Use of 8-anilino-1-naphthalene sulfonate as a fluorescent indicator in colloid titration

Abstract

The use of 8-anilino-1-naphthalene sulfonate (ANS) is recommended as the fluorescent indicator in the titration of dilute colloid. Potassium polyvinylsulfate (PVSK) and methylglycolchitosan (MGch) are used for the standard substances of the negatively and positively charged colloid, respectively.The fluorescence intensity was measured with the spectrofluorophotometer Hitachi 204 equipped with magnetic stirrer in cell compartment. Titrations were carried out in a 300 ml beaker using a megnetic stirrer. ANS is practically nonfluorescent in water but becomes strongly fluorescent when it is bound to cationic colloid such as MGch (λex=374 nm, λem=480 nm).The fluorescence of ANS-MGch complex is diminished by the titration with PVSK standard solution. The end point of the fluorometric titration was determined from a plot of fluorescence intensity against the volume of the titrant added.A typical example is shown in Fig. 1 where 40 ml of aqueous MGch solution (5.5×10-5 M) was titrated with 2.5×10-4 M aqueous PVSK standard solution. A clear equivalence point was obtained by this method. Transition metals, especially, iron(III), quench the fluorescence of ANS-MGch complex and must be removed by dialysis or masked with proper reagents.