A new chromoionophore sym -(decyl)(2-hydroxy-5-nitrobenzyloxy)dibenzo-16-crown-5 (1) is found to selectively respond to Na+ with a detection limit of 10(-3) M (1 M = 1 mol dm(-3)) and no interference from K+ up to 0.05 M for detection of 10.0 mM Na+ in aqueous 1,4-dioxane solution.