagents. 80
Journal of Chemical Education
better match between my expectations and the current class. Here is last year's version. Qualitative Analysis Lab #1
In your last few labs and activities you haw sccn many n h r changes and preripitntes. In this lah you will examme some specific prcrlptates that can t w uscd to idcnt~iycertain cntims. These rcartions are sufficiently complex that you are not asked to write the equation or classify the reaction type; rather, you will he polishing your powers of observation and discrimination. Your objectives will be to determine which test or sequence of tests unambiguously identifies each cation and to use your results to identifyunknowns I will show you. An important first step is to record your obsewations so clearly that someone else could use them for his or her own analysis of an unknown in spite of the subjective nature of color perception. Begin by designing a useful table for recording your data. Read the procedure carefully and refer to it often so that you will be sure to provide room for all the observationsyou will need to record. There are solutions of four cations and five reacents. - You should describe the color of the cations, although description ofthe reagenrs can bcomitted. Runeachpossible test by miringahout one-thirdof a 13 x 100-mmtent tube of cation with an equal amount of each reagent. You can free-pourso that you do not have to wash a graduated cylinder repeatedly. However, try to keep the volume used constant since it will affect the amount of any. .precipitate and the intensity of color. For each test oak. describe the color and texture of the result after it has been shakA A d has sat undisturbed foraminute. Note whether the result is clear or cloudy. Is it gelatinous or does it settle out? Since colors are often alike, these particulars can help distinguish systems that appear to be similar. Save your results as you go and line them up so you can compare them. If you donot want tolabel each test tube, try lining them up on a labeled strip of paper. The idea is to compare results. chmine the details that d i s t i i s h one svstem from another. You do not w&t to lose track of your&st pairs bifore you can record the comparisons! Finally, pour eachtest pair into a clean 600-ml beaker and dilute it gradually with water until it is close to overflowing. Note the "holding power" of a color while the solution is diluted. Record your observation for each test pair on a relative scale. This test tells you the intensity of the colors and gives some information about the test sensitivitv. it takes to show u a .. that is. how little vrecivitate . . It ispcrfcctly ali right for you todiscuss your observationsand interpretation with others. Just rcmcmbcr that ultimately you will comhine your dam with y~~r~bser~ationstod~t~rminetheidentity of the unknowns yourself. Cations (0.1 M): Fe3+,Cu2+,Co2+, Ni2+;nitrates or chlorides. Test reagents (0.1 M): KSCN, dimethyl glyoxime, &Fe(CNk (potassium ferrocyanide),NaOH, NHdaq). bv dissolvine The dimethvl elvoxime (DMG) solution is vrevared . . 0.6 g in 500 miofi5% ethanol.' ~
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New eclures an0 aboraton, exper ments an0 dmctlons m team ng cnem sw mroqn the s e a l the laooralary are pro" ow in mm f e a l ~ l e Experments w 11 ~e Idly delalled and I oe field tested before mey are
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published. Contributions should be sent to the feature editor.
