Use the plotting and linear regression tools in Excel to calculate K and β from the data in the table.

Question

For the question below I am having difficulty getting the right answer and I don't know if I am wrong or if my homework software is marking it wrong when it is right. I added my answers at the bottom and just want to see if your answers are similar or the same as mine! Thank you.

Fluoride ion selective electrodes obey the following equation:

      E(V) = K - β(0.0592)log[F^-]

where the fluoride concentration is expressed as mol/L and the temperature coefficient β is normally equal to 1.

In a normal calibration procedure you dilute a standard fluoride solution to between 10 and 20 mg/L fluoride ion and then measure the response of the electrode. You then plot the data and do a linear regression to obtain the fitting coefficients K and β. Use the plotting and linear regression tools in Excel to calculate K and β from the data in the table. The coefficient β is close to 1, and can be determined from a calibration curve.

F- concentration (mg/L)	E(V)
10.0	-0.273
12.0	-0.277
14.0	-0.281
16.0	-0.285
18.0	-0.288
20.0	-0.290

What value of K do you obtain from your plot?

I got -0.46397 and that is supposedly wrong.

What value of β do you obtain from your plot?

I got 0.988515 and that is also supposedly wrong. (0.058321/0.0592)

Answer 1

Well, let's see where you might have gone wrong. Did you use: 18998 mg/mol as the molar mass of F- ion, to transform the mg/L into mol/L -- looks as if you did. Did you transcribe correctly from your Excel sheet? Nope, 0.988515 was/is not what you get from 0.058321/0.0592, it's 0.985156 (you wrote an extra 8). That's error 1.

-0.46397 is fine for intercept, except for the final rounding. Since you are only supplied with 3 sigfigs, after doing the calcs to as much precision as possible, you still need to round (both slope and intercept) values off appropriately, when reporting. That's errors 2 and 3. Try that and see if you don't get the green light.

My favorite are the "new" solid-state pH electrodes. Just immerse and read, then squirt off. No filling solutions, no reference electrodes. Maybe a temperature correction, if that.

-- Cheers, --mr. d.