Wurster's blue is the trivial name given to the chemical ''N'',''N'',''N''′,''N''′-tetramethyl-''p''-phenylenediamine, also known as TMPD. It is an easily oxidised

phenylenediamine Phenylenediamine may refer to: * ''o''-phenylenediamine or OPD, a chemical compound C6H4(NH2)2 * ''m''-phenylenediamine or MPD, a chemical compound C6H4(NH2)2 * ''p''-phenylenediamine or PPD, a chemical compound C6H4(NH2)2 * ''N,N''-dimethyl-''p'' ...

, which loses two

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...

s in one-electron

oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...

steps; the

radical cation In organic chemistry, a radical anion is a free radical species that carries a negative charge. Radical anions are encountered in organic chemistry as reduced derivatives of polycyclic aromatic compounds, e.g. sodium naphthenide. An example of a ...

is a characteristic blue-violet colour, which gives the compound part of its name. The remaining part of its name comes from its discoverer, the German

chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin ''alchemist'') is a scientist trained in the study of chemistry. Chemists study the composition of matter and its properties. Chemists carefully describe t ...

Casimir Wurster (7 August 1854 – 29 November 1913). The hydrochloride salt finds use as a

redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...

indicator in the

oxidase test The oxidase test is used to determine if an organism possesses the cytochrome c oxidase enzyme. The test is used as an aid for the differentiation of ''Neisseria'', ''Moraxella'', ''Campylobacter'' and ''Pasteurella'' species (oxidase positive). It ...

and is also used in electron transport chain analysis as it is capable of donating electrons to cytochrome c. TMPD HCl solution for oxidase test

The term "Wurster's blue" is often reserved for the radical cation, the colorless diamine being called tetramethylphenylenediamine (TMPD). The midpoint potential for titration of the first electron is given as 0.276 V vs

NHE The standard hydrogen electrode (abbreviated SHE), is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials. Its absolute electrode potential is estimated to be at 25 °C, but to form a basis ...

, and this transition is useful in potentiometric titrations as both a redox mediator and indicator. The two electron-oxidized form (di-iminium) is unstable in aqueous solutions, therefore highly oxidizing conditions should be avoided in titrations relying on TMPD, or reached only during the final stage of the titration. The second oxidation step is not well separated from the first on the redox scale, so some instability will be encountered on the oxidizing side of 0.276, and it is impossible to prepare pure aqueous solutions of Wurster's Blue due to its

dismutation In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. More generally, the term can b ...

to the unstable diaminium and TMPD.

References

{{Reflist
Wurster's original paper (in German)
Anilines Diamines Redox indicators