Invisible inks

Student Sheet

In this practical I will be:

• Writing messages with a variety of substances which can be used as invisible inks.
• Changing the conditions of the invisible inks to see if messages become visible.
• Using my scientific knowledge to explain how changing the conditions the inks are under affects their visibility. 

Introduction:

It is 410BCE and the Ionian (or Decelean) war, between Athens and Sparta has been raging for four years. You have been tasked by King Agis of Sparta to develop a way of delivering secret messages to his armies, which would be unreadable if they fell into the enemies’ hands. You wonder if there is a way of writing with an ink that is only visible under certain conditions. Like all good science-artists, you decide to investigate further…

Wear eye protection.
Wear disposable plastic gloves.

Use one of the methods below to write a question on a piece of white paper. Pass the message to another group who will write an answer to the question and pass it back to you to develop. It would be good to use different types of ink as advised by your teacher.
How you reveal the message depends on the ink you used. 

Heat-activated invisible inks

Equipment:

Any of the following for your invisible inks:

• Any acidic fruit juice (e.g. lemon, apple, or orange juice) 
• Onion juice 
• Baking soda (sodium bicarbonate) (or 1 M sodium hydrogen carbonate)
• Vinegar (or 1 M ethanoic acid)
• Dilute cola 
• Diluted honey 
• Milk 
• Soapy water (or 0.1 M sodium carbonate)
• Sucrose (table sugar) solution (1 tsp in 10 cm³  water)

Other Equipment:

• Iron or oven at 230 °C or hotplate
• Hairdryer
• A4 paper or filter paper
• Paintbrushes (and/or straws, cotton buds, spills)
• White tiles (For using on the hotplate. The tiles heat up but protect the paper from burning.)

Method:

1. Use the ink by writing your message with it using a paintbrush, spill, cotton bud, straw with a cut end to make a nib, or toothpick with cloth wrapped around it. Let the message dry in the air or use a hairdryer. 
2. Write another (not secret) message on the paper with ballpoint pen, pencil or crayon, so it doesn’t appear to be blank and meaningless. Do not use a fountain pen since this ink could run into your invisible ink and make it visible. Avoid using lined paper to write your invisible message, for the same reason.

Most invisible inks are made visible by heating the paper. Take extra care when heating the paper. An easy way to reveal these types of messages is to iron the paper gently. The paper containing the message can be placed onto a white (ceramic) tile and the white tile can then be placed onto a hotplate. This is a reasonably safe way to heat the paper without risk of burning and it works well to reveal the message.

The paper can also be placed directly onto the hotplate on a low heat but greater care must be taken because it is more likely to burn this way if not supervised carefully.

Going further:

Inks developed by chemical reactions
Many of them work as pH indicators, so when in doubt, paint or spray a suspected message with a base (like sodium carbonate solution) or an acid (like lemon juice). 

Equipment:

Any of the following for your invisible inks:

• Phenolphthalein (pH indicator) HIGHLY FLAMMABLE & HARMFUL, developed by 0.1 M sodium carbonate (Solid is IRRITANT – solution is LOW HAZARD) (or another base).
• Thymolphthalein HIGHLY FLAMMABLE & HARMFUL, developed by 0.1M sodium carbonate (or another base) 
• Vinegar or  1 M acetic (ethanoic) acid, developed by red cabbage water 
• Ammonia(1 M ammonium hydroxide) developed by red cabbage water 
• 1 M sodium hydrogen carbonate sodium bicarbonate (baking soda), developed by grape juice 
• 1 M sodium chloride (table salt), developed by 0.1 M silver nitrate 
  • M copper sulfate, developed by 1 M sodium iodide, 0.1 M sodium carbonate, or 1 M ammonium hydroxide 
• 0.005 M lead(II) nitrate TOXIC, developed by 1 M sodium iodide 
• M iron sulfate IRRITANT, developed by 0.1 M sodium carbonate, 1% starch solution (e.g. corn starch or potato starch), or by 0.1 M iodine solution 
• Lemon juice, developed by 0.1 M iodine solution 

Other Equipment:

• Hairdryer
• A4 paper or filter paper
• Paintbrushes (and/or straws, cotton buds)

Method:

1. Use the ink by writing your message with it using a paintbrush, spill, cotton bud, straw with a cut end to make a nib, or toothpick with cloth wrapped around it. Let the message dry in the air or use a hairdryer.
2. Write another (not secret) message on the paper with ballpoint pen, pencil, or crayon, so it doesn’t appear to be blank and meaningless. Do not use a fountain pen since this ink could run into your invisible ink and make it visible. Avoid using lined paper to write your invisible message, for the same reason.
3. Your message will be developed by spraying, wiping or painting the paper with a second chemical such as an acid. 

Inks developed by ultra-violet light (black light)

Equipment:

Any of the following for your invisible inks:

• Dilute laundry detergent (the bluing agent glows) or 0.1 M sodium carbonate hydrated (Check packets for safety notes)
• Tonic water (quinine glows) 
• Vitamin B-12 dissolved in vinegar 

Other Equipment:

• Hairdryer
• A4 paper or filter paper
• Paintbrushes (and/or straws, cotton buds)

Method:

1. Use the ink by writing your message with it using a paintbrush, spill, cotton bud, straw with a cut end to make a nib, or toothpick with cloth wrapped around it. Let the message dry in the air or use a hairdryer or warm radiator. 
2. Write another (not secret) message on the paper with ballpoint pen, pencil, or crayon, so it doesn’t appear to be blank and meaningless. Do not use a fountain pen since this ink could run into your invisible ink and make it visible. Avoid using lined paper to write your invisible message, for the same reason.
3. Messages are revealed by shining an ultraviolet light on the paper.

Theory:

Heat-activated invisible inks

Some of these are organic substances that oxidize when heated, which usually turns them brown or black because the compounds break down. For this type of “heat fixed” ink, any acidic fluid will work in helping to develop the ink. The most secure way to use any of the following substances for invisible ink is by dilution, usually with water, close to the concentration point when they become difficult to develop. This can only be determined by trial and error.

Inks developed by chemical reactions

In most cases, the substance changes colour when mixed with an acid or base because the pH changes and the liquid acts like an indicator and changes colour.
In some cases specific indicators have to be used during which a chemical reaction takes place in which there is a colour change or a precipitate is formed.

Inks developed by ultra-violet light (black light)

Some inks glow faintly or fluoresce when under an ultra-violet lamp. This is a property of many substances, particularly organic substances and body fluids.
Other inks work in a near opposite way by absorbing ultra-violet light but without fluorescing. When these are used on fluorescent paper, the inked areas fluoresce less than the surrounding paper area when under an ultra-violet lamp. This is especially a property of inks with a yellow tint.
Some UV-visible inks may be detected on a photocopy, due to the relatively strong ultra-violet component in light from the photocopier scanning head.

Teacher and Technician Sheet

In this practical students will:

• Write messages with a variety of substances which can be used as invisible inks.
• Change the conditions of the invisible inks to see if messages become visible.
• Use their scientific knowledge to explain how changing the conditions the inks are under affects their visibility. 

Introduction for teachers:

(This topic could start with a group discussion or a short story woven around the facts given below, during which the teacher introduces the ideas especially the words in bold. This is a fun activity but does have some real chemistry behind it. However, for younger children probably the best are the heat-stimulated inks.)

Invisibleink is any substance that you can use to write a message that is invisible until the ink is revealed. Anyone can write an invisible message, assuming you have paper, because body fluids can be used as invisible ink.

Invisible ink has a history that goes back in time. Ovid (43 BCE-17/18 CE), a Roman poet wrote in his Ars Amatoria in 18BCE:

“Characters written in fresh milk are a well-known means of secret communication. Touch them with a little powdered charcoal and you will read them.”

At the other extreme, Mary, Queen of Scots (1542-1587), used invisible ink and ciphers to communicate with Catholic supporters. Her supporters wrote to her using two common substances: alum (hydrated potassium aluminium sulfate) or gall ink (tannic acid from parasitic wasps in oak trees galls). The alum letters would be developed by soaking the letter in water, and the gall ink would be developed by soaking the letter in iron sulfate solution.

During times of secret love, war or in the world of spies in the cold war, highsecurity is important and there is often the need for invisible ink with which a message can be written, be invisible, and when it reaches the recipient be brought back so it can be seen.

During World War II the Special Operations Executive (SOE) were a group of secret agents operating behind enemy lines. This meant they had to operate without being detected. Hence, messages were often passed using invisible ink. In their training manual the following properties of an ‘ideal’ invisible ink were identified.

1. It needs to mix with water so it can be widely used.
2. Non-volatile, i.e. no pronounced smell to the ink.
3. Must not deposit crystals on the writing paper, i.e. not easily seen in glancing light.
4. Be invisible under ultraviolet light.
5. Does not decompose or discolour the paper, as with photo paper using silver nitrate.
6. Non-reactive with iodine, or with any of the other usual developers.
7. Potential developers for the ink should be as few as possible.
8. Should not develop under heat.
9. Easily obtainable and has at least one plausible innocent use by the holder.
10. Not a compound of several chemicals.

SOE agents were trained not to risk their lives so they used invisible inks as a back-up method of communication when other, more secure communication techniques were unavailable. The agency supplied special inks to its field agents. When agents were forced to improvise, they were advised to dilute their invisible ink as much as possible to reduce chances of detection. 

The properties identified by the SOE could be discussed by students as to why they are important criteria for an invisible ink.
The students will enjoy the practical activity and the explanation is best given after they have results they can talk about. If they work in groups using different inks to write questions they can pass those messages to another group who can develop the message and send the answer to the question back.
A good way of recording this work is to get the pupils to write a story involving the ink they have used for their experiment.

Curriculum range:

All ages can take part in this activity but at different levels since the aim is to gain some understanding of the thinking of the scientist and artist with regard to the properties of materials. It links with:

• setting up simple practical enquiries, comparative and fair tests;
• reporting on findings from enquiries and observations, including oral and written explanations, displays or presentations of results and conclusions;
• using straightforward scientific evidence to answer questions or to support their findings;
• comparing and grouping together everyday materials on the basis of their properties such as solubility and transparency;
• knowing that some materials will react to produce a visible product;
• building a more systematic understanding of those types of materials by exploring and comparing the properties of a broad range of materials;
• asking questions and develop a line of enquiry based on observations of the real world, alongside prior knowledge and experience;
• using appropriate techniques, apparatus, and materials during laboratory work, paying attention to health and safety;
• making and recording observations and evaluate the reliability of methods and suggest possible improvements;
• presenting observations using appropriate methods;
• interpret observations, identifying patterns to draw conclusions;
• presenting reasoned explanations, including explaining data in relation to predictions and hypotheses;
• the concept of a chemical change, oxidation, pH and decomposition; and
• knowing about the idea of a chemical change, mixture and solution, including dissolving.

Hazard warnings:

Students should wear eye protection and disposable gloves.

Supervision may be necessary when the students are using the heating equipment.

Ammonia is TOXIC 

Concentrated ammonia is CORROSIVE & DANGEROUS FOR THE ENVIRONMENT

Suggest not using. Ammonia gas is TOXIC. Ammonia concentrate is CORROSIVE & DANGEROUS FOR THE ENVIRONMENT. This would need a very well ventilated room and student behaviour assessment also CLEAPSS advises a maximum of 2M for Year 7.

Sodium carbonate (solid) is IRRITANT

Phenolphthalein is HIGHLY FLAMMABLE & HARMFUL

Thymolphthalein is HIGHLY FLAMMABLE & HARMFUL

Copper (II) sulphate (solid) is HARMFUL & DANGEROUS FOR THE ENVIRONMENT

Lead (II) nitrate is TOXIC & DANGEROUS FOR THE ENVIRONMENT.

Iron sulphate (solid) is HARMFUL 

Sodium sulphide is CORROSIVE & DANGEROUS FOR THE ENVIRONMENT

Cobalt chloride is TOXIC & DANGEROUS FOR THE ENVIRONMENT

Iodine is HARMFUL (will stain skin but is low hazard at 0.1M)

Iodine solution should not be present with Ammonia solution. An explosive mixture can be formed.

Potassium ferricyanide should not be present with Ammonia because an explosive mixture can be formed. It should also not be present where concentrated acid could be added because it would produce TOXIC gases.

Potassium ferricyanide & phenolphthalein should not be used where there are naked flames (do not use Bunsen burners).

Some of the metal compounds such as copper, cobalt, and lead can be toxic in varying quantities and can be an irritant if the liquid is spilt or put on the skin.

Ultra-violet light can be harmful to eyes so pupils should be supervised when handling the lamp or they can observe as an adult uses the lamp. Portable counterfeit money detectors can be used for this and are easily sourced and reasonably priced.

Use one of the methods below to write a question on a piece of white paper. Pass the message to another group who will develop your message and then write an answer to the question and pass it back to you to develop. It would be good to use different types of ink as advised by your teacher.

Heat-Activated Invisible Inks:

These inks are best for younger children since most of the chemicals are household and generally less hazardous. They can be developed by heat generally ironing them with a hot iron or placing them in an oven or (on a white tile) on a hotplate.

Equipment:

Any of the following:

• Any acidic fruit juice (e.g. lemon, apple or orange juice) 
• Onion juice 
• Baking soda (sodium bicarbonate) solution or 1 M sodium hydrogen carbonate
• Vinegar or 1 M ethanoic acid
• Dilute cola 
• Diluted honey 
• Milk 
• Soapy water or 0.1 M sodium carbonate hydrated
• Sucrose (table sugar) solution (1 tsp in 10 cm³ water)
• Iron or oven at 230 °C or hotplate
• Hairdryer
• A4 paper or filter paper
• Paintbrushes (and/or straws, cotton buds, spills)
• White tiles (For using on the hotplate. The tiles heat up but protect the paper from burning)

Inks Developed by Chemical Reactions:

These are best used with secondary students and most could be used comfortably with lower secondary students. Many of them work as pH indicators, so paint or spray a suspected message with a base (eg sodium carbonate solution) or an acid (e.g. lemon juice). Some of these inks will also reveal their message when heated (e.g. vinegar). The substances in milk weaken the paper and may be more susceptible to heat than the paper, so although the message dries clear, the paper weakens and darkens where the milk was applied.

Equipment:

Any of the following:

• Phenolphthalein (pH indicator) HIGHLY FLAMMABLE & HARMFUL, developed by sodium carbonate (Solid is IRRITANT – solution is LOW HAZARD) (or another base). Suggest not using. Ammonia gas is TOXIC. Ammonia concentrate is CORROSIVE & DANGEROUS FOR THE ENVIRONMENT. This would need a very well ventilated room and student behaviour assessment also CLEAPSS advises a maximum of 2M for Year 7.
• thymolphthalein HIGHLY FLAMMABLE & HARMFUL, developed by sodium carbonate (Solid is IRRITANT – solution is LOW HAZARD) (or another base) 
• vinegar or 1 M (ethanoic) acetic acid, developed by red cabbage water 
• ammonia (1 M ammonium hydroxide) , developed by red cabbage water 
• 1 M sodium hydrogen carbonate, sodium bicarbonate (baking soda), developed by grape juice 
• 1 M sodium chloride (table salt), developed by 0.1 M silver nitrate 
• 1 M copper sulfate, developed by sodium iodide, sodium carbonate, or 1 M ammonium hydroxide 
• 0.005 M lead (II) nitrate, developed by 1 M sodium iodide (Lead (II) nitrate is TOXIC at = > 0.01M and HARMFUL in solutions less than this.)
• 1 M iron sulfate IRRITANT, developed by 0.1 M sodium carbonate
• 1% starch solution (e.g. corn starch or potato starch), developed by 0.1 M iodine solution 
• Lemon juice, developed by 0.1 M iodine solution
• Hairdryer
• A4 paper or filter paper
• Paintbrushes (and/or straws, cotton buds) 

Inks Developed by Ultraviolet Light (Black Light):

These can be used with younger children but the chemistry is more complex so the theory is best left to secondary age students. Most of the inks that become visible when you shine an ultra violet light on them also would become visible if you heat the paper.

Equipment:

Any of the following:

• Dilute laundry detergent (the bluing agent glows) or 1 M sodium carbonate hydrated (Lead (II) nitrate is TOXIC at => 0.0 M and HARMFUL in solutions less than this.)
• tonic water (quinine glows) 
• vitamin B-12 dissolved in vinegar 
• Hairdryer
• A4 paper or filter paper
• Paintbrushes (and/or straws, cotton buds) 

Technical notes:

Because of the wide range of materials, teachers should select either a few of the materials or group students to allow a wider range of materials to be investigated. If using groups then sufficient time needs to be allowed for whole class reporting on the results and evaluation of the materials being used.

Provide solutions of solids if preferred. Solutions made of one spatula measure or teaspoon mixed with 10 cm3 of water is sufficient.

The discussion can be approached using the Special Operatives Executive (SOE) properties for ‘ideal’ invisible ink as the evaluation criteria.
The invisible ink could be dried with a hairdryer or on a warm radiator initially.

The hotplate can be used on a low heat without the white tile, but it will have to be supervised as the paper could burn. It is safer to use a white tile on the hotplate to place the paper onto. It takes about 20–30 seconds on average on a low to medium heat setting for the message to appear. This means that a class practical could be carried out using a hotplate without congestion.

The oven method takes longer and there is less control over when to discontinue heating.

There is more control over the chemicals if paintbrushes, etc. are used instead of spray bottles.

Results:

After writing, the message should not be visible either by inclining the paper and using oblique incident light or seeing any residue on the surface of the paper.

When developed the message should be visible and readable so students should be able to inform others of the message. The message is given to another group who develops the message and reports to the group on what the message said.

Good results can be obtained from the heat method on milk, sodium hydrogen carbonate solution, vinegar (ethanoic acid), sugar solution and soapy water (sodium carbonate solution). 

The following work well from the chemical reaction method, copper sulphate and ammonium hydroxide, copper sulphate and sodium carbonate, phenolphthalein and sodium carbonate.