Cyanotype

Create an image using light!

Difficulty:
Danger:
Duration:
25 minutes
Experiment's video preview

Reagents

Safety

  • Conduct the experiment on the plastic tray.
  • Put on protective gloves and eyewear.
General safety rules
  • Do not allow chemicals to come into contact with the eyes or mouth.
  • Keep young children, animals and those not wearing eye protection away from the experimental area.
  • Store this experimental set out of reach of children under 12 years of age.
  • Clean all equipment after use.
  • Make sure that all containers are fully closed and properly stored after use.
  • Ensure that all empty containers are disposed of properly.
  • Do not use any equipment which has not been supplied with the set or recommended in the instructions for use.
  • Do not replace foodstuffs in original container. Dispose of immediately.
General first aid information
  • In case of eye contact: Wash out eye with plenty of water, holding eye open if necessary. Seek immediate medical advice.
  • If swallowed: Wash out mouth with water, drink some fresh water. Do not induce vomiting. Seek immediate medical advice.
  • In case of inhalation: Remove person to fresh air.
  • In case of skin contact and burns: Wash affected area with plenty of water for at least 10 minutes.
  • In case of doubt, seek medical advice without delay. Take the chemical and its container with you.
  • In case of injury always seek medical advice.
Advice for supervising adults
  • The incorrect use of chemicals can cause injury and damage to health. Only carry out those experiments which are listed in the instructions.
  • This experimental set is for use only by children over 12 years.
  • Because children’s abilities vary so much, even within age groups, supervising adults should exercise discretion as to which experiments are suitable and safe for them. The instructions should enable supervisors to assess any experiment to establish its suitability for a particular child.
  • The supervising adult should discuss the warnings and safety information with the child or children before commencing the experiments. Particular attention should be paid to the safe handling of acids, alkalis and flammable liquids.
  • The area surrounding the experiment should be kept clear of any obstructions and away from the storage of food. It should be well lit and ventilated and close to a water supply. A solid table with a heat resistant top should be provided
  • Substances in non-reclosable packaging should be used up (completely) during the course of one experiment, i.e. after opening the package.

FAQ and troubleshooting

I stained my table with the blue dye. How can I wash it off?

This dye can easily be removed using a solution of NaHCO3. Dissolve 1 teaspoon of baking soda (sodium bicarbonate) in a glass of water, moisten a cloth with the resulting solution, and wipe the stains off the affected surface.

The mixture that I obtained isn’t yellow. What should I do?

If, at this stage of the experiment, you notice blue stripes on a yellow background, take a new absorbent and keep applying the mixture to the paper.

However, if the whole mixture has turned blue, this likely means that you’ve overexposed it to light. But don’t worry! Just discard this plastic cup and start over. When repeating the experiment, keep in mind that the experiment must be performed in very dim lighting.

And if the mixture turned greenish? It’ll still work, but the drawing won’t be as contrasted.

Which lamps will work for this experiment?

For this experiment, you can use:
- 75 W incandescent light (including halogen);
- 11 W LED or fluorescent light.
Your lamp must be positioned approximately 12 in. (30 cm) from the paper. Leave the paper exposed to light for 10-15 min. If your light source is more powerful than the references listed above, keep it farther away from the paper. A 100 W light source should be positioned approximately 16 in. (40 cm) away.

It’s been 10 minutes, but the drawing is still very pale. What should I do?

If you haven’t washed your drawing yet, leave the paper under the light for 5-10 more minutes.

Step-by-step instructions

As we will be dealing with light-sensitive compounds, we need to avoid bright light, such as direct sunlight or strong lamps.

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Prepare the plexiglas plate.

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First, prepare light-sensitive ammonium iron(III) citrate by causing an ion exchange reaction between ammonium iron(III) sulfate and citric acid.

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Mix ammonium iron(III) citrate with potassium hexacyanoferrate(III). This mixture is the key to making our print visible.

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Now apply the photosensitive mixture to a paper.

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Use the transparent negative image to protect some parts of the paper from light.

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Under the impact of light, iron ions in ammonium iron(III) citrate are reduced to iron(II). Which in turn react with potassium hexacyanoferrate(III) to produce a stable blue compound–Prussian blue.

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After removing the excess of the photosensitive mixture, we are left with a nice blue and white image.

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Disposal

Dispose of solid waste together with household garbage. Pour solutions down the sink. Wash with an excess of water.

Scientific description

What reactions occur in the plastic cup?

We obtain ammonium iron(III) citrate solution. Though ammonium iron(III) citrate has a rather complicated formula (Fe(NH4)3(C6H5O7)2), it’s actually not as complex as it seems:

  • Fe3+ ions come from ammonium iron(III) sulfate NH4Fe(SO4)2;

  • ammonium NH4+ comes from ammonium carbonate;

  • citrate ions C6H5O73- come from citric acid C6H8O7.

A bit more about how ammonium iron(III) citrate forms

Ammonium carbonate is not very stable in the solution, existing there in a tentative balance with some other products:

(NH4)2CO3 + H2O = NH4HCO3 + NH4OH

The last item in the equation, NH4OH, is nothing but ammonia dissolved in water. That is why ammonium carbonate solution smells strong - our noses sense the ammonia escaping from it:

NH4OH → NH3↑ + H2O

Meanwhile, when ammonium carbonate meets citric acid, carbon dioxide bubbles out of the solution because the citric acid adds some H+ to the solution, making it acidic:

H+ + NH4HCO3 → NH4+ + CO2↑ + H2O

But these processes aren’t our main focus. In parallel, some ammonium citrate forms:

2NH4+ + C6H8O7 →(NH4)2C6H6O7 + 2H2O

And when we add ammonium iron(III) sulfate, the mixture becomes greenish-yellow:

  • 2(NH4)2C6H6O7 → Fe(NH4)3(C6H5O7)2 + (NH4)2SO4

Why should we avoid bright light when working with this solution?

Since this is a photosensitive mixture, some of its processes are heavily influenced by light. We exploit this property when using a lamp or sunlight to make our picture. Thus, before we start exposing the picture, we need to avoid bright light such as direct sunlight or strong lamps. Otherwise, all the mixture will turn blue even before we begin working in earnest!

What happens when we expose the picture under the source of light?

No matter if we use sunlight or a lamp, what happens is the following: light makes the iron(III) ions from ammonium iron(III) citrate take electrons from the citrate ions:

Fe3+ + e- → Fe2+

The Fe2+ ions react with [Fe(CN)6]3– ions, yielding the dark blue precipitate Fe4[Fe(CN )6]3, also known as Prussian blue.

To make the picture contrast and stable, we need to wash the unreacted mixture from the areas previously shielded by the negative image. The technique we used is also known as cyanotype.

Where did cyanotype come from and how we use it?

The cyanotype process was developed by Sir John Frederick William Herschel in the 19th century. It was mostly used in the 19th and early 20th centuries to make copies of technical drawings.

For example, one of the first known scientific photobooks, “British Algae” by Anna Atkins, was printed using the cyanotype method.

Nowadays, the cyanotype process is used mostly by hobbyists to print photos or decorate clothes.

That's interesting!

Cyanotype and blueprints

Cyanotyping was historically used in some industrial fields to create what are known as blueprints. The term ‘blueprint’ usually refers to engineering-specific technical drawings. The blueprint was mostly used in the 19th and 20th centuries to copy technical drawings of buildings and ships. However, with the advent of industrial printers, this method gradually lost popularity, and now is most often used by hobbyists.

The method’s “blue” name is, indeed, because the result is blue. A bit later, another method was developed, which differs mostly in the resulting color - the images range from pinkish to pure white. Such a picture is known as a whiteprint. The reagents that contribute to this process differ vastly from those used to create a blueprint, the key substances being organic compounds containing diazo compounds.