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Science Education Section, EDB
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
When [Ph-(aq)] = [HPh(aq)],
pH = pKIn
= -log (7 x 10-10)
= 9.15
When 1/10≦
pH pK≦ In + log 1/10
≦ pKIn - 1
≦ 8.15
When 10≧
pH pK≧ In + log 10
≧ pKIn + 1
≧ 10.15
[HPh(aq)]
(aq)][Ph
When [Me(aq)] = [HMe+(aq)],
pH = pKIn
= -log (2 x 10-4)
= 3.7
When 1/10
pH pK≦ In + log 1/10
≦ pKIn - 1
≦2.7
(aq)][HMe
[Me(aq)]
When 10≧
pH pK≧ In + log 10
≧ pKIn + 1
≧ 4.7
(aq)][HMe
[Me(aq)]
Methyl orange
Acid-base Titrations
Acid (CH3COOH or HCl)
Oven cleaner (NaOH) + indicator
Strong Acid-Strong Base TitrationStrong Acid-Strong Base Titration
Sharp change in pH at the equivalence point: 3-11
Sharp change in pH at the equivalence point: 3-11
pH Titration Curves
Sharp change in pH at the equivalence point: 2-6
Sharp change in pH at the equivalence point: 2-6
Strong Acid-Weak Base TitrationStrong Acid-Weak Base Titration
Sharp change in pH at the equivalence point: 8-12
Sharp change in pH at the equivalence point: 8-12
Weak Acid-Strong Base TitrationWeak Acid-Strong Base Titration
NO Sharp change in pH at the equivalence point.
NO Sharp change in pH at the equivalence point.
Weak Acid-Weak Base TitrationWeak Acid-Weak Base Titration
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
pH2
pH3
pH4
pH5
pH6
pH7
pH8
pH9
pH10
pH11
pH12
1
2
3
4
5
1.red cabbage 2.red rose petal
3.red cherry 4.red grape
5.beet root
• Beet root showed a sharp colour change between pH 12 and pH 11
Back
pH12 pH2
As it showed a sharp colour change at alkaline region, it is suitable for being used as indicator in acid-base titration.
Freshly prepared beet root solution
Over-night
beet root solution
pH12 pH2
pH12 pH2
Colour of the indicators (overnight and fresh beet root
solution)in different pH solutionspH 2
pH3
pH4
pH5
pH6
pH7
pH8
pH9
pH10
pH11
pH12
1
2
1.Overnight 2.Fresh
Indicator Phenolphth-alein
Over-night beet root indicator
Fresh beet root
indicator
Average volume of HCl required for the neutralization
9.68cm3 9.58cm3 9.10cm3
Titration between HCl and oven cleaner(NaOH) using phenolphthalein and beet root as indicator
Indicator used phenolphthalein Over-night beet root solution
Fresh beet root solution
Concentration of NaOH in the oven cleaner
1.936M 1.916M 1.820M
The concentration of NaOH in the oven
cleaner with using different indicators
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