An Inorganic Spectrophotometry Experiment for General Chemistry A Simplified Graphical Approach for Determining ~ i ~ a n d - t o - ~ 'Complexation etal Ratio Henry N. Po and Kenneth S.-C. Huang California State University, Long Beach, CA 90840 There are several spectrophotometric methods ( I , 2) for determining the stoichiometric ratio of ligand coordinated to transition metal ion. These methods include the molar ratio method ( 1 4 ) , Job's method of continuous variation (1,2, 4, 51, the slope-ratio method (2) and the Bjerrum method ( I , 6). I n the Job's method of continuous variation, absorbance is plotted against the mole fraction of the metal and the ligand. This method is adopted by several genI n this paper eral chemistry laboratory textbooks (7,8,9). we describe an approach that simplifies and reduces the calculations for determining the stoichiometric ratio within a metallo complex. A complexation reaction for a metal ion, M, with a n organic ligand, L, is shown below. For simplicity, the charges are not included.
The stoichiometric ratio, bla, corresponds to the number of moles of ligand coordinated to one metal ion. Procedure and Results The following solutions are needed for the Fe(phenhh spee trophotometry experiment: 1.00 x 1@M (NHdzFe(S0dz (with 3 mL wnc. HzS04added to 1L solution), 1.26 x lo3 M 1,lO-phenanthroline (phen), 2 M sodium acetate, and 3 M and NHzOH. lhnsfer exactly 0,2,3,4,6,8,10,12,14,16,18, 20 mLFe2+solutionto 100-mLvo1umetricflasks. Add in serial order to the above solutions 20, 18,17,16, . . .8,6,4,2, and 0 mL phen solution. 'lb each flask add 5 mL of 2 M sodium acetate. about 4 mL of 3 M NH?OH. dilute to mark with distilled wa&, and mix. Wait five to i 0 mjnutes. S d u m acetate buffer is used to maintain the solution at pH 5 while hydroxylamine ensures that iron is in +2 oxidation state. lhe absorbance of ..510 nm usinaa each Fe(I1,-ohen solution is measured at L Shimadm lh160 spectmphotometer. (other models of s 6 e trophotometers can be used just as well.) The data points for three Fez+concentrations are plotted and fitted with straight Stoichiometry of Fe(ll) Complexes of Terpy, Bipy, and Phen mole ratio UFe(ll) 1.39x 10" bipy 1.39 x bipv 1 . 2 6 104ph;n ~ 196~10~phen 1 . 2 6 103phen ~ 1.00 x lo4 terpy 1O . O x 10" terpv
3.02 2.95 3.07 2.94 3.00 2.13 1.OO 2.12 Absorbance measured at the following Lax: Fe(bipy)z2*= 521 Fe(phen)s2*= 510 nm: ~e(ter~~)2~'=552 nm. 1.50 1.OO 1.50 I .OO 0.70 1.50
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Journal of Chemical Education
mL Fe(ll) Absorbance versus volume Fe(ll) plot for ~e(phen)?+complex. + symbol, 1.5 x lo3 M Fe(ll) ;squares, 1.0 x 10"M Fe(ll);and triangies, 0.70 x 10" M Fe(ll). lines a s shown in the figure. For each series of Fez+, a "peak" volume of Fez+is obtained a t the intersedion of the two data lines. This corresponds to a fully wmplexed imn(I1)-phen. The "peak" volume of phen is by difference because the total volume of Fez+and phen is kept constant a t 20 mL. Alternately, one can obtain "peak" milliliters of phen directly from a double abscissa plot where the volume of phen (in reverse order of 20, 18, 16, . . .2,O mL)is the second abscissa. Mole ratios calculated as phen/Fe(II) are listed in the table. Two other polypyridines, 2,Z'-hipyridine (bipy) and 2,2',2"-terpyridine (terpy), were used in the experiment. These results are also listed in the table. The stoichiometric ratios determined by this simplified approach for the three iron(I1)-polypyridine complexes are precise and accurate. The deviation from the theoretical value is less than 1.3% for bipy and phen and 6.2 % for terpy. I n the figure, one observes that the peak location is changed when Fez+concentration is varied. Thus, it is possible for students to prepare different concentrations of Fe(I1) and ligand for the spectrophotometry experiment. The peak volume may change for a series of M-L combination but the same stoichiometry should be obtained. Literature Cited 1. Bauer, H.H.;Christian. G. D.; O%illy, J.E.InsfrumnfolAn~lysU;AllynandBam: Boston.1978;Chapter 7.
nm:
2. Wlllard. H. H.: Memitt, L. L.,Jr; Dean,J. A Instrumntd Methtbods ofAndysis, 5th sd.; 0. Van Nostrand: New York,1974:Chapter 4. 3. Jakubiec, R.;BolU,D. EAm1. Chem. 1969,41,13.
4. Cheng, K h lnSpPetmekmimlMethodpofAnalyais; Wlnefordner J. D.. Ed.; WileyInterstienee: New York, 1971: Chapter& 6. a. Job, P A M I Chim 1988,9, 113. b.Vo8burgh. W C.: Cooper, G. R.J h r Chem. SN. 1941, M , 437. 6. Bjermm, J. Kg1 Don& Vhnskab. S d ~ k a b Mot3ys. , MPdd l s M , Z 1 H ) . Olemp, H. JmKlori&mos Komplerilel Thesis: Lund, 1944
7. Whitten, K W.; Gailey, K D.;Bishop, C. B.; Bishop, M . 8. Exprimenfs in & m r d Chemistry: Saundem: Philadelphia, 1988; Experiment 25. 8. WahLey, J. A ; Walmsley, E Chemical Principles, P m p w t b , and &mtions in f k Lobomfory: Addison-Wealey: Reading MA, 1986: Experiments 12 and 14. 9. BaLer, A. D.:Glies, L. F;Nsridi, M. H. Labomtory Monild to Accompany Chemistry: West: St. Paul MN,1990; Experiment 32.
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