Exp 7 Lab Report

Exp 7 Lab Report - Application of the method of flame...

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Application of the method of flame emission spectroscopy to determine the concentration of calcium in a fruit juice sample Jubilee Ngo, Nora Roberts TA: Christine Huang Section: A00 Chemistry 100A at University of California, San Diego 9500 Gilman Drive, La Jolla, California 92093 31 July 2006 Abstract The concentration of calcium (Ca 2+ ) in fruit juice with the brand name Tang drink was determined using the method of flame atomic emission spectroscopy. First, a 100-mL of 100-ppm solution was created using solid CaCO 3 , 6 M HCl, and deionized water and a blank solution was also created with radiation buffer, 6 M HCl, and deionized water. Then, a set of standard solutions with different, but known concentrations of Ca 2+ was prepared by mixing an appropriate amount of 100-ppm calcium solution (diluted with the blank solution) that would result in 25-mL solutions with 4 ppm, 8 ppm, 16 ppm, and 20 ppm concentrations of Ca 2+ . The diluted (1:4 dilution) and undiluted fruit juice samples were then made using ~5mL of Tang drink, 1% (NH 4 ) 2 C 2 O 4 , 1 M NH 4 OH, 6 M HCl, and a centrifuge. The emission spectroscopy readings of all of these solutions were measured using a photometer and the data obtained was plotted on a graph of reading versus Ca 2+ concentration. From the trendline of this plot, interpolation was possible to back calculate the Ca 2+ concentration of the diluted sample, which is 11.81 ppm (or mg/L). Furthermore, the Ca 2+ concentration was then determined to be 47.25 ppm while the Ca 2+ concentration of the original 5-mL fruit juice is calculated to be 236.23 ppm and the mass of Ca 2+ in a 250-mL serving of fruit juice is 1.181 x 10 4 mg (109.84% of the RDA value for calcium). The class average for the 1:2 diluted fruit sample is 18.05 ± 6.39 ppm. Result The following table shows the emission spectroscopy readings of four solutions with known concentrations of Ca 2+ ions (standard solutions). Table 1. Emission spectroscopy readings of solutions with known Ca 2+ concentrations ppm (mg/L) Ca 2+ Reading 4.00 0.084 8.00 0.147 16.00 0.257 20.00 0.304 Based on Table 1, a plot of readings versus Ca 2+ concentrations (in ppm or mg/L) is constructed (along with a trendline line and the R-squared value), shown below in Figure 1.
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Figure 1. Plot of emission spectroscopy readings versus concentrations of Ca 2+ Emission Spectroscopy Readings for Known Solutions of Ca2+ y = 0.0138x + 0.033 R 2 = 0.9979 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.00 5.00 10.00 15.00 20.00 25.00 [Ca2+] (ppm) Reading From both Table 1 and Figure 1 shown above, the upper and lower limits for the readings have been determined to be 0.084 and 0.304, respectively. Hence, the diluted fruit juice solution should fall within these values in order to determine its Ca 2+ concentration. The reason is that when performing experiments, it is preferable to interpolate the results from a trendline when the unknown values fall within the range of the known rather than to extrapolate the results. The linear regression R-squared value is 0.9979, which
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Exp 7 Lab Report - Application of the method of flame...

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