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Exp 6 Lab Report - Fluoride analysis in samples of...

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Fluoride analysis in samples of toothpaste and tap water using the fluoride-ion selective electrode and methods of direct potentiometry and variable volume standard addition Jubilee Ngo, Nora Roberts TA: Christine Huang Section: A00 Chemistry 100A at University of California, San Diego 9500 Gilman Drive, La Jolla, California 92093 28 July 2006 Abstract Using a fluoride-ion selective electrode in two different methods of fluoride analysis, direct potentiometry and variable volume, the concentrations of fluoride in toothpaste and tap water samples were determined. Direct potentiometry involved preparing fluoride standards at various concentrations (in mg/L or ppm) and measuring their voltages in order to create a plot of potential versus log C. Then, the trendline obtained from this plot was used to interpolate the log C of the two samples and after the linear regression analysis and error analysis, the fluoride concentrations resulted in 11.62 ± 5.68 mg/L (or ppm) with a 48.87% relative error for toothpaste and 0.50 ± 0.26 mg/L with a 52.39% relative error for tap water. In standard addition, voltage readings of each sample after adding standard 300-ppm F - solution in ~0.5 mL increments were taken. Then, a plot of response (mL) versus (C std V std ) was constructed for each sample and after linear regression analysis and error analysis were performed, the fluoride concentration of toothpaste sample was determined to be 2.02 ± 0.04 mg/L and 0.32 ± 0.03 mg/L for the tap water sample. The differences in the fluoride concentrations obtained using both methods indicate occurrences of errors and a comparison of the results from the methods shows that standard addition is the better method between the two because of the smaller standard deviations in the fluoride concentrations resulting from it and due to other important reasons as well. Result Part A. Direct Potentiometry Table 1 below shows the concentrations (in mg/L or ppm) of the standard fluoride solutions prepared in a background matrix of TISAB (Total Ionic Strength Adjustment Buffer)” 2 and their respective potential readings from the voltmeter. Table 1. Concentrations and potential readings of standard fluoride solutions C logC E (mg/L or ppm) mV 0.5 -0.30 108.3 1.0 0.00 99.4 2.5 0.40 81.7 2 Zellmer, David L. “The Fluoride Ion Selective Electrode Experiment Direct Potentiometry and Standard Addition Methods.” 15 Feb. 1999. CSU Fresno. 22 July 2006. < http://zimmer.csufresno.edu/%7Edavidz/Chem102/FluorideISE/FluorideISE.html >.
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5.0 0.70 49.2 100.0 2.00 -8.5 300.0 2.48 -65.6 Based on this data, a plot of potential versus log C is made (Figure 1) and the trendline and R-squared value are also included. Figure 1. Plot of potential versus log C of various standard fluoride solutions by direct potentiometry Potential vs. Log C y = -60.525x + 97.275 R 2 = 0.976 -80.0 -60.0 -40.0 -20.0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 -0.50 0.00 0.50 1.00 1.50 2.00 2.50 3.00 Log C (C in mg/L) Potential/E (mV From both Table 1 and Figure 1 shown above, the upper and lower limits for the potential readings have been determined to be 108.3 mV and -65.6 mV, respectively. Hence, the voltages of the toothpaste and
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