Dissolved Oxygen (DO) refers to the level of free, non-compound oxygen present in water or other liquids. Oxygen is perhaps the most well-established indicator of water quality. It is an important parameter in assessing water quality because of its influence on the organisms living within a body of water. A dissolved oxygen level that is too high or too low can harm aquatic life and affect water quality. DO can be measured through classical method of azide modification or by membrane-electrode DO meters.