An acidity function has been determined spectrophotometrically in aqueous sulphuric acid, at 298 K, by using seven secondary amine indicators. This function is the same as that based on primary amines and so differs from that relative to tertiary amines. Therefore, the first two types of amines are the indicators of the same acidity function. It is concluded that for a given acidity function, the Hammett postulate is only verified for all thecompounds characterized by the same value of a r structural parameter: apparent ratio of solvation.

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