Chang Pui Chung Memorial School
Miss Soo May Kei
Target group: S6 Student
Background knowledge:
1) Acid-base titration
2) Chemical equilibrium
(titration curve)
Making Your Own Acid-Base Indicator
• Date: 3 July, 2006 (3 hours)
• Searching information and planning
investigation
• Writing the first page of proposal form
Making Your Own Acid-Base Indicator
• Date: 6 July, 2006 (4 hours)
• Discussing the first page of the
proposal form with the students
• Writing the proposal
Making Your Own Acid-Base Indicator
• Date: 17 & 18 July, 2006 (6 hours)
• Conducting the investigation
1) Making natural indicators
2) Choosing a suitable indicators for titration
3) Determining the concentration of oven cleaner
Making Your Own Acid-Base Indicator
• Date: 19 July, 2006 (4 hours)
• Organizing and analyzing data
Making Your Own Acid-Base Indicator
• Date: 21 July, 2006 (3 hours)
• Presenting the findings
Making Your Own Acid-Base Indicator
END
How to extract the pigments
from the plant materials?
For cherries and grapes:
Remove their peels
For red cabbage, beet
root and rose petal:
Cut them into small
pieces
• Add some water to
them and heat the
mixtures using a
heater
• Filter the solutions
using pieces of filter
paper
• Five plant indicator solutions are made!
Red cherry
Red cabbage
Beet root
Red grape
Red rose petal
Five natural indicators
Back
Phenolphthalein
≦ 1/10
When
pH ≦ pKIn + log 1/10
≦ pKIn - 1
≦ 8.15
When [Ph-(aq)] = [HPh(aq)],
pH = pKIn
= -log (7 x 10-10)
= 9.15

When [Ph (aq)] ≧ 10
[HPh(aq)]
pH ≧ pKIn + log 10
≧ pKIn + 1
≧ 10.15
Methyl orange
When
[Me(aq)]
[HMe (aq)]
1/10
pH ≦ pKIn + log 1/10
≦ pKIn - 1
≦2.7
When [Me(aq)] = [HMe+(aq)],
pH = pKIn
= -log (2 x 10-4)
= 3.7
When
[Me(aq)]
[HMe (aq)]
≧ 10
pH ≧ pKIn + log 10
≧ pKIn + 1
≧ 4.7
Acid-base Titrations
Acid
(CH3COOH or HCl)
Oven cleaner (NaOH)
+ indicator
pH Titration Curves
Strong Acid-Strong Base Titration
Sharp change in pH at the
equivalence point: 3-11
Strong Acid-Weak Base Titration
Sharp change in pH at the
equivalence point: 2-6
Weak Acid-Strong Base Titration
Sharp change in pH at the
equivalence point: 8-12
Weak Acid-Weak Base Titration
NO Sharp change in pH at
the equivalence point.
Results
• Red cabbage showed different colours at
different pH solutions.
pH12
pH2
• Red grapes showed insignificant colour change
pH12
pH2
• Red cherries also showed insignificant colour
change.
pH12
pH2
• Red rose petals showed a lot of colour at
different pH solution but less significant than
red cabbage.
pH12
pH2
• Beet root showed a sharp colour change
between pH 12 and pH 11
pH12
pH2
Colour of the indicators in
different pH solutions
pH
2
pH
3
pH
4
pH
5
pH
6
pH
7
pH
8
pH
9
pH
10
1
2
3
4
5
1.red cabbage
3.red cherry
5.beet root
2.red rose petal
4.red grape
pH
11
pH
12
• Beet root showed a sharp colour change
between pH 12 and pH 11
pH12
pH2
As it showed a sharp colour change at alkaline region,
it is suitable for being used as indicator in acid-base
titration.
Back
Freshly
prepared
beet root
solution
pH12
pH2
Overnight
beet root
solution
pH12
pH2
Colour of the indicators
(overnight and fresh beet root
solution)in different pH solutions
pH pH pH pH pH pH pH pH pH pH pH
2 3 4
5
6
7
8
9 10 11 12
1
2
1.Overnight
2.Fresh
Titration between HCl and oven cleaner
(NaOH) using phenolphthalein and beet
root as indicator
Indicator
Average
volume of
HCl required
for the
neutralization
Phenolphth Over-night Fresh beet
-alein
beet root
root
indicator
indicator
9.68cm3
9.58cm3
9.10cm3
The concentration of NaOH in the oven
cleaner with using different indicators
Indicator used phenolphthalein Over-night Fresh beet
beet root
root
solution
solution
Concentration
of NaOH in the
oven cleaner
1.936M
1.916M
1.820M
Back