The purpose of this experiment is to analyze the efficacy and accuracy in determining mass composition of copper in a penny using two methods: atomic absorption spectroscopy and ultraviolet-visible spectroscopy.
The “Copper” Penny
The penny is a common form of currency used in the United States, holding the monetary value of one cent. From 1793 to 1837 the penny was composed of pure copper. As the years passed, the percent composition of copper in the penny decreased. From 1837 to 1857, the penny was bronze; containing 95% copper and 5% zinc and tin. In 1857, it was 88% copper and 12% nickel for a brief period of time then went back to bronze until 1962, with the exception in 1943 when it was zinc-coated steel due to the demand of copper for the war effort. In 1962, the tin was removed making the penny 95% copper and 5% zinc until 1982 when the composition was drastically changed to 97.5% zinc and only 2.5% copper, commonly known as copper-plated zinc. In this experiment we will analyze the use of atomic absorption and ultraviolet-visible spectroscopy to determine the mass composition of copper in pennies before and after the drastic change in 1982.
Atomic Absorption Spectroscopy
Atomic absorption spectroscopy is a technique used to identify the presence and concentration of substances by analyzing the spectrum produced when the substance is vaporized and absorbs certain frequencies of light. In atomic absorption spectroscopy, the sample is decomposed into atoms by a source of heat, such as a flame. The flame not only atomizes the sample, but also gets rid of the solvent used to dissolve the compound so we can analyze only the desired atoms. The concentration of atoms in the vapor is measured by the absorption of characteristic wavelengths of radiation. Atomic absorption spectroscopy is particularly useful because it has the ability to distinguish one element from another in a complex sample and it can perform simultaneous multi-element analyses. An atomic absorption spectrometer, is generally composed of a hollow-cathode lamp, an atomizing source, a monochromator, and a detector. After atomization of the sample, a light is passed through the flame which excites the electrons. When vaporized, different elements absorb light of specific frequencies. The hollow-cathode lamp, which produces the light, contains vapor of the same element as that being analyzed; this allows it to be specific to the analyte being observed. The hollow-cathode lamp will allow the absorbance to be selective for the copper content of the penny as opposed to zinc. Gaseous atoms excited by the collisions of high-energy electrons will emit photons. This atomic radiation has a frequency that is specific to the analyte in the flame. The monochromator will direct one band of wavelength that has passed through the sample to the detector. The detector will measure the amount of light that passes through the flame and will convert the photons to a function of...