Diazotisation titrations

The determination of primary amines on the macro scale is most conveniently carried out by titration in non-aqueous solution (Section 10.41), but for small quantities of amines spectroscopic methods of determination are very valuable. In some cases the procedure is applicable to aromatic amines only, and the diazotisation method described for determination of nitrite (Section 17.38) can be adapted as a method for the determination of aromatic primary amines. On the other hand, the naphthaquinone method can be applied to both aliphatic and aromatic primary amines. 

Discussion. The formation of coloured compounds by coupling phenols with diazotised primary aromatic amines has long been recognised as a method of determining phenols, and procedures have been evolved whereby the phenol solution is titrated with a diazonium solution which has been calibrated against known concentrations of the phenol. The resultant reaction products are coloured, but many are only sparingly soluble in water and organic solvents and do not therefore lend themselves to colorimetric determination. 

As obtained commercially solapsone is not of constant composition but is chiefly tetrasodium 4,4 -di-(3-phenyl-l,3-disulphopropylamino)-diphenyl sulphone it contains water of hydration. The compound is decomposed in acid solution to diaminodiphenyl sulphone (dapsone), the hydrolysis not being quite quantitative even with chemically pure material, it can then be diazotised and determined by titration as above. The acid hydrolysis should not be done under reflux as this would prevent the removal, by volatilisation, of the cinnamic acid produced in the reaction. 

The official sulphonamides are all derivatives of sulphanilamide and the assay for all members of the group by diazotisation follows that of the parent substance either directly or after preliminary hydrolysis or reduction (para-nitrosulphathiazole) except for sulphafurazole and sulphafurazole diethanolamine. Sulphafurazole cannot be diazotised quantitatively since the product consumes more than one equivalent of nitrous acid. Non-aqueous titration can also be applied. 

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