A method of making a pyranine monomer composition comprises the step of reacting pyranine with a functionalizing agent that functionalizes the pyranine molecule with a polymerizable functional group to provide a composition of functionalized pyranine monomers, said functionalization reaction taking place in an aqueous reaction medium, and in the presence of a molar excess of said functionalizing agent, such that the functionalization reaction product is a monomer composition substantially free of unfunctionalized pyranine compound. The pyranine monomer compositions made by the method can be used to make fluorescent tagged water soluble polymer compositions that advantageously are substantially free of unpolymerized pyranine. The fluorescent tagged water soluble polymer compositions can be used in a method of inhibiting scale in industrial water systems.