A New Oxidation-Reduction Experiment At the present time, the beginning chemistry student is most often introduced to stoichiometry via an oxidation-reduction titration. Several of the experiments available in the literature ask the student to determine the electron change for a given titration r e a c t i ~ n .Other ~ , ~ exercises involve reduction of a soecies. under a varietv of conditions. to different oxidation states and the subseauent reoxidation b y a titration reacti~n.~~nfortunatel;, in most of these experiments, the student i's not introduced to new techniques and only a few experimental results can be obtained during one laboratory periad. A new experiment has been developed t o introduce a new technique, speetmphotometry, and to increase the number of determinations possible during a single laboratory period. The student is first expected to prepare a calibration curve far the absorbance of an oxidant. sueh as cerate or permanganate, over a variety of concentrations. Good linear plats have consistently been obtained by students using only graduate cylinders and disposable graduated pipets. Cerate solutions are preferable to permangsnate stock solutions which are somewhat unstable. After the calibration has been completed, a solution of a reducing agent, sueh as arsenite perioxide or iodide, is added to a fixed volume of cerate. The new absorbance measurement is made and the student calculates the males of cerate reacted from the change in absorbance as a result of adding the reducing agent. The importance of dilutions is clearly brought home to the student in this calculation. By using the initial concentration and volume of the reducing agent and the moles of cerate consumed by the reaction, the student calculates the mole ratio of cerate to reducing agent. Finally, the student writes an equation for the chemical reaction. It is quite feasible to determine the stoichiometry of several reactionsduring a3-hrlaboratoryperiod.
Experimental Procedure Standard 0.001 M solutions of cerium(N) are provided for each student. This solution is used to prepare four diluted solutions in the concentration range from 7.5 to 1.25 X M. The absorbance of these solutions was measured a t 375 nM. The calibration curve is plotted and then the absorbance of a 5.0 ml sample of 0.001 M Ce(IV) is measured. The 5-ml aliquot is next treated with a 5.00 ml sample of 0.001 MI-. After the solution is thoroughly mixed, the absorbance of the reacted solution is measured. Finally, the student is asked to calculate the moles of C e W ) reacted, the moles of I- used, and thus the mole ratio for the reaction of thesespecies. Other reducing agents sueh as AsOJ3- are also reacted in a similar manner to determine the stoichiometry. The arsenate reaction required osmium tetroxide as a catalyst. It should be possible to employ this general procedure for other oxidizing agents and to expand the number of reducing agents used. Results indicate that the majority of the students were able to obtain the proper mole ratio for the three reactions investigated. Care must be taken to account for the dilution processes or meaningless results are obtained. 'Tietz, H. R., J. CHEM. EDUC., 40,344 (1963). ZLatimer, George W., and Ragsdale, Ronald O., "Modem Experimental Chemistry," Academic Press,New York, 1971, p. 91. 4 Conroy, Lawrence E., and Tobias, R. Stuart, "General Chemistry Laboratory Operations," The Macmillan Company, New York, 1965, p. 155.
T. R. Williams J. D. Van D o r m College of Wooster Wwster, Ohio 44691
Volume 51. Number 6. June 1974
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