Answer Key Post ‐ lab Quiz Second Week of Co ‐ Ni Lab Chemistry 223 October 22, 2010 1. You did not add quinoxoline to the metal ion samples; instead you added quinoxaline ‐ 2,3 ‐ dithiol. Why? (1) Quinoxaline ‐ 2,3 ‐ dithiol is stable at room temperature. The complexing agent itself is unstable in aqueous solution and rapidly decomposes. 2. It was emphasized repeatedly that the order of addition in the volumetric flasks was buffer, then complexing agent, then metal ion. What would have happened if the order of addition was metal ion, then buffer, then complexing agent? (1) Because of the high pH, the metal ions would precipitate, meaning that reaction with the quinoxoline would be slow (and require re ‐ dissolution). This would either make quantitative reaction more difficult or prevent it altogether. 3. Cu 2+ can also complex with quinoxoline. The peak absorbance wavelength is different than for Co 2+ or Ni 2+ . If you wanted to assay these three ions spectrophotometrically, based on your experience in
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peak absorbance wavelength, metal ion samples, Key Post‐lab Quiz, Co‐Ni Lab Chemistry, separate working curves