A compound represented by the following general formula (I): ##STR00001## [R.sup.1 represents hydrogen atom, or 1 to 4 monovalent substituents; R.sup.2, R.sup.4, R.sup.5, and R.sup.7 represent hydrogen atom, or a monovalent substituent; R.sup.3 represents .dbd.O or .dbd.N.sup.+(R.sup.8)(R.sup.9).M.sup.- (R.sup.8 and R.sup.9 represent an alkyl group, and M.sup.- represents a counter ion); R.sup.6 represents --OH or --N(R.sup.10)(R.sup.11) (R.sup.10 and R.sup.11 represent an alkyl group); n represents an integer of 1 to 3; and X represents --SH or --OH], which is useful for selective measurement of hypochlorite ion and the like.