Water Quality - Oxygen

Water quality can be characterized by three parameters: hydrology, physico-chemistry and biology. Here, we present methods to establish water quality based on the amount of dissolved oxygen, and nitrogen level

Dissolved Oxygen

Dissolved oxygen (DO) is the amount of gaseous oxygen dissolved in water. Oxygen diffuses into water from the surrounding air, by aeration and as a by-product of photosynthesis. Dissolved oxygen levels are used as an indicator of water quality.

In this lab, we use a colorimetric method to measure the dissolved oxygen levels in the water sample. The dissolved oxygen test utilizes indigo carmine. In an acidic solution, oxygen oxidizes the yellow-green leuco form of indigo carmine to form a brightly colored blue dye. The resulting blue color is proportional to the dissolve oxygen levels in the water sample.

Eight tubes, each with a solution of a different shade of blue, measuring the dissolved oxygen, expressed in P P M milligrams per liter. From tube number one to the tube number 8, the blue shade changes from very pale, almost sea like blue colour, to very dark, ink like colour. Under the scale, the undergoing chemical reaction in the tubes is presented.The chemical structure of leucoindigo gets oxygenated at two hydroxyl groups, resulting in a new chemical structure of indigo, with two ketone groups. The indigo compound is marked in dark blue colour.

Figure 1. First image - The darker the blue in the tubes, the higher the dissolved oxygen concentration. Second image - Indigo carmine reaction.