Dissolved Oxygen (DO) Measurement Explained

Water quality impacts all life on Earth, and dissolved oxygen levels in water compose one of its most important quality indicators. Oxygen levels in water must fall within a specific range to be considered healthy and capable of sustaining life. If dissolved oxygen (DO) levels reach 2.0 mg/L, the water becomes hypoxic and unable to support life, while DO levels around 6.5-9.0 mg/L are considered healthy.

The primary source of DO in water comes from oxygen diffusing into the water through the atmosphere and the aeration of water caused by turbulent flow. A second source of DO is plants and algae that produce oxygen as a by-product through photosynthesis..

Water Testing
Oxygen Requirements

Impacts on Dissolved Oxygen Levels

DO levels depend on three critical variables: temperature, pressure, and salinity. The amount of DO in water can differ. Oxygen levels are affected by various factors. They include the temperature of the water, the altitude of the sample, and the concentration of dissolved salts.

Each value is related to its ability to affect the solubility of oxygen in the water. Evaluating these variables helps interpret the DO data and understand each variable's impact on DO. For example, DO levels observed will decrease as the water temperature increases due to reduced oxygen solubility.

A biological example of life affected by DO is trout, found most commonly in cooler water. Trout require a high oxygen content and seek cooler water because they can thrive best in that environment.

It is important to understand the factors that affect DO levels. This will help ensure aquatic life can thrive in its environment. It is important to note that low DO levels can be fatal for fish. If they are unable to get enough oxygen from the water, they can die.

Environmental scientists and water management personnel monitor DO levels in order to ensure water quality and the health of aquatic life.

It is also important to note that DO levels fluctuate as the seasons change due to temperature, pressure, and salinity changes. The variables mentioned make it crucial to consider the solubility fluctuations that occur as the seasons change. DO levels obtained for the same area in the summer will drastically differ from those measured in the winter.

The concentration of dissolved salts can increase. This reduces the free water molecules available to dissolve oxygen, making it less soluble in water. Atmospheric pressure can also affect dissolved oxygen (DO) levels. An increase in pressure increases DO, while a decrease reduces it.

Each variable is proportional to the DO level. However, temperature and salinity are inversely proportional to the amount of DO, while the pressure is directly proportional.

Optical Dissolved Oxygen Probes

DO levels are frequently measured using optical and electrochemical probes. Optical dissolved oxygen probes employ a lumiphore composed of a fluorescent material that emits red light when exposed to blue light.

The blue light excites electrons in the material. As the electrons relax back to the ground state, the material emits red light. This is because collisions with the material and oxygen present cause a reduction in the emission window known as fluorescence quenching.

The Stern-Volmer equation can calculate DO concentrations. This is based on the relationship between the amount of oxygen and the time for red light emission. The initial rate of red light emission is used as a reference. The higher the DO level, the shorter the red light emission will be due to increased oxygen-material collisions.

The use of optical probes to measure DO concentrations is fast, reliable, and cost-effective. The process requires no calibration and the probes can be deployed in both static and dynamic conditions. Additionally, the probes are highly sensitive, making it possible to detect small fluctuations in DO concentrations over time. Optical probes are a valuable tool for monitoring water quality and ensuring that aquatic ecosystems remain healthy and balanced.

Electrochemical Probes

Electrochemical probes are economically advantageous, allow for continuous sampling of DO levels, and are used for various applications. There are several options for these kinds of probes, including polarographic, pulsed-polarographic, and galvanic probes.

A polarographic probe typically uses a gold or platinum cathode as the working electrode. The counter electrode is a silver anode, submerged in a potassium chloride (KCl) internal solution. Samples must be stirred while taking measurements. This prevents a low signal caused by full oxygen consumption in non-flowing water around the electrodes.

A constant voltage capable of reducing oxygen is applied to the electrodes, which causes them to become polarized with charge. The cathode collects a negative charge, while the anode collects a positive charge. Oxygen in the sample will then get reduced at the cathode, which causes an increased voltage reading.

Pulsed-polarographic probes function similarly to a regular polarographic probe, except that the need for stirring has been eliminated. The pulsed-polarographic probe turns on and off every few seconds. This is to allow oxygen to replenish around the electrodes. A third silver electrode keeps the electrodes properly polarized during on-off cycles.

Galvanic DO probes function similarly to the other two probes, but instead employ dissimilar metals. Silver is often used as the cathode or working electrode. The anode or counter electrode is usually lead.

The internal solution is usually potassium hydroxide (KOH). A constant voltage does not need to be applied because the electrodes will self-polarize and reduce oxygen in the sample.

Dissolved Oxygen Applications

Several industries use DO levels to analyze and monitor water health. Examples include water pollution assessments, wastewater treatment, fish farming, aquariums, water quality testing for wells and potential potable water sources, and groundwater surveys. Human, plant, and animal health depend on the accuracy and reliability of the information discovered during DO testing.

The Lab Depot offers multiple lines of the newest technologies available for measuring DO, such as optical, polarographic, and galvanic DO probes, in addition to meters used in conjunction with the probes. We also supply required cleaning and storing solutions that support accurate DO analysis. Review product suggestions for these applications.

For an interesting look at the effects of dissolved oxygen on natural water sources, click here.

Sources:

Wei, Y.; Jiao, Y.; An, D.; Li, D.; Li, W.; Wei, Q. Review of Dissolved Oxygen Detection Technology: From Laboratory Analysis to Online Intelligent Detection. Sensors 2019, 19 (18), 3995. https://doi.org/

Carpenter, J. H. THE ACCURACY OF THE WINKLER METHOD FOR DISSOLVED OXYGEN ANALYSIS1. Limnol. Oceanogr. 1965, 10 (1), 135–140. https://doi.org/

Please view the reference images below. Image #1: DO and Organisms Freshwater & Image #2: DO and Organisms Saltwater

Image Credits: Fondriest Enviornmental, Inc.

Request A Quote

Customer Information
Shipping Address
Billing Address

Will you need a lift gate? *

(For deliveries requiring a LTL truck and the customer does not have a dock door)

Would you like to add shipping amount to quote? *

Are you tax exempt? *

(If you are tax exempt, please email your tax exemption form to [email protected])

Upload PDF: