HOW GOOD IS RED CABBAGE AS A DYE?
Introduction
As Hong Kong has an active textiles industry, we thought that we might investigate the chemistry
of dyes and the dyeing process.
We examined the possibility of combining the properties of red cabbage as a dye and as an
indicator to investigate what colours we could dye different fabrics using the coloured material
extracted from red cabbage. In this project we tested the dye on two synthetic (silk and cotton) and
two natural materials (nylon and polyester). We also investigated the effect of pH and mordants on
the dyeing process and then investigated the fastness of the dyes.
Principles
Colour in plants
Anthocyanins are natural plant pigments which are responsible for the large variety of colours in
flowers: red-violet-blue. Different anthocyanins have been shown to contain the same carbon
skeleton and differ only in the nature of the substituent groups.
One anthocyanin thought to be responsible for both the blue colour of cornflowers and the red of
roses is shown below:
OH
OH
HOv
O
ss
0Ft
OH
Red cabbage is thought to contain an anthocyanin with three OH groups.
Indicators
A good acid-base indicator will change its colour as the concentration of hydrogen ions in a
solution changes. Often an indicator will have one or more atoms that can gain or lose protons
(H+).
Mordants
Mordants often consists of metallic cations such as Al3+ or Fe3+ ions help to make the dyes fast .
The Investigation
1 Extraction of the red cabbage dye
The dye was extracted by boiling chopped red cabbage in water.
2 Testing with acids and alkalis
Cabbage extract was added to various acids and alkalis and the colours were recorded.
3 Chromatography
Chromatography of the red cabbage extract was carried out in order to determine whether it
contained more than one coloured component. The solvents used were ethanol, propanone and
water.
4 Determination of the pH range(s) of red cabbage dye
The purpose of this experiment was to try and determine the pH range(s) over which the red
cabbage extract changed colour.
3
To determine the pH range(s), 0.1M NaOH was added, 1.0 cm at a time, to a solution containing
an indicator and a buffer. The pH was measured, using a pH meter, after each addition. The pH
values at which a colour change begins and ends defines the pH range.
The experiment was performed using a buffer solution to measure the pH range(s) rather than water
because the buffer meant that the pH would only change gradually. This enabled more accurate
values of pH range(s) to be determined.
5 Dyeing without a mordant
1M citric acid or 1M NaHCO3 were added to red cabbage extract. Each of the four fabrics to be
tested were boiled in the acidified or alkaline extract for 20 minutes. They were removed and dried
using a hair dryer.
This was repeated using a neutral solution of the dye.
6 Dyeing with a mordant
1% solutions of the following compounds were prepared and used as mordants:
aluminium (III) potassium sulphate-12-water (alum)
iron (III) sulphate, Fe2(SO4)3
potassium dichromate (VI), K2Cr2O7
Pieces of fabric were boiled in mordant solutions for 15 minutes and then boiled in the appropriate
dye solutions for 20 minutes. They were removed and dried.
7 Fastness of the dye
Different pieces of dyed fabric were boiled in water for 15 minutes and exposed to UV light for 2
hours in order to investigate the fastness of the red cabbage dye.
RESULTS
1 Colour and the pH range(s) of red cabbage extract
Volume (cm3)
0
10
18
24
43
Colour
red
purple
blue
green
yellow
Recorded pH Calculated pH
3.3
3.3
5.5
5.3
7.0
7.1
8.5
8.5
11.4
12.7
2Chromatography
The photograph below shows the chromatograms obtained.
Note. In the original written account the results were shown either as photographs or as actual
samples of dyed material which therefore cannot be shown in this abstract.
Discussion
Red cabbage extract as an indicator
The results show that the red cabbage dye behaves as a fairly good universal indicator in the
pH range 3 -14.
To try and explain the different colours obtained from red cabbage dye at different pH's we
assumed that the three -OH groups on the anthocyanin molecule were present on carbon atoms
3. 5 and 7.
HO
o
r)N
~H
We would expect the anthocyanin to ionise in a similar way to phenol but as it has three -OH
groups it could ionise three times to give three different anions with different colours at
alkaline pH's, e.g. blue (pH= 7), green (pH = 8.5) and yellow (pH = 11.4).
HO.
O H (4q
~'
)
HO
OH
>O
yellow
H -,~ s
hlue
b
HO
>
O~ green
Chromatography
The chromatography experiment suggested that there were at least two coloured constituents
in the dye.
Dyeing
The cotton and silk seemed to take the dye well whereas the polyester and nylon did not.
Different conditions of pH and different mordants gave different colours and shades. The 'red'
colour obtained in acid conditions was deeper when carried out after the use of alum or
iron (II) ions as mordant.
The dye was not fast to boiling in water under any conditions. Most of the colour was lost in
each example tested. UV light however did not seem to have any effect on the dye.
We think that cotton and silk took the dye better than nylon and polyester because they could
form more and stronger bonds between the fibres and the dye. The structure of cotton and silk
is such that they have -OH and -NH groups respectively which can hydrogen bond with the
dye. Nylon also has -NH groups but not as many per unit length as silk.
Effect of mordants
We expected the cations in the alum (Al3+) and the iron (III) sulphate (Fe3+) to help bond the
dye by attracting the electronegative oxygen atom and the delocalised benzene rings more
strongly. However this did not work and some colour of the dye was lost on washing with
boiling water.
It is possible that the mordant ions may have been washed out of the fabric when it was boiled
in the dye solution. It may be possible to improve the efficiency of the mordant by
precipitating it out in the form of a hydroxide . This precipitate would be more likely to be
held by the fibres than the free cations.
The mordants did change the colour of the dye. This not unexpected as the cation may have
formed a complex salt with the dye. This is a characteristic property of transition metals such
as iron and chromium.
To summarise, we think that red cabbage has much potential as a source of a dye as it is cheap,
safe to use and can produce a variety of colours.
It could be especially useful as dye for natural fibres such as cotton and silk, both of which are
in common use in Asian countries such as Hong Kong.
However its use is dependent on it being fast. The main problem we found was that fabric
dyed with red cabbage dye faded when boiled in water. We think this could be overcome by
dipping the untreated fabric in weak alkali such as aqueous ammonia and then in a mordant
such as alum solution This will precipitate out aluminium hydroxide which may be better held
by the fibres and may improve the bonding of the dye to the fabric.
Al3+ (aq) + 30H-(aq) = Al(OH)3(s)