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Chemistry 223-Spectrophotometric Co/Ni 1 SPECTROPHOTOMETRIC DETERMINATION OF COBALT AND NICKEL References: 1. C, Chaps. 13 (pp. 371-376), 16 (pp. 451-455). 2. H7, Chaps. 19, 23, and 25 3. S & W, Chaps. 15 (especially pp. 443-49), 17 (pp. 487-89). 4. SWH, Chaps. 18, 20 (pp. 505-518), 25 (pp. 652-653), 29 (pp. 715-716). 5. J. S. Fritz & G. H. Schenk, Jr., Quanta. Analyt. Chem., 4th ed., (pp. 582 & 621). 6. J. A. Dalziel and A. K. Slawinski, Talanta 15 , 367-372 (1968). 7. J. Bjerrum, A. S. Halonin, and L. H. Skibsted, "Studies on Cobalt(II) Halide Complex Formation. I. A Spectro-photometric Study of the Chloro Cobalt(II) in Complexes Strong Aqueous Chloride Solutions," Acta Chem. Scand. 29A, 326-332 (1975). Purpose Determination of multiple components in a single unknown is frequently required. An analyst has a choice of attempting a simultaneous determination or performing a preliminary separation. Each approach has its advantages and drawbacks. Because few techniques are truly specific, any simultaneous determination usually suffers from some mutual interference between the combined components, with resulting loss of accuracy and/or precision. Frequently, however, the resulting uncertainties are less than those introduced by the extra sample handling required for preliminary separation. Furthermore, simultaneous determinations are often faster, and therefore more efficient, than those relying on separations. Generally, the choice of a preferred method must be based on the analyst's assessment of the balance among these factors. In this experiment, the amounts of Co(II) and Ni(II) in a mixture will be determined by two different approaches. In one procedure, the Co(II) and Ni(II) will first be isolated by ion exchange and the separated components assayed. In the other procedure, the Co(II) and Ni(II) will be assayed simultaneously without separation. In both cases, spectrophotometry will be used for quantitation. The separation employs an ion exchange column, and depends on the selective formation of an anionic complex of cobalt(II). In 9 M HCl the principal equilibrium forms of the two metal ions are CoCl 4 2- and Ni 2+ (or NiCl + ). The Co(II) ions are exchanged onto the resin: 2 Res-NR 3 + Cl - + CoCl 4 2- (Res-NR 3 + ) 2 CoCl 4 2- + 2Cl - and the Ni(II) passes through the resin. The R group is often -CH 3 . In 4 M HCl the CoCl 4 2- ion breaks down into cationic forms of cobalt(II) and all the cobalt(II) passes through the resin. Determinations of the separated components (and of the ions in the mixture) will employ a spectrophotometric method based on the absorbance of the Co(II) and Ni(II) complexes of quinoxaline-2,3-dithiol. This reagent is unstable as a solid, and is only stable for a matter of hours in solution. However, its precursor, S-2-(3- mercaptoquinoxalinyl)-thiuronium chloride, is quite stable. In aqueous ethanol solution, the precursor is readily hydrolyzed to form quinoxaline-2,3-dithiol when an ammonia-ammonium chloride buffer (pH 10) is added,
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This note was uploaded on 01/06/2011 for the course CHEM 223 taught by Professor Scheeline during the Fall '08 term at University of Illinois at Urbana–Champaign.

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