Theory and Measurement
Dissolved oxygen (DO) is a measure of how much oxygen is dissolved in a system. Measurements are usually taken in water using a DO probe and meter. Measurements taken follow Henry’s Law, which states that the concentration of gas in a solution is directly proportional to the partial pressure of that gas above the solution. Henry’s Law constant is a factor of proportionality, and so is specific to the gas in the solvent being measured.
The partial pressure of oxygen is in fact a measurement of the thermodynamic activity of its molecules. The rate at which oxygen dissolves, diffuses, and reacts is not determined by its concentration, but by its partial pressure. The Earth’s atmosphere is composed of 20.9% oxygen, and at sea level the atmosphere is 100% saturated with oxygen.
Percent saturation is the amount of DO present per amount of DO possible at a given temperature and pressure. Percent saturation is a common unit for DO measurement since it is based upon the partial pressure of a gas; thus it is correct for determination in any solvent.
Concentration measurements of DO can also use the units of parts per million (ppm) or milligrams per liter (mg/L). In meters that report DO concentration in ppm or mg/L, the solvent is always assumed to be water. In other solvents such as oils or acids, the Henry’s Law constant would be different. In those cases, percent saturation should be used as it is incorrect to use ppm or mg/L.
Effects of Temperature and Pressure
As the temperature of a solution increases, the particle movement within that solution increases. With greater particle motion, dissolved gases escape more readily from solution. In warm water oxygen is less soluble, while in cold wateroxygen is more soluble. DO concentration in air saturated waters decreases with increasing temperature.
Water quality measurements are vital to environmental monitoring. In quiescent lakes and rivers, the decay of organic matter can cause bacteria levels to increase. The aerobic bacteria consume oxygen, triggering a deficiency that can cause a water body "to die," killing aquatic plants and animals.
Aquaculture is the breeding, rearing, and harvesting of plants and animals in all types of water environments. Dissolved oxygen is needed by fish, zooplankton, and plants to survive and reproduce. DO measurements are used to monitor and control the environment required for success.
Wastewater treatment plants rely on bacteria to break down the organic compounds found in water. If the amount of dissolved oxygen in the wastewater is too low, these bacteria will die and septic conditions will occur. The amount of DO must be consistently monitored to ensure proper waste treatment.
Wine and beer are both affected by oxygen at various stages during production and storage. DO is an important parameter to monitor for those who wish to produce consistent, high quality products.
Hanna Instruments offers a variety of methods to measure dissolved oxygen. Products include portable and benchtop meters that use either a Clark-Type Polarographic, Galvanic, or Optical probes.
Dissolved Oxygen can also be measured photometrically with reagents. Photometric analysis is based on the Beer-Lambert principle of absorbance. Photometric analysis products include portable and benchtop photometers, and spectrophotometers. Photometric methods include reagent chemistries based on an adaptation of Standard Methods for Examination of Water and Wastewater (23rd edition) Azide modified Winkler method in which there is a reaction that causes a yellow tint in sample.
Chemical Test Kits (CKT) are also available and are simple titrations using a modified Winkler method.
Below are the most common products used for measuring dissolved oxygen.
Benchtop meters are available with Clark-Type Polarographic Probes. Benchtop meters are available with manual entry of salinity and altitude for compensated measurement. Meters are also available to perform BOD (Biochemical Oxygen Demand), OUR (Oxygen Uptake Rate), and SOUR (Specific Oxygen Uptake Rate). Parameters including bottle volume and seed are used to calculate the BOD based on two measurements taken 5 days apart.
Portable meters are available with probes that use either the Clarke-Type Polarographic, Galvanic, or Optical technologies. The portable meters include those with manual calibration and manual selection of altitude and salinity compensation values. Portable meters also include versions that have a barometric pressure transducer for automatic pressure compensation along with versions that have an EC sensor for salinity compensation. Meters are available with the BOD, OUR and SOUR programs built-in for automatic calculation.
Portable Meters with Polarographic Probe
The following portable meters use Clark-Type Polarographic probe. Polarographic DO probes consist of a working electrode (cathode) and a counter electrode (anode). A polarizing voltage is applied to these electrodes that is specific for the reduction of oxygen. A thin, gas permeable membrane isolates the sensor elements from the water sample but allows oxygen to pass through. The oxygen that passes through the membrane is reduced at the cathode, causing a current from which the oxygen concentration is determined. Two-electrode polarographic probes use the anode as a reference electrode.
Portable Meters with Galvanic Probe
The following portable meters use galvanic technology. Galvanic DO probes consist of a working electrode (cathode) and a counter electrode (anode) that act as a battery to produce a voltage specific for the reduction of oxygen. A thin, gas permeable membrane isolates the sensor elements from the water sample but allows oxygen to pass through. The oxygen that passes through the membrane is reduced at the cathode, causing a current from which the oxygen concentration is determined.
OPDO Portable Meters
Portable meters are available that use optical technology to measure dissolved oxygen. Optical DO sensing probe is based on the principle of fluorescence quenching. The sensing method features an immobilized Pt based luminophore that is excited by the light of a blue LED and emits a red light. Dissolved oxygen quenches this excitation. When there is no oxygen present, the lifetime of the signal is the greatest; as oxygen hits the sensing surface, the lifetime becomes shorter. The intensity and lifetime are inversely proportional to the amount of oxygen present; as oxygen interacts with the luminophore it reduces the intensity and lifetime of the luminescence. The lifetime of the luminescence is measured by a photodetector, and is used to calculate the dissolved oxygen concentration. This is in turn reported by the meter as a % saturation or mg/L reading of Dissolved Oxygen.
Dissolved Oxygen Probes and Sensors
The following are replacement probes and sensors for benchtops and portable meters that can measure dissolved oxygen.
Process controllers are available for the continuous monitoring of solutions. Process controllers include a programmable set point that can activate a relay connected to a solenoid valve, dosing pump, or other device to make adjustments to a process. The process controllers offer a analog output for connection to a remote device such as SCADA, PLC or other compatible devices.
The following category is for the probes used with process controllers.
Solutions for probe style measurements include fill solutions for polarographic and galvanic probes. In this category is zero oxygen solution for calibrating meters to 0 mg/L (ppm).
The solution category also include the CAL Check standards for verifying portable and benchtop photometers. Each CAL Check standard is supplied with a Certificate of Analysis stating the accuracy and traceability of the standard
Spectrophotometers are available that have a dissolved oxygen method pre-programmed into the meter. The spectrophotometer also has oxygen scavenger methods built in for boiler maintenance. The spectrophotometer offers the highest precision due to the quality of the optical system that has a wavelength accuracy of +/- 1.5 nm. The spectrophotometer allows for custom methods.
Benchtop photometers include multiparameter versions for water quality, wastewater and Environmental. Each photometer is customized to have the parameters used by a specific industry. Some models, like the version for boilers and cooling towers, include the oxygen scavenger methods. All benchtop have a digital pH electrode input allowing it to be used as a traditional pH meter.
A portable photometer is available for measuring dissolved oxygen.
Chemical Test Kits
A dissolved-oxygen chemical test kit (CTK) is available as a single parameter or available with multiparameter CTKs. The dissolved-oxygen CTK uses a modified Winkler titration method. All accessories are included with the CTK to measure dissolved-oxygen.
Reagents include the reagents used with the photometers and chemical test kit to measure dissolved oxygen. Also included in this category are the oxygen scavenger reagents.
Accessories include cuvette cleaning solution, microfiber cloths, and replacement cuvettes and caps for photometers. Also included are accessories for the meters that use probes.