Why does potassium permanganate decolorize ink?
Potassium permanganate KMnO4 is a strong oxidizing agent. It means that permanganate reacts with various substances and takes their electrons away. Dyes are specific substances, which are responsible for the ink color. Usually, dyes are complex organic compounds. Thanks to their specific structure, dyes provide the ink with bright and deep color, even when added in very small amounts. Potassium permanganate oxidizes dyes while changing their structure. That is why dyes, and ink at the same time, decolorize.
Let us start with the fact that permanganate oxidizes double bonds in dyes molecules. This reaction destroys the conjugated system:
R2C=CR2 + KMnO4 + H2O + H+ → R2COH–CR2OH + MnO2 + K+
What do we need a reductant for?
We have successfully decolorized the ink with potassium permanganate. However, some brown manganese dioxide MnO2 is formed in this reaction. It is very poorly soluble in water, so it is impossible to wash it off from paper. That is why we need a reductant – disodium disulfite Na2S2O5. Disulfite reduces brown MnO2 to colorless Mn2+. The reaction is as follows:
MnO2 + Na2S2O5 + NaHSO4 → MnSO4 + Na2SO4 + H2O
MnO2 precipitate is very hard to remove from paper. However, it is not the only reason why we use disulfite. Paper has partially absorbed excess of purple KMnO4 solution we used in the first step. Disodium disulfite reduces purple permanganate to colorless Mn2+ salts:
KMnO4 + Na2S2O5 + NaHSO4 → MnSO4 + Na2SO4 + H2O
What do we use sodium acetate for?
To be honest, we need aqueous solution of acetic acid for this experiment. Unfortunately, European safety standards for children science kits prohibit including acetic acid (or its solution in water). Luckily, this is all about chemistry, so there is no problem in obtaining acetic acid aqueous solution during our experiment.
When dissolved in water, NaHSO4 releases H+ and makes the solution quite acidic. Sodium acetate CH3COONa in this acidic medium forms acetic acid:
CH3COONa + H+ → CH3COOH + Na+
Acetic acid is an organic compound by nature, and it is also highly soluble in water. In turn, ink consists mostly of organic compounds that are poorly soluble in water. Acetic acid helps these compounds to migrate from paper into aqueous solution where the reaction with an oxidizer takes place.
Without acetic acid, there would be very few dye molecules in the solution. Then, the majority of them (absorbed into paper) would not decolorize.
Reactions proceed much faster if reagents are mixed thoroughly. When substances are separated (or mixed not well enough), they can exchange molecules only through the surface they are separated with. In such cases reactions proceed very slowly.
Why does the ink become colorless during the oxidation?
Let us talk a little about the nature of color. It is evident that we see almost nothing in the dark. All the things seem more or less black. Therefore, we need a light source (sunlight, electric lamp or flashlight) to distinguish colors. A light source sends an immense amount of light particles called photons. Every photon possesses certain energy. A light beam may be compared with a traffic stream on a big and busy road where numerous cars move in the same direction. However, photons move much faster than cars, and there are incredibly more of them. Actually, one cannot even imagine this amount!
Some of these photons are absorbed by the material while some are reflected. Most photons pass through transparent surfaces such as glass. Human eye catches the reflected part of light beam (photon stream). Our brain interprets it as a color.
It is the structure of a particular substance that determines how many and which photons (depending on their energy) would be absorbed. Usually, dyes are organic compounds of complex structure. They are spread equally throughout the colored material. Dyes actively and selectively absorb photons of certain energy. That is why in case of dyes we recognize the reflected photons as bright and deep color.
The bond structure of dyes often includes a long conjugated system of alternating single and double bonds. Such electronic structure allows the substance to absorb photons actively and selectively. Consequently, it makes dyes so bright.
An oxidizing agent promptly destroys the conjugated system of dye. Substance starts to absorb less and less photons and eventually becomes colorless. At that point, we can only see white paper, which absorbs light poorly. White color of an object means that it mostly reflects the photon stream.
What else can we decolorize ink with besides KMnO4?
There are two different ways to complete this task.
The first method has already been demonstrated in our experiment. We have oxidized dyes in the ink with a strong oxidizing agent: permanganate. One can try other oxidizing agents. For example, fresh hydrogen peroxide solution (first aid kit disinfectant) should be able to decolorize ink. However, we do not recommend you to try experiments like that. Most strong oxidizers are quite dangerous substances, which should be handled with knowledge and care.
The other method is to dissolve the ink or the dye from it. You can use nail polish remover for this purpose. It usually contains a mixture of organic solvents such as acetone and isopropanol. Using a cotton swab, carefully apply a little bit of nail polish remover on a paper with ink. Then wipe it off with another cotton swab. Repeat this procedure several times. Be careful! Nail polish remover is toxic and flammable. Do not breathe it in, avoid skin contact, and wear splash goggles to protect eyes.
Why is KMnO4 a strong oxidizing agent?
Manganese atom in KMnO4 has a significant positive charge: Mn+7. It means that a maximum amount of electrons has been removed from its outer electronic shell. The latter participates in various chemical reactions. Obviously, Mn+7 readily oxidizes other substances to obtain electrons from them. The reaction produces much more stable Mn+2 or Mn++4(which precipitates in form of MnO2).
Generally, the power of oxidizer (or oxidizing ability) is determined by its tendency to possess more electrons than they already have (in other words, to take electrons from other atoms). Strong oxidizers usually feature an atom with a significant positive charge (such as Mn+7 in our case).