Question

Fluoride ion selective electrodes obey the following equation:

      E(V) = K - $\beta$(0.0592)log[F^-]

where the fluoride concentration is expressed as mol/L and the temperature coefficient $\beta$ 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 $\beta$. Use the plotting and linear regression tools in Excel to calculate K and $\beta$ from the data in the table. The coefficient $\beta$ is close to 1, and can be determined from a calibration curve.

F- concentration (mg/L)	E(V)
10.0	-0.333
12.0	-0.338
14.0	-0.342
16.0	-0.346
18.0	-0.349
20.0	-0.352

What is the value of K?

Please show work!

Answer 1

F Molar mass = 19 g/mol  

Excel: insert E and conc. in columns A and B  

Column C:            c(mol/L) = c/19 / 1000)

Column D:            log c (M)

Column E:            pF = -log c

E (mV)	c (mg/L)	c (mol/L)	(-log c)
-0.333	10.0	0.000526	3.28
-0.338	12.0	0.000632	3.20
-0.342	14.0	0.000737	3.13
-0.346	16.0	0.000842	3.07
-0.349	18.0	0.000947	3.02
-0.352	20.0	0.001053	2.98
Molar mass F =		19
SLOPE		0.06309
INTERCEPT		-0.539825
beta		1.065708

Insert Chart XY ( E and log c) . The graph reprezent the equation E = K + S log c, where K is INTERCEPT and S is SLOPE. (SLOPE and INTERCEPT are in Excel, Statistical Function)

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beta = SLOPE/0.0592

K= INTERCEPT = standard potential