A method using a multi-purpose reagent composition to simultaneously determine concentrations of at least two analytes in a water sample. In one embodiment, the multi-purpose reagent composition used is for simultaneously determining free chlorine concentration and pH and comprises a free chlorine sensitive dye and a pH indicator. In another embodiment, the reagent composition used is for simultaneously determining of free chlorine and anionic polymer and includes a free chlorine sensitive dye, a pH buffer, a cationic dye, and an organic co-solvent. Thus, the pairs of free chlorine and pH or free chlorine and anionic polymer are determined using a single reagent composition. The reagent composition can also be the main reagent for one analyte analysis when it is used alone and also function as an ancillary reagent when it is combined with a second reagent composition for the determination of another analyte.