Primary, secondary, and tertiary alkylamines react with tetrabromophenolphthalein ethyl ester dye (HTBPE) to form reddish charge transfer (CT) complexes in 1,2-dichloroethane (1,2-DCE). Absorption maxima of the complexes with primary amines occur at ∼560 nm, with secondary amines at ∼570 nm and tertiary ones at ∼580 nm. CT complex formation constants between amines and HTBPE in 1,2-DCE at 25 °C increase in the order of primary, secondary, and tertiary amines but decrease with an increase in temperature. This phenomenon (thermochromism), which depends on the basicity of amines, was thermodynamically studied and used for the selective analysis of alkylamines. For example, tri-n-butylamine could be determined without interference from n-butylamine.

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