An embodiment of a method for measuring an amount of manganese in an aqueous sample includes: reducing, using a dechlorination reagent, wherein the dechlorination reagent comprises iron(ll) and potassium iodide; oxidizing, under an alkaline condition using sodium hydroxide, Mn(ll) to Mn(IV) in the aqueous sample, chelating, using etidronic acid (HEDP), Fe(ll) and Fe(lll) in the aqueous sample, oxidizing an amount of the colorimetric indicator 3, 3', 5,5'- tetramethylbenzidine (TMB) with Mn(IV); and measuring the amount of manganese within the aqueous sample, by measuring an absorbance intensity at a wavelength of the oxidized amount of 3,3',5,5'-tetramethylbenzidine (TMB). Other aspects are described and claimed.