Luminescent optical indicator for measuring the activity of alkali metal ions in a sample solution.

In the case of a luminescent optical indicator, for determining the activity of alkali metals on the basis of decay time measurement, provision is made for the indicator to have at least three components chemically bonded together, namely a fluorophore, a functional group by means of which the luminescence decay time of the fluorophore can be influenced, and an ionophore which selectively and reversibly binds the alkali metal ions to be measured, by means of which the action of the functional group on the decay time of the fluorophore can be altered as a function of the activity of the alkali metal ions. <IMAGE>