Fluoride ion selective electrodes obey the following equation: where the fluorid

Question

Fluoride ion selective electrodes obey the following equation: where the fluoride concentration is expressed as mol/L and the temperature coefficient 13 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 B. Use the plotting and linear regression tools in Excel to calculate K and B from the data in the table. The coefficient 13 is close to 1, and can be determined from a calibration curve. What value of K do you obtain from your plot? What value of 13 do you obtain from your plot?

Explanation / Answer

Calculate the log[F-] value from the F- concentrations values from the table given.

The table will change as:

Now, plot a curve taking E(V) value on negative y-axis and log[F-] on x-axis. The curve comes out to be a straight line.

From the given equation-

E(V) = K - B(0.0592)log[F-]

E(V) = - B(0.0592)log[F-] + K ............................(1)

Comparing this equation with y = mx +c

Slope m = - B(0.0592) & c = K

Calcute slope of the equation from the curve

m = y2 - y1/ x2 - x1

= (-0.312 + 0.295)/ (1.30 - 1)

= - 0.0566

- B(0.0592) = - 0.0566

B = 0.956

Sustituting value of B in above equation (1)

- 0.295 = - 0.956*0.0592*1 + K

K = - 0.2385

log[F-] E(V) 1 - 0.295 1.07 -0.299 1.14 -0.303 1.20 -0.307 1.25 - 0.310 1.30 -0.312

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