1. The Winkler's method is used to determine the amount of dissolved oxygen (DO) in water samples. In this method, DO oxidizes manganese sulfate to form brown hydrated manganese dioxide precipitate in an alkaline medium. 2. The manganese dioxide then dissolves and oxidizes iodide ions to form iodine. The iodine is then titrated with a standardized sodium thiosulfate solution. 3. The volume of thiosulfate solution used corresponds to the amount of DO originally present in the water sample. Calculations are done to determine the concentration of DO in the sample water in units of mg/L or g/L.