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IDENTIFICATION ANALYSIS, A STIMULATING REVIVAL OF QUALITATIVE ANALYSIS' RICHARD E. PRANK University of North Dakota, Grand Forks, North Dakota
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AT NO point is the student forced t o make a real 'hoice as to what he should try next." R. K. Summerhell's2 statement, quoted here, well characterizes the main shortcoming of our usual qualitative analysis courses. Strangely enough, it has not received much attention from critics and reformers in recent years. Rather, the role of the chief villain was undoubtedly played by hydrogen sulfide. We have seen a number of expurgated versions proposed to deodorize the classical but, oh so smelly Bunsen-Fresenius cation separation scheme, by the use of such compounds as thioacetamide which release gradually, upon heating, the same H2Sstill needed to effectthe group separation. More recently, a new nousulfide cation separation has been worked out by P. W. West and co-workers,3 based upon selective precipitation of hydroxides from buffered solutions. I n every instance, the cation separation and classification scheme has remained the central theme of the qualitative analysis course. Since most students, by 'Presented a t the 130th Meeting of the American Chemical Society, Atlantic City, September, 1956. R. K., G. LESTINA,L. C. KING, H. M. NEUSUMMERBELL, MANN, J. CHEM. EDUC., 32, 475 (1955). WEST, P. W., M. M. VICK, A. L. LEROSEN,"Quditative Analysis and Analytical Chemical Separations," The Macmillan Co., New York, 1953.
VOLUME 34, NO. 8, AUGUST, 1957
far, are taking only one semester, or even one quarter of qualitative analysis, they never get much beyond that cation analysis scheme in a standard course. They find themselves pretty much in the situation of the well-shepherded group of tourists who dare not step off the trail laid out for them, lest they he eaten by head hunters or-Heaven forbid-miss one of the "required" three-star sights. Just as the thoughtful and imaginative traveler has long discovered that there is more fun to be had off the beaten path, the intelligent chemistry student deserves to be liberated from his all too narrowly guided tour through inorganic chemistry. Let us admit that he has been only mildly enthusiastic about his analytical exploits: they start with a nondescript liquid, dumped together for him by an assistant and hearing no easily recognizable resemblance to any reallife analytical sample; they continue with that leftfoot, right-foot, left-foot, right-foot march along the narrow pavement of the textbook's separation scheme, relieved with occasioual sightseeing when one of the cations is being separated and confirmed. At the end of these doings, our chemistry student formulates his findings that the sample contains, among possible others, such ions as he tested for and found present. A fragmentary result, he feels, without much significance to him or to anybody else.
THE CONTRAST OF PRACTICAL ANALYSIS
Any chemist who has performed a fair number of practical qualitative analyses, or has watched others doing so, will know that this activity proceeds in a much freer style than the usual course work just discussed. The analyst examines any record, label, or letter connected with the sample and inspects the material as to looks, smell, feel, weight, etc. If, by this time, he has already an idea what to look for, he may be seen grabbing a reagent bottle and making some specific test which, according to orderly procedure, he is only entitled to try two hours later. If he has no leads yet, he will look for them by subjecting the unknown substance to a few simple tests and closely following everything that happens. This is usually when onlookers stop talking, because they become aware of the suspense of the situation and the concentrated mental effort being made to solve the problem. What happens from here on is largely a question of the analyst's ability to observe keenly, to interpret correctly, and to make the right decisions as to which elimination and confirmation tests he should carry out next. If he is a good man and has a lucky day, chances are high that one or two little tests, well performed and fully exploited, will tell him most of the story, and he has only to insure himself against error with a careful follow-up of confirmatory tests. This necessarily sketchy, much too generalized and somewhat overoptimistic picture of the activities of a practical qualitative analyst was drawn here, not to try to explain away the intricacies and pitfalls of qualitative analysis, but rather to show how much the actual praxis of qualitative analysis and the academic course content of the same name have grown apart, and to give an idea how much could be gained on the academic side from adopting a similar lively and unprejudiced approach. Naturally, the time available for an elementary course in qualitative analysis is limited, and the adoption of a new course content necessitates the elimination of a large portion of the time-honored curriculum we are having now. This author contends that the part most worthy to go is precisely the systematic cation separation scheme, with or without H2S, either not to he taught a t all in the elementary course, or to be relegated to the end of it, mayhe as an extra project for the superior student. I cannot see too great a loss in that. Interesting chemical facts are where we look for them. We have become used to so mauy hitching posts along that one road that we doubt there will ever be one like it again. Those teachers to whom the hitching-on of extraneous materials has become the sole excuse for traveling the road of qualitative analysis a t all, can, of course, not be expected easily to let go from the scheme. But the many others who see enough intrinsic values in qualitative analysis itself to justify its teaching will not lightly dismiss this alternative. It would be futile, indeed, to argue that a full knowledge of qualitative analysis can ever be acquired without a workable cation separation scheme being included. But, what the one-term qualitative analysis course imparts to the student today has no resemblance to a "full knowledge" of this art, and what the decision a t hand amounts to is simply what to leave out: either a
wealth of interesting anion testing, including the use of modern spot-test methods for which nobody finds time today, or the pretentious, tradition-laden, outmoded, time-consuming, and pedantic cation separation scheme. As indicated by the choice of adjectives, the latter is what this writer suggests. THE USE OF STARTING TESTS
Once the cation scheme is out the window, a reappraisal of other segments of qualitative analysis seems in order, particularly of such items as may have been pushed into the corner in years past. In this category are the so-called "preliminary tests" which acquired the bad reputation of wasting an inordinate amount of the student's time. And, indeed, they were hard to justify, when followed and duplicated by as systematic and holeproof a procedure as the cation separation. Now they appear in a very different light, and a careful re-examination in the author's laboratory led to the rehabilitation of some of these quickies. As the sample analyses given below will prove, the function of these tests in the new analysis setup is such an essential one that continuing to call them "preliminary" would he misleading. Instead, the name starting tests is suggested here, indicating that the analysis proper is started with these tests, and that they should be regarded as an intrinsic part of the procedure. The six starting tests taught in the author's course are: (1) physical inspection of the solid sample, (2) ignition test, consisting of gradual heating of a wheat-grain sized sample in a 10- X 75-mm. test tube or ignition tube, (3) flame test, (4) action of water, (5) action of dilute sulfuric acid, (6) action of concentrated sulfuric acid. Bead tests and blowpipe work may also be included in this category, depending on the individual preferences of the instructor. The nature of the starting tests just described reveal another departure from common usage: solid samples rather than solutions are handed out. If there is any virtue a t all in simple observations and their intelligent interpretation, this rich opportunity to practice these abilities should not be overlooked. Students are amazed and fascinated by the unexpected wealth of sensory impressions they learn to record and put to use in the inspection of the sample and the other starting tests. The same person who yesterday described a sample as "white powder (period)," will soon be found writing down something like this: ‘ t coarse, even-grained, crystalline white powder, freeflowing; individual crystals are needlelike, up to 2 mm. long, clear, colorless, and show exceptionally high luster; they are hard and brittle and seem not to be hygroscopic." RECORDING OBSERVATIONS
This is another feature of primary importance in the new type of qualitative analysis course here proposed. The examples given below will show one of many possible patterns for such records, hut this author would never consent to making any such pattern, even the best possible one, into a preprinted straitjacket. Here is much too precious an opportunity t o teach the student how t o express a correlation of facts by means of their spatial arrangement, and make him do the arranging himself, and the logical JOURNAL OF CHEMICAL EDUCATION
thinking that goes with it. Even his aesthetic sense gets a chance to express itself in the creation of a beautiful, easily read page. ' Our grade and highschool education, under pressure of high student enrollment and low teacher's pay, has become so set on easy-grading features, that such chances for teaching intellectual self-expression in tabulations, diagrams, and hand sketches are sadly overlooked in favor of single words, figures or just pencil marks, to he plopped into preprinted spaces. Certainly nobody has ever learned to arrange his thoughts logically by having somehody else do the arranging for him. In t,he tearhing of qualitative analysis, there is even more to be gained from an insistence upon complete, detailed, and orderly records. The analytical process consists of a sequence of observation-interpretation-confirmation-conclusion. Thought and action sequences which are exceedingly similar to, if not identical with this one, mill he expected from the fnture research chemist, physicist, development engineer, medical diagnostician; in other words, from a majority of the students taking qualitative analysis. Pointing out to an embryo doctor that he declared a substanre to be calcium carbonate, which, according to his o\\x notes, he had found to be completely volatile on heating, may go a long way towards making him appreriate keen observation and strict,ly honest, thorough interpretation of disease symptoms later in his highly responsible career. IDENTIFICATION ANALYSIS
The term "identification analysis" refers to work with individual chemicals which the student is supposed to identify rompletely. At least this is what he will be doing throughout a large first portion of the course, with progress provided and interest maintained by a gradually more and more comprehensive list of ions from which the instructor chooses the unknown individual compounds. I t is hard to believe the amazing improvement in student motivation that even this one change brings about. No longer is the student tracking down which three or four liquids the assistant dumped together for him, no longer has he to content himself with the ,fragmentary finding that such and such ions were tested for and found present. Instead, he explores the behavior of a definite chemical entity and finds out quickly what it is. Modest as it may seem in hindsight, momentarily, the statement that "the sample i s lead carbonate" is a complete analysis and a highly satisfactory result to him, particularly if he achieved it, quickly, and with renewed zest he attacks the next, sample. Theoretically, of course, a student working on cation separat,ions is supposed to study the reactions of individual ions as "knowns," e.g., by taking a ferrous sulfate bottle from the shelf and seeing what that substance will do with ammonia. Unfortunately, the way humans are built, half of them do it listlessly, the others not a t all. Anybody who doubts this statement should ask how many chemistry students can tell him the characteristic ohsenrations made in the case just given as an example. The best way to make a mid-twentieth century youngster curious about the nonexplosive part of
VOLUME
34, NO. 8,
AUGUST, 1957
chemistry is to give him the incentive of a puzzlesolving situation. Make it simple and quick at first, keep adding complications until he has, quasi playfully, acquired a respectable knowledge of chemical facts and some experimental facility. Such intense preoccupation with single chemicals, as recommended here for the beginning of the course, should give way to some well-selected separations at a later time, because separations are important fundamental operations of analysis. Objections are only raised here against making students separate what they don't know, and have them depend on a textbook writer's prescription for every step they take. After enough identification work, they can handle mixtures intelligently in a much freer style, with shortcuts not only permissible but encouraged, and then, a t last, a qualitative analysis as taught in school and a qualitative analysis as practiced in commerce and industry will come somewhat closer to being the same thing. An identification analysis course, as here suggested, also sets the stage for successful introduction of one of those less common elements which have a way of fast becoming tomorrow's common elements. They are most logically introduced in form of a pure compound, first, for study of their "chemical personalities," unconfused by other substances. Summerbell's "simulated research" project2can serve as an ideal crowning feature of this phase of the course. Fortunately, the instructor's chances to branch out into some pet subjects of his are absolutely unlimited a t this point. To mention just a few ideas: mixtures with very unequal proportions, trace analysis, spot tests, determination of test sensitivity and limit of identification, chemical microscopy-without or, better, with use of polarized light-handling of insoluble substances, ores, minerals, alloys, complex salts, etc., spectroscopy, chromatography. Whatever the tune may be, the class will respond to it well after the allegro uiuace of the first movement. THE PLACE OF THEORY
There can be as much or as little theory in such a qualitative analysis course as the individual instructor wishes to present. He may have to search a while for new hitching posts. Soon he will find, e.g., that the hydrogen peroxide test for titanium which is masked by fluoride and unmasked again by beryllium4 is no meaner subject for exemplifying stability of complexes than any other more time-honored one. The instructor will also find that, stimulated by the more researchlike atmosphere of the laboratory, his better students and some he did not think were so good will come more often asking for the theoretical explanation of some phenomenon. And so, not only learning will be fun in this lab, but teaching too. And, dedicated teaching, more even in the laboratory than in the lecture room, is the critical ingredient that decides the success of this course, certainly not equipment which can be kept ridiculously inexpensive.5 The most nowerful tool of education is ~ersonal FEIGL, FRITZ, "Spot Tests," 4th English ed., Elsevier, Hauston, Tex., 1954, Vol. 1, p. 186. "n unexpected little gain in form of a.loss may hit the ohemicsl storage room: those "antiques" that have possibly been sitting there ior years and gathering dust are perfectly good for identification analysis. "~~nknowns" 385
example, and a delightful way of starting the course is a little personal demonstration in identification analysis, given by the instructor to the students directly, or to the assistants where the classes are big. The author lets his audience pick the unknowns for him from a large choice of chemicals, and then just goes ahead with the analysis, not forgetting to take notes in the same fashion as he expects his students to do it. A few examples of typical identification analyses follow, recorded in one of many possible tabular arrangements.
b i a n white aohlimste. .4mmonia evolved with No residue. alkslies. C o w . H&O. Colorlese, sharpsmeili?a White curdy p t with =as, forms fumes with A ~ N O sol. ~, air, dense fumes with re-ppt. by HNO,. NHs. REBOGT: The aamglc is ammonium chloride.
NHli
RLT.OH CI-
(2)
No ppt. with NarCOa. N o ppt. with NHaOH. White oryet. pot. with HCLOa. Dil. &SO, No brown fumes. Brown ring with FeSOd Cone. HISO, Brown fumes. and cooo. HaSO.. nnsvm: The aamplc ia potosaium nitrota. Flame
Violet flame.
}
It+ NOz-
(3)
Yellow-green flashes. H C I - s o l n . o f s * m ~ l e ~ i t hBatt more steady after CaSO&solution: nmquick dip in HCL medlste white ~ p t . Water Inaaluble White precipitate and Gas causes turbidity in COs-Dil. HtSOa coldera gas. n drop of Ca(0Hh. REBULT: Th. somp1r is barium corbonotc. Flame
smmr: Tha aomvls is !nod a c d d c .
(5)
Ioweotion
Pale green crystals.
HCI forma white ppt. Fruit-ester test positive.
Pb++ C2H801-
Fet+
Ignition. Light:
Crystals turn white. oolorlesa, neutral disl tillate. . . . . . . . . . . . . .,....................... Hydrate knition strong: Turna rust-brown, Fine white preoipitate so.-so, evolved. then with BaCI.. denae white h m e a . '
Comments: (1) What could be quicker and more straightforward than this detection of the ammonium ion? Whenever an ignition test is performed, ammonium can hardly be overlooked.
(2) Notice the judicious use of ~everal(negative) elimination tests along with the (positive) confirmation tests. (3) The beginner possibly will overlook the barium Aame, but the white precipitate with ~ulfuricacid will give him 8, choice hetween barium or lead, snd one more ooniirmstion test will establish the identity. (4) One single starting test mpplies strong clues for both cation and anion in this case. The cacodyl test may be substituted for the f r u i k s t e r test for those who do not insist on loveliness. ( 5 ) T h a t dirty-green precipitate with the rusLbrown fringe is something any chemist will recognize from s distance of ten feet after having seen i t once. But, where did he have a chance to see i t even once in his standard qua1 course? Notice how much information that one ignition test reveals: At low temperature, it shows that the substance is a hydrate; a t high temperature, almost certain evidence of a sulfate is given, plus strong suspicion of iron-all from hesting 8. vheaGgrain sine sample in a little glass tube and carefully watching everything that happens. I t does not seem such 8. bad waste of time after all. (6) This, too, is an interesting ignition test. I t may well, for a moment, arouse the wrong suspicion of s, hydrate but then points strongly towards a phosphate because of the viscoua melt, releasing no acid fumes, ending up as a clear, water soluble, neutral glass (of metaphosphate). Silicates and borates behave similarly, but aslong as they are not on his ion list yet, the student may as well reaoh for tho ammonium molybdate bottle right away. The acid reaction of the aqueous solution decides the choice among three sodium phosphates in favor of monosodium phosphate.
SUMMARY
A new type of qualitative analysis course is proposed which emphasizes rapid and complete identification of individual chemicals handed out as solid samples. No rigid analysis procedure has to be followed. Rather, a choice of six starting tests is suggested which the student learns t o perform and interpret properly. The leads gained from the starting tests he follows up with elimination and confirmation tests, until the complete identity of the compound is established. Careful notetaking is essential. Advantages are many: Encountering distinct chemical entities as unknowns, the student has a chance to study their "chemical personalities," unconfused by other substances. He is strongly motivated to do so to the best of his ability by the tension of the puzzle-solving situation. He quickly finds out that keen observation and intelligent interpretation give results, and feels greatly stimulated by the freedom of action he is given. The instructor keeps this interest alive by stepwise expansion of the list of ions from which he chooses the unknowns. Later, he can branch out into many directions. If work with mixtures is introduced a t this more advanced stage, more benefit is derived from it, as the student can apply to it a sound working knowledge of the chemistry of individual ions and compounds which he has previously acquired. He can afford and will enjoy t o take shortcuts, and, in his hands, a separation scheme, too, is a live proposition.
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