1 Determination of Copper Concentration in Pennies Authors Courtney Adams Lab Partners: Michael Armstrong, Ben Arias Teaching Assistant: Mitchell Instructor: Professor Yasmin Jessa CHM 144 - Section E Grade Submitted: November 12, 2015 Introduction To conduct a quantitative analysis of the metal content of a substance such as a coin, the metals in the substance must be dissolved. In this experiment, nitric acid is used as an oxidizing agent to dissolve the copper and zinc present in the coins. The oxidation of copper and zinc produces nitrogen dioxide gas and the complex ions Cu(H 2 O) 6 2+ and Zn(H 2 O) 6 2+ . These complex ions consist of a metal (copper or zinc) that is covalently bonded to two or more anions or molecules that are called ligands. The H 2 O ligands in these complex ions can be replaced by ammonia to produce two new complex ions Cu(NH 3 ) 4 (H 2 O) 2 2+ and Zn(NH 3 ) 4 (H 2 O) 2 2+ . The Cu(NH 3 ) 4 (H 2 O) 2 2+ ion has a deep blue color and the Zn(NH 3 ) 4 (H 2 O) 2 2+ ion is colorless. This allows for an opportunity to determine the concentration of copper in each coin through the use of a spectrometer without interference from the zinc. Spectroscopy is used for identification purposes by determining the absorbance of a solution. The concentration of the light-absorbing species in the solution can then be found because the concentration will be directly proportional to the solution’s absorbance. The spectrometer does this through the use of a lamp which strikes a diffraction grating. The diffraction grating works like a prism by separating the light into its components. These component wavelengths then strike the sample. Some of the light is absorbed by the substance and some of the light passes through it. A detector determines the amount of light that is transmitted through the sample and converts this information into a digital display of either transmittance or absorbance. Absorbance will be the quantity used in this analysis because of its direct proportionality with the concentration of a light-absorbing species. The absorbance data
2 collected from the calibration solutions will be plotted versus concentration of Cu 2+ to form a calibration curve which will then be used to find the concentration of Cu 2+ in the three solutions containing the dissolved pennies. The objectives for this experiment include learning how visible and UV absorbance of a solution is related to the concentration of a specific element or compound in the solution, learning what a spectrometer does and how to use it, utilizing a calibration curve based on the Beer-Lambert law, and learning how less active metals are oxidized and what complex ions they form once oxidized. The main purpose is to combine all of these objectives to experimentally determine the concentration of copper in three pennies. This is done by utilizing the absorbance
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