Diphenylbenzidine Violet

Mention should be made of one of the earliest internal indicators. This is a 1 per cent solution of diphenylamine in concentrated sulphuric acid, and was introduced for the titration of iron(II) with potassium dichromate solution. An intense blue-violet coloration is produced at the end point. The addition of phosphoric(V) acid is desirable, for it lowers the formal potential of the Fe(III)-Fe(II) system so that the equivalence point potential coincides more nearly with that of the indicator. The action of diphenylamine (I) as an indicator depends upon its oxidation first into colourless diphenylbenzidine (II), which is the real indicator and is reversibly further oxidised to diphenylbenzidine violet (III). Diphenylbenzidine violet undergoes further oxidation if it is allowed to stand with excess of dichromate solution this further oxidation is irreversible, and red or yellow products of unknown composition are produced. 

The use of diphenylamine in concentrated sulphuric acid to identify cellulose nitrate was originally used in forensic investigations in the 1930s to detect the presence of explosives on suspects hands (Gowan and Purdon, 1967). As a forensic test, wax casts were taken of the suspect s hands and the diphenylamine reagent applied to the wax. Sulphuric acid reacts with cellulose nitrate to form nitronium ions (N02 ) which then oxidize diphenylamine to form the dark blue dye, diphenylbenzidine violet. 

Diphenylbenzidine is then able to act as a two-electron reversible redox indicator to give purple-violet diphenylbenzidine violet ... 

Similar considerations apply to redox titrations, where the indicator dye should possess differently coloured oxidation states and a redox potential which is appropriate for the reaction to be studied. A large number of dyes are known which are suitable for this purpose. Ferroin (Scheme 7), a complex between iron(ll) and phenanthrolin with a deeply red colour, upon oxidation forms a pale blue iron(lll) complex. Another redox indicator is diphenylamine (Scheme 8), which is colourless in the reduced state. It is oxidized in acidic solution irreversibly to diphenylbenzidine (Scheme 9) which can be further oxidized reversibly to the intensely coloured diphenylbenzidine violet (Scheme 10). 

The interest in the application of indicators in oxidation-reduction titrations has followed on the discovery that the familiar color change undergone by diphenylamine on oxidation could be used to determine the end-point of the titration of ferrous ion by dichromate in acid solution. Diphenylamine, preferably in the form of its soluble sulfonic acid, at first undergoes irreversible oxidation to diphenylbenzidine, and it is this substance, with its oxidation product diphenylamine violet, that constitutes the real indicator. ... 

Reduction to quinol can also be carried out with powdered zinc. In 75% sulfuric acid, oxidation of diphenylbenzidine with quinones is accompanied by the formation of a blue-violet color. Anthraquinone, diacetyl, and benzil do not react, but with all other organic oxidants the reaction is smooth (32). In a similar manner the reaction with p-dimethylaminodiphenylmethane can also be used for the detection of chloranil (33). Quinones, similar to 1,2-diketones, catalyze the slow reaction of formaldehyde with o-dinitro-benzene, in which the violet salt of the acid form of o-isonitrosobenzene (34) is formed. It seems, however, that certain phenols react as well (35). 

Procedure A crystal of the tested substance is added to a solution of diphenylbenzidine in H2SO4. A violet-blue color appears. 

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