A Freshman Laboratory Program without Chromium Mary F. Wilcox and Judith G. Koch lthaca College, Ithaca, NY 14850 For a number of years Ithaca College has offered a fivesemester sequence of projecboriented laboratory courses. While this program has been strikingly successful, the evolving safety and disposal standards have led us to revise the first course of the sequence, given in the second semester of the freshman y e a r . ~ ~ ~ ~ ~ ~ l chmmiumassifies VI compounds as carcinogenic, and the EPA requires the expensive disposal of chromium compounds as "hazardous wastes". The first eight weeks of this course have been a modified version of the course designed by Gray, Swanson, and Crawford ( I ) and published in the book, ProjectACAC:An Experimental Investigation in Synthesis and Structure. In our course, the students prepared and purified their "research compounds" tris(acetylacetonato)chromium(III) and the corresponding cobalt compound and determined their compositions using standard colorimetric, volumetric, and gravimetric analytical techniques. In recent years it became clear that the toxicity and the cost of disposal of the hazardous chromium compounds required that experiments involving this element should be eliminated.-The challenge was to preserve the solid pedagogy provided by these exoeriments while eliminatine chromium. The experiments involving the chromium compounds focused on the characterization of newly synthesized compounds and served to convey the nature of a typical research topic and how a chemist proceeds to answer questions about it. The bonding and structure in these acac comoounds were investieated bv analvsis of their infrared sdectra as well as by The abiorptiln bands in the ultraviolet and visible reeion. The students learned not only basic analytical tech&ues, but also they learned how to use instrumentation to find meaningful answers to chemical questions. Other metal acetvlacetonates were investieated for their ease of synthesis, 6 w toxicity level, and adaptability to titrimetric and colorimetric methods of analvsis. We found that the chromium acac could be replaced by two environmentally more benign complexes: the red crystalline tris(acetylacetonato)iron(III)and the dark brown crystalline tris(acetylacetonato)manganese(III). The iron acac is synthesized from iron(II1) chloride in an aqueous sodium acetate solution containing 2,4-pentanedione.After recrystallization from 2-pmpanol, students typically get a 60-70 percent yield. The manganese(II1) acac is formed in an aoueous solution of maneanese(I1) chloride. sodium acetate, potasslum permangnnate (as an rn s ~ t uoxldant) and the acac llrnnd. The dark sohd can be recmstallized from cyclohexane. The detailed procedures for the preparation ~ v e in n the exverimental secof these two complexes is tion. The oereent maneanese is determined with ease colorimetri/ally by treatment of a sample with acid and sodium ~eriodateuntil the ~ u r u l ecolor of MnOn-is developed. For class of 25 students, 22 students (3 students were clearly outliers) produced an average value of 15.2% (3% error). The actual value is 15.6%. This was the first exposure of the students to analytical techniques and in particular their fxst use of a pipet. The iron(II1) acac compound is analyzed titrimetrically by using an iodometric method.
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Journal of Chemical Education
The Fe(CsH70z)3is dissolved in acid and treated with an excess of potassium iodide in the presence of urea (to remove oxides of nitrogen) followed by titration with standard thiosulfate solution. Before beginning the titration NaHC03 is added to blanket the solution with COz to prevent reoxidation by air. Students prepare and standardize their thiosulfate solution. In another class of 15 students the average error was 3.0% and the average deviation was 1.3%. Experimental Details Add with stimng 12 mL (0.12 mol) of 2,4-pentanedioue and 0.55 g (0.0035 mol) of KMnOl to 2.50 g (0.0071mol) of MnC12.4Hz0 and 7 g of NaHCz&02.3Hz0 (0.051 mol) in 100 mL of water. Add a n additional 7 g of NaHC2H3O2.3Hz0and stir the mixture for 5 min. Then heat the mixture a t 70 O C for 10 min. Collect the dark solid and wash with a 0.1 M aqueous solution of 2,4-pentanedione (to remove impurities, but with suppression of the dissociation of the complex). The air-dried solid can be recrystallized from cyclohexane with a 70% recovery, mp 169. Colorimetric Analysis of Mn(C5H702)3 Add 2 mL of 6 M HzS04and 5 mL of concentrated HN03 to a 0.1-g sample of Mn(CsH702)3.Heat the solution gently until it is colorless (25 min). Add 25 mL of distilled water and make the solution uo to volume in a 250-mL volumetric with water. %eat a 2 4 - m ~ aliquot with 50 mL of water, 6 mLof6.M HBO. and 1.0eof NalOa and heat the solution a t 80 T for i 5 &in in o d e r to develop the M n 0 4 color completely. Dilute the solution quantitatively to 250 mL with water and read the absorbance a t 535 nm. Preparation oftris(Acetylacetone)iron(lll) Add with stirring 5.0 g (0.037 mol) of NaCzH4Oz.3Hz0 dissolved in 25 mL of HzO to a solution of 2.7 g (0.010 mol) of FeCIq.6H.O in 25 mL of HqO. Add 5.0 mL of 2.4-oen.. . tanedione to the solution and stir for an additional 10 min. Recrvstallize the crude oroduct from 2-orooanol (70%
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Titrimetric Analysis of Fe(C5H702)~ Heat gently for 25 min (in triplicate) 0.17 g of Fe(CsH70z)3in 5 mL of 6 M HCI and 2.5 mL of mncentrated HN03. Cool the solution and add 40 mL of HzO, 10 mL of 3 M HC1 and 0.5 g of urea and then heat the solution to 60 'C for 5 min. Add 2-2.5 g of KI along with a small portion (-0.1 g) of NaHC03. Keep the flask sealed with Parafilm until you are ready to titrate the solution. Titrate with standard thiosulfate. Literature Cited 1. Gray,H.; Swansan,X.; Craufmd, X. P m j d ACACAn &perimonfdlnmstig.fion in synrheaia and stnreturs; ~ o g d e nand wgky, ~srrytorryto-on-~udson: NY,1912.
