Lo Fong Him(19) Chan Lik Sang(2)
Yim Yuet Chit(29)
Fung Hon Ho(5)
Title: Making your own acid-base indicator
Date: 26th April , 2010
Objective: To determine what substances can be used as acid-base indicator
Theory:
Acid-base indicators are organic dyes that change colours
at different pH values. Most indicators are synthesised from
chemical reactions, but some indicators can be easily extracted
from plant materials e.g. red cabbage, beetroot, blackcurrant
and black bean using water or an organic solvent.
Materials and Apparatus:
0.5 M sodium hydroxide solution
Ethanol
0.5 M ethanoic acid
0.5 M hydrochloric acid
Buffer solutions of pH 1-12
Distilled water
10 times diluted oven cleaner solution
Common titration apparatus and laboratory glassware
Microscale chemical apparatus (plastic pipette and well-plate)
pH meter (or data logger with pH sensor)
Cutting board
Hotplate
Precautions:
Avoid direct contact with chemicals. Wash immediately with plenty of tap water
if chemicals spill on your skin. Do not touch any hot objects with your bare hands.
Flammable solvent should be heated in a hot water bath on a hotplate instead of
direct heating using naked flame.
Never ingest any plant materials used in experiments. Never use plants which are
known to contain irritants that induce allergy or dermatitis on skin contact. Plant
pigments may stain your hands. Wear disposable plastic gloves when handling the
plant materials. When dealing with plant specimens which bear spines or thorns, care
must be taken. Wear suitable protective gloves when necessary. Use sharp tools
carefully. Wash hands after handling plant materials.
1
Lo Fong Him(19) Chan Lik Sang(2)
Yim Yuet Chit(29)
Fung Hon Ho(5)
Procedures:
Part A: Extraction of plant pigments
1. Cut coloured plant materials into small pieces.
2. Place a measured amount of materials in a beaker and add 30 cm3 of distilled water.
3. Use a hotplate to heat the materials for 10 minutes. Stir the materials occasionally
while they are being heated.
4. After cooling down, obtain the plant extracts by filtration.
Part B: Preparation of buffer solutions and making choice of suitable acidbase
Indicator
1.Prepare buffer solutions of pH 1-12 according to this table.
pH Chemical composition
1
50 cm3 0.1M HCl
2
5 cm3 0.1M HCl
3
11.2 cm3 0.10M HCl + 0.51 g potassium hydrogen phthalate
4
0.05 cm3 0.10M HCl + 0.51 g potassium hydrogen phthalate
5
11.3 cm3 0.10M NaOH + 0.51 g potassium hydrogen phthalate
6
2.8 cm3 0.10M NaOH + 0.34 g potassium phosphate monobasic
7
14.6 cm3 0.10M NaOH + 0.34 g potassium phosphate monobasic
8
23.4 cm3 0.10M NaOH + 0.34 g potassium phosphate monobasic
9
2.3 cm3 0.10M HCl + 0.24 g sodium tetraborate
10
9.2 cm3 0.10M NaOH + 0.24 g sodium tetraborate
11
11.4 cm3 0.10M NaOH + 0.11 g sodium hydrogencarbonate
12
5 cm3 0.10 M NaOH
2. Measure and record the pH value of the buffer solutions using pH meter. If the pH
is too high, adjust the pH to the target value by adding 1 M HCl; if the pH is too low,
adjust the pH to the target value by adding 1 M NaOH. Dilute each of the mixtures
with distilled water to 50 cm3.
3. Label 12 clean test tubes with the pH values of the solution.
4. Fill each tube one-third full with the corresponding solution of known pH. Add
approximately 1 cm3 of the plant extract to each test tube and mix the solution.
5. Record the colours of the plant extract at different pH values.
6. Repeat steps 1-4 with other plant extracts.
7. Determine which plant extract is suitable as an indicator for titration of strong
alkalis with strong and/or weak acids.
2
Lo Fong Him(19) Chan Lik Sang(2)
Yim Yuet Chit(29)
Fung Hon Ho(5)
Observations:
Dragon fruit peel (10.017g)
pH Color
1
Light pink
2
Light pink
3
Light pink
4
Light pink
5
Light pink
6
Light pink
7
Light pink
8
Light pink
9
Light pink
10
Very light pink
11
Nearly colourless
12
Nearly colourless
Red cabbage(11.90g)
pH Color
1
Red
2
Reddish purple
3
Reddish purple
4
Purple
5
Light pale purple
6
Pale purple
7
Violet
8
Blue
9
Greenish blue
10
Bluish Green
11
Green
12
Greenish yellow
From left to right : pH12-1
From left to right : pH1-12 (no 9&10)
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Only for 9&10(because they were fresh but the other were not)
Lo Fong Him(19) Chan Lik Sang(2)
Yim Yuet Chit(29)
Fung Hon Ho(5)
Grape juice
pH Color
1
Red
2
Red
3
Red
4
Red
5
Nearly Colourless
6
Light pink
7
Greyish green
8
Purple
9
Violet
10
Light red
11
Green
12
Green
From left to right : pH1-12
Yellow ginger powder
pH Color
1
Dark brown
2
Brown
3
Orange
4
Orange
5
Very pale yellow
6
Very pale yellow
7
Pale yellow
8
Pale yellow
9
Light yellow
10
Light yellow
11
Ligh yellow
12
Yellow
From left to right : pH1-12
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Lo Fong Him(19) Chan Lik Sang(2)
Yim Yuet Chit(29)
Fung Hon Ho(5)
Discussion:
Error:
1. Error on measuring the volume of solution using pipette.
2. The color of the extracts is difficult to distinguish by eyes.
3. Some chemical substances such as potassium hydrogen phthalate is used for many
times so these chemical may be polluted.
4. The plant material may contain impurities which will affect the final color in the
result.
Improvement:
1. Using apparatus with higher precision .
2. Colorimeter can be used instead of our eyes for observation of the color of the
extracts.
3. Polluted chemical substances can be replaced by new one and handle it carefully in
order to prevent pollution of it.
4. The plant material can be cleaned by distilled water before the experiment.
Questions and Answers:
1.What is an acid- base indicator?
A: Acid- base indicators are substances which change colour with the pH of their
environment .
They are usually weak acids or weak bases.
2.What is the chemical principle involved in acid- base indicators?
A: Acid-base indicators are substances which appear as characteristic colours in
differentpH.
They are usually weak organic acids or bases which have different colours in their
ionized form and non-ionized form.
In aqueous solution, the two forms of the indicator are in equilibrium.
HIn (aq) H+(aq) + In-(aq)
In an acidic solution, the equilibrium shifts to the left and the colour of the
indicator is mainly the colour of the non-ionized form. In alkaline solution,
the equilibrium shifts to the right and the colour of the ionized form
predominates.
5
Lo Fong Him(19) Chan Lik Sang(2)
Yim Yuet Chit(29)
Fung Hon Ho(5)
3.What substance inside plant materials can be used as acid-base indicator?
What are their basic chemical structures?
A: Many plants including from leaves (red cabbage); flowers (geranium,poppy,
or rose petals); berries (blueberries, blackcurrant); and stems (rhubarb) contain
chemicals from the naturally-colored anthocyanins family of compounds. They are
red in acidic solutions and blue in basic.
4.How to determine whether the indicator is suitable for a titration between a
strong alkali and a weak acid?
A: Indicators should be chosen in such a way that the pH at equivalence point lies
within the pH range of indicator. For a titration between a strong alkali and a weak
acid, the pH at equivalence point is below 7 ,so the pH range of the indicator should
below 7 to give a sharp colour change at end-point.
5. What are the pros and cons of using your indicator over acid-base indicators
commonly found in the laboratory?
A: The indicators we used are natural. They are all non-hazardous and they can
be prepared easily. These advantages make both red cabbage and blueberry
extracts decent teaching materials for junior form students. But both plant
extracts cannot be stored for a long time and they are not suitable for precise
pH measurement.
Conclusion:
Both red cabbages and Grape juice can be used as acid-base indicator, but red
cabbage is the better one.
6
Lo Fong Him(19) Chan Lik Sang(2)
Yim Yuet Chit(29)
Fung Hon Ho(5)
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