What is Dissolved Oxygen?
Dissolved oxygen is the term used for the amount of gaseous oxygen, which is dissolved or trapped in a liquid medium.
The solubility of oxygen changes for different liquid substances and different temperatures.
When a liquid solution holds all the dissolved oxygen it can hold at a given temperature, it is said to be 100% saturated. If the liquid substance holds half-as-much oxygen as it can hold at a given temperature, it is 50% saturated.
The units can also be reported in ppm if solubility information is available.
Methods of Dissolved Oxygen Determination
Dissolved oxygen determination methods have been found to vary widely in their sensitivity, susceptibility to electrode poisoning by the presence of undesirable compounds, and, in some instances, sensitivity to a slow-flowing or static aqueous media. Some methods require a very clean sample, devoid of extraneous chemical compounds, but only a few are adaptable to continuous online measurement.
The various methods can be divided into three principal categories:
- Laboratory methods (Winkler Method).
- Electrochemical analysis (conductimetric, voltammetric, and galvanic).
- Membrane electrode methods (galvanic membrane electrodes and amperometric membrane electrodes).
The Amperometric Membrane Technique used is the most popular method used industry-wide.
In this technique, the electrodes are driven by an externally applied polarizing voltage. The current flow between the electrodes can be directly correlated to the amount of oxygen present in the stream.
In a typical dissolved oxygen sensor, two electrodes, a gold cathode, and a silver anode are immersed in a specially prepared electrolyte solution and separated from the sample to be measured by a gas-permeable membrane.
The transfer of oxygen across the membrane is proportional to the partial pressure of oxygen in the fluid. The chemical reactions
that accompany this process are as follows:
Gold cathode: O2 + 2H2O + 4e- 4OH
Silver anode: Ag + HCI AgCI + e- + H+
The reaction that takes place at the anode is the oxidation of silver to form silver chloride. This reaction is offset at the gold cathode by the reduction of oxygen molecules to hydroxide ions. The resulting current flow is directly proportional to the dissolved oxygen content of the stream.
Why is Dissolved Oxygen Measured?
Dissolved oxygen is a measurement of water quality in Waste Water, Drinking Water & Natural bodies of water
It is a measurement of hydrocarbon co-monomer quality Hexene, Decene, Styrene, Xylene, Octane, etc.
Used in Industries like:
- Brewing Industry
- Wine Industry
- Biotechnology Industry
- Boiler Feed-Water (ppb levels)
Factors Affecting Dissolved O2 Measurement:
Temperature changes: As temperature increases, the amount of dissolved oxygen in a particular solution decreases. In addition, temperature changes affect the permeability of the membrane and thus the diffusion current.
Salinity or concentration of dissolved salts in the solution: Detector sensitivity increases exponentially as salt concentration increases.
Atmospheric pressure or altitude: Pressure changes with a change in altitude and changes in pressure affect the calibration of the partial pressure of oxygen that is being measured.
The flow rate of the solution: There are several different components present in the total probe current from the detector. These components change with time if additional dissolved oxygen from the solution is not passed through the membrane. As a result, the probe must be moved through the solution if accurate results are desired.
Dissolved Oxygen -Measurement in Water Sources (Fig. 2):
Pollutants in the waterways block sunlight and poison and kill aquatic plants and animals, thus interfering with the biological balance of the waterways. The body of water that formerly generated oxygen from plants and animals now demands oxygen from the atmosphere. An accepted indicator of overall water pollution is the level of dissolved oxygen, which is measured in parts per million (ppm).
- The dissolved oxygen sensor is immersed in water. The water sample must flow past the sensing membrane at a speed of at least 1 ft per second for good measurement.
- The principle of measurement is amperometric: oxygen diffuses through a semi-permeable membrane and is reduced to hydroxyl ions by a small polarizing voltage. The resulting current produces a readout on the analyzer.
- For applications with low flow or no flow conditions, the DO sensor is the ideal solution. An oscillating sample chamber on the end of the sensor pumps the sample past the measuring electrodes to provide an accurate measurement.
Dissolved Oxygen -Brewery Process (Fig. 3):
- Brewing is an art that is becoming increasingly scientific in its approach to the perfect beer.
- There are several critical control points throughout the brewery. One critical measurement is dissolved oxygen (DO).
- DO is measured in water purification, fermentation, aging, filtration, and filling.
- Dissolved oxygen monitoring throughout various stages in the brewery is critical to quality control as well as increasing final product shelf life.
- Too much oxygen in the purification, aging, filtration and filling stages will affect quality by producing dimethyl sulfide (DMS), or “skunk beer”. Too much oxygen may also affect the beer’s shelf life.
- Too little oxygen in the fermentation stage will affect the yeast’s effectiveness. Sterile air or pure oxygen may be added to ensure the yeast strain is in the ideal environment.