What is a Dissolved Oxygen Test and How Do You Perform It?

Water quality testing is crucial for a variety of industries, from fish farming to wastewater management. One of the most important and informative tests is the dissolved oxygen test. Several methods and tools can be used to perform a dissolved oxygen test, all of which offer valuable insights into a water sample's oxygenation level.

What is Dissolved Oxygen?

Dissolved oxygen (DO) represents the amount of gaseous oxygen that has been dissolved into an aqueous solution, usually water. Oxygen diffuses into water from the surrounding air or via rapid disruption of the water, known as aeration. Plants that live in the water can also transfer oxygen as a waste product of photosynthesis, much in the same way they add oxygen to the atmosphere on land.

The DO levels in a sample or body of water can be affected by factors like temperature, pressure and salinity. As temperature rises, transferring more energy to molecules and causing their bonds to break, DO levels decrease. Conversely, as temperatures fall, the DO levels of a sample or body of water will rise again.

Pressure, meaning atmospheric pressure, weighs down on water and causes greater DO concentration — in other words, DO concentration might be lower in a mountain lake than another lake at sea level.

Salinity refers to the salt concentration of a water sample. Salt molecules dissolve into a solution with water much easier than oxygen, and when a high concentration of salt is present, it becomes much harder for oxygen molecules to dissolve into the water, meaning that the higher a sample's salinity, the lower its dissolved oxygen will most likely be.

Why is DO Important?

Maintaining a consistent DO level is critical for aquatic life to thrive. For example, if dissolved oxygen levels are too high, the fish living in the body of water may suffer from "gas bubble disease," in which tiny bubbles form in the gills, eyes and fins of the animals. Extensive exposure to this disease can damage a fish's tissues enough to be lethal. On the other hand, if DO levels drop too low, "dead zones" where aquatic plants and animals can no longer survive will form. Performing dissolved oxygen tests can help you form a plan of action to create a healthy aquatic environment, whether it's in a large river or a small aquarium.

Methods for Performing a Dissolved Oxygen Test

There are three main methods used to perform the dissolved oxygen test, all of which can be done in the field or in the lab, although some are more difficult to execute accurately on-site than others.

Sensor/Electroanalytical Method

Probably the most popular method of measuring dissolved oxygen is by using dissolved oxygen meters and sensors. Some sensors have analog output to quickly log data for further analysis and others are even designed to remain in water in order to record data over extended periods of time.

The two main types of dissolved oxygen sensors are optical DO sensors and electrochemical DO sensors. Optical sensors use light and luminescent dyes to measure DO levels — the concentration of dissolved oxygen is inversely proportional to luminescence lifetime, meaning that the higher the DO level, the less light will be returned to the sensor.

Electrochemical DO sensors, which are broken down into galvanic and polarographic subgroups, use electrodes and electrolyte solution to measure oxygen levels in water. Oxygen reacts with the solution to create an electrical current, which, when measured, is proportional to the oxygen concentration in the water.

Colorimetry

This simple and easy test uses chemicals and the level of color they produce to measure DO. The tests are performed using either the indigo carmine or the rhodazine D method. The indigo method is best for measuring oxygen concentrations between 0.2 and 15 mg/L, but ferric iron, ferrous iron, nitrate and sodium hydrosulfite can interfere with the results. The higher the oxygen concentration of a sample or body of water, the darker the blue will be.

Rhodazine D can measure oxygen concentrations as small as parts per billion (ppb) rather than the traditional parts per million (ppm), although chlorine, ferric iron and cupric copper, along with other oxidizing agents, can interfere. If the oxygen concentration is high, the sample will turn a deep, vibrant pink.

Overall, colorimetry is a quick, easy and inexpensive way to get a preliminary read on dissolved oxygen content. However, additional elements in the water and outside factors like temperature, pressure and turbidity can affect the accuracy of the outcome. Additionally, these tests are time sensitive: you generally must analyze the results within 30 seconds of mixing the reagent.

Titrimetric Method/Iodometry

Also known as the Winkler method, iodometry is the most traditional way to perform a dissolved oxygen test, although it works best with a sample in a lab setting. It uses the oxidizing property of dissolved oxygen to deliver some of the most accurate and precise results of any DO test. There are seven variations of the Winkler method, which can be used if there is prior knowledge of additional elements present in a sample.

As a form of titration, iodometry uses a titrant with a known concentration to determine the unknown concentration of dissolved oxygen in a sample. The Winkler method, which is the titration used to determine DO, studies the appearance and disappearance of iodine to signal the end of the process so the DO level can be measured.

Find Dissolved Oxygen Test Equipment and More at Certified MTP

Certified Material Testing Products carries all the water quality testing meters you need to perform dissolved oxygen tests and keep tabs on your water or other fluids. Get dissolved oxygen meters, turbidity sensors, pH probes and more in our high-quality selection.