In the Laboratory
Trisethylenediaminecobalt(III) Chloride Sulfate as a Subject Material for Widely Different Chemistry Laboratory Courses Yoshiki Moriguchi Department of Chemistry, Fukuoka University of Education, Akama Munakata, Fukuoka 8114192, Japan
I previously reported on new chemistry laboratory curricula that unify several courses by use of a single substance that can be used in a variety of experiments. Such substances include Mohr’s salt (ammonium iron(II) sulfate hexahydrate),
cis-,trans-[CoCl2(en)2]Cl
The material is quickly but sufficiently washed with a little hot water, ethanol, and ether, using suction. After airdrying for one hour at 90 °C in an oven, the trihydrate product is obtained as yellow-orange powder: Mr = 424.48 for [Co(en)3]ClSO4⭈3H2O; [α]589 = 0. In this lab (lab I-1 in Table 1), students learn the role of activated charcoal in this procedure. They also learn about complexation in a typical inorganic reaction and the difference in lability between cobalt(II) and cobalt(III) complexes.
where en represents ethylenediamine. These substances have been used for courses that unify inorganic, analytical and physical chemistry laboratory courses. The curricula have been implemented at our college by shifting teams of 2 or 3 students from one lab to others. The curricula have provided enhanced educational effects on the chemical lab classes despite limited lab hours, equipment, and staff (1–6 ). In this report, I describe an expanded lab curriculum that includes organic chemistry (stereochemistry) by using rac-tris(ethylenediamine)cobalt(III) chloride sulfate ((±)589[Co(en)3]ClSO4), as shown in Table 1. The curriculum could be adapted to sophomore or junior chemistry major laboratory courses that have 15 lab sessions in a half or full semester. Credit is 2 or 3 credit hours for a full semester of one 200minute lab class per week or a half semester of two 200-minute lab classes per week.
Electronic Spectrum In the UV–visible region, the spectrum should have two peaks (λmax) at 340 and 469 nm with log ε = 1.9, almost the same as tris(ethylenediamine)cobalt(III) chloride, [Co(en)3]Cl3. These peaks are assigned to the cobalt(III) in a crystal field of octahedral symmetry. We have students prepare a 0.01 M aqueous solution of the material they made and measure its electronic spectrum in the range λ = 300–600 nm. They identify the cobalt(III) complex ion and determine its optical purity by the absorbance at maximum wavelength (λmax). In this lab (I-2), students learn about absorption spectra of metal complexes in the UV-visible region and the basis of the absorption spectra on crystal field theory. They also learn the use of the Beer–Lambert law in spectrophotometric determination.
Fe(NH4)2(SO4)2⭈6H2O and dichlorobis(ethylenediamine)cobalt(III) complexes,
Preparation and Identification of the Material to Be Tested
Preparation In the first stage, the students individually prepare tris(ethylenediamine)cobalt(III) chloride sulfate as a common material for analysis. The preparation and purification of this material can be achieved as described in the first part of ref 7 except for the method of separation of the solid-state material, [Co(en)3]ClSO4, from the reaction mixture. activated C
4CoSO4 + 12en + 4HCl + O2 → 4[Co(en)3]ClSO4 + 2H2O
Elemental Analysis Students individually analyze their material for cobalt, chloride, and sulfate. Here, they learn the principles and techniques of volumetric and gravimetric analyses. Cobalt The sample is pretreated by acid decomposition to make possible the determination the metal content of such chelate compounds by chelatemetry (8). The solution of the pretreated material is adjusted to pH 8 by 0.1 M NaOH and 0.5 M NH4OH, and the cobalt content is determined chelatemetrically with 0.005 M EDTA, using 0.1 g of a 1:100 (w/w) Murexide–potassium sulfate mixture
Table 1. Syllabus for Unified Inorganic, Analytical, Physical, and Organic Chemistry Lab Classes Title
Techniques
Learning Objectives
I. Preparation of Testing Material 1. Synthesis of [Co(en)3 ]ClSO4 Oxidation of Co(II) by activated charcoal and air; separation and purification Catalytic reaction; lability of complex 2. Electronic Spectrum
Identification by UV–vis spectrometry
Crystal field theory; Beer–Lambert law
II. Elemental Analysis 1. Cobalt
Chelatometric titration after acid decomposition by HNO3 –H2 SO4
2. Chloride
Precipitation titration by Mohr method after pretreatment by cation exchanger Ion exchange; solubility product
Pretreatment in chemical analysis; chelatometry
3. Sulfate
Gravimetric analysis as BaSO4 after pretreatment by cation exchanger
1. Optical Resolution
Fractional recrystallization of diastereoisomer for tartrate
Optical isomer; racemate; diastereoisomer
2. Racemization
Measurement of racemization rate by polarimeter
Kinetics of reaction rate
Instrumentation of weighing by balance; solubility
III. Optical Isomers
IV. Discussion and Examination on the Results
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In the Laboratory
as a metal indicator (9). Calcd for [Co(en)3]ClSO4⭈3H2O: Co, 13.87%. In this lab (II-1), the students learn the pretreatment technique, a fundamental analytical process. They also learn the principle of chelatemetry and relate it to the stability constants for the complexes of the metal ion (cobalt) with the complexane (EDTA) and with the metal indicator (Murexide). Chloride The sample is pretreated with a cation-exchange resin in order to prevent interference by the complex ion, [Co(en)3]3+, in the precipitation titration (Mohr’s method). The chloride content is determined in the usual manner by Mohr’s method, that is, titration of the effluent with 0.01 M silver nitrate using 10% potassium chromate as an indicator. Calcd for [Co(en)3]ClSO4⭈3H2O: Cl᎑, 8.35%. In this lab (II-2), students learn (i) the principle of separation by ion exchange and its application, and (ii) solubility product in relation to the principle of precipitation titration. Sulfate As in the chloride determination, the sample is pretreated by cation exchange resin to avoid interference from a complex ion, [Co(en)3]3+. Sulfate is then precipitated as barium sulfate. Calcd for [Co(en)3]ClSO4⭈3H2O: SO42᎑, 22.60%. The objectives of this lab (II-3) are to have students use the fundamental procedures of gravimetric analysis to learn about solubility, coprecipitation, aging, and peptization in the formation of a precipitate. Optical Resolution
Diastereomers of Tartrate and Their Separation The procedure is almost the same as described in the latter half of ref 7, as shown in Scheme I: (+ −)-[Co(en)3 ]Cl·3H2O Ba·L(+)-tart filtration
filtrate(1)
precipitate(1)
(+)-[Co(en)3 ]Cl·L(+)-tart (−)-[Co(en)3 ]Cl·L(+)-tart
(BaSO4)
filtrate(2)
precipitate crystal(1) (+)-[Co(en)3 ]Cl·L(+)-tart [α]589 = +102
conc NH4OH NaI conc NH4OH NaI
crystal(3) (−)-[Co(en)3 ]I3·H2O [α]589 = −90
crystal(2) (+)-[Co(en)3 ]I3·H2O [α]589 = +89
Scheme I
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N N
N N
N
Co
N
Co N
N N
N
N
N
Λ-form
∆-form
(+)589-[Co(en)3 ]3+
(−)589-[Co(en)3 ]3 +
In this lab (III-1), students learn the stereochemistry of metal complexes and the meaning of symbols such as (d,l ), (D, L), ((+),(᎑)) and (∆, Λ).
Racemization The optically active tris(ethylenediamine)cobalt(III) complex has been reported to completely racemize in the presence of activated charcoal in 2 min at 90 °C in spite of there being no change after 24 hours on a steam bath without the activated charcoal (11, 12). The recommended method for measuring the racemization rate is as follows. The sample solution, which has activated charcoal suspended in it, is placed in a three-necked flask fitted with a reflux condenser and a thermometer. It is allowed to stand at 60 °C on a water bath with stirring. Samples of the solution are removed with a pipet at 15- or 20-min intervals and filtered through a glass filter using suction, and their optical rotation is measured using a polarimeter. The racemization goes essentially to completion within 1.5 hours. Students may carry out a blank experiment in the absence of activated charcoal in same manner if there is enough time and equipment in the lab. The result shows that the reaction rate is first order. In this lab (III-2), students learn the kinetics and rate law for a chemical reaction. Acknowledgment I gratefully acknowledge emeritus professor John W. Hill of the University of Wisconsin–River Falls for help with my English in the manuscript.
concentraion cooling filtration
(−)-[Co(en)3 ]Cl·L(+)-tart
Thus, the racemic form of the sample material is separated into the optically active isomers (+)589- and (᎑)589-[Co(en)3]3+I3 (Crystal(2),(3))by fractional recrystallization of the diastereomers (Filtrate(1)). The specific rotations are [α]589 = +89° and ᎑90° for (+)589- and (᎑)589-[Co(en)3]I3⭈H2O, respectively. The absolute conformations of (+)589- and (᎑)589- isomers are determined by X-ray diffraction to be Λ and ∆ forms, respectively, as shown in the structures below (10).
Literature Cited 1. Moriguchi, Y. J. Chem. Educ. 1987, 64, 955. 2. Moriguchi, Y.; Muraishi, H. Kagaku To Kyouiku (Chem. Educ.) 1990, 38, 336–337. 3. Moriguchi, Y.; Muraishi, H. Bull. Fukuoka Univ. Educ. 1992, 41 (Part IV), 475–479. 4. Moriguchi, Y.; Muraishi, H.; Akiyama, E.; Mine, Y. Kagaku To Kyouiku (Chem. Educ.) 1991, 39, 444–445. 5. Moriguchi, Y. Presented at The 8th International Conference on Chemical Education, Tokyo, Aug 1985; Abstract 4C160p. Moriguchi, Y.; Muraishi, H. Presented at The 11th International Conference on Chemical Education, York, UK, Aug
Journal of Chemical Education • Vol. 77 No. 8 August 2000 • JChemEd.chem.wisc.edu
In the Laboratory 1991; Abstract P3.25. 6. Moriguchi, Y.; Muraishi, H.; Akiyama, E.; Mine, Y. Bull. Fukuoka Univ. Educ. 1992, 41, Part III, 87–94. 7. Broomhead, J. A.; Dwyer, F. P.; Hogarth, J. W. Inorg. Synth. 1960, 6, 183–186. 8. Tuchitani, Y.; Tomita, Y.; Ueno, K. Talanta 1962, 9, 1023.
9. Flaschka, H. Mikrochem. Mikrochim. Acta 1952, 39, 38. 10. Iwata, M.; Nakatsu, K.; Saito, Y. Acta Crystallogr., Sect. B 1969, B25, 2562. 11. Douglas, B. E. J. Am. Chem. Soc. 1954, 76, 1020–1021. 12. German, W. G.; Fernelius, W. C. J. Inorg. Nucl. Chem. 1959, 9, 71–81.
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