In lieu of the bucket science symposium, I thought of doing up a blog post with regards to the topic that I will be showing on the day itself. >.<
History of invisible ink (all things need a history right? 😀 )
More than 2000 years ago, invisible inks are used by ancient Greeks and Romans as one form of transmitting secret messages, though the inks that are used might differ from what we use nowadays. The inks usually fall under 2 categories: organic fluids and sympathetic inks, where organic fluids are considered naturally synthesized. Examples include lemon juice, vinegar, milk, sweat, saliva, onion juice, and even urine and diluted blood and the method to reveal the messages is by physical means (heating, shining UV light etc…). The sympathetic inks are based on chemical reactions to reveal the messages. We will see both types of ink today!
A little background about light and colours
E = hc/λ
E is the energy of the light, h is Planck constant, c is speed of light and λ is the wavelength of light
From this equation, we can see that light is energy and the phenomenon of colour is a product of the interaction of energy and matter. When an object absorbs energy, it will get excited and move faster. However, the object will also quickly lose that energy as a form of light. (see gif)
If the wavelength of the light emitted corresponds to the visible spectrum, we will be able to see the light, or even the colour of the light emitted.
Molecules exist in different energy levels. At room temperature, most of the molecules exist in the ground state, which is the state with lowest energy level. Objects are invisible/colourless is because they allow light to pass through without any absorption. In other words, the energy gap between the ground state and excited state is larger than the energy of the visible light.
Invisible ink with fluorescence, featuring UV light Pen
The UV light pen works by the excitation of the molecules in the “invisible” ink with the use of UV light (black light), where the energy source now is from the UV light.
At normal lighting, the ink is invisible as there is no absorption. However, when UV light (higher energy) shines on it, the energy from the UV light is absorbed by the molecules in the invisible ink and the molecules become excited. However, since the excited state is unstable, the molecule will quickly lose that extra energy as light and return to the ground state. This phenomenon is also known as fluorescence.
Invisible ink with chemical reactions
To make invisible ink appear with chemical reactions, the general equation is as shown:
A(colourless) + B –> C (coloured) + D
Some coloured compounds include:
- Transition metals compounds
- Organic compounds that have high conjugation (e.g. methylene blue, methyl orange and phenolphthalein). They are often used as indicators as the colour changes with pH
Since alkali is commonly found in labs, it is chosen to be the reagent to make inks ‘appear’. First invisible (not really) ink is copper sulfate. When copper sulfate reacts with alkali, the OH- ions will form a precipitate (solid) with the Cu2+ ions which is blue in colour.
CuSO4 + 2NaOH → Na2SO4 + Cu(OH)2
light blue blue solid
Though copper sulfate solution is blue, the colour is rather faint (forbidden dd* transition, but too much content for today) and so we can’t really see the colour and it is rather invisible when dried on paper.
Second invisible ink is phenolphthalein. In acidic or neutral conditions, phenolphthalein is colourless and thus ‘invisible’. But when an alkali is added, it loses a hydrogen ion (proton) and turns bright pink when pH >8.2.
The colour in organic compounds is dependent on the energy gap between the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital), where HOMO is similar to ground state and LUMO is similar to excited state. When the H+ ion is lost, the energy gap decreases due to increase conjugation (higher stability of ion). Thus, the energy gap falls in the visible light spectrum and bright pink solution results.
Invisible ink with friction, featuring FriXion Pen
Disclaimer: *This is not an advertisement for FriXion Pen*
The FriXion pen is a very new technology (introduced in 2006 in Europe)that allows ink to be erasable with friction. The ink is the one that makes it special! The pigment is formed by special microcapsules that contain a combination of 3 components. These components are sensitive to heat (thermosensitive) generated by friction and the ink becomes transparent when subjected to heat.
The Leuco dye (A) is the main component to determine the colour of the ink, but it only does so if it is chemically bonded to the colour developer (B). When the ink is “erased” using the rubber tip on the FriXion pen, the heat generated from friction break the bonding between A and B, and B is bonded to C instead. Since A is no longer bonded to B, the colour disappears and thus the ink is “erased”. To make the ink appear again, simply place that piece of paper under a very cold environment such that the bond between B and C will break and B will form back a bond with A.