1. Determining the ka of an acid-base indicator
Experiment 6

2. Indicator HIn + H2O ↔ In- + H3O+ Red blue Ka = [H3O+] [In-]/[HIn]
[H3O+] = Ka [HIn]/[In-]
(-log [H3O+]) = (-log Ka) + (log [In-]/[HIn])
pH = pKa + log [In-]/[HIn]

3. Determining Ka Ka = [H3O+] [In-]/[HIn]
Adjust pH slightly and measure the concentration of all three species, independently
H3O+ (pH meter)
In- and HIn (spectroscopy)

4. Ka = [H+][In-]/[HIn]

5. Quantitative Spectroscopy
Beer’s Law
Al1 = el1bc
e is molar absorptivity (unique for a given compound at l1)
b is path length
c concentration

6. Standard Practice Prepare standards of known concentration
Measure absorbance at lmax
Plot A vs. concentration
Obtain slope
Use slope (and intercept) to determine the concentration of the analyte in the unknown

7. Typical Beer’s Law Plot

8. Spectroscopy Indicators give fairly broad visible absorption spectra
Blue solution absorbs in the red region
Red solution absorbs in the blue region
But there is usually some significant overlap

11. Absorption measurement
Must make measurements at two wavelengths
Al1 = el1HInbcHIn + el1InbcIn-
Al2 = el2HInbcHIn + el2InbcIn-
Must have the slopes of four Beer’s law plots. Then you have two equations with two unknowns.

12. The four slopes
Measure standard solutions of Indicator adjusted to high pH (three pH units above pKa)
[In-]/[HIn] = 1000
Measure standard solutions of Indicator adjusted to low pH (three pH units below pKa)
[HIn]/[In-] = 1000
These solution will be prepared and measured to produce the four Beer’s Law Plots (each set at both wavelengths)

13. In the lab Prepare Indicator solution adjusted to the proper pH
Measure its pH and Absorbance at 430 nm and 565 nm
Adjust pH with dilute HCl or NaOH by .2 pH units and remeasure
Continue until you have five measurements within 1.0 pH units above and below the starting pH

14. Spreadsheet Use data to produce four Beer’s Law Plots
Determine slopes of Beer’s Law Plots
Calculate [H+] from the pH
Use slopes to solve the system of two equations for [HIn] and [In-]
Solve for Ka, Ka = [H+][In-]/[HIn]
Solve for pKa, pKa = -logKa
Calculate averages and 95 % CL