J. E. Packer University of Auckland Auckland, New Zeoland
Research Topics from Qualitative Inorganic Analysis
For a large first year chemistry class it is not difficult to design a laboratory course to acquaint students with the facts of chemistry and to illustrate the principles underlying the subject, these being the primary functions of such a course. Because of the organization and planning required it is considerably more difficult to train students to be scientists by allowing them to plan and carry out individual investigations on a wide variety of topics and to create a realistic "research" atmosphere. Ihrther, in a well established elementary course, there is the inherent danger that the teacher will be able to give a satisfactory explanation of most questions that arise during the experiments and thus disguise the important truth that chemistry is still an experimental science and that theories and explanations should be checked by experiment before being accepted. O'Donnelll in a recent article summarizes the common arguments for and against traditional qualitative inorganic analysis. We have found an additional use for such analysis, because without additional organization we can provide the students in a large class with the opportunity to plan and carry out individual investigations, emphasizing the importance of experiment to check possible theories and explanations. Our flow sheet is very similar to that given by Vogel in his first tables2 Thestudents initially work through five solutions made up as follows: 1. Hg'; Ag', Pb" 2. Hgr5 Birr', Cu", Cdl' 3. Cd", Asn1, Sbul, Sn"
Fe"', Mn", Cr"', Al"' 5. Cu", Nin, Cotl, Zn1[
4.
What is present in the solutions soon becomes common knowledge and the challenge is provided by an extensive questionnaire on the chemistry underlying the separations. On completion of the five solutions, when the students have had the opportunity to "see" all the chemistry, there is a class discussion on the questionnaire. Care is taken by the teacher to put up seemingly logical answers as postulates only, and many more auestions arise. I n some cases the validitv of the assumptions underlying the questions is queried. From this discussion, and by mentioning the "peculiar" results obtained by some students, it is easy produce thirty or more topics requiring experimental investigations, some of these being verification of postulated theories, others being exploration of optimum conditions, etc. A topic arouses extra interest if students O'DONNELL, T. A,, J. CHEM.EDUC.,42,434 (1965). VOGEL,A. I., "A Text-book of Macro and Semi-micro Q~talitative Inorganic Analysis," 4th ed., Longmans Green, l on don, 1954. Ch. 7.
have disagreed on a possible argument. The ingenuity required to make progress on the topics differs considerably and it is advisable to take the students' ability into account when assigning the topics. Below is a small selection of questions and the modest topics that arise from them. 1. Why is dilute HCI and not NaCl used to precipitate the chlorides of Gr. I? Tonic: Investieate solutions 2 and 3 usine NaCI solution. 2.' Why shoull dilute and not caneentGted HCI be used to precipitate the chlorides of Gr. I? Topic: Investigate solutions 1, 2, and 3 using cone. HC1. 3. Would it matter (aside from the inherent danger) if excess chlorate were used in dissolving the HgS in cone. HC1 prior to the final mercury test? Discussion could lead to an answer of yes as excess chlorate may oxidise the added SnrL,or no as it should decompose on heating. Topic: Investigate the validity of the answer and the ease of decomposition. 4. Why is lead listed in both Gr. I and Gr. I I ? In discussion, the question of chloride enhancing the soluarises. , bility of lead over thst calculated from S P ~ Topic: Investigate the amount of lead not precipitated as . chloride. 5. If the nitric acid used in separating HgS from Bi& CuS, and CdS is too strone and dissolves the HeS where would vou ex-
test? Topic: See what happens if i t is omitted or replaced by another acid. 7. In the final test for tin, what is the function of the einc? Why is it necessary for all the zinc to dissolve before adding HgCl.? Could the solution be decanted from excess einc? Topic: Investigate the latter questions. 8. I n the separation of the hydroxides (or oxides) of Mn, Fe, Cr and Al, why does the sluminurn dissolve in an aqueous solution of Na20p? How would subsequent tests be affected if NaOH were used in place of Na.O.? Topic Verify postulated result experimentally. 9. In the final test for chromium, why is it necessary to acidify before precipilating PbCr04? Why is acetic acid used? Topic: Is acidification necessary? Does it have to be acetic ~
m i.d~ ? -
10. Why is ammonia rather than sodium hydroxide used to treat the acidified nickel solution before adding dimethylglyorime. Topic: Investigate the conditions necessary for the formation of nickel dimethylglyoxime. Examples of topics arising from students' findings on working through solutions 1-5 are: 1. Some students find mercury coming into the arsenic subgroup in solution 2. Find the conditions necessary for this t,o occur. 2. In the final t,est for antimony some students obtain a greywhite precipitate. Find the conditions necessary for this to occr~kand determine what it is. 3. I n sohtt,ion 2 many students miss cadmium. Investigate the possibility of chloro-complexes forming, thus making precipitation of CdS more difficult. Volume
43, Number 4, April 1966
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By handing out these topics to students either individually or in small groups, we have found that great interest is aroused and the laboratory really becomes a place of active research for this period. Some students wish to return to continue their research after the period is over. For a new teacher who has not read the authoritative account of this work by King,3 many of the topics may yield new information, while the experienced teacher, to he fully successful, may have to feign a little ignorance and treat students' findings as though they were quite new to him. There is the saying that: "A chemist who can successfully carry out an experiment is worth his weight in lead; one who can improve the experiment is worth his 8 KIKC, EDWARD, J., "Qualitative Analysis and Electrolytic Solotions," TIarroirt, Brace, & World, Inc., N e w York, 1959.
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journal of Chemical Educofion
weight in silver; hut one who can show that the experiment is not even necessary is worth his weight in gold." While this is true, and we should eucourage students to consult available literature before carrying out experiments, the benefits of this approach excuse us on this occa3ion. The student has the opportunity to plan his experiments himself, to practise recording data clearly, and to dram conclusions unaided from his results. It provides the teacher with a further met,liod of assessing the students' ability and originality. oft,en wit,h surprising results. Acknowledgment
The author wishes to acknowledge the enthusiast,ic collaboration of his teaching colleagues, B. R. Davis, B. A. Grigor, and G. R. White in this extension of the qualitative a~~alysis assignment.
