Dissolved Gases:
Water contains various gases such as nitrogen, methane, hydrogen sulphide, carbon dioxide and oxygen which are normally get dissolved in water due to its contact with the atmosphere and ground surfaces. Hydrogen sulphide gives bad taste and odour even in small quantity. Carbon dioxide indicates biological activities; causes corrosion, increases the solubility of many minerals in water and thus gives taste to water.
Oxygen in the dissolved state is generally obtained from atmosphere and pure natural surface water is usually saturated with it. The oxygen is also being consumed by organic substances present in water for their oxidation, hence if dissolved oxygen content in natural water is less than saturation it indicates the presence of organic matter. The amount of dissolved oxygen in water can be found by exposing water sample for 4 hours at a temperature of 27oC with 10% acid solution of potassium permanganate. The quantity of oxygen absorbed is calculated. Its permissible limit for potable water is about 5 to 10 ppm. The extent of organic matter present in water sample can also be easily estimated by supplying oxygen to this sample and finding the oxygen consumed by the organic matter present in water. This oxygen demand is known as Biochemical Oxygen Demand (BOD).