A-level Applied Science/Colour Chemistry/Experiments

Onion skin and turmeric dyes

The methods for using these two dyes are very similar. The raw material is either the outer, papery skin of the onion or turmeric powder. The polyphenols in onion skins (kaempferol, quercetol and quercetin-3-glucoside) are more soluble in water than those in turmeric (curcumin, demethoxycurcumin and bis-demethoxycurcumin). The turmeric polyphenols are highly soluble in polar organic solvents such as propanone ('acetone') and butan-1-ol.

The solid plant material was boiled in water for 1 hour.

The mixture was cooled to room temperature and filtered to remove solids. The filtrate was used as the dyeing solution.

The fibre material was pre-mordanted or left without a mordant as a control. Mordants were found to have limited effect on the fastness of the dyes, but some affected the colour.

A material-to-liquor ratio of 1:40 was used. The liquor was a 1% extract of onion.

The fibre was added to the liquor at room temperature and then boiled for 45 minutes.

The fibre was then cold-washed and treated with a fixative. After fixative treatment, the fibre was washed with detergent, rinsed and dried.

The pH of the dye bath was found to be important: alkaline pH worked much less effectively than neutral or acidic pH. It was speculated that under acidic conditions the amine and amide groups of the nylon were ionised and attracted the dye molecules. The dye molecules would be un-ionised and relatively insoluble, so they may bind to the nylon like a disperse dye (a solid-solution mechanism).

Hiding power

Determination of hiding power using a colorimeter.

The hiding power of a paint can be measured as the thickness of paint required to obscure 98% of the colour of the substrate.

On a colorimeter, this corresponds to a transmission of 2% or an absorbance of 1.70.

[Absorbance = -log₁₀ (transmission)].

If we can dilute the paint so that its absorbance is 1.70, then we can say that paint this dilute will need to be applied 1 cm thick to achieve 98% coverage.

More helpfully, if we have diluted the paint by a factor of 500, we can say that the undiluted paint needs to be 1/500 cm thick to achieve 98% coverage. 1/500 cm is 0.002 cm or 20 μm (microns).

Try this example. You will need to draw a graph of dilution (horizontal axis) against absorbance (vertical axis). Use the graph to find the dilution required for 98% coverage. Calculate the thickness of pure paint that is required.

Dilution	Absorbance
700	2
1400	1.86
3500	1.44
7000	0.74

Manufacturers of paint usually give the number of square metres that can be covered by 1 litre of paint. We can calculate this value too. The volume 1 litre is 1 x 10⁻³ m³. A thickness of 20 μm is 20 x 10⁻⁶ m. The area that can be covered is the volume divided by the thickness:

1 x 10⁻³ m³

20 x 10⁻⁶ m

= 50 m²

Try the same calculation with your answer for the example above.

98% coverage is not aesthetically pleasing – patterns under the paint can still be seen – so paint manufacturers recommend five times the thickness of paint required for 98% coverage. They would recommend 10 m² per litre instead of 50 m², for example.

Density

Paint is very viscous, so it is easier to measure small masses of paint than small volumes. To find the density:

Weigh a 5 cm³ measuring cylinder.

Drip paint into the measuring cylinder, taking care that paint drips directly to the bottom without sticking to the sides.

Weigh the cylinder with its paint contents.

Allow the paint to settle and measure its volume.

Calculate the density.

Dilution 1

Weigh a 100 cm³ conical flask.

Add a small mass of paint – less than 0.5 g.

Weigh the flask and its paint contents.

Add 50 cm³ of solvent and mix thoroughly. It is very difficult to disperse all the paint. This 50 cm³ is dilution 1. Calculate the volume of paint used (using the density measured previously).

The dilution factor is the volume of solvent used (50 cm³), divided by the volume of paint used.

Measure the absorbance or transmission of this sample. When taking samples of diluted paint, make sure the paint is properly stirred before sampling.

If the transmission is more than 2%, or the absorbance is less than 1.7, then the sample is too dilute and you need to make a new dilution using more paint.

Further dilution

To a 10 cm³ measuring cylinder, add;

1 cm³ dilution 1

Solvent up to the 10 cm³ mark.

Mix thoroughly and measure the absorbance/transmission as before. This is dilution 2.

If the absorbance is still greater than 1.7, or the transmission is less than 2%, you will need to make a more dilute sample.

Take 1 cm³ of dilution 2 and make it up to 10 cm³ – this will give a dilution of 100 x dilution 1.

If the absorbance is now less than 1.7 (or the transmission is greater than 2%), you will need to make some less dilute samples. Try taking 1 cm³ of dilution 1 and making it up to 5 cm³, or 2 cm³ of dilution 1 to 5 cm³, etc.

Use your initiative to make several dilutions which have absorbance around 1.7.

Plot the results on a graph and find exactly what dilution would give an absorbance of 1.7.

Calculate the thickness (in microns) of pure paint required for 98% coverage.

Calculate the area (in square metres) that 1 litre of paint could cover.

Obtaining Indigo From Woad (Isatis tinctoria)

Woad plants in their first year.

Drawing of woad plant.

See w:Woad

These are basic suggestions for practical work. You will have to prepare your own handouts and schedules and deal with health and safety including COSHH regulations.

Woad should be sown in Spring (March–May) in fertile soil (with plenty of nitrogen fertiliser) and weeded as it grows.

When the leaves are 15 cm (6”) or more, cut the leaves off (or twist them off by hand) leaving the growing point at the top of the stem (so that the plant will continue to make more leaves). Collect as many leaves as you can cram into a pan or beaker or other vessel in which you can boil water.

Remove the leaves from the vessel or pan (and weigh them) and fill it ²/₃-³/₄ full with water and raise it to the boil.

Add a salt spoon full of cream of tartar, citric acid, or some other weak acid (to neutralise any hardness in the water).

When the water is boiling, with heat turned up to maximum, add leaves a handful at a time, twisting off hand sized portions from the leaves (so that they go into the pan easily), and stir; only add more leaves when the water has returned to the boil.

When the pan is full (cannot stir it any more), or all the leaves are added, leave for a further 1 min to boil. (Weigh any leaves left over.)

Then strain off all the water extract from the boiled leaves into another vessel. Throw away (compost) the boiled leaves.

Cool the extract down to room temperature (body heat or lower) as quickly as you reasonably can. This can be done readily by pouring the extract back to a metal pan (especially if one is used for the boiling) and floating it in a sink full of cold water, stirring both the extract and the water in the sink.

You now have a solution containing, among lots of other things, isatin B which is an unstable glycoside of indoxyl which readily breaks down to release indoxyl. This can be done with weak alkali.

When cool, add a table-spoon (5–10 cm³ to a litre) of strong aqueous ammonia (ammonium hydroxide) (or strong solution of sodium carbonate or sodium hydroxide).

There is an instant indicator reaction in the extract that should immediately change colour from a tea colour to a deep browny-yellow and should rapidly (after a few seconds) turn green. If this does not occur add more alkali.

You now have a solution containing, among lot of other things, indoxyl.

Now aerate the extract. Pouring it from one vessel to another can easily do this, or you can beat in, or bubble in, air.

A significant amount of foam should develop. If necessary, this can be quelled by adding some hexanoic (caproic), octanoic (caprillic) or decanoic (capric) acid; or other short chain fatty acid. A small bit of old cheese or rancid butter will also do the trick!

The colour of the extract should go an increasing dark green. This is the result of indigo forming within the brownish-yellow extract. To demonstrate that indigo is really being formed, remove a small aliquot and neutralise the alkali with some weak acid solution.

The oxidation of the indoxyl by aeration is an asymptotic process after about 5 mins most people are fed up with doing it and it is probably well over 90% complete.

To demonstrate the need for air in the oxidation process: take a portion of the extract immediately after adding the alkali after cooling, then leave it in a separate beaker. After a few minutes, you will see a scum developing which, if picked up with the corner of a bit of white paper, is intense blue. This is indigo forming at the surface where the indoxyl in solution is coming in contact with the air.

When you have finished the aeration, you can add acid to neutralise the extract, if you want, although this is not necessary.

Whether neutralised or not, if left standing for long enough, e.g. overnight, the indigo will settle as a precipitate. Most of the supernatant can be decanted off (and thrown away down the drain) and the precipitate filtered off the remaining liquid with ordinary filter paper (weighed) which can be dried to make dry indigo (weigh again to get weight of indigo).

Some purification can be achieved by rinsing the precipitate by adding plenty of distilled water after decanting, allow to settle again, decanting again – repeating the process until the supernatant is quite clear and clean. There can be a loss of some of the indigo in doing this.

You can easily find the amount of indigo produced per kilogram of fresh leaves.

Projects can be done:

Which produces more indigo, larger older leaves or the younger smaller leaves? Express the amount of indigo obtained by weight and per leaf (count leaves before extracting them). Which produces the most indigo?

Grow plants with and without added nitrogen fertiliser. (2-4g nitrogen per m². (You will have to calculate the amount of fertiliser giving this amount of nitrogen.) Does added nitrogen fertiliser increase the amount of indigo produced?

Try putting the plants in the dark for a few days (cover with black plastic). Does this have an effect on the amount of indigo produced? What can you conclude about the metabolism of isatin B in the plant?

Try other treatments to the plants or cut leaves.

Try varying the method of the extraction – e.g. using different kinds of alkali etc.

How to dye with woad leaves (freezing method)

The easiest method is to put the fresh leaves from non-flowering plants in a freezer (-40 °C) for one day (to kill them by freezing).

Remove the frozen leaves and spread them out so that they dry. They can than be crushed into a powder.

Add a small amount of boiling water to wet a handful of this powder than add 1 litre water with a spatula of sodium dithionite (sodium hydrosulphite) plus a little alkali e.g. enough ammonium hydroxide to give the solution a faint smell of ammonia. The sodium dithionite needs to be reasonably fresh as it can become ineffective if kept too long - alternatively use formamidine sulphinic acid.

Heat to 50 °C and leave for about 15 min.

Add wool (previously wetted) and stir briefly. Leave for 5 min.

Remove and the wool should go blue in the air.

D J Hill (University of Bristol)

9 July 2006

(Reproduced with his full permission and our gratitude.)