SIMPLE IN-SITU DETERMINATION OF DISSOLVED-OXYGEN IN WATER USING METHYLENE-BLUE

A simple visual method has been proposed for the in situ determination of dissolved oxygen in environmental waters, based on spectrophotometric studies with Methylene Blue. The reduced Methylene Blue was prepared in a graduated syringe by the reduction with glucose in aqueous-ethanol solution containing sodium hydroxide. Then, the sample was introduced into the syringe. The oxidation of the reduced Methylene Blue with the dissolved oxygen was completed almost instantaneous, the color changing from colorless to blue. The color intensity was proportional to the concentration of dissolved oxygen. The color remained stable for 90 min. Up to 13 mu g/ml dissolved oxygen can be determined visually and also spectrophtometrically at 600 nm with a relative standard deviation of 3.9%. The visual method can be applied to in situ determination of dissolved oxygen in seawater and brackish water as well as fresh water.