Dissolved oxygen (DO) refers to the concentration of non-compound oxygen gas (\(text{O}_2\)) physically dissolved within a volume of water. This free oxygen is distinct from the oxygen atoms that make up the water molecule (\(text{H}_2text{O}\)). The amount of oxygen water can hold is determined by physical laws, primarily temperature and atmospheric pressure. Measuring DO is a fundamental practice in environmental science, aquaculture, and regulatory monitoring, providing a direct assessment of water quality.
Why Measuring Dissolved Oxygen Matters
Dissolved oxygen is required for the survival and health of virtually all complex aquatic life, including fish, invertebrates, and aerobic microorganisms. Aquatic organisms use DO for respiration, a process analogous to breathing air. When DO concentrations drop too low, aquatic life experiences stress, which can impair growth, interfere with reproductive cycles, and lead to suffocation or mass die-offs.
Levels below 5 milligrams per liter (\(text{mg}/text{L}\)) are considered stressful for most fish species; levels below 2 or \(3 text{ mg}/text{L}\) create hypoxic conditions that cannot sustain populations. Microbes rely on DO to decompose organic material, which is important for nutrient recycling. However, excessive decaying organic matter, often from pollution or large algal blooms, can cause microbes to rapidly consume oxygen, leading to sudden drops in DO levels.
The Classic Method: Winkler Titration
The Winkler titration, also known as the iodometric method, is a precise wet chemistry technique developed in 1888. It remains the benchmark for calibrating modern electronic sensors. The method requires the oxygen to be chemically “fixed” immediately upon sample collection to prevent its escape or consumption. This is achieved by adding manganese sulfate and an alkaline-iodide-azide reagent directly to the water sample in a sealed bottle, pipetting the reagent beneath the surface to exclude atmospheric air.
In the alkaline environment, dissolved oxygen oxidizes manganese(II) ions (\(text{Mn}^{2+}\)) into a brown precipitate containing a higher oxidation state of manganese. Once the precipitate settles, a strong acid is added to dissolve it and release iodine (\(text{I}_2\)) from the iodide ions. The amount of liberated iodine is chemically equivalent to the amount of oxygen originally dissolved in the sample.
The final step involves titration. A standardized solution of sodium thiosulfate is slowly added until the color changes from yellow-gold to a pale straw color. A starch solution is then added as an indicator, turning the solution dark blue. Titration continues until the blue color disappears completely. The precise volume of thiosulfate used corresponds directly to the concentration of dissolved oxygen.
The Modern Method: Digital DO Meters
Modern electronic meters offer a simpler, faster, and more portable alternative to the labor-intensive Winkler method, allowing for real-time field measurements. These instruments rely on probes containing specialized sensors, primarily electrochemical and optical types. Electrochemical sensors (including polarographic and galvanic types) use a semi-permeable membrane. Oxygen diffuses into an electrolyte solution where it is electrochemically reduced, generating a measurable electrical current proportional to the DO concentration.
Optical sensors, often called luminescent dissolved oxygen (LDO) sensors, operate using luminescence quenching. These probes use a blue light to excite a luminescent dye applied to a sensing foil, causing it to emit red light. The presence of oxygen molecules quenches this luminescence, reducing both the intensity and the decay time of the red light. The sensor measures this reduction, which is inversely proportional to the amount of dissolved oxygen.
Both electronic methods require regular calibration against an oxygen-saturated water standard. This process ensures the accuracy of the reading before deployment in the field.
Understanding Your Results
Dissolved oxygen results are reported in one of two ways: concentration or percentage saturation. Concentration is expressed in milligrams per liter (\(text{mg}/text{L}\)), equivalent to parts per million (ppm), representing the actual mass of oxygen present. Percentage saturation reports the DO concentration as a percentage of the maximum amount of oxygen the water can physically hold at a given temperature and pressure.
Concentration determines if aquatic organisms have enough oxygen for respiration (readings below \(5 text{ mg}/text{L}\) indicate stress). Percentage saturation provides context because oxygen solubility depends highly on temperature and barometric pressure. Colder water naturally holds a greater concentration of oxygen than warmer water, and water at higher altitudes holds less due to lower atmospheric pressure. For example, a \(10 text{ mg}/text{L}\) reading might be \(100%\) saturation in cold water but \(120%\) (supersaturated) in warmer water, showing why both metrics are necessary for evaluating water quality.