New Perspectives in Science Education

Aniline black (polyaniline) is one of the oldest, if not the oldest, synthetic dyes, produced since 1834 through the oxidation and polymerization of aniline. [1] Aniline black was initially used for dyeing cotton and for lacquering, but is still of interest today as it may be used in the construction of batteries and as a medium for energy storage. [2]

This makes aniline black a polymer that is both historically and chemically interesting, but one that has rarely been used in school lessons due to safety concerns:

The handling of aniline itself is not the primary concern, as it can be used in schools under controlled conditions, such as in a fume hood and with appropriate protective equipment. However, the oxidizing agents such as potassium chromate or potassium dichromate, which are needed to oxidize the aniline, are much more dangerous. These oxidizing agents are highly toxic, mutagenic and carcinogenic and therefore strictly prohibited in schools.

To make chemical and industrial processes safer, to reduce the need of dangerous chemicals, and to thereby achieve a more sustainable chemistry, electrochemical alternatives have been increasingly developed in recent decades. [3] This means that dangerous oxidizing or reducing agents can often be fully replaced by electrical current.

In this contribution, we present simple school experiments to produce aniline black from aniline electrochemically. The oxidation process shows good results even at room temperature and using simple carbon electrodes, enabling an interesting and unexpected link between organic chemistry and electrochemistry. The experiments can also be used to discuss organic catalysis: nitrate is added as a catalyst, is oxidized at the electrode to a nitrate radical, and subsequently reacts with aniline, causing the oxidation and polymerization. Nitrate thus acts as a catalyst in an electrochemical reaction (a so-called mediator) – a phenomenon that is usually only known from non-electrochemical experiments.

We also present the electrochemical generation of polymethylpyrrole from n-methylpyrrole as an even safer alternative. Unlike aniline, methylpyrrole can be used in schools without any safety precautions and it reacts in the same way as aniline to form a black polymer. The use of nitrate as a catalyzing mediator can also be easily demonstrated in this experiment.

These new, safe experiments open up opportunities for students to investigate historic synthetic dyes while also engaging with modern, sustainable synthesis methods.

Keywords: Aniline black, synthetic dyes, electrochemical ozidization, mediators, sustainable electrochemistry 

References: [1] Zubiri, I. B., Technè 2023, 55, 96-105.

[2] Goto, H. et al., Journal of Chemical Education 2008, 85, 8, 1067-1070.

[3] Nikl, J. et al., Nature Communications 2023, 14, 4565.