Abstract: The indigo carmine was reduced by using cheap, easily attainable and non poisonous reagents. Therefore, a special reagent was developed to measure the concentration of dissolved oxygen in water in situ. The molar absorption coefficient of the method is 1.35×10⁴, and its dynamic linear range is 0-27 mg/Llinearity coefficient of 0.997. The most oxidizing species, such as Cl₂, ClO^-, Cu²⁺, Fe³⁺, V⁵⁺, and As(Ⅴ), do not interfere the determination. It takes no more than 1 m in to analyze one sample. The method was applied to determining some practical samples and all the results obtained were satisfactory.

Key words: Dissolved oxygen, In situ determination, Portable photometer