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Exp. 3 Lab Report

# Exp. 3 Lab Report - Experiment 3 Determination of Fluoride...

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Experiment 3 Determination of Fluoride in Water with Ion Selective Electrodes (ISE) Jennie Senter Locker H-10 January 30, 2008

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Senter In this experiment, an ion-selective electrode was used to determine the concentration of fluoride in an unknown sample and a Knoxville city tap water sample using the standard addition method. 0.2569 g NaF was used to make the standard solution. The concentration of this solution is calculated as follows: M mol gNaF gNaF C STD 0612 . 0 / 98 . 41 2569 . 0 = = To correct for the HF/F - equilibrium at pH 5.0, the following calculation is needed: M M 0603 . 0 69 68 0612 . 0 = × This is the concentration used in future calculations. In order to calculate the unknown sample concentration after the first spike of standard solution, the potential difference must be measured. V V V E 051 . 0 ) 012 . 0 ( 063 . 0 - = - - - = Since the equation used to determine the concentration is Nernstian, the value of RT/nF must also be known. The room temperature (T) was measured at 19°C (292K) so it follows: V eq C mol eq K mol K C V nF RT 004433 . 0 ) 1 96485 ( ) / 1 ( ) 292 ( ) / 314 . 8 ( = - × × = With all of this information, the concentration of the unknown sample after the first spike of standard solution can be determined as follows: M e mL mL e M mL C V V V V F 8 004433 . 0 051 . 0 004433 . 0 051 . 0 10 25 . 5 50 55 0612 . 0 5 - - - × = × - × × = 2
Senter Four total spikes were added to the unknown solution, with an average concentration of 1.49x10 -8 M. To get the total concentration, this average must be multiplied by 69/68, yielding a corrected average concentration of 1.51x10

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