EXPT 11.
pKa of pH Indicators
[Key Contents]
- acid dissociation constant, pKa
- buffer, Henderson-Hasselbalch equation
- spectrophotometry, Beer's law
- pH indicator
[References]
Principles of Modern Chemistry, 6th Ed. (Oxtoby et al.)
Ch 15. Acid-Base Equilibria
Chemistry for Life, Chemistry for Better Life (Kim et al.)
Ch 9. Equilibrium Reactions
[Goal]
- to understand basic principles of acid dissociation equilibrium
-
to
learn
to
use
Henderson-Hasselbalch
equation
in
acid
dissociation equilibrium
- to learn basic principles of spectrophotometry
[Background]
A typical pH indicator (HInd) is a weak acid that undergoes acid
dissociation in solution with an accompanying color change.
HInd ⇄ Ind- + H+
The color of the solution is determined by the concentration ratio
between HInd and Ind-. The concentration of HInd and Ind- can be
measured using Beer's law.
A
= εbC
ε: molar extinction coefficient,
b: path length,
A: absorbance
You will first obtain absorption spectrum of the indicator at acidic
and basic pH and select a wavelength where HInd absorbs strongly
and
Ind-
absorbs
little.
Then
you
will
measure
the
decreased
absorbtion at this wavelength as the pH of the indicator solution is
increased gradually using buffers of different pH. The decrease in
absorbance corresponds to the concentration of Ind-. The pKa of HInd
is the pH at which [HInd] and [Ind-] are the same. You can
determine pKa of the indicator by plotting log([Ind-]/[HInd]) against
pH.
[Apparatus and Chemicals]
spectrophotometer, cuvette, test tubes, 10 mL pipet, Pasteur pipet
0.01 mM bromophnol blue (BPB) solution, 0.02 mM phenolphthalein
solution
0.1 N HCl solution, 0.1 N NaOH solution, 10 mM buffer solutions (pH
3.4, 3.7, 4.0, 4.3, 4.6, 7.0, 8.5, 9.0, 9.5, 9.8)
[Procedure]
Expt 1. pKa of Bromophenol Blue
1) In separate test tubes, prepare following mixtures.
mixture 1 : 2.0 mL 0.05 mM BPB soln + 2.0 mL 0.1 N HCl soln
mixture 2 : 2.0 mL 0.05 mM BPB soln + 2.0 mL pH 3.4 buffer
mixture 3 : 2.0 mL 0.05 mM BPB soln + 2.0 mL pH 3.7 buffer
mixture 4 : 2.0 mL 0.05 mM BPB soln + 2.0 mL pH 4.0 buffer
mixture 5 : 2.0 mL 0.05 mM BPB soln + 2.0 mL pH 4.3 buffer
mixture 6 : 2.0 mL 0.05 mM BPB soln + 2.0 mL pH 4.6 buffer
mixture 7 : 2.0 mL 0.05 mM BPB soln + 2.0 mL pH 7.0 buffer
2) Using a spectrophotometer, obtain absorption spectrum from mixture
1 and 7. Read absorbance between 450-650 nm at 25 nm intervals. Read
absorbance at 5 nm intervals at the absorption peak. Put the solution
back to the test tube. Determine wavelength for maximum absorption (λ
max)
for HInd from mixture 1. Determine λmax for Ind- from mixture 7.
3) From mixture 2~6, read absorbance at both λmax.
4) Visually, estimate the pH where the color of the solution is the mixed
color of mixture 1 and 7 in equal volume.
5) Test of Beer's Law : In a test tube, mix 2 mL of mixture 1 with 2 mL
of 0.1 N HCl solution to make 0.025 mM BPB solution and read
absorption at both λmax. Next, mix 2 mL of the 0.025 mM BPB solution
with 2 mL of 0.1 N HCl solution to make 0.0125 mM BPB solution and
read absorption at both λmax. Finally, mix 2 mL of the 0.025 mM BPB
solution with 2 mL of the 0.0125 mM BPB solution to make 0.0188 mM
BPB solution and read absorption at both λmax.
HO
Expt 2. pKa of Phenolphthalein
OH
O
1) In separate test tubes, prepare following mixtures.
O
phenolphthalein
mixture 1 : 2.0 mL 0.02 mM phenolphthalein + 2.0 mL pH 7.0 buffer
mixture 2 : 2.0 mL 0.02 mM phenolphthalein + 2.0 mL pH 8.5 buffer
mixture 3 : 2.0 mL 0.02 mM phenolphthalein + 2.0 mL pH 9.0 buffer
mixture 4 : 2.0 mL 0.02 mM phenolphthalein + 2.0 mL pH 9.5 buffer
mixture 5 : 2.0 mL 0.02 mM phenolphthalein + 2.0 mL pH 9.8 buffer
mixture 6 : 2.0 mL 0.02 mM phenolphthalein + 2.0 mL 0.1 N NaOH
2) Using a spectrophotometer, obtain absorption spectrum from mixture
1 and 6. Read absorbance between 450-650 nm at 25 nm intervals. Read
absorbance at 5 nm intervals at the absorption peak. Put the solution
back to the test tube. Determine λmax for Ind- from mixture 6.
3) From mixture 1~5, read absorbance at λmax.
4) Visually, estimate the pH where the color of the solution is the mixed
color of mixture 1 and 6 in equal volume.
5) Test of Beer's Law : In a test tube, mix 1.5 mL of mixture 6 with 1.5
mL of 0.1 N NaOH solution to make 0.01 mM phenolphthalein solution
and read absorption at λmax. Next, mix 1.5 mL of the 0.01 mM
phenolphthalein solution with 1.5 mL of pH 7.0 buffer solution to make
0.005 mM phenolphthalein solution and read absorption at both λmax.
Finally, mix 1.5 mL of the 0.01 mM phenolphthalein solution with 1.5 mL
of
the
0.02
mM
phenolphthalein
solution
to
make
0.015
mM
phenolphthalein solution and read absorption at both λmax.
[Data Analysis]
Expt 1. pKa of Bromophenol Blue
1) Draw an absorption spectrum for mixture 1 and determine λmax and ε
at λmax. Calculate [HInd] from absorbance at λmax.
2) Plot [HInd] and [Ind-] againt the molar concentration of BPB. Is
the Beer's law valid?
3) For mixture 2~6, calculate
[HInd]
at
different
pH's
from
absorbance at λmax.
4) Calculate [Ind-] as difference between [HInd] for mixture 1 and
[HInd] at different pH's.
5) Plot log[Ind-]/[HInd] againt pH and determine pKa of BPB as the pH
where log[Ind-]/[HInd] is zero.
6) Draw an absorption spectrum for mixture 7 and determine λmax and ε
at λmax. Calculate [Ind-] from absorbance at λmax.
7) For mixture 2~6, calculate
[Ind-]
at
different
pH's
from
absorbance at λmax.
8) Calculate [HInd] as difference between [Ind-] for mixture 7 and
[Ind-] at different pH's.
9) Plot log[Ind-]/[HInd] againt pH and determine pKa of BPB as the pH
where log[Ind-]/[HInd] is zero.
10) Compare pKa values obtained by both methods. How good is the
agreement? Compare the average of the two pKa values with the
literature value of Ka, 1.5 x 10-4.
Expt 2. pKa of Phenolphthalein
1) Draw an absorption spectrum for mixture 6 and determine λmax and ε
at λmax. Calculate [Ind-] from absorbance at λmax.
2) Plot [Ind-] againt the molar concentration of BPB. Is the Beer's
law valid?
3)
Plot
log[Ind-]/[HInd]
againt
pH
and
determine
pKa
of
phenolphthalein as the pH where log[Ind-]/[HInd] is zero.
4) Compare pKa values obtained with the literature value of 9.3. Discuss
why phenolphthalein is a good indicator for titrating strong acid with
strong base.
[Additional Material]
Phenol Red and Phenolphthalein
pH 5.5
8.0
phenol red
phenolphthalein
10.0
12