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||-42.|
|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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