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If you are a student and your projec
t is to call about eraser you will notice
that there is a lack of information on
internet notably for the composition
of the eraser. In this topic I will bring
you useful informations for the
advancement of your project.

Most of modern ink pen are constituted
of aniline compound derivatives as for
example the aniline blue. these are
complex structures that generally
contain a sulfonic acidic group.

The stain of interest is the aniline blue because we recover it in the blue ink of our pens. You can easily prove it while making a thin layer chromatography (I won't treat the topic here. Nevertheless it is relatively simple). The formula of the blue aniline is represented below

warning : On wikipédia it is mentioned that the aniline is a mixture of two compounds that is the methyl blue (do not to confound with the methylene blue !!!) and the water blue. It is not true ! this mixture constitutes the aniline blue WS. It is necessary to notice also that the water blue is simply the aniline blue. Therefore the aniline blue WS is composed of aniline blue and methyl blue

Inks contain also the glycol ethylene or glycerin (that permits to adjust the inkviscosity ), phenolic compounds to prevent the bacterial development, surfactants and other additives for the stability of the ink pH.

The ink formulas are secret and therefore are not all patented. And yes if they are secret we doesn't publish them. It is for these reasons that it is difficult to find ink compositions

Why aniline blue  ?

Yes ! Why this molecule is colored ? It is due to the resonance structure of the molecule that spreads of nitrogen 1 to nitrogen 2 (picture below).

The important electronic delocation observed in this molecule gives a wave length in the visible that corresponds to the blue color.

How does act the eraser?

The eraser contains a reducing substance that is the sodium bisulfite Na2S2O5 . You have just understood it is going to have an oxydoreuction reaction between the aniline blue and the sodium bisulfite. The resulting compound of this reaction will be colorless.

in a first time sodium bisulfite is decomposed in 2 molecules of sodium hydrogenosulfite in presence of water according to the reaction :

The 2 redox equation are

Equation 1

Equation 2

total reaction

Why reduced aniline are colorless ?

Look at the molecule. the electronic delocation is not important as in the the aniline blue ( see below) and this compound doesn't give a wave length in the visible but in the ultra purple, it is for this reason that the molecule is colorless.

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