Naphthol derivatives, method for their preparation and use therefor

The invention relates to naphthol derivatives of the general formula I in which R , R and R are identical or different and each represents hydrogen or an alkyl or alkoxy group, where at least one of the radicals is a (C8-C30)-alkyl or alkoxy radical, R denotes hydrogen or an alkyl group, R denotes a nitro group, an alkyl group substituted by halogen, a cyano group, a sulphonamide group or an alkylsulphonyl group, X denotes nitrogen or the radical CR and Y denotes sulphur or the radical CR = CR , in which R , R and R are identical or different and each denotes hydrogen, halogen, a nitro group, a cyano group, an alkyl or haloalkyl group or an alkylsulphonyl group, a method for their preparation and their use as a pH indicator or in methods and compositions for determining an ion in an aqueous liquid.