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 studentsDiscussing 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 indicatorsMaking natural indicators 2) Choosing a suitable indicators for titration Choosing a suitable indicators for titration 3) Determining the concentration of oven cleaner 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

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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 cherryRed cabbage

Beet rootRed grape

Red rose petal Five natural indicators Back

Phenolphthalein When [Ph - (aq)] = [HPh(aq)], pH = pK In = -log (7 x ) = 9.15 When ≦ 1/10 pH ≦ pK In + log 1/10 ≦ pK In - 1 ≦ 8.15 When ≧ 10 pH ≧ pK In + log 10 ≧ pK In + 1 ≧ 10.15

When [Me(aq)] = [HMe + (aq)], pH = pK In = -log (2 x ) = 3.7 When 1/10 pH ≦ pK In + log 1/10 ≦ pK In - 1 ≦ 2.7 When ≧ 10 pH ≧ pK In + log 10 ≧ pK In + 1 ≧ 4.7 Methyl orange

Acid-base Titrations Acid (CH 3 COOH or HCl) Oven cleaner (NaOH) + indicator

Strong Acid-Strong Base Titration Sharp change in pH at the equivalence point: 3-11 pH Titration Curves

Sharp change in pH at the equivalence point: 2-6 Strong Acid-Weak Base Titration

Sharp change in pH at the equivalence point: 8-12 Weak Acid-Strong Base Titration

NO Sharp change in pH at the equivalence point. Weak Acid-Weak Base Titration

Results Red cabbage showed different colours at different pH solutions. pH12 pH2

Red grapes showed insignificant colour change pH12pH2

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 pH 11 pH red cabbage2.red rose petal 3.red cherry4.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

Colour of the indicators (overnight and fresh beet root solution)in different pH solutions pH 2 pH 3 pH 4 pH 5 pH 6 pH 7 pH 8 pH 9 pH 10 pH 11 pH Overnight2.Fresh

IndicatorPhenolphth -alein Over-night beet root indicator Fresh beet root indicator Average volume of HCl required for the neutralization 9.68cm cm cm 3 Titration between HCl and oven cleaner (NaOH) using phenolphthalein and beet root as indicator

Indicator usedphenolphthaleinOver-night beet root solution Fresh beet root solution Concentration of NaOH in the oven cleaner 1.936M1.916M1.820M The concentration of NaOH in the oven cleaner with using different indicators Back