Posts Tagged ‘Dissolved Oxygen’

Dissolved Oxygen and Importance of its measurements

May 2nd, 2014

Dissolved oxygen (DO) is the physical distribution of oxygen in water (as the oxygen does not react with water at all). A water body seldom has more than 10 ppm dissolved in water. Concentrations above this level can be harmful to aquatic life; fish in water body containing excessive dissolved gases may suffer from “gas bubble disease”.

Dissolved oxygen concentrations below 3 ppm stress most warm water species of fish and concentrations below 2ppm will kill some species. Often fish that have been stressed by Dissolved Oxygen concentrations in the range of 2 or 3 ppm become susceptible to diseases. The main sources of Dissolved Oxygen in water are

Atmosphere; wave action and splashing allow more oxygen to be absorbed into the water.
Photosynthesis; in the presence of light aquatic plants including algae; produce oxygen as a by-product during photosynthesis.

Factors affecting Dissolved oxygen in water are

Water temperature; Cold water can hold more oxygen than warm water.
Salinity; fresh water can hold more oxygen than salt water
Atmosphere; the greater the atmospheric pressure the more the oxygen water will hold.

Dissolved Oxygen increases with the decrease in temperature, salinity and an increase in atmospheric pressure.

As it is well known that,

5-6 ppm; sufficient for most species
<3 ppm; stressful to most aquatic species
<2 ppm; Fatal to most species

Because of its importance to fish survival, aqua culturists, or fish farmers and aquarists measure the Dissolved Oxygen as a primary indicator of their system’s ability to support healthy aquatic life.

A Dissolved Oxygen Meter is used to measure the amount of oxygen present in a unit volume of water; it indicates if the water is useful for a specific application like water treatment plants, sewage treatment works, river monitoring and fish farming. It is a measure advantage for the aqua culturists, or fish farmers and aquarists.

Types of dissolved oxygen meter are

Polarographic (Clark cell method) sensor oxygen meter; use an external voltage keeping the potential difference between the anode and cathode to less than 0.5 volts. The working electrode uses a noble metal (Pt), and the opposite electrode uses Ag.
For the electrolyte, a potassium chloride solution is used, and for the membrane, a Teflon membrane is used. Voltage is applied between the two electrodes so that the threshold diffusion current for oxygen is generated there. The oxygen which has passed through the membrane is reduced with the working electrode. A reduction current in proportion to the Dissolved Oxygen is generated, and then the Dissolved Oxygen is measured.
Galvanic Sensor Oxygen Meter; does not use an external voltage and the difference between the anode and the cathode is greater than 0.5 volts. These are more stable and accurate when compared to the Polarographic oxygen meters. The working electrode uses a noble metal (Ag), and the opposite electrode uses a base metal (Pb).
For the electrolyte, an alkaline solution is used, and for the membrane, a highly oxygen-permeable Teflon membrane is used. Oxygen which has passed through the membrane is reduced with the working electrode. A reduction current in proportion to the concentration of the Dissolved Oxygen is generated, and then the Dissolved Oxygen is measured.
Optical Fluorescence Meter; are extremely suitable for long-term measurements in ground water as it is not sensitive to contaminats or for that matter, aging.