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HShin_Lab_Report_7

HShin_Lab_Report_7 - Author Hoon Shin Austin Kwon Branden...

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Author: Hoon Shin, Austin Kwon, Branden Heilman Group: F TA: Eileen Chen Section: A05 Class: Chem100A, UCSD (University of California San Diego) Date: 5/29/08 Ion Selective Electrode Abstract The main goal of this experiment is for one to find the concentration of fluoride in unknown toothpaste. To find this, method of direct potentiometry was introduced and used. It is to measure the voltage of each solution with different concentration which is explained in introduction. As a results, concentration of fluoride in unknown, number 120, toothpaste sample was determined as 2.82 ± 0.43mg/L where the percentage of Fluoride in the toothpaste was 0.14%. In an addition method, which is to add unknown sample to the known sample and finding the difference in potential, the concentration was 1.27 ± 0.11 mg/L. Introduction Purpose of this lab is for one to determine the concentration of Fluoride in toothpaste. To do this, the method of electrochemistry is used including the Nernst equation. Before describing Nernst equation, one must know ISE. ISE, Ion selective electrode, is show below:
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To find the concentration of a solution, ISE is able to measure (ai) which a i = γ i C i where γ i is activity coefficient. Also, γ i will be equal to: -logγ i = 0.51 Z i 2 µ 1/2 Where = = all i i i Z C I 1 2 ) 2 1 ( *I= Ionic strength Overall Nernst Equation is: The concentration, Ci, and ionic strength u will decrease when the γ i increase. This experiment can be done by measuring each solution’s potential measured. Making the graph of E vs log C, equation S = K + S log C will be the equation to obtain straight line. Then the unknown samples created will be measured their potential by seeing the concentration of each solutions. To know this concentration, one must obtained the slope of the electrode which is “S,” as well as the intercept. This method is widely used in the world which is called a direct potentiometry. Theoretically, slope should be -59.16 mV at 25°C. Presence of OH- and the other ions are the main cause to change the pH values. Thus, using TISAB, total ionic strength adjustment buffer, pH range can be kept between 4 to 8.
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