advanced applied science: GCE A2 UNITS
© The Nuffield Foundation 2008
ACTIVITY BRIEF
Extracting and testing a natural
plant dye
The science at work
Most fabrics you buy are coloured using synthetic dyes. However, before these dyes were
developed, many natural dyes were obtained from plants. There is an increasing interest in
the application of these natural dyes as environmental awareness and public concern about
pollution increase.
Your brief
You need to extract a natural plant dye, then investigate its effectiveness to dye three
different fabrics and write a detailed report of your work. The plant is likely to contain a
mixture of dyes.
Task 1 Getting prepared
Complete Study sheet: Natural dyes. Keep the completed sheet in your portfolio. Then select
a plant material from which to extract a dye and
ƒ
identify the types of plant dyes likely to be present
ƒ
write structural formulae for these dyes
ƒ
identify the functional groups responsible for the colours.
Select three fabrics to dye. In your portfolio:
ƒ
write the names of the chosen fabrics, together with the structural formulae of the
molecules that make up the fabric
ƒ
suggest how the dyes might be attached to the fabrics.
Task 2 Extracting the dye
Suggest a method for extracting the dyes from the plant material. The method should
include isolating a solid product as well as a solution of the dye. You may find it helpful to
look at Information sheet: Extraction and dyeing methods. In your portfolio:
ƒ
outline the method
ƒ
list the chemicals and apparatus needed
ƒ
make a risk assessment.
Check the method with your teacher. After making any necessary modifications, carry out
some trials using the suggested extraction method. In your portfolio make a note of any
changes to the method, with your reasons, and write out in full the method to be used.
Extracting and testing a natural plant dye: page 1 of 19
advanced applied science: GCE A2 UNITS
© The Nuffield Foundation 2008
Carry out two extractions:
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extract a small sample of solid dye
ƒ
extract sufficient dye to make 100 cm3 of dyeing solution (you do not need to isolate the
dry dye).
In another activity you will look at scaling up production to an industrial scale.
Task 3 Applying the dye and testing its effectiveness
This task needs to be planned carefully. There are many different things you could try, but
your time will be limited. You need to decide how to apply the dye. Use Information sheet:
Extraction and dyeing methods to help you. Then make notes about the apparatus and
method you will use. There are a number of conditions you could vary, e.g. temperature,
dye concentration and drying the fabric between successive dyeings. Most natural dyes are
only effective if a mordant is used.
Apply your dye to the three chosen fabrics and evaluate its suitability for each fabric.
Example criteria are the appearance of the fabric, the depth of colour and the fastness of the
dye (does it wash out easily?).
Task 4 Writing your report
Write a report of your work. This should describe the purpose of the work, the key findings
and your conclusions. Key data may be taken from your portfolio and presented in a form
that makes it easier for the reader to understand.
You do not need to write out all your work again. You may cross-reference information you
found (together with their source) and data recorded during your practical work in your
portfolio, during the investigation.
Extracting and testing a natural plant dye: page 2 of 19
advanced applied science: GCE A2 UNITS
© The Nuffield Foundation 2008
STUDY SHEET
Natural Dyes
You do not need to remember all the information on this sheet.
However, working through the questions will give you a good background for the practical
work to come.
Substances with intense colours are often called colorants. Colorants may be natural or
synthetic. Plants are a source of natural colorants.
ƒ
Colorants used for textile and food are often called dyes.
ƒ
Colorants used for inks, paints and cosmetics are often called pigments.
Colorants from natural materials may be used with textiles, food, inks, paints and cosmetics.
So the two terms are often used interchangeably.
Strictly speaking, the difference is what happens to their crystal or particle structure during
use: a pigment keeps its structure; a dye loses its structure, e.g. by dissolving in a solvent.
Natural dyes in plants
A plant usually contains a mixture of natural dyes. This mixture is often extracted and used
to dye textiles. However, a particular dye may be extracted from the mixture to be used
alone.
Natural dyes may be grouped according to their chemical structures. Here are some
important groups.
Carotenoids
There are two major types of carotenoids: carotenes (orange or red-orange) and
xanthophylls (yellow).
Carotenoids
Carotenes
Xanthophylls
Carotenes are found in, for example, carrots, red peppers, oranges and tomatoes. They are
also responsible for the colour of most yellow and orange flowers. Xanthophylls are found in,
for example, nettles, French marigolds, annatto and saffron.
Most carotenes are hydrocarbons though some also contain oxygen atoms. The most
well-known is often simply called carotene (yes, there is a compound called carotene and a
group of compounds called carotenes). It’s found in carrots and has the chemical structure:
Extracting and testing a natural plant dye: page 3 of 19
advanced applied science: GCE A2 UNITS
© The Nuffield Foundation 2008
H3C
CH3
CH3
H3C
CH3
CH3
CH3
H3C
CH3
CH3
All carotenoids are based on this carotene structure. The central carbon chain remains the
same, but the groups at either end differ slightly. The central chain consists of 18 carbon
atoms bonded alternatively with single and double covalent bonds with four methyl groups
attached (always in the same positions). The sequence of alternative single and double
covalent bonds is called a conjugated system.
Some carotenoids have molecules containing oxygen atoms. These are xanthophylls. Here
is an example:
O
H3C
CH3
HO
CH3
H3C
CH3
CH3
CH3
CH3
H3C
OH
CH3
O
All carotenoids are fat-soluble. They dissolve in organic solvents that are not too polar. They
do not dissolve in water.
Questions
1
Explain why carotene is a hydrocarbon.
2
Write the molecular formula of carotene.
3
Identify the part of the molecule responsible for its colour. Hint: What’s unusual about
the structure of the carotene molecule compared with other hydrocarbons you have
come across?
4
On the chemical structure of carotene given above, highlight the backbone common to
all carotenoids and circle the four methyl groups attached to it.
5
List plant products containing carotenoids that you can obtain easily and not too
expensively.
6
If you want to extract a carotenoid to investigate its potential as a dye, what solvents
might you use and why?
Flavonoids
There are four main types of flavonoids and other less common ones.
Flavonoids
Flavones
Flavonols
Extracting and testing a natural plant dye: page 4 of 19
Anthocyanidins
Anthocyanins
advanced applied science: GCE A2 UNITS
© The Nuffield Foundation 2008
Flavonoids are water-soluble compounds with molecules derived from
2-phenyl-1,4-benzopyrone.
O
O
Flavones and flavonols have yellowish colours. The colours are sensitive to pH. The
yellow becomes much deeper in solutions of high pH. The colours of flavones tend not to
fade in strong light as flavonols do, but they are paler.
quercetin, kaempferol and myricetin
found in many families, including onions,
horse chestnuts and tea
fisetin
found in fustic, nettles etc
morin
found in fustic, Osage-orange etc
apigenin
found in members of the daisy family
luteolin
found in daisy, pea and weld
flavone
found in primrose
The yellow colour of onion skins is due to a mixture of flavones and flavonols: quercetin,
kaempferol and quercetin-3-glucoside.
OH
OH
HO
O
OH
OH
O
quercetin
OH
HO
O
OH
OH
O
kaempferol
Extracting and testing a natural plant dye: page 5 of 19
advanced applied science: GCE A2 UNITS
© The Nuffield Foundation 2008
OH
OH
HO
O
O
OH
O
OH
O
HO
CH2
OH
OH
quercetin-3-glucoside
Questions
1
Highlight the 2-phenyl-1,4-benzopyrone backbone in each of the flavonoids found in
onion skins.
2
Highlight the glucoside part of quercetin-3-glucoside.
3
Here is the chemical structure of glucose:
OH
OH
HO
HO
OH
or
HO
CH2
O
HO
O
OH
representation A
OH
OH
representation B
a
Explain what representation B tells you about the glucose molecule that
representation A doesn’t.
b
Try to explain how quercetin-3-glucoside might be formed from quercetin and
glucose.
4
Why are flavonoids water soluble while carotenoids are not?
5
Why do you think flavonoid colours are sensitive to pH while carotenoids are not?
Anthocyanidins and anthocyanins are the
most highly coloured of the flavonoids. They are
responsible for the scarlets, reds, violets and
blues in many flowers, fruits and vegetables.
R
R
HO
O
OH
OH
2
+
R
The general structure for an anthocyanidin is
given on the right.
Extracting and testing a natural plant dye: page 6 of 19
1
3
advanced applied science: GCE A2 UNITS
© The Nuffield Foundation 2008
This positive charge on this cation is balanced by an anion, e.g. chloride, Cl-.
The chromophore (functional group responsible for colour) is:
O
+
The most common anthocyanidins:
R1 R 2 R 3
cyanidin
red-purple OH OH H
delphinidin
blue-purple
pelargonidin orange-red
OH OH OH
H OH H
malvidin deep purple OCH3 OH OCH3
peonidin red
petunidin purple
OCH3 OH H
OH OH OCH3
When anthocyanidins bond to sugar molecules they become anthocyanins.
Anthocyanins are found in plants far more commonly than the parent anthocyanidins. They
are found in:
ƒ
pigmented leaves and flowers of many plants
ƒ
fruit such as blackcurrants, aubergines, oranges, blackberries, raspberries, cherries and
redcurrants.
Anthocyanins are soluble in water and easily extracted into weakly acidic solution. However,
the colour is pH dependent.
O
+
OH
Other minor flavonoids include
ƒ
chalones - coreopsidoside and mareoside (daisy family)
ƒ
aurones - sulfuroside (fustic, daisy family)
ƒ
isoflavones - tend to produce strong, permanent colours e.g. genistein (pea family),
osajin and pomiferin (Osage-orange (Maclura pomifera)).
Questions
1
Sketch structural formulae for (a) cyanidin, (b) pelargonidin and (c) peonidin. In each
case highlight the chromophore.
Extracting and testing a natural plant dye: page 7 of 19
advanced applied science: GCE A2 UNITS
© The Nuffield Foundation 2008
2
Now sketch the anthocyanins that would form when glucose reacts with (a) cyanidin, (b)
pelargonidin and (c) peonidin. Hint: Look back at question 3 in the section Flavones and
flavonols.
3
Try finding plants that contain some of anthocyanins listed above. Hint: Think which
names of flowers the anthocyanin’s names remind you of.
4
Outline how anthocyanins might be extracted from plants.
5
What techniques might be used to identify a mixture of anthocyanins?
Anthracenes
There are two major groups: anthraquinones and napthoquinones. They contain several
well-known dyes.
Anthracenes
Anthraquinones
Napthoquinones
Some examples:
Anthraquinones
alizarin
mungistin
madder
purpurin
Napthoquinones
juglone
walnut
alkanin
hypericin
St. John’s wort
Questions
Here is the structure of a napthoquinone called juglone:
O
O
OH
It can be extracted from walnut rind:
Cut the rind from six walnuts in small pieces. Place them in a round bottomed flask, add
100 cm3 ethyl ethanoate and fit a condenser. Set up a hot water bath. Raise the temperature
to 70 ºC and reflux the solution for 45 minutes. Filter and evaporate the solvent. Juglone is
obtained as a brown solid.
Extracting and testing a natural plant dye: page 8 of 19
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© The Nuffield Foundation 2008
1
Why do you think ethyl ethanoate was used for the extraction rather than water?
2
What other solvents might you try?
3
Why is the extraction mixture refluxed?
4
How could you determine the yield of juglone?
5
How could you calculate the possible yield of juglone from 50 kg of walnut rinds?
Some other dyes from plants
There are several other types of plant dyes. Here are some examples you might come
across.
The colour of red beetroot is due to betanin, which can be extracted easily from the
vegetable. It makes up 95% of the red pigments in the extract. Betanin is a betacyanin – a
group of red dyes once thought to be flavonoids. Like anthocyanins they are glycosides.
However, their molecules contain nitrogen. Also, they do not change colour reversibly when
pH changes as anthocyanins do.
Tannins are present in most plant tissues, being produced from the flavonoids, especially
the anthocyanins, when tissues break down. They are found, for example, in the bark of oak
and willow.
Chlorophyll molecules consist of (a) a porphyrin head (four pyrrole rings containing
nitrogen arranged in a ring around a magnesium ion) and (b) a long hydrocarbon tail, which
is soluble in fats and oils. There are four types of chlorophyll. One of them, chlorophyll b,
can be extracted from green plants such as stinging nettles and spinach. It is used
extensively as a colorant, but rarely to dye textiles. Commercial extraction from plant
material involves many steps using solvents such as petroleum ether and propanone.
Dyeing with natural dyes
Most natural dyes are acid dyes, which is why they bond to wool, but not to cotton.
1
Explain the term ‘acid dye’.
The use of natural dyes is much improved by using a mordant. The fabric is treated with a
solution of metal ions; these form a bridge between the fabric molecules and the dye, a
process called mordanting.
2
Give some examples of mordants that are used with natural dyes.
Extracting and testing a natural plant dye: page 9 of 19
advanced applied science: GCE A2 UNITS
© The Nuffield Foundation 2008
INFORMATION SHEET
Extraction and Dyeing Methods
This sheet contains extracts of articles about extracting natural plant dyes and using them to
dye, for example fabrics.
You may do your own research and add to this list.
Please be aware that these articles do not contain heath and safety information. You will
have to do a risk assessment before you carry out any work. Make sure risk assessments are
checked by your teacher before you begin work.
Extraction of natural juglone dye from the rind of walnut
Juglone is chemically 5-hydroxy-naphthoquinone. It is known as natural brown 7.
O
O
OH
It can be extracted from the rind of the walnuts (Juglans regia). It represents an important
landmark in the use of natural silk dyeing since ancient times.
Procedure
Cut the rind from six walnuts in small pieces. Place them in a round bottomed flask, add
100 cm3 ethyl ethanoate and fit a condenser.
Set up a hot water bath. The temperature is raised to 70 ºC and the solution is maintained in
reflux (45 minutes). Filter and evaporate the solvent. The brown solid obtained should be
dried and weighed.
Using a microwave oven?
http://www.bgci.org/educationcongress/proceedings/Workshops/Public%20awareness/Chest
ers%20Sarah%20-%20PA.pdf explains:
Dyers usually use large quantities of boiling water … which can be difficult, if not impossible
for teaching … However, the microwave can produce wonderful results in a few minutes.
Pure untreated wool, either in its natural state or spun, is first treated with a mordant, which
holds the dye onto the wool. There are numerous mordants which can themselves alter the
final shade of a plant dye - an alum mordant will give clear, bright colours, chrome will dull
or ‘sadden’ the colour. A local dyer can be employed to mordant your wool beforehand, but
if you choose to mordant yourself, make sure you wear gloves, as some, for example,
chrome, can burn the skin when wet. Short lengths of wool are then treated with a plant dye
in the microwave. Orange onion skins, mordanted with alum and cooked in 1-2 cm water for
3 minutes on a medium heat, will release orange coloured water. Strain the water, add the
wool and cook for a further 3 minutes on medium. Remove the wool, rinse with cold water
and the threads should have turned bright yellow.
Extracting and testing a natural plant dye: page 10 of 19
advanced applied science: GCE A2 UNITS
© The Nuffield Foundation 2008
There are numerous other dyes available from different parts of plants – roots, bark, stems,
leaves, fresh/dried flowers (fresh fruit juices tend to fade easily). Some plants give better
results than others and it is best to make precise notes of everything you try out. Some plant
dyes, such as powdered madder root are available from plant dye websites.
Three dye plants
Adapted from http://www.gallica.co.uk/celts/dyes.htm
Madder
The part of the plant used for the dye is the tuber type roots. The plant should be pulled
from the ground after loosening the soil. The leaves can then be stripped of the plant and
the roots put in a sheltered place to dry out. When dry the roots can be ground up into a
powder and put in a pot with some water. This should be heated to extract the bright red
dye. The use of alum as a mordant will give a deep red on wool. If a copper dye vat is used,
the colour will be brighter.
Weld
The whole of the plant can be harvested and dried. You can chop it up to make storage
easier. The whole of the plant is put in boiling water to extract the yellow dye. Use alum as
a mordant with wool, and you will get a lemon-yellow colour.
Woad
Drop the leaves into fast boiling water with a pinch of cream of tartar.
Boil for 2 to 3 minutes. Remove the leaves and cool the liquid as quickly as possible. Add
ammonia until the Liquid is yellowish in colour. Bring the liquid to 60 degrees Centigrade
while adding ammonia until yellow again. Add sodium dithionite to deoxidise the liquid. Stir
gently without splashing until a bronze scum appears. Put in the hot wet wool then remove
it and shake in the air. Dip it again if not blue enough.
Blueberries, onions and spinach
Adapted from an article by Dr Beth Calder, University of Maine, USA
The pigments can be prepared in a saucepan over a stove or electric hot plate. Water can be
heated in an electric kettle and poured onto fruit and vegetable skins to leach out plant
pigments, as well.
ƒ
Simmer 1 pint of fresh or frozen blueberries in ½ cup of water in a small saucepan over
medium heat until a deep blue colour is leached from the skins. Strain out blueberry
skins with a fine kitchen strainer. (A tea strainer works well).
ƒ
Simmer yellow onion peels (the yellow papery skins, not the onion flesh) in ½ cup of
water over medium heat until the water turns a golden yellow colour. Pour liquid through
a strainer to remove onion peel.
ƒ
Simmer cooked spinach in ½ cup of water over medium heat until water turns a dark
olive green. Strain out spinach. (Fresh spinach greens do not release chlorophyll pigment
well and are not recommended for this exercise.)
The pigment solutions can be stored at room temperature in air-tight glass containers.
Disposable plastic bowls and spoons are best for dying materials in the classroom.
Example of use:
Extracting and testing a natural plant dye: page 11 of 19
advanced applied science: GCE A2 UNITS
© The Nuffield Foundation 2008
Divide the blueberry solution into three bowls. Add vinegar to one bowl in small amounts
until the blueberry solution turns pink and add baking soda to a second bowl in small
amounts until the blueberry solution turns dark purple. Students will then have three
separate anthocyanin shades to dye materials over a range of colour. After adding vinegar
and baking soda to blueberry pigment, the dye will be weak.
Making natural dyes from plants
Adapted from http://www.pioneerthinking.com/naturaldyes.html
Gathering plant material for dyeing: Blossoms should be in full bloom, berries ripe and
nuts mature.
It's best to use an old large pot as your dye vessel. Wear rubber gloves to handle the fabric
that has been dyed, the dye can stain your hands. It's also important to note, some plant
dyes may be toxic.
To make the dye solution: Chop plant material into small pieces and place in a pot.
Double the amount of water to plant material. Bring to a boil then simmer for about an hour.
Strain. Now you can add your fabric to be dyed. For a stronger shade, allow material to soak
in the dye overnight.
Preparing the fabric for dyeing: You will have to soak the fabric in a colour fixative
before dyeing. This will make the colour set in the fabric.
Colour Fixatives
ƒ
Salt Fixative (for berry dyes) 1/2 cup salt to 8 cups cold water
ƒ
Plant Fixatives (for plant dyes) 4 parts cold water to 1 part vinegar
Add fabric to the fixative and simmer for an hour. Rinse the material and squeeze out
excess. Rinse in cool water until water runs clear.
Dyeing: Place wet fabric in dye bath. Simmer together until desired colour is obtained. The
colour of the fabric will be lighter when it’s dry. Also note that all dyed fabric should be
laundered in cold water and separately.
Muslin, silk, cotton and wool work best for natural dyes and the lighter the fabric in colour,
the better. White or pastel colours work the best.
Home-made plant dyes
Adapted from http://tilz.tearfund.org/Publications/Footsteps+21-30/Footsteps+21/Homemade+plant+dyes.htm
There is a whole variety of plants which can be used to make plant dyes. We can suggest
some well known plants found in many countries. However, each area will have its own
plants which can be used to make dyes. Ask older people for their advice.
Different parts of plants are used to make dyes – for example, the leaves, the skins of fruit,
the bark, roots or wood.
Wool and silk are fairly easy to dye. Cotton, however, is much harder. Because cotton is the
most common material that people will want to dye, we will only look at recipes for cotton
dyes in this article.
Extracting and testing a natural plant dye: page 12 of 19
advanced applied science: GCE A2 UNITS
© The Nuffield Foundation 2008
The quantities that follow will be sufficient to dye 0.5 kg of dry cotton material. This is about
three to four T-shirts. You will need at least two large pans which you do not plan to use for
cooking food and a stove to heat these pans.
Washing. Place the cotton in boiling water, adding washing soap and 2– 3 tablespoons of
washing soda. Boil for an hour and then leave to soak for 24 hours before rinsing the cotton
well.
Mordanting. Mordants prepare the cotton fibres and help them to absorb the dye better. It
is possible to dye without using mordants, but using mordants will generally give much
better, brighter and more permanent colours. Many different chemicals can be used as
mordants, most of which are very poisonous. Before you put the wool or cloth into the dye,
you must soak it in a mordant. The most common mordants are alum, copper sulfate,
potassium dichromate(VI), iron(II) sulfate and tannin. Often two mordants are mixed
together for the best results.
ƒ
Alum mordant: Add 1 small teacup of alum and, if available, 2 level tablespoons of cream
of tartar to 5 litres of water (quarter of a large water bucket). Dissolve the chemicals in
warm water and leave cotton to soak for 24 hours.
ƒ
Tannin mordant: Tannin powder can be bought but you can also use bark from trees
known to contain high levels of tannin. Use 2 heaped tablespoons of the powder or 500 g
of bark from African wattle, gum tree (eucalyptus species) or mimosa thorn tree. Bring to
the boil in 5 litres of water and leave cotton to soak for 24 hours.
ƒ
Copper sulfate mordant: Mix 500 g of gum tree bark with 1 heaped teaspoon of copper
sulfate. Again, bring to boil in 5 litres of water and leave cotton to soak for 24 hours.
ƒ
Metal mordants: You can make mordants from iron, chrome or tin by boiling some of
these metals with water. To make iron mordant, for example, boil 5 litres of water with 2
cups of vinegar and 1 cup of rusty nails for one hour. Leave it to stand for 24 hours and
then pour off the solution. This solution is the mordant.
Different mordants will give different colours from the same dye. Again, you can experiment
to see what results you get. Alum mordant usually gives the best results as it is cheap, very
reliable and gives bright colours. Salt, vinegar and wood ashes can also be tried as mordants
if nothing else is available.
Plant materials. Collect the flowers, leaves, roots, bark and berries you plan to use. About
500 g of plant material is needed, about enough to fill half a 20 dm3 water bucket. You need
less for wood or bark. Boil the plant matter with 10 dm3 of water (half a full water bucket)
for about one hour until the dye is dark in colour. Then strain out all the plant material. Now
you are ready to dye the cotton.
Dyeing. Rinse off the mordant with clean water. Now add the wet fabric to the hot dye.
Bring the dye to just under boiling temperature and let the mixture simmer, but not actually
boil. Keep the material moving gently to ensure that the dyeing will be even. Keep the
material in the dye for at least half an hour – depending on how strong you want the colour
to be. Take it out and rinse the material several times until the water becomes clear. Give a
final wash with soap, then rinse and dry.
Extracting and testing a natural plant dye: page 13 of 19
advanced applied science: GCE A2 UNITS
© The Nuffield Foundation 2008
Extracting colours
Adapted from The Viking Deception, Nova, Original broadcast: February 8, 2005;
www.pbs.org/nova/vinland
Follow the recipe below for your berry or onion skin type to make your dye. Take the
following precautions:
ƒ
wear goggles
ƒ
use potholders or mitts when touching hot pots
ƒ
be careful not to spill dyes on your skin or clothing because they stain.
Make a chart on a separate sheet of paper, with areas large enough to record your results.
After you have made the dye, record its colour and intensity (e.g. blue, very light) in your
chart.
Dye recipe
ƒ
Boil about 500 cm3 of water.
ƒ
For onions: Add red or yellow onion skin preparations. (For each type of onion skin, place
a large handful of onion skins in knee-high nylon stocking. Make a knot in the stocking at
top.)
ƒ
For berries: Add 1 cup blueberries or blackberries.
ƒ
Simmer for 30 minutes.
ƒ
Cool, then carefully pour dye through sieve into the spouted measuring cup.
ƒ
For onions: Discard nylon sacks and pour dye into jar. Seal jar.
ƒ
For berries: One student should hold the cheese cloth over the jar so that the cheese
cloth dips inside while another student slowly pours the dye into the jar. The cheese
cloth will catch berry particles. Discard berries. Seal jar.
Red onion anthocyanin
Yellow onion quercitin
Blueberry anthocyanin
Blackberry anthocyanin
Utilisation of orange by-products - orange peel carotenoids
G Aravantinos-Zafiris, V Oreopoulou, C Tzia and C D Thomopoulos; Department of Chemical
Engineering, National Technical University of Athens, Greece
Carotenoids were extracted from fresh orange peel with various solvents. Propanone was the
most efficient of the solvents tested. Two successive extractions with propanone after an
initial washing with either propanone or methanol were adequate to remove 89% of the total
carotenoids. The extracts were concentrated, the carotenoids transferred to hexane and a
crude pigment concentrate was obtained by hexane evaporation. Water washings prior to
propanone extraction eliminated the solvent-solvent transfer to hexane. The extraction
residue was used for pectin recovery. Carotenoid removal from the peel did not affect the
yield and quality of the pectin.
Extracting and testing a natural plant dye: page 14 of 19
advanced applied science: GCE A2 UNITS
© The Nuffield Foundation 2008
Teacher notes
This activity links to AQA A2 Unit 13 Colour chemistry.
One assessment requirement for the unit is that students must produce:
a report detailing the extraction of a natural (plant) dye together with details of its
application to three different fabrics including:
ƒ
identifying your chosen dye and researching a method for its extraction
ƒ
detailing the type of dye and the method used to make it, the scientific principles
underlying the process together with the apparatus used and any appropriate risk
assessments
with the highest marks gained if they show that (this has been taken from the specifications)
ƒ
Thorough research has produced comprehensive and workable method of extracting a
natural (plant) that is firmly rooted in science.
ƒ
Initial plan into a method of extracting a natural (plant) dye was comprehensive with no
error and yielded workable methods. Appropriate modifications to the method of
extraction of the dye are suggested.
The awarding body teacher guidance notes suggest:
Candidates will need to complete the small scale extraction of a natural (plant) dye. Natural
dye extraction could be extraction of lizarin from the common madder plant or indigo from
the indigo plant.
Presumably ‘lizarin’ should be ‘alizarin’. It is difficult to imagine that students will have easy
access to madder plants or woad (‘the indigo plant’).
More likely options are onion skins, nettles, flowers such as marigolds and geraniums,
carrots and fruit such as blackcurrants, blackberries, raspberries, cherries and redcurrants.
Extraction
Students are provided with several examples in Information sheet: Extraction and dyeing
methods. They may be asked to find further information and add this to the examples given.
Students then need to consider variables such as solvents, time, temperature, pre-treatment
of plant material and so on.
Health and Safety
It is important that students carry out detailed risk assessments. You must check these
before they begin practical activities. Some of the health and safety issues in the articles
listed include:
Eye protection should be worn for all practicals
Ethyl ethanoate [HIGHLY FLAMMABLE] [IRRITANT]
Chrome alum [IRRITANT]
Regarding the article ‘Three dye plants – Woad’ it states ‘…with a pinch of’. Students should
realise it is bad practice to handle chemicals with their fingers. For example they might adapt
it to ½ a spatula full.
Extracting and testing a natural plant dye: page 15 of 19
advanced applied science: GCE A2 UNITS
© The Nuffield Foundation 2008
Sodium dithionite [HARMFUL]
References to gloves should be substituted for ‘protective disposable gloves, preferably
nitrile’
Washing soda [IRRITANT]
Mordants most are [TOXIC]
Copper sulphate [HARMFUL]
Potassium dichromate(VI) [OXIDISING] [VERY TOXIC] Students should look for a safer
alternative
Iron(II) sulphate [HARMFUL]
Thermal protective gloves should be worn when touching hot pots, not potholders or mitts.
Propanone [HIGHLY FLAMMABLE] [IRRITANT]
Methanol [HIGHLY FLAMMABLE] [TOXIC]
Hexane [HIGHLY FLAMMABLE] [HARMFUL] It is preferable to use cyclohexane which is also
highly flammable and harmful, but there are less serious long term effects.
Dyeing
Again, students can get ideas from Information sheet: Extraction and dyeing methods. One
possibility for streamlining the dyeing tests, however, is to use multifibre strip (see below). It
is relatively inexpensive and will reduce the number of experiments to set up to investigate
dyeing effectiveness.
SDC Multifibre Strip
The SDC Colour Experience (Museum) handles the supply to educational establishments at a
greatly reduced rate. The price is £2/metre plus VAT plus a charge of £1/order to cover
postage and packing.
The SDC multifibre test fabric is made up of 6 different fibres, each is a band of about
15 mm. The fibres are: secondary cellulose acetate (Dicel); bleached unmercerized cotton;
nylon 6.6; polyester (Terylene); acrylic (Courtelle); wool worsted.
SDC Colour Experience, Perkin House, 1 Providence Street, Bradford, BD1 2PW
telephone 01274 390955; fax 01274 392888; www.colour-experience.org
Answers to questions
Carotenoids
1
Explain why carotene is a hydrocarbon.
Answer: Its molecules are made up of carbon and hydrogen atoms only
2
Write molecular formula of carotene.
Answer: C40H56
3
Identify the part of the molecule responsible for its colour. Hint: What’s unusual about
the structure of the carotene molecule compared with other hydrocarbons you have
come across?
Answer: The chain of alternating single and double covalent carbon-carbon bonds
Extracting and testing a natural plant dye: page 16 of 19
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© The Nuffield Foundation 2008
4
On the chemical structure of carotene given above, highlight the backbone common to
all carotenoids and circle the four methyl groups attached to it.
5
List plant products containing carotenoids that you can obtain easily and not too
expensively.
Answer: Examples are listed in the text.
6
If you want to extract a carotenoid to investigate its potential as a dye, what solvents
might you use and why?
Answer: Carotenoids are non-polar molecules and do not dissolve in highly polar solvents
such as water. They dissolve in non-polar organic solvents such as hexane or
cyclohexane.
Flavonoids
1
Highlight the 2-phenyl-1,4-benzopyrone backbone in each of the flavonoids found in
onion skins.
Answer: See black solid ovals on diagrams below
OH
OH
HO
OH
O
HO
O
OH
OH
OH
O
OH
O
OH
OH
HO
O
O
OH
O
OH
O
HO
CH2
OH
OH
2
Highlight the glucoside part of quercetin-3-glucoside.
Answer: See above (black dash oval)
3
a
Explain what representation B tells you about the glucose molecule that
representation A doesn’t.
Answer: It shows the three-dimensional shape of the molecule.
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b
4
Try to explain how quercetin-3-glucoside might be formed from quercetin and
glucose.
Answer: It is formed by a condensation reaction in which a water molecule is
eliminated.
Why are flavonoids water soluble while carotenoids are not?
Answer: Unlike carotenoids, flavonoids have ionisable –OH groups attached to benzene
rings (phenoxy groups).
5
Why do you think flavonoid colours are sensitive to pH while carotenoids are not?
Answer: Flavonoids have phenoxy groups which are weakly acidic. In alkaline solution
these will ionise while in acid solutions they will exist as unionised molecules. The colour
of the ionised and unionised forms may be different.
Anthocyanidins and anthocyanins
1
Sketch structural formulae for (a) cyanidin, (b) pelargonidin and (c) peonidin. In each
case highlight the chromophore.
Answer: Students need to substitute the correct groups for R1, R2 and R3, using the table
to identify them.
2
Now sketch the anthocyanins that would form when glucose reacts with (a) cyanidin,
(b) pelargonidin and (c) peonidin. Hint: Look back at question 3 in the section Flavones
and flavonols.
Answer: Students need to show the condensation product between the phenolic -OH and
a hydroxyl group in a glucose molecule.
3
Try finding plants that contain some of anthocyanins listed above. Hint: Think which
names of flowers the anthocyanin’s names remind you of.
4
Outline how anthocyanins might be extracted from plants.
Answer: With water, preferably slightly acidic. Would need to check importance of
temperature and pH.
5
What techniques might be used to identify a mixture of anthocyanins?
Answer: Chromatography. Because their structures are so similar, HPLC is needed rather
than paper or thin layer (though some separation can be achieved with the latter).
Anthracenes
1
Why do you think ethyl ethanoate was used for the extraction rather than water?
Answer: Juglone is only slightly soluble in hot water. It is more soluble in organic
solvents that are slightly polar.
2
What other solvents might you try?
Answer: Ethanol and propanone.
3
Why is the extraction mixture refluxed?
Answer: To allow the extraction to take place at the boiling point of ethyl ethanoate
(solubility increases with temperature), but to avoid loss of ethyl ethanoate by
evaporation.
4
How could you determine the yield of juglone?
Answer: Weigh the product.
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© The Nuffield Foundation 2008
5
How could you calculate the possible yield of juglone from 50 kg of walnut rinds?
Answer: Weigh the walnut rinds and express the yield in terms of grams of juglone per
100 g of walnut rinds. Then scale up to 50 kg.
Dyeing with natural dyes
Most natural dyes are acid dyes, which is why they bond to wool, but not to cotton.
1
Explain the term ‘acid dye’.
Answer: Acid dyes are used mainly for wool and polyamides. They are anionic and
applied in an acid dye bath. The anionic group has a negative charge. This can form
ionic bonds with those parts of the fabric molecules that have a positive charge.
The use of natural dyes is much improved by using a mordant. The fabric is treated with
a solution of metal ions; these form a bridge between the fabric molecules and the dye,
a process called mordanting.
2
Give some examples of mordants that are used with natural dyes.
Answer: The most common mordants are alum, copper sulfate, potassium
dichromate(VI), iron(II) sulfate and tannin. Often, two mordants are mixed together
for the best results.
Extracting and testing a natural plant dye: page 19 of 19