Week 6: Coordination Chemistry 2 Overview Getting Started Techniques Procedure FAQ Full Lab Manual Introduction & Goals Chemistry & Background Key Questions Prelab Problems Safety Procedure In Your Write-up Experiments Index ChemLab Home		Colorimetric Cobalt Analysis In order to determine the total amount of cobalt in a sample colorimetrically, it is necessary to convert all of it into the same form. Ideally, the form chosen should have a large extinction coefficient at a convenient wavelength so that the method will be sensitive. In fact, one of the prime virtues of colorimetric methods is that they often are more sensitive than techniques based on titrations or gravimetric methods. High sensitivity is essential for many analyses of practical importance since, for example, reagents present in natural water systems or in clinical samples may have profound impact even though they only occur in trace quantities. The cobalt analysis involves converting your cobalt complex to a bright blue thiocyanate complex of Co(II), [Co(SCN)4]-2. Thiocyanate (SCN-) is an anion that behaves much like the halides as a ligand. Thus, for instance, [CoCl4]2- is also a bright blue cobalt(II) complex which could also be used as a basis for colorimetric analysis. The thiocyanate complex [Co(SCN)4]-2 is chosen in this analysis because its extinction coefficient, at the wavelength of peak absorbance, is much larger than that of [Co(NH3)5 (H2O)]3+. A colorimetric analysis of the thiocyanate complex will be able to detect lower concentrations of cobalt-containing material than a direct analysis of your synthesized compound. Nearly all cobalt(III) amines can be converted to [Co(SCN)4]2- by the following series of reactions: [Co(NH3)5 (H2O)]3+ + 3 OH- CoO(OH)(s) + 5 NH3 + 2 H2O 3 H+ + 4 H2O + CoO(OH)(s) + Cl- [Co(H2O)6]2+ + 1/2 Cl2 [Co(H2O)6]2+ + 4 SCN- [Co(SCN)4]2- + 6 H2O The key step involves heating the amine complex in concentrated base (the first reaction above), the one set of conditions under which the coordinated ammonia is quite easily lost from cobalt(III). The product is CoO(OH), a highly insoluble, finely divided brown solid, the hydroxyoxide of cobalt(III). In the second reaction, it dissolves readily in hydrochloric acid and the chloride ion reduces the cobalt(III) to cobalt(II). Finally, in the third reaction, the Co(II)hexaaquo complex is converted to the thiocyanate complex [Co(SCN)4]2-. The formation of the thiocyanate complex is carried out in a mixed solvent system made up of 50% water and 50% acetone. Acetone (CH3COCH3) is a polar organic ketone that is an exceptionally good solvent for many kinds of organic and inorganic compounds. It is used here to lower the concentration of water while still providing a polar enough solvent for the ionic solutes. The competition between water and thiocycanate for coordination of the cobalt(II) favors thiocyanate more strongly in aqueous acetone than in pure water because of the lower water concentration. The absorbance measurement is made at the wavelength of maximum light absorption for [Co(SCN)4]2-. Since the complex is blue, it is not surprising that the maximum absorbance is in the red-orange region of the spectrum at 625 nm. Figure 2 shows the full visible absorption spectrum of the complex. The colorimetric cobalt assay should establish that the synthesized material is indeed a cobalt complex salt (or mixture of such salts) with a relative molecular weight about 350. Figure 2 Absorption Spectrum of [Co(SCN)4]2- Sample concentration is 2.5 x 10-4 M in 50% aqueous acetone and path length is 1.0 cm. The blank is 50% aqueous acetone. Gravimetric Analysis To further test for the purity of your cobalt complex, you will try to detect the possible presence of ligand-substitutional impurities of about the same molecular weight with two additional, more specific analyses. These assays illucidate the detailed structure of aquapenta-amminecobalt(III) nitrate. Next week, you will perform an assay to examine the acidity of the water ligand. This week you will perform a "gravimetric" assay for the presence of a weakly bound H2O ligand coordinated to the Co atom. In this experiment, the complex will be heated until the water ligand is removed and the dehydrated mass compared to the initial mass, to find the % H2O by mass in your sample. The gravimetric assay is a simple substitution of one of the "outer" nitrate counter-ions for the "inner" H2O ligand: [Co(NH3)5(H2O)][NO3]3(s) [Co(NH3)5(NO3)][NO3]2(s) + H2O(g) The conversion occurs quantitatively in a 100-110 °C oven in about an hour.
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