This test is an azide modification of the Winkler method where oxygen, in the presence of a strong alkali, oxidizes manganese, which in turn reacts with iodide to form iodine. This is titrated with a standard thiosulfate solution in the presence of a starch indicator to enhance the endpoint. Azide eliminates the nitrite interference. Test kit and refills contain enough reagents for 50 tests at an oxygen level of 10 ppm.