Introduction
In nature, leaves, flowers and fruits of many plants have very attractive
colours. It is interesting that these colours may change. In this project, we try
to extract the colours from four plants (i.e. red cabbagef blueberries, tea and
beetroot) and investigate two of their applications.
In the first part, we try to investigate whether the colour extracts from two
food plants, red cabbage and blueberries, can be used as indicator.
In the second part, we try to find out whether tea-leaves and beetroot extacts
can be used as dyes. Further studies on the effects of temperature, pH and
the presence of mordant on dyeing quality are also carried out on these two
kinds of extracts.
P.1
Part I: Plant Extracts as Homemade Indicators
Principle
Acid-base indicators are substances which appear as characteristic colours in
different pH. 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.
H+(aq) + In-(aq)
ionized
base form
HIn (aq)
non-ionized
acid form
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.
As we suspected that some colour pigments in red cabbage and blueberries
might change colours with pH, we added the plant extracts into solutions
with different pH to investigate the possibility of using them as indicators.
The investigation
The plant extracts were prepared by boiling the plant in a suitable solvent
(distilled water for red cabbage and ethanol for blueberries). Solutions with
pH 1 - 14 were prepared. Then 5 drops of the plant extract were added to the
solutions with different pH. The two sets of colours obtained by using red
cabbage and blueberry extracts were observed and compared.
P.2
Result
Blueberries as indicator
pH range
colour
1-3
pink
4-9
nearly colourless
10-14
brownish green
Red cabbage as indicator
pH range
1
2-3
4-6
7-9
10
11
12-14
colour
deep red
reddish purple
pale violet
blue
green
yellowish green
yellow
Discussion
As both extracts changed colours with pH, they could be used as acid-base
indicators. But of course red cabbage extract, which gave more colours,
worked better and had a wider field of application.
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.
P.3
Part II Plant Extracts as Dyes
Principle
Dyeing is an adsorption phenomenon and there are three essential components
needed for dyeing namely a pigment, a solvent and a mordant, which binds
colours on cloths. In the project, colour extracts form beetroot and tealeaves
are sources of pigments; ethanol and water are used as solvents and alum is
chosen as the mordant.
We immerse cloths into dyes kept at different temperatures, pH, with or
without alum to find out how the three factors affect dyeing quality. The result
of dyeing is studied by soaking the dyed cloth in distilled water for 15 minutes
and then comparing the intensity of the colours of the cloths before and after
soaking.
The Investigation
Beetroot and tea-leave extracts were prepared and some cleaned cloths were
boiled in alum solution. To investigate the effect of temperature, dyeing
solutions were kept at 0oC, 30oC,60oC and 80oC. Cloths were then added and
dyed for 15 minutes. For the effect of pH, cloths were immersed into dyes
containing hydrochloric acid, distilled water or sodium hydroxide solution. For
the factor of mordant, ordinary cloths and those soaked in alum solution were
used for dyeing. After the dyed cloths had been dried, they were soaked in
distilled water for 15 minutes.
Results
Dye
Optimal conditions for dyeing
Temperature
pH medium
With or without alum
Beetroot
o
30 C
Neutral (distilled water)
With alum
Tea-leaves
o
Neutral (distilled water)
With alum
60 C
P.4
Discussion
According to our results, every dye had its own optimal temperature range for
dyeing. In both cases, extreme temperatures had adverse effects on dyeing, for
example, beetroot extract changed from pink to brown at 80oC. For both
beetroot and tea-leave extracts, the best dyeing quality was given when
distilled water was added to the dyeing solution. The dyeing quality,
especially the ability to retain colour, was greatly improved with the use of
mordant. This matches a mordant's function to bind colours on cloths
There are limitations on the uses of natural plant dyes because the dyes may be
degraded by light and heat. Nevertheless, natural dyes have some advantages
over artificial dyes. Natural dyes are non-allergic and they are more suitable
for children and babies. They are also less likely to cause hazards to health.
Moreover, they are biodegradable and pose less pollution problems.
To conclude, the pigments used gave quite attractive colours. However,
further research has to be carried out before they can be used as dyes in real
life.
P.5
-End-