In the Laboratory
Alternative pH-Shift Ion-Exchange Chromatography: Quantitative Spectroscopic Monitoring of the Progress of a Reaction Concepción López, Manuel Martínez,* Mercè Rocamora, and Laura Rodríguez Departament de Química Inorgànica, Facultat de Química, Universitat de Barcelona, Martí i Franquès 1-11, E-08028 Barcelona, Spain; *
[email protected] Chemical reactions are based on the changes that reactants undergo to become products. Although thermodynamic and static measurements are a good indicator of the existence of such transformations, monitoring the progress of these changes represents a better approach to their kinetico-mechanistic characteristics. This monitoring, which includes time as a non-chemical variable, is usually carried out via different spectroscopies. Some educational articles on this subject have appeared at different levels and with varying degrees of difficulty (1–5). Here we propose an experiment where the monitoring is carried out using two spectroscopic measurements. The aquation of complex [Co(H2PO4)(NH3)5]2+ to produce [Co(H2O)(NH3)5]3+ and free H2PO4− can be followed by UV–vis and 31P NMR spectroscopies (6–8). The parallelism of the exponential changes of the concentrations measured using both techniques is a good indicator of the quantitative measures involved as well as the concentration–time profiles expected for simple reactions (9). The preparation of the initial complex is also interesting and revealing for the students. A solvolysis process is carried out in highly acidic medium to displace a chloro ligand in the form of its volatile molecular acid (HCl) (10). The use of a simple aqueous cation-exchange chromatography in an alternative pH-shift ON/OFF manner is based on the deprotonation of a cationic complex. The deprotonation leads to the formation of a neutral complex showing different behavior on the ion-exchange column (7). The experiment was developed for an advanced inorganic chemistry laboratory and can be carried out in four laboratory sessions of four hours. This experiment can be easily completed with some other experiences, already published in this Journal (4, 5), that will lead to a proper laboratory project including different preparations and quantitative reaction monitoring via the study of concentration–time profiles. Procedure The reaction sequence and experimental monitoring are shown in Scheme I. Compounds The complex [CoCl(NH3)5]Cl2, I, is prepared by the standard literature procedure (11). The cation [Co(H2PO4) (NH3)5]2+, II, is obtained in solution from the solvolysis of the [CoCl(NH3)5]Cl2 in concentrated H3PO4 at 85 °C (10). Cation-Exchange Chromatography Column cation-exchange chromatography with an Amberlite IR120 resin of the concentrated H3PO4 reaction solution
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retains the cationic complexes from the mixture. Treatment of the retained complexes with 0.15 M NaOH produces the neutral [Co(PO4)(NH3)5] species, III, which is not retained by the resin (6, 7). Addition of NH3 and ethanol to the washed out solution produces, on cooling, the precipitation of the neutral [Co(PO4)(NH3)5] complex. Hydrolysis Reaction The neutral complex [Co(PO4)(NH3)5], III, is dissolved in 0.1 M HClO4 and the resulting solution of [Co(H2PO4) (NH3)5]2+, II, is heated at 75 °C to produce the hydrolysis to [Co(H2O)(NH3)5]3+ (7, 12–14), IV. Sample aliquots are taken at regular intervals, quenched by cooling, and their UV–vis and 31P NMR spectra recorded to monitor the progress of the hydrolysis process.
solvolysis reaction
[CoCl(NH3)5]2á
H3PO4
[Co(H2PO4)(NH3)5]2á II
I
HCl
cation-exchange chromatography
[CoCl(NH3)5]2á retained [Co(H2PO4)(NH3)5]2á NaOH
[CoCl(NH3)5]2á
[Co(PO4)(NH3)5]
I retained
III not retained
Há
[Co(H2PO4)(NH3)5]2á II
UV–vis and 31P NMR monitoring
ź
H2PO4
[Co(H2O)(NH3)5]3á Scheme I. The reaction sequence.
IV
Journal of Chemical Education • Vol. 85 No. 3 March 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory Table 1. Relevant Spectral Characterization Data for Complexes [Co(H2O)(NH3)]3+ and [Co(H2PO4)(NH3)5]2+
A
100 90
Species
[Co(H2O)(NH3)]3+
H2PO4−
[Co(H2PO4)(NH3)5]2+
λ/nm [ε/(M‒1 cm‒1)]a
δ (ppm)b
493 [47], 346 [44]
—
—
0.1
516 [64], 356 [48]
a UV–vis maxima in 0.1 M HClO ; from ref 6. 4 M HClO4; from ref 7.
b 31P
Extent of Reaction (%)
8.0 resonance in 0.1
80 70 60 50
Mobs at the 500 nm zone
40
Mobs at the 350 nm zone
30 20 10 0 0
200
400
B
Concentrated acids (H3PO4, HClO4) and alkalis (NaOH, NH3) are corrosive and irritants. Concentrated HClO4 is potentially explosive and should be manipulated carefully under a fume-hood. Cobalt(II) chloride hexahydrate causes eye, skin, and respiratory tract irritation and may cause cancer based on animal studies.
1000
1200
1400
1600
1400
1600
100
Extent of Reaction (%)
90
Results and Discussion
Hydrolysis Reaction The progress of any reaction can be monitored by any means capable of distinguishing the initial and final complexes and their relative concentration at any time within the process (Table 1) (9, 17, 18). In UV–vis spectroscopy the electronic spectra of the different complexes do not usually show separated bands. Only the knowledge of the molar absorptivities of the compounds at different wavelengths allow for the determination of the absolute concentration of the species in solution. If the absorbing species are only the reactants and products of the process, and provided the molar absorptivities of the samples are comparable, the wavelength of the maximum absorbance in the electronic spectrum represents a reliable measure of the progress of the reaction even if the individual concentrations cannot be determined spectrophotometrically. That is, the plot of the ratio (λobs − λmax,II)∙(λmax,IV − λmax,II) versus time indicates the extent of the reaction with time (Figure 1A). The shift to lower wavelengths of the two maxima observed agree with the higher field of H2O versus H2PO4− in the spectrochemical series.
800
Time / min
Hazards
Cation-Exchange Chromatography Preparative column ion-exchange chromatography usually involves the absorption of a complex sample on a resin from which the different ions are eluted at variable ionic strength, depending on their charge (15). In this respect the use of simple ON/OFF cation- or anion-exchange resins is normally limited to simple processes such as water deionization (16). In this experiment a different approach to the technique is used, the system is tuned from an ON to an OFF position by a pHshift. While the alkaline medium does not affect the charge of the [CoCl(NH3)5]2+, which stays ON the column, the [Co(H 2PO 4)(NH 3) 5] 2+ cation is changed to the neutral [Co(PO4)(NH3)5] species, which is washed OFF the resin.
600
80 70
coordinated H2PO4ź 31 P NMR peak (8.0 ppm)
60 50
free H2PO4ź 31 P NMR peak (0.1 ppm)
40 30 20 10 0 0
200
400
600
800
1000
1200
Time / min Figure 1. Monitoring the extent of the aquation reaction of complex [Co(H 2PO 4)(NH 3) 5] 2+ in 0.1 M HClO 4 at 75 ºC with (A) UV–vis analysis: (λobs − λmax,II)/(λmax,IV − λmax,II) versus time and (B) 31P NMR analysis: relative intensities of the signals at 0.1 and 8.0 ppm versus time.
31 P
NMR spectroscopy is capable of identifying all phosphorus-containing species in the reaction medium. Although the intensity of the 31P signals in different environments does not have the same response to concentration for Fourier-transformed spectra, the chemical characteristics of the 31P nuclei in coordinated and free phosphate anion are very close, and the relative quantification of their resonance signals is possible (19). The relative integration of the signals produces, as before, a good measure of the extent of the reaction with time (Figure 1B). The relative 31P chemical shift positions of the free and coordinated H2PO4− ligand can also be easily related with the shielding of the phosphorus nucleus in the different environments (8). Fitting of any curves to a single exponential produces the observed rate constant, k, according to the general first-order rate equation,
Xi = Xt= ∞ + (Xt =0 − Xt =∞)e–kt
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 3 March 2008 • Journal of Chemical Education
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In the Laboratory
where X is the extent of the reaction and t is the time. The practically equivalent values of the rate constant (0.0065 s‒1) obtained for the four sets of data are a good indication of the reliability of the methods used. Literature Cited 1. González, G.; Martínez, M. J. Chem. Educ. 2005, 82, 1671. 2. Orvis, J. A.; Dimetry, B.; Winge, J.; Mullis, T. C. J. Chem. Educ. 2003, 80, 803. 3. Martínez, M.; Muller, G.; Rocamora, M.; Rodríguez, C. J. Chem. Educ. 2007, 84, 485. 4. Moody, G. J.; Thomas, J. D. R. J. Chem. Educ. 1963, 40, 151. 5. Phillips, W. M.; Choi, S.; Larrabee, J. A. J. Chem. Educ. 1990, 67, 267. 6. Coronas, J. M.; Vicente, R.; Ferrer, M. Inorg. Chim. Acta. 1981, 49, 259. 7. Schmidt, W.; Taube, H. Inorg. Chem. 1963, 2, 698. 8. von Seel, F.; Bohnstedt, G. Z. Anorg. Allg. Chem. 1977, 435, 257. 9. Espenson, J. H. Chemical Kinetics and Reaction Mechanisms; McGraw Hill Book Co.: New York, 1981. 10. Martínez, M.; Pitarque, M. A. J. Chem. Soc., Dalton Trans. 1995, 4107. 11. Schlessinger, G. G. Inorganic Laboratory Preparations; Chemical Publishing Company Inc.: New York, 1962. 12. Ferrer, M.; González, G.; Martínez, M. Inorg. Chim. Acta. 1991, 188, 211.
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13. Martínez, M.; Ferrer, M. Transition Met. Chem. 1984, 9, 395. 14. Martínez, M.; Ferrer, M. Inorg. Chem. 1985, 24, 792. 15. Szafran, Z.; Pike, R. M.; Singh, M. M. Microscale Inorganic Chemistry. A Comprehensive Laboratory Experience; John Willey and Sons: New York, 1991. 16. Kunin, R.; Downing, D. G. Chem. Eng. 1971, 78, 67. 17. Perkampus, H. H. UV-Vis Spectroscopy and its Applications; Springer: Berlin, 1992. 18. Wilkins, R. G. Kinetics and Mechanisms of Reactions of Transition Metal Complexes; VCH: Weinheim 1991. 19. Ebsworth, E. A. V.; Rankin, D. W. H.; Cradock, S. Structural Methods in Inorganic Chemistry; Blackwell Scientific: Boston, 1991.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Mar/abs426.html Abstract and keywords Full text (PDF) Links to cited JCE articles Supplement Detailed preparative procedure for the students
Instructor notes
Spectral data
Journal of Chemical Education • Vol. 85 No. 3 March 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
