The level of dissolved oxygen in natural waters is often a direct indication of quality, since aquatic plants produce oxygen, while microorganisms generally consume it as they feed on pollutants. At low temperatures the solubility of oxygen is increased, so that in winter, concentrations as high as 20 ppm may be found in natural waters; during summer, saturation levels can be as low as 4 or 5 ppm. Dissolved oxygen is essential for the support of fish and other aquatic life and aids in the natural decomposition of organic matter. Waste treatment plants which employ aerobic digestion must maintain a level of at least 2 ppm dissolved oxygen. This is usually accomplished by mechanical aeration.

At elevated temperatures, oxygen is highly corrosive to metals, causing "pitting" in ferrous systems such as high-pressure boilers and deep well oil recovery equipment. To prevent costly corrosion damage, the liquids in contact with the metal surfaces must be treated, usually by a combination of physical and chemical means. Deaeration can reduce the dissolved oxygen concentration of boiler feedwater from several ppm to a few ppb. Chemical reducing agents such as hydrazine, DEHA, or sodium sulfite are sometimes used instead of or in conjunction with deaeration.

The Colorimetric Methods. Test kits for environmental and drinking water applications (ppm range) employ the indigo carmine method. The reduced form of indigo carmine reacts with D.O. to form a blue product. The indigo carmine methodology is not subject to interferences from temperature, salinity or dissolved gases such as sulfide, which plague users of D.O. meters. Results are expressed as ppm (mg/L) O₂.

Test kits for boiler waters and applications requiring trace levels of D.O. (ppb range) employ the Rhodazine D methodology. Developed by CHEMetrics, Inc., the Rhodazine D compound in reduced form reacts with dissolved oxygen to form a bright pink reaction product. This method is not subject to the salinity or dissolved gas interferences which plague dissolved oxygen meters. Oxidizing agents, including benzoquinone, can cause high results. Reducing agents such as hydrazine and sulfite do not interfere. Results are expressed as ppm (mg/L) or ppb (µg/L) O₂.

The dissolved oxygen products provide fast, accurate colorimetric oxygen determination. Test kit K-7512 is used to monitor surface waters. ULR CHEMets^TM ampoules detect oxygen to 1 ppb. Test kit K-7540 is widely used to monitor boiler feedwater.

Low range dissolved oxygen test kits include a special "sampling tube" for use with boiler feedwater. This device allows the user to break the tip of the ampoule in a flowing sample stream in order to preclude error from contamination by atmospheric oxygen.