(...) What is Dissolved Oxygen?

Some oxygen is dissolved in practically every liquid. For example, at a temperature of 68°F (20°C) and an atmospheric pressure of 1013mbar saturated water contains about 9mg/L oxygen. Ethanol can contain up to 40mg/L whereas glycerol only has about 2mg/L.

Each liquid takes up oxygen until the partial pressure of oxygen in the liquid is in equilibrium with the air or gas phase in contact with it. The actual oxygen concentration depends on a number of factors, such as temperature, air pressure, oxygen consumption by microorganisms in a biodegradation process or oxygen production by algae, etc.

The oxygen concentration is important for the:

• Living conditions for fish and microorganisms in waters;

• Degradation processes in wastewater treatment;

• Corrosion processes in pipelines;

• Shelf life of beverages, etc.

The determination of the dissolved oxygen concentration was formerly carried out by the WINKLER titration method. Today electrochemical measurement is a recognized method in numerous standard procedures.

In its simplest form a dissolved oxygen electrode contains a working electrode and a counterelectrode. Both dissolved oxygen electrodes are located in an electrolyte system which is separated from the sample by a gaspermeable membrane. The working dissolved oxygen electrode reduces the oxygen molecules to hydroxide ions. In this electrochemical reaction a current flows from the counterelectrode to the working dissolved oxygen electrode. The more dissolved oxygen present in the sample, the larger the current signal generated by the dissolved oxygen electrode. A meter connected to the dissolved oxygen electrode calculates the oxygen concentration in the sample from this signal.

Why Measure DO?

Dissolved oxygen (DO) is the amount of oxygen (O²) dissolved in water. Dissolved oxygen measurements provide one of the best indicators of the health of a water ecosystem, as oxygen is a necessary element for all forms of life, including aquatic life. Oxygen enters water at the water surface through direct exchanges with the atmosphere. It is also produced as a byproduct of plant and phytoplankton photosynthesis. A decrease in dissolved oxygen levelsis typically associated with an organic pollutant. Dissolved oxygen is used by plants and animals for respiration, and by aerobic bacteria in the process of decomposition. When organic matter (such as animal waste or improperly treated wastewater) enters a body of water, algae growth increases. As the plant material dies off and decomposes, dissolved oxygen levels decrease. If the water at the surface is not mixed with deeper water layers, the water's dissolved oxygen levels can become stratified. Dissolved oxygen levels can also vary according to the time of day, weather, and temperature. Dissolved oxygen in water can range from 0-18 parts per million (ppm), but most natural water systems require 5-6ppm to support a diverse population. As dissolved oxygen levels drop below 5.0mg/L, aquatic life is put under stress. As dissolved oxygen levels decrease, pollution-intolerant organisms are replaced by pollution-tolerant worms and fly larvae. If dissolved oxygen levels fall below 1- 2mg/L for a few hours, large fish kills can result. (...)