Ise Determination of Fluoride Content in Various Substances

Introduction: The ion-selective electrode used was made up of a silver/silver chloride reference electrode and a glass electrode. The pH of the electrochemical cell is determined by monitoring the electrical potential difference between the half-cells. The glass electrode is an ion-selective electrode (ISE), which only responds to the hydronium ion concentration/activity in a solution under a large range of conditions.
In this experiment, we are measuring the fluoride ion concentration, which means our electrode responds to fluoride ion concentrations.

Standard Solutions
Concentration (M)
Potential (mV)

0. 9967 x 10-6
15. 0

0. 9967 x 10-5
1. 4

0. 9967 x 10-4

0. 9967 x 10-3
-101. 1

0. 9967 x 10-2
-160. 8

0. 9967 x 10-1
-219. 0

Unknown #6 was used for this lab group. It produced a potential difference of -196. 8 mV. This was then used in the equation derived from our standard solutions and found to have a [F-] = 0. 0618 M. Preparation of the toothpaste sample was done by a sonicator. Then, the electrical potential difference was measured again, just as in the standard solutions and unknowns. It was found to have a potential of -85. 6 mV.
Inserted into the equation, [F-] = 3. 31 x 10-4 M.
Calculations: See attached sheets.
Results: Toothpaste [F-] = 3. 31 x 10-4 M
Unknown #6 [F-] = 0. 0618 M Label Value in Toothpaste 0. 15% w/v Calculated Value in Toothpaste 0. 8 mg / g 0. 8 mg/g converted to w/v (g/100 mL) is approximately 0. 1144% w/v. In comparison with the label, the calculated error is found to be about -24%. A potential source of error could be found in the method of dissolving the toothpaste. There were a lot of bubbles formed, thus allowing for a greater possibility of misreported volumes of solution. Also, the pH / V meter had difficulty settling on values during the standard measurements, which would alter the regression line. The ISAB seemed to work better with stronger concentrations of [F-], as the regression line fit better toward the latter part of the plot. However, the pH region seemed to be less important than the [F-] concentration.


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