Question

Determination of Fluoride in Toothpaste with a Fluoride Ion-Selective Electrode lab.

1. The electrode also responds to OH-. Why doesn't this cause an error in the analysis, or does it?

2. What is the theoretical value of the slope of the graph?

3. What is the significance of the intercept?

4. Is the Oxford-University water supply fluoridated? Should it be? Why?

Answer 1

1.The measured values depend upon the pH value and the total ionic strength of the solution. As the ionic strength decreases, the activity coefficient increases. At pH values below 5, HF and HF2 – are formed. Both of these are not detected. At pH values over pH = 8 the electrode becomes cross-sensitive to OH– ions and hence do not cause error in analysis.

2.

3.The significance of the intercept is that it helps in getting the value of standard reduction potential which is usually calculated from experiment.