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LPasok_The_Ion_Selective_Electrode_Fluoride_graded

LPasok_The_Ion_Selective_Electrode_Fluoride_graded - The...

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The Ion Selective Electrode – Fluoride Author: Lesley Pasok Lab Partners: Helen Lu & Thien Vu TA: Rachel Tsui Section: Chemistry 100A, UC San Diego May 24, 2010 Abstract: The purpose of this experiment is through the use direct potentiometry and standard addition method, the concentration of fluoride can be found in toothpaste and tap water. For direct potentiometry the concentration of fluoride in toothpaste was found to be 11.40 +/-0.88. for tap water is found to be 0.57 +/- 0.87. For standard addition method the unknown concentration of fluoride was found to be 5.27 +/-0.13 and the unknown concentration for tap water was 0.62 +/-0.21. The class histogram showed the unknown concentration of tap water to be around the mean of 0.46. Introduction: Direct potentiometry is the method of using ISE (ion-selective electrodes) to measure the electrode response in an unknown solution. Ion-selective electrodes are defined to be indicators in measuring electrodes with high degree of specificity for a single ion or class of ions. It will be used with a potentiometer which is a device used to measure the circuit, which will be attached to the pH- millivoltmeter. The emf, E of a cell can be observed using the measuring circuit through the Nearnst equation: E = E o +(2.3 RT / nF )log a i where E ° is a constant, F is Faraday’s constant, a i is the activity being measured. For this experiment we will be using a fluoride ion-selective electrodes in order to determine the fluoride concentration in an unknown toothpaste sample and water. It’s Nernst Equation can be expressed as E = E - 59.16log a F - The fluoride ion sensitive electrode consists of fluoride ion-sensitive membrane sealed over the end of an inert and usually plastic tube that contains an internal electrode filling solution. The membrane consists of a single rare earth fluoride like lanthanum fluoride, which will be doped up with divalent ions. For this experiment series of fluoride standards will be prepared in a background matrix of TISAB (Total Ionic Strength Adjustment Buffer). TISAB is used so that the fluoride ion concentration may be deduced from measurement of fluoride ion activity. In equal volumes TISAB are mixed fixing the ionic strength high enough to make activity coefficients of unknowns and standards virtually identical. Thus, because E = K + S log C E = K + S log C A simple plot of E vs. log C should yield a working curve that can measure the unknown concentration of fluoride in a toothpaste solution as well as the unknown concentration of fluoride in tap water. Then through standard addition methods, which is a method, used for large volume of unknown to which small amounts of standard solution are added. Although the total volume is not changed greatly, it is the volume changes produced that will produce the data needed.
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