Gravimetric Analysis-The Synthesis of Tin Iodide George G. Hickling University of Manitoba. Winnipeg, MB, R3T 2N2, Canada Most students entering university have heard of gravimetric analysis in their high school, but few have actually quanextitatively prepared a compound from its elements. periment that we have used t o further our students' understanding of quantitative analysis and to introduce some new laboratory techniques involves the preparation of tin(IV) iodide. The procedure that we have developed not only reinforces concepts about a simplest formula and mole ratios but also emphasizes the idea-of a limiting reagent. In addition to develodng - - .recision and accuracy in determining masses and transferring chemicals the techniques of refluxing and uacuumfiltration, new to most students, are introduced in a meaningful manner. The starting materials, metallic tin granules and solid diiodine crystals, are transformed into a new compound with readily examinable properties. The nonaqueous solvent we have employed, VM&P1, has no known health hazards when handled with standard laboratory precautions. Certainly, i t is safer and easier to work with than carbon tetrachloride or carbon disulfide, which are the recommended s o l ~ e n t s . ~ Experimental Between 1and 2 g of granular tin are weighed into a 100mL round-bottom flask. A similar amount of diiodine is added, and the mass is recorded t o the accuracy of the balance. Note: Safe handling techniques must be reuiewed. Although quantities are small, students should be warned about the hazard of iodine and potential toxicitv of tin i t e r s O~VM&P solvent is obiained in compounds. ~ i fmth ~ a graduated cylinder, with 25 mL being poured into the round-hottom flask and the rest saved for washing. The apparatus for refluxing is assembled, and the water is turned on. After the instructor has checked to see that there is no blockage in the reflux column and there is an adequate flow of water, heating is begun and the temperature r e d a t ed so that the volatile components condense no further than halfway up the reflux column. Note: I t is important that all condensate remains in the system from both a quantitative and safetv versoectiue. Past ex~erienceindicates that it takes abo;t 50 &in for complete reaction, a t which time the purple color, due to dissolved diiodine, disappears and an orange color, characteristic of the product, dominates. The reflux system is disassembled and the hot solution is passed through a preweighed filter paper in a Buchner funnel in afilter flaskattached toan operatingvacuum line. The round-bottom flask is rinsed with three separate portions of the remaining 25 mL of VM&P solvent, passing all solvent through the filter system. This usually removeiall product and any remaining tin from the flask. If tin remains, the flask is inverted in the Buchner funnel, and, when dry, i t is tapped and the tin falls out. At the same time virtually al! product passes through the filter paper and begins to crystallize in the filter flask.
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V.M.&P. naohtha Is obtained in Canada from Stanchem. adlvision of C.I.L. In the ljnited States it Is available from UNOCAL. 425 James Ave.. St. Paul. MN 55102. MmIIer, T.; Edwards, D. C. Inorganic Syntheses, 1953, IV, 119.
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The dry filter paper containing any unreacted tin is removed and the mass of tin remaining is determined. The diiodine, the limiting reagent, has obviously heen consumed completely. By knowing the mass of diiodine and the initial and final masses of tin, the moles of each reactant consumed can be calculated. The simplest whole number ratio of the moles of reactants used permits a determination of the simplest formula of the The crystals of product are separated by vacuum filtration and dried by su&ion. students are askkd to describe the appearance of the substance and perform several tests on it. The results of these tests are summarized in the discussion. Cleanup involves placing all solvent containing unrecovered product in the waste container provided so that it can be recycled and saving all tin iodide product not used in testing in a labelled container.
Results and Dlscusslon This experiment has been performed by thousands of students with excellent results and few difficulties. In part, the success depends upon the reflux solvent selected. Of course, the solvent must be unreactive to all possible constituents. Diiodine must dissolve readily, but the product must be considerablv more soluble when hot than when cold. So that the reaction-occurs in a reasonahle length of time, the boiling temperature must be fairly high yet easily regulated by inexperienced students. VM&P solvent, a safe naphthalike hydrocarbon with a boiling temperature of about 120 OC, has proven to be the most successful solvent that we have found. Since i t is a commercial solvent i t is relatively cheap and readily available. Toluene is a barely acceptable and expensive substitute. We expect students t o c o m ~ a r ereactant and ~ r o d u c t charactehstics. The diiodine cr;stals, almost black;n color as a solid, dissolve readily in the solvent to form a purple solution. The tin does not dissolve and any remaining after reaction is virtually unchanged. In contrast, a briaht oranee solution remains after reaction. When cooled, distinct re& orange crystals form. ~ i o students d consistently obtain a mole ratio of four iodine atoms for each tin atom, i.e., the formula for tin iodide is SnIa when prepared by this technique. In fact, the technical ability of a student, or the lack thereof, is quite apparent by the result. The product, which is quite pure, exhibits all expected nrooerties of SnIa. When students heat i t vieorouslv in a test tub;, i t sublimes. The orange crystals, wken in a limited amount of water, form a white gellike substance. In excess water the product dissolves to form a clear solution. This solution is acidic t o litmus paper. When a few drops of silver nitrate solution are added, a cream precipitate forms; in the presence of lead nitrate a bright precipitate - yellow . forms. From such observations i t is expected that ail students will discover that the initial hydrolysis of tin iodide, forming the gellike t i n ( N ) hydroxide precipitate, goes further in excess water to form hydrated tin(1V) hydroxide, hydrogen ions and iodide ions. While we do not have our students do a melting point determination, i t has been
checked for numerous samples and is between 142 and 144 OC. . Aversion of the experiment, designed to he completed and re~ortedin a three-hour laboratow oeriod. is available from thk author.
Acknowledgment
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I would like to thank W. G. Baldwin for thevalued suggestions of a colleaeue and. for the extensivetestine.manv -. - eaeer " University of hianitobi students.
Volume 67
Number 8
August 1990
703
