James H. DeLap
stetso" University Deland, Florida 32720
First Semester Qualitative Analysis
I n 1965 t,he chemistry department a t Stetson University (enrollment of approximately 100 in general chemistry) revised its curriculum, separating the laboratory and lecture into two concurrent courses during the freshman year. The traditional first-year general chemistry laboratory was dropped; quantitative analysis was moved to the spring semester of the freshman year; and a two-hour laboratory course entitled "Chemical Periodicity" was initiated for the fall semester of the freshman year. It is with this laboratory course that this paper is concerned. Chemical Periodicity meets one hour weekly as a lecture-discussion and has one three-hour laboratory. There is no textbook or laboratory manual and no discussion of chemical equilibrium. Discussions center primarily on the orderly assimilation of inorganic facts, including acid-base theory, complexion theory, and chemical periodicity. I n addition, the mechanics of writing net ionic equations is stressed. I n the laboratory each student works a t his own rate in completing four sequences of experiments, each with an unknown, and a final laboratory exam, which is a fifth unknown. The first three sequences of experiments are similar in approach. The student studies the properties
of a group of ions in terms of color, precipitate formation, complex ion formation, and gas formation and uses these properties to devise a scheme of analysis (from memory) for a unique group of these ions as a written examination and in the laboratory. This method of assigning unknowns is the strength of the course in developing student independence and research thinking. If the unknown could contain all ions, eventually a "cookbook" scheme would be devised and distributed through student channels. Having a unique set of ions for each student makes it necessary for him to know the properties and devise a scheme extemporaneously. For example, there are 462 different unknowns for the group B cations (see bclow). Group A is concerned primarily with precipitation reactions of cations; group R expands this group to include complex ion formation and effects of acidity on precipitation; and group C reverses the process to identify anions. Group D exemplifies the use of physical properties (spectra) to identify cations.
Cation Group A
Pb2+, Ba1+, Bi3+, Fe'+, Hg2+, and Mn2+
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Test reagents: NaCI, HCI, (NH~)zSOI, H2SO4, NazCOa, K2Cr04, NH3, NaOH, HzS (acid). The student adds each test reagent to each cation, records the color if a precipitate forms, and tests the solubility of the precipitate in nitric acid. He is given a set of solubility rules and compares his ohsewed results with the rules. When the student feels he knows these properties of the elements, he presents himself for examination. He draws from a hat a slip of paper which may look like Name Unknown Number 1% Possible Ions: Pb", Ba2+,Fez+
He creates from memory a scheme of analysis for these (and only these) three ions and turns in the scheme for grading. He then takes the slip of paper to the stockroom, is given unknown #I06 which may contain only Pb2+, Ba2+, and/or Fe3+ and investigates the unknown using the same or another scheme. The written scheme and the unknown are weighted equally in his grade. Disagreement among students in the results of some tests provides a basis for useful discussions. For example, in testing with KsCrOa, a variety of results will be observed. Some observe a precipitate with iron; others do not. The precipitation dissolves in acid. Therefore, the precipitation depends on the acidity of the solution. The precipitate forms in a neutral solution but not in an acidic solution. Similarly, BiOCl precipitates from a NaCl solution but not from HCI. When Mn2+and C r O P are mixed, some observe a brown precipitate, others do not. MuOl precipitates very slowly under these conditions.
Cation Group 6
All of the ahove cations and Ag+, Cuz+, NiZ+,Zn2+, AP+ Test reagents: excess (cone.) NH,, excess NaOH, H,S(base), Na3POnin addition to those listed for Group A ahove. The student observes the effect of acidity on precipitation by comparing the precipitation of sulfides from an acid (HNOa) solution as in Group A and from a basic (NH3) solution. I n addition he studies complex ion formation observing that an excess of reagent (NH, or OH-) is necessary to redissolve the insoluble hydroxide. The solubilities of the phosphates are studied as in Group A. Following the same procedure as for Group A, the
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student prepares from memory a scheme for five of the eleven cations, hopefully using complex ion formation and acidity control to separate the ions. From eleven ions, there are ,462 different unknowns containing five ions so each student is truly working with his own scheme of analysis. Anion Group C
This group follows more conventional approaches in that specific tests are recommended to the student. These include: a) the test solution is acidified and the vapors are passed into saturated Ca(OH)2 and Pb(C2H302)2 paper; b) the relative solubilities of silver halides in NHI are measured; and c) the reactions of halides with Clz water and with PbClz are investigated. I n addition, the student has available the data collected in Groups A and B. Once again, the student prepares a scheme for three of these ions and tests his scheme on an unknown in the laboratory. Cation Group D
Li+, Na+, Mg2+, Ca2+, Sr2+, Ba2+ The line emission spectrum of an unknown solution is compared to those of "known" solutions containing the possible ions, and a positive identification can be made. The ions are excited by an ac voltage of 80-100 V passing between platinum electrodes along the surface of the solution which is about 10% HNOI, 1% ',NOa, and 0.3 M in each ion. Bunsen spectroscopes are used to analyze the emitted light. Final Examination
The student is given a single soluble salt (one cation from Group B and one anion from Group C) and identifies it in two hours using all available test solutions but no notes. This course has stimulated the research thinking of both science and non-science majors. I t was designed for the beginning laboratory as an expediency to the curriculum. However, it is recommended as a replacement for the standard qualitative analysis scheme. The logical sequence of the precipitation, acid-base, and complex ion formation should facilitate the correspondence between equilibrium studies and solubility products, acid and base ionization constants, and instability constants. The student-initiated scheme of analysis results in an increased retention of inorganic facts over the routine "cookbook" schemes. Finally, the introduction to research thinking or logical deduction based on experimentation represents chemistry as the experimental science that it is.
