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Q: Dissolved Oxygen Info

Dissolved Oxygen Info

Basic Theory
Dissolved oxygen analysis measures the amount of gaseous oxygen (O2) dissolved in an aqueous solution. Oxygen gets into water by diffusion from the surrounding air, by aeration (rapid movement), and as a waste product of photosynthesis.

Dissolved oxygen is consumed in the water by respiration and decomposition. It is replenished mainly by the atmosphere and photosynthesis. Water temperature is a key factor in the regulation of water's oxygen level. Warm water contains a lower oxygen concentration than cold water. If DO concentrations get too high, though, the dissolved oxygen can become toxic to plant and animal life.

The Dissolved Oxygen Probe can be used to measure the concentration of dissolved oxygen in water samples tested in the field or in the laboratory. Since dissolved oxygen is one of the primary indicators of the quality of an aquatic environment, you can use this sensor to perform a wide variety of tests to determine changes in dissolved oxygen levels, especially in aquariums, photosynthesis and respiration, and on-site testing. In a stream or lake the evaluation of the capability of the water to support different types of life, the Biochemical Oxygen Demand (BOD) tests water samples containing organic matter that consumes oxygen as it decays and to determine the relationship between dissolved oxygen concentration and temperature of a water sample.

DO sensors use a thin membrane to cover a layer of electrolyte over a platinum cathode and silver anode, generally the cathode is almost in direct contact with the membrane.Oxygen will diffuse across the membrane at a rate proportional to its partial pressure—the greater the oxygen partial pressure, the more oxygen diffuses through the membrane.Then DO meters measure the current as oxygen is reduced at the cathode while more oxygen diffuses through the membrane. Since the diffusion current is directly proportional to the concentration of dissolved oxygen, the calibrated meter simply converts measured current into concentration units.

The concentration of dissolved oxygen is usually expressed in milligrams of oxygen per liter of water (mg/L) or parts per million (ppm). Some meters compare calculated oxygen content with observed concentration and report percent saturation (O2% sat.).

There are two types of methods for determining dissolved oxygen, polarographically and galvanically. Polarographic probes require a voltage input from the meter to polarize the electrodes.Since the voltage from an external source may take up to 15 minutes to stabilize, polarographic probes usually need to warm up before use to ensure proper polarization of the electrodes. Galvanic probes have electrodes made from two different metals that spontaneously polarize to generate the voltage. Since the voltage is spontaneous rather than supplied by an external source, galvanic probes are always operable and do not require the "warm up" time that polarographic probes need for polarization.

Environmental Impact
Adequate dissolved oxygen is necessary for good water quality. Oxygen is a necessary element to all forms of life. Natural stream purification processes require adequate oxygen levels in order to provide for aerobic life forms. As dissolved oxygen levels in water drop below 5.0 mg/L, aquatic life is put under stress. The lower the concentration, the greater the stress. Oxygen levels that remain below 1-2 mg/L for a few hours can result in large loss of aquatic life.

Dissolved oxygen measurements are used to monitor processes where oxygen content affects reaction rates, process efficiency, or environmental conditions:
Environmental testing (lakes, streams, oceans)
Water / Wastewater treatment
Wine production

Temperature Compensation
Temperature compensation is necessary for standardized DO measurements, as temperature affects both the solubility and diffusion rate of oxygen.

Salinity Correction
The presence of dissolved salts limits the amount of oxygen that can dissolve in water. The relationship between the concentration of oxygen and partial pressure varies with the salinity of each sample, so most meter manufacturers supply the ability to manually correct for salinity to compensate for variations in ionic concentration.

Biochemical Oxygen Demand (BOD)
The BOD test is typically perfomed in wastewater treatment plants, where it is important to understand the amount of oxygen that microorganisms consume from the water when they break down organic matter. This test allows the plant to determine the effectiveness of their water treatment, or the amount of pollution that still exists. By measuring the amount of oxygen dissolved in samples at the beginning and end of a specified incubation period, the relative oxygen requirements of wastewaters,effluents, and polluted waters can be determined. You can calculate BOD by measuring DO at time 1 (T1) and subtracting the DO at time 2 (T2); multiple that number by the final volume (VF) and dividing that number by the initial sample volume (V):
BOD (mg/L) = (T1 – T2)VF/V

Troubleshooting Tips
When using a polarographic style probe, allow the probe at least 15 to 30 minutes before calibrating or measuring. To ensure that the membrane has no air bubbles in the electrolyte solution, the ASI membrane caps are designed to release all air while the module is being installed. Do not allow any air bubbles to be trapped on the membrane surface, as it will read an air bubble as an oxygen-saturated sample. Calibrate your electrode at temperatures close to the sample temperature, even when using a meter with automatic temperature compensation (ATC). Always calibrate your DO electrode dry 
using air as the 100% test point it is important during measurement to stir the solution as oxygen consumption by the probe can momentarily reduce the oxygen oncentration at the probe surface.

Membranes wear out, you should replace the membrane membrane as needed.

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