Lab AA - Part E and modifications of Parts B and C A. Omitted. B. Quantitative Determination of Copper in a Penny by Atomic Absorption Spectroscopy Preparation of Penny for Analysis After weighing penny, put it in a 125 mL Erlenmeyer flask. Dissolve penny by using 1:1 HNO3 to oxidize the Cu and Zn to Cu2+ and Zn2+. Pennies minted between 1962 and mid 1982 are 95% Cu and 5% Zn. Pennies minted after mid 1982 are 2.5% Cu and 97.5% Zn.1 This information is provided to help you decide how to dilute your unknown and to serve as "true" values for accuracy determinations. Find limit of linearity for copper determination on this instrument from the instrument manual. Use this information to dilute your unknown so that its concentration is within the linear response range. Ordinary Calibration Method - Prepare a stock solution of 1.000 g of Cu per liter (1000 ppm) from an appropriate amount of Cu metal. Do not prepare a whole liter of stock solution! Use 1:1 HNO3 to oxidize Cu to Cu2+. From this solution, prepare seven solutions in 100 mL volumetric flasks that contain 1.00, 2.50, 5.00, 7.50, 10.00, 12.5, 15.0, 17.5, 20.0, and 22.5 ppm Cu. Note: This concentration range goes above the limit of linearity. You will have to choose the best points to use in the calibration curve. The 3110 AA spectrometer you are using routinely exceeds the published limit of linearity. Plot curve with all points and with straight line going through linear ones to show negative deviation at higher concentrations. Put some distilled water into a small, clean beaker and aspirate the water into the flame. Adjust the absorbance reading to zero. Aspirate one of the solutions into the flame until an absorbance maximum is reached and record this value. Aspirate remaining known solutions into flame (rezeroing the absorbance between each one) and record their absorbances. Aspirate the unknown solution into the flame last and record its absorbance. Standard addition Method - Prepare a 100.0 ppm Cu solution from the 1000 ppm Cu stock solution. Pipet 10.00 mL of unknown into each of five 25 mL volumetric flasks. Add 0, 0.10, 0.20, 0.50, and 1.00 of mL 100 ppm Cu to flasks and then fill flasks to the line with deionized water. Record absorbance of each solution. Report: Ordinary Calibration Method Construct a calibration curve by plotting absorbance vs. concentration in micrograms per mL. Determine concentration of unknown Cu in solution from calibration curve and then determine % Cu in the penny. Determine sc and relative %sc of Cu in solution and of the %Cu in the penny. Report: Standard Addition Method Plot absorbance vs. mL of known solution. Using information on standard addition on pages 15-18 in your lecture textbook, find the concentration of the Cu (g/mL) in the solution and the % Cu in the penny. Determine sc and relative %sc of Cu in solution and of the %Cu in the penny. C. Sensitivities Cu, Mg and Cr will be done. Ignore concentrations called for in this part and determine reasonable concentrations (must be in linear response range) from the instrument manual. Use both lean and rich flames when analyzing Cr. D. Omitted.
E. Comparison of Flame & graphite furnace AA and their Detection Limits (LOD) Several Cu solutions (prepared by instructor) will be run on flame AA and graphite furnace AA (GFAA) instruments to compare approximate LODs and sensitivities between flame AA and GFAA. Because the GFAA ...