experiment#7.docx - Experiment#7 Spectrophotometric Determination of a Mixture of Cobalt(II and Nickel(II Jared Cox Lab partner Alex Vogel CHEM 3360-001

experiment#7.docx - Experiment#7 Spectrophotometric...

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Experiment #7 Spectrophotometric Determination of a Mixture of Cobalt (II) and Nickel (II) Jared Cox Lab partner: Alex Vogel CHEM 3360-001 TA: Kasun Ratnayake Performed: February 25, 2019 Due date: March 13, 2019 1
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ABSTRACT: The purpose of this experiment was to determine the concentration of cobalt and nickel in an unknown solution. This was accomplished using a UV-visible spectrophotometry to obtain calibration curves of known concentrations. From these curves, Beer’s law was used to calculate the concentration of nickel and cobalt in the unknown solution. To produce one specific species of nickel and cobalt, the metal ions were complexed with EDTA. The absorbance values at different wavelengths were recorded by the UV-Vis spectrophotometer for each metal-EDTA complex. From these absorbance values, calibration curves were plotted. Using Beer’s law, the concentrations of cobalt (II) and nickel (II) in the unknown solution were determined to be 3.3592 M and 5.9082 M, respectively. The determinations were in % error and % error from the true value of 0.06000 M Ni 2+ and 0.04000 M Co 2+ , respectively. These high percent errors indicate poor accuracy. INTRODUCTION: In order to remain healthy, humans need to have clean drinking water available. Water can be purified by several processes such as ion exchange. However, once the water has been purified, it must be tested to ensure its purity. Spectrophotometry is one method that can be used to determine the concentrations of ions in water [1]. Although some ions present in drinking water are not harmful such as the cations sodium and potassium, other ions such as cadmium and mercury can be deadly. Therefore, these harmful ions need to be detected and removed from all drinking water so humans can drink healthier water. The purpose of this experiment is to determine the concentration of cobalt and nickel in an unknown solution. This was accomplished using a UV-visible spectrophotometry to obtain calibration curves of known concentrations. From these curves, Beer’s law was used to calculate the concentration of nickel and cobalt in the unknown solution. The addition of EDTA is very important in this experiment because it forms specific colored complexes with Co 2+ and Ni 2+ . These complexes can be determined separately because they have different atomic absorption spectra. This allows UV-visible spectrophotometry to determine the concentrations of these ions. When EDTA is added to each solution, the following reactions occur [2]: 2 ¿ ( aq ) EDTA ¿ ¿ 4 ¿ ( aq ) ¿ 2 + ¿ ( aq ) + EDT A ¿ N i ¿ Stability Constant: 10 18.4 (Reaction 1) 2
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2 ¿ ( aq. ) EDTA ¿ ¿ 4 ¿ ( aq. ) ↔Co ¿ 2 + ¿ ( aq. ) + EDT A ¿ C o ¿ Stability Constant: 10 16.45 (Reaction 2) (Stability constant is the equilibrium constant calculated by the log of K f in n + ¿ M ¿ ¿ 4 ¿ Y ¿ ¿ K f = [ M Y n 4 ] ¿ ) The stability constant is proportional to the stability of the metal-EDTA complex. Therefore, the Ni-EDTA complex is slightly more stable than the Co-EDTA complex. However, since the stability
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