I QUAL
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"Qual" From a Different Viewpoint
Michael Laing University of Natal, Durban 4001, South Africa For years we have done qualitative analysis according to the 150-year-old scheme of Carl Remegius Fresenius ( I ) , and what should be wrong with doing that? The students see insoluble chlorides, sulfides, hydroxides, and carbonates. Afew remember which precipitate was which color. A few more, ifwe are to believe the textbooks (2),will acquire a knowledge of the properties of the elements by gaining an understanding of the interaction between complex equilibria and K,'s. Fresenius set up this classic scheme of analysis to enable a person to analyze an ore, a mixture of minerals, and identify which elements were present. Nowadays we have better methods: X-ray fluorescence to identify the elements (31,X-ray diffraction to identify the mineral species, and crystalline phases (4). Moreover, it is well to remember that Fresenius' scheme was developed about 1850, well before Mendeleyefflaid out the final 1871 version of his periodic table (5) (a format that is still in use). Aim It is our task as teachers of inorganic chemistry to help our students learn the facts of inorganic chemistry and thus to acquire a feel for the periodicity in properties of the elements. Doing "old fashioned Quay does not do this, although there are many who swear that it does (in a rather round-about way) (6). They are over-optimistic when you consider that historical report of over 50 years ago (7). Those authors carried out a detailed survey of the aims of "Qual" and concluded that the emphasis was on the basic training of analysts, the learning of good technique, and the acquiring of problem-solvingskills. If our aim as inorganic chemists is to teach the reactions of the metals and thus demonstrate group periodicity, then this should be done directly, because this will make it easier for the student to learn the facts. The approach is straightforward. Look at the periodic table (81, do the simple reactions of consanguineous elements, block by block, and only then show how these simple reactions can be used to separate and identify each element in any block. The strategy is to start with the "simplest", and then proceed to the right of the periodic table. Elements to be Studied The blocks of metals are 1. Magnesium, Calcium, Barium, Lead 2. Aluminum, Chromium, Manganese, Iron 3. Cobalt, Nickel, Copper, Silver 4. Tin, Zinc, Cadmium, Mercury 5. Arsenic, Antimony, Bismuth
The metals in each block have been chosen deliberately to emphasize: groups ofclosely allied metals in the periodic table, metals whose compounds are found together in nature, and metals that are used in industry or are found together in commonly encountered alloys. Block 1is composed ofthe Group I1 alkaline earth metals as the cations M e , Cazt, Ba2+.Lead is included as Pb2+
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Journal of Chemical Education
because it. like barium. forms an insoluble sulfate (which is unusual). Block 2 contains metals whose cations A13+. Cd+. Fe3+ form insoluble hydroxides with aqueous ammkia.'~hese metals are "oxygen lovers". Block 3 is composed of transition metals whose cations Co2+,Nizt, Cu2+,Ag+ are of the "soft acid" variety that readily form stable coordination complexes with ammonia. They include the "coinage metals". Block 4, apart from representing the well-known p u p XI1 in the periodic table, includes tin because Sn, Zn, and Cd are used industrially as protective metal coatings. The ionic species are Snz+,Sn(N), Zn2+,Cd2+,Hga, and HgsZ+. which makes this the most complex of these blocks of metals in this series of exercises. Block 5 is periodic group XV, which exemplifies the change from amphoterism (As(II1) and AsW)) to basicity (Bi3+)on descending a group in the periodic table. The metals in blwks 4 and 5 are the heavy dl0 metals in the lower right-hand side of the periodic table with peculiarities like the inert oair effect (9). All fall toeether as colored sulfides, inso~ud~e in acid (with zinr being'lust -out of line"., It is imwrtant that students encounter the sulfides because it Is as sulfides that these metals occur in nature: Wurtzite (ZnS),Galena (PbS)as well as CuzS,NiS, HgS and CdS (which is commonly used as a yellow pigment in paints and inks). In addition, the different blocks can in turn be linked by the classical comoounds of the traditional aualitative scheme, e.g., Pb inblock 1 links with Ag in block's as insoluble PbCI, and ApCI. and to blocks 3 and 4 as the acid-insoluble suifides P~s,'CuS, and CdS; while Mn in block 2 links with Ni in block 3 and with Zn in block 4 as the sulfides MnS, NiS, and ZnS, precipitateable only in base medium. By doing this, we are reliving Fresenius, 1850, but our true aim is not to set up a qualitative analytical scheme, it is to look at systematic trends and similarities in the periodic table. However, it is well to remember that between 1850 and 1890, the criterion for identifying a new element was that i t did not fit into the qualitative scheme of Fresenius! (This worked well for gallium, scandium, and germanium.) Principles That Can Emerge As the precipitates form, one might ask the students "Why is it insoluble ?"The answer is not "because K., is so small", but rather "because the lattice energy is so large." This automatically leads to a discussion of the Group I1 sulfates. Why is it that MgS04.7Hz0is soluble; yet BaS04 is insoluhle?The concepts ofionic radius, heat ofhydration and hydrogen bonding (in the crystal) now emerge. These could be followed by questions like: "What is the hldrated species in solution?"; [Mg(OHz)e12+,[CU(OHZ)~I +,and IAI(OH9).Pt. In turn these could lead to a discussion of &nphot&ism and roncepts like Cartledxe's ionic potential (10, and the diagonal relationshio (111.with the arcomoanying similarities in values of q/? forcations of char& q
and radius r. Many are the questions "why" that can be asked and the explanations that can be given (12).It may pay, however, to ensure that the students first know t h e fads, e.g., that silver chloride is a white solid. which is insoluble in water; and darkens i n sunlieht before mucking about with too many questions concerning "why" this should be so (13).This series of exercises is predominantlv Piagetian concrete in $haracter. Conceptual sophistleation always can be saved for the new sophomore course (14).
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+ 57 Lanthanides La
Actinides
58
59
W
Ce
Pr
Nd
61
62
Pm Sm
89
90
91
92
93
94
Ac
Th
Pa
U
Np
Pu
63
64
65
66
67
68
69
70
71
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
95
96
Am Cm
97
98
99
100
101
102
103
Bk
Cf
Es
Fm Md
No
Lw
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Outline of the Work The method is to test a solution of each cationic spec i e s w i t h e a c h of t h e s t a n d a r d dilute bench reagents (HC1, NH3, H z S 0 4 , N a O H ) a n d a few e x t r a (more s ~ e c i f i c r) e a e e n t s : t h i o a c e t a m i d e , d r o p by d r o p , o b s e r v i n g for a n y precipitate formed, and t h e n adding excess where appropriate. (For details of t h e procedures, see t h e Appendix.) The students then have to identify the cations in two "unknowns". The first is simply one cation. The second solution contains two of the four possible metals that have just been studied. This requires that the students construct a scheme for separation and identification. This approach is like that of Drago and Brown of 20 years ago (151, but now instead of attempting to test two dozen cationic species in a n afternoon and from this to "create" a scheme of analysis, the student has only four metals to contend with in each practical, and he repeats the same mental process on five different occasions-reinforcing a scientific approach-
! 54
Xe
ns
Rn
Xe
carry out tests, make observations, look for patterns, look for differences,and create a systematic scheme for identification. Figure 1. A series of periodic tables, showing the different blocks of elements as they are studied in the method. (top)The full set of metals studied in the scheme. (center)Block 1. (bottom)Block 2.
Note t h a t in each threehour practical period, the students study never more than four metals and their reactions. This greatly reduces
Volume 70 Number 8 August 1993
667
the problem of "white noise" and "detail overload" (16) that bedevils t h e usual "qua1 scheme". A similar approach of working with smaller numbers of related metals is found in the laboratory manual by Marcus, Sienko, and Plane (17). There are some remarkable spin-offs, e.g., the reactions of Bi3+with 1- and SCN-. Documenting these poses quite a challenge for the student. The classic text by Vogel will solve the problem (18).I n addition, it soon becomes evident that one can separate and identify the various metals without recourse to thioacetamide and the classic sulfide method. When all five blocks have been studied, with the support of formal lectures about the reactions of the elements, i t now becomes possible for the student to tie the different regions of the periodic table together with his own observations ( t h a t were deliberately designed to achieve this end). There is a bonus. It also is possible for the student to separate and identify the metals from a n admixture by using the simple reactions that he has met. At this stage the students are presented with "Sherlmk Holmes' Problem of the Six Mvsterious Bottles" (19). . . They a r e given six bottles, each containing a solution of a pure compound (from a list of 18 ~ossibilities).Their task is to :dentify the six compounds bv usine the reactions learned in the Cve previous practical sessions. A final exercise is to analyze some real materials for their constituents; e.g., an "aluminum" pencil sharpener for Al, Mg; some solder for Sn, Pb: a 5-cent piece for Ni and Cu; a new 1 cent and an old 1cent far Cu, Zn, and Fe (separately, by stripping the surface of the new win); a brass screw for Cu and Zn (not as easy aa it sounds); a piece of a can for Sn and Fe; and (if you can still find Figure 1. Continued. (top) Block 3. some!) printer's type (center)Block 4. metal for Pb and Sb (20). (bottom)Block 5.
For convenience these unknOwns can be presented to the students a s a solution containing the two cationic species. 668
j: CI
Ar
Br
Kr
Note how Aluminum ( ~ 1 % ) linksthe hard acids on the left-hand side to the 3+ transition metal cations; and how the Class B heavy metals inevitably group themselves as acid-insolublesulfides. This "duplicate"form of the periodic table shows the links of ~g to both Ca and Zn, and that the reactions of AI will hint at the properties of Be and Ti by the diagonal relationship.
Journal of Chemical Education
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Instruction Sheet for Block 2 The commonly found compounds of these metals are: Aluminum (Ai3+):Bauxite A1203 2H20; Corundum A1203; Alum (Potassium:KAI(S08,)2 12Hz0 Chromium: Chrome green Cr203; Chromite FeCrzO4; Zinc chromate ZnCrOa. Manganese: Potassium permanganate (Condy's crystals) KMn04; Pyrolusite Mn02. Iron: Haematite FezOs: Magnetite Fe3Oa; Mohr's salt (NH4)zFe(S04)26H20.
1. Color 2. NaOH 3. Excess NaOH
instructions Test the solutions of the metals systematically to determine which compounds are colored or insoluble. Record your observations on the table at the right and give the formulae of the insoluble a n d h colored snecies in the soaces orovided. Note {hat redox reactions occur for these transition metals, for example: the solution becomes dark brown, and the iodine can be extracted into dichloromethane. cr3++ OH- + H202-+ croqZ
Tests for AI~', c?',
~ e ~ +
i
I
I
I
I
I
I
I
I
I
5. Na bismuthate
8. KSCN
M&+,
1
I
I
(2)
the solution becomes bright yellow.
~ 2+ Bi03+ + H30t + M n 0 4
(3)
the solution becomes a bright purple
Comments on the Reactions above 1. The colors of solutions of C?+ and ~
e will ~ vary + depending on the identity and concentration of the anion present. 2,3. BothA13+and C?+ are clearly amphoterie. 4. Mn(OHI2does not form because [OH-] is too low in aqueous NHB. 5. Sodium bismuthate is a very powerful oxidizing agent, one of the few capable of oxidizing hfn2+to Mn(VI1). 6. This deep Prussian (Turnbull's) blue is a classic compound, characteristic of iron. 7. The iodide is oxidized to iodine. Extract this into dichloromethane:note the change from brown (Iz in water) to purple (I2in nonpolar organic). 8. The dark red brown compound [Fe(SCN)I2+is quite characteristic of iron. A. This absomtion of the alurninon dve onto the aluminum hvdroxide is used in dyeing cioth :the aluminum hydroxide is said to be a "mirdant", and the colored material is a "lake". B. This oxidation of Cr3+to bright yellow Cr04=is quite characteristic C. Of all these metals, only Mn2+will form a sulfide, insoluble in water; its pink wlor is unique : MnS. The H2Sforms as the thioacetamide hydrolyzes with heating. + , ~ n ' + . ~e". This is typical of the instruction sheek for each block: (a) Figure 2. The set of tests for each of the species in block 2, i.e.. A I ~ crS. a list of well-known wmpounds of the metals, (b) the instructions, calling for colors and formulas,and (c) some comments on the reactions. (The student is expected to find further information in the textbook, i.e. ref. 2). This series of exercises costs o u r freshman students seven three-hour practical sessions i n the second semester. I like to believe t h a t i n t h e process they have learned a little chemistrv of these metals and somethine" about the periodicity of properties of t h e elements. If I a m to believe the answers on class questionnaires, quite a few actually enjoy it!
Acknowledgment
I thank our freshman chemistry class whose hard work i n the laboratory identified the shortcomings i n t h e preliminary version of the practicil instructions. I also thank m y friends Dulcie Mulholland, Karl Pegel, and Roy Osborne (UND) for useful comments about t h e approach; Volume 70 Number 8 August 1993
669
Dean Skovlin and Bill Harrison of Cal. State University, Northr~dgewho tirst s t a r t e d me thinlung about t h l s problem: and K n s t ~ nW v n n ~ J o n e sfor a copy of the* Instructions for her Nine shution Puzzle. Appendix: Details of Procedure This series of exercises can be carried out on two scales: (a) classical centrifuge
tube semi-micro (181,or
(b) spat plate and dropper, to minimize reagent use lems of waste disposal.
and prob-
(We currently use (a) for most parts, but use (b)to show, for example, that Co2+,Ni2+.and Fe3+can he simultaneously identified in one drop of solution). The concentrations of the unknowns are uniformly 0.1 M (nitrate, sulfate, or chloride in acid medium, as appropriate); the specific reagents are generally 2 M, while the bench reagents are 4 M. These concentrations give good visible precipitates with less than 10 drops of the reactants. We use 5%thioacetamide and follow the standard practice for precipitation of the metal sulfides.
CAUTION:Students always must be informed of the high toxicity of the H,S that is produced.
However, we are totally convinced that the importance of seeing the various colored sulfides far outweighs the slight potential danger of any H2S that is liberated. Generally, the procedures and reagents are minor adaptations of what is in the well-known books (6), especially that by Vogel(18). Waste disposal problems are no different from those encountered in the standard Fresenius scheme (21). Copies of the complete set of instructions for the seven practical sessions are available on request.
Literature Cited 1. Fresenius,C. R. QuolifotiueChemlcolAnolysis:loth ed: Churchill: landan, 1881;pp 291-317 dextipfion of the "claseicalscheme". 2. Maeller 2;Bailar. J. C.: Kleinbelg,J.:Guss,C.O.;CasteUion,M.E.;Meh,C.ChamInorganic Quolitofiue Amlysis; 3rd ed: Harcourt Brace Jovanovich: istry, ~ i l h on 891-964. New York. 1989: ... 3. Jenhs. R A n Infmdudion to X-Ray Specfmmhy;John Wiley: New York. 1983. 4. mug, H. P.;Alexander,L. E.X-rayDl/fmclionPmcedures; J. Wiley: New York, 1954. 5. van Spronsen,J. W The Petidic System of ChemM Ekmmb; Elsevier:Amater dam. 1969:p 137. 6. Bmum, T L.; IeMay,H. E. QuolitdiueImrsanieho1~is:Prentice-Hell:New Jer. sey, 1983. 7. Red, R D.: Cortelou,W P; Calandra , A. J Chrm. Edue. 1940,17,220.See also Bendeni-Pichler, A. A; Sehneider.F.; Steinbach,0. F.;J. Chem. Edue 1951.34,
em
8. Ling. M.J Cham. Edue 1984.66.746. 9. Laing, M. S. A. J kience 1991.87.285-287. 10. MaeUer, T. Inorganic Chrmislgv;John Wiley: New Vork. 1982;p 247. n. ~ouglae.B E.; ~ c ~ a n i eD.l .H.; ~lexander, J. J. concepts ond ~odoisoflnorganic New York. 1983:pp 616617,620-621. Chemistry, 2nd *John W~lw: , 12. Wulfsberg,G. P~incipks ofDosctip.5~ Ino?pg.nir Chamistry; BmokssiCole: Monterey.1987: especially Chapter 3. 13. Davenport,D. A. J. Cham. Educ. lW0,47,271;famoushrtheterrible warningthat ens'.. and for remindins us that alwavs "Silver chlo"de ia a ,ode ereen . %e #rim elknee of facts remainam. 14. Verksde, J. G. J Chrm. Edue 1881.68.911414. 15. Drago, R. S.; Bmwn, T L. Expe?imenls in Oenerd Chamisfry, 3rd ed; Allyn and Bacon: Boston,1970;pp 215-228. 16. (a1 Johnstone,A J C h m . Educ 1984.61.847.ibl 6UI lnternati0n.dConferen- on Chemical Edueation,Maryland,1981;h e e d i n s p 116. 17. Marcus,S.; Sienko, M. J.; Plane, R. A. Ezpetimntol General Chmdry; MMeCrawHill: New Yark, 1988:pp 37742. 18. Voge1.A. I. MacmandSemimicro Quolihtim hoganleAnolysis.4th ed; hqmans Green: Landon, 1954;p 222. 19. (a1 Steig.3.J. Chem. Educ 19&5,65,3M.lbl Finhalt,J. E. J Chem. Educ 1984.61, 849. (cl MacWod, G. E.; Lassetfre, E. N.: Breen, G . J Cham Educ 1940,17,520. (dl Slowinski,E. J.; WolPey. W. C.; MesteMn, W. L. Chamicol Principlas in tha Lobomfogv.4th ad.;Saunders:New Ymk 1985;pp 311319. (el WymneJones,K 11th Internotional Conkmncr on Chemrml Edumtion. York, 1991;Proeeedinga, p 1C9,Abshaet P2.41. 20. Alexander, W.; Street, A. Metals in tha Seruin ofMan 8th ed: Penyio: London, 1982. 21. Petty, J. T J. Cham. Edue. 1991.68,942943,a truncated amawment ofqualitative analwli. which emohadzes the mmimization of the ~rablemsof waste dir-
Nominations Being Solicited for the Fourth Brasted Memorial Award Nominations for this Award, which is adminstered by the ACS Division of Chemical Education through its International Activities Committee, are now being solicited. The fint Brasted Memorial Award was presented to Professor David Waddington (of the United Kingdom) at the Division's Tenth Biennial Conference on Chemical Education, held in West Lafayette, IN, in 1988. The second Brasted Award was presented to Professor Aleksandra Kornhauser (of Yugoslavia)at the Eleventh Biennial, held in Atlanta, GA, in 1990. The third Brasted Award was presented to Professor Ernesto Giesbrecht (of Brazil ) at the Twelfth Biennial, held in Davis, CA, in 1992. The fourth Brasted Award is to be presented at the Thirteenth Biennial, to be held at Bucknell University in Levisburg, PA, in 1994. The primary criterion for eligibility far the award is significant cantribution to the advancement of chemical education internationally-a criterion that clearly described a n important part of Bob Brasted's life. Anyone wishing to nominate an eligible person should send a nominating letter (in English), not t o exceed two typewritten pages in length (single spaced), which describes the candidate's contributions to chemical education, with particular reference to the international aspects of these contributions. A curriculum vitae and other supporting documentation (10 pages, maximum) should be attached to the nominating document, and a seconding letter is required. The nominee must not he a citizen or a resident of the United States, but the nominator and seconder may he from any country. The nominating documents should be sent to the Committee Chair, Rofessor Zafra Lerman, Institute for Science Education and Science Communication, Columbia College, 600 S. Michigan Avenue, Chicago, IL60605 U.S.A., in time to reach her by November 30,1993. She will forward all nominating documents to the Brasted Award Subcommittee of the DIVCHED International Activities Committee. It is intended that the name of the Award recipient shall be announced by March 31,
1994. The Brastrd Mcmonal Award mnsisrs of rconnmy class air farr fmm the recrpltnt's humc city ta the lliennial .Lewisbury, PA, in this rase,, l w n g expenses at the Biennial C'onfrrmrc, n one-yearrnembcrsh~pin r h r D ~ s l o nof Chemical Education, and a one-year subsrr~pr~on to the Journnl ofChpn,,~alEdumlio,,. Thc rcriplrnt is expected to present the Brasted Memorial Award Lecture a t the Biennial Conference on Chemical Education.
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