# Post Lab #10.docx - Post Lab#10 Spectrophotometric...

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Post Lab #10: Spectrophotometric Determination of Manganese and Chromium in Mixture Introduction: The goal of this week’s lab was to oxidize two metals, manganese and chromium, to Cr 2 O 7 2- and MnO 4 - . Once this was done, their concentrations were able to be determined simultaneously by measuring the absorption at two wavelengths. The absorbances were measured using a spectrophotometer. A spectrophotometer works by shining a beam of light that is at a specific wavelength, through a sample onto a light meter, which allows the absorbance to be found (Michigan State University, 2011). Beer’s law proved that if the solutions were in at least 0.5 M H 2 SO 4 , then chromium would have an absorption maximum at 440 nm, and manganese would have a maximum absorption at 545 nm. Beer’s law allowed us to calculate how much light was absorbed in each sample, using a simple equation that is directly proportion to other parameters (Sheffield Hallam University, 2016). However, this information was easily obtained using the spectrophotometer, which is an instrument used to measure how much light a substance absorbs (Foist, 2016). Four constants were determined using the absorbances and pure solutions of a known concentration in order to solve for the unknown concentrations. A calibration curve for each of the two metals provided slopes that were used to obtain the average k value. Cr 2 O 7 2- oxidized by peroxydisulfate: Ag + 2Cr 3+ + 3S 2 O 8 2- + 7H 2 O  Cr 2 O 7 2- + 6SO 4 2- + 14H + Mn 2+ oxidized by peroxydisulfate: 2Mn 2+ + 5IO 4 - + 3H 2 O  2MnO 4 - + 5IO 3 - + 6H + Mn 2+ oxidized by periodate: Ag + 2Mn 2+ + 5S 2 O 8 2- + 8H 2 O  2MnO 4 - + 10SO 4 2- + 16H +
Procedure and Materials: 1. 500 mL unknown solution 2. DI water 3. MnSO 4 H 2 O 4. K 2 Cr 2 O 7 5. H 2 SO 4 6. KIO 4 7. K 2 S 2 O 8 8. AgNO 3 9. Spec20D 10. Kimwipe 11. Parafilm 12. 140 mL beakers 13. 100 mL volumetric flask 14. 250 mL Erlenmeyer flasks 15. Graduated cylinder Qualitative Observations: This experiment indicated numerous important qualitative observations. The manganese solutions were pink, but the color got more intense as the concentration increased. Meaning, the solution with the least manganese was a light pink, and the solution with the most manganese was almost purple. This was the same situation for the