1. Determination of I- in Salt (Ion-Selective Electrode)
CALCULATIONS

2. Steps to determine µg of I-/g of salt
Calculate calibration curve
Determine [KI]
Plot log [KI] vs. Voltage
Calculate Least-Squares Regression Line (LSR)
Use slope to calculate analyte in sample
Determine A
Determine X
Use stoichiometric expressions to determine µg of I-/g of salt

3. Calculating the calibration curve
Determine [KI] stock; each subsequent dilution x 10-1
Determine log [KI] – This will be your “x” variable
Measured Potentials – This will be your “y” variable
Trial 1 2 3 4
Standard Concentration (M)
9.87E-03
9.87E-04
9.87E-05
9.87E-06
Log of Concentrations
-2.01
-3.01
-4.01
-5.01
Potential (mV)
-271.5
-216.2
-190.9
-160.0

4. Calculating the calibration curve
Plot the log[KI] (M) vs. Potential (mV) and fit a LSR Line

5. Calculating the calibration curve
How do you calculate the LSR line?
Least-Squares Analysis
Xi, log[KI]
Yi, Potential
XiYi
Xi^2
=sum[B12:B6] Total:
2.760E+03
5.416E+01
=yi-mxi-b
Least-Squares Analysis
Xi, log[KI]
Yi, Potential
XiYi
Xi^2 di
di^2
-2.006
5.446E+02
4.023E+00
-7.879
62.082
-3.006
6.499E+02
9.035E+00
11.441
-4.006
7.647E+02
1.605E+01
0.761
0.579
-5.006
8.009E+02
2.506E+01
-4.319
18.656
=sum[B12:B6] Total:
2.760E+03
5.416E+01
0.003
Formulas

6. The Calculated Calibration Curve

7. Determining µg of I-/g of salt
Sample
ΔE (f-i)
A = 10 (ΔE/m)
X=S(.091/A-.909)
μg I- / g salt 1
-93.3
392
2.30 x M
4.8528 2
-91.8
356
2.53 x M
5.3547 3
-91.9
358
2.51 x M
5.2812
(sample + standard potential) – (sample potential) = ΔE

8. Determining µg of I-/g of salt

9. Question:
What is the theoretical slope for a measured calibration curve of a standard ion-selective electrode for a single charged (monovalent) ion, and how is it derived?
The theoretical value of the slope is -59 mV / log[ax] and this is derived from the Nernst Equation at 298°K for a single charged ion