Dissolved oxygen (DO) represents the amount of free, non-compound oxygen gas dissolved in water, distinct from the oxygen that is chemically bonded within the water molecule itself. This measurement, typically expressed in milligrams per liter (mg/L) or parts per million (ppm), is a fundamental indicator of water quality. Oxygen dissolves into water through direct diffusion from the atmosphere and as a byproduct of photosynthesis by aquatic plants.
The concentration of DO supports nearly all aerobic aquatic life, including fish, invertebrates, and microorganisms, which rely on it for respiration. Low DO levels, known as hypoxia, can severely stress or kill aquatic organisms, leading to reduced biodiversity. Furthermore, in industrial applications like wastewater treatment, maintaining specific DO levels is necessary for aerobic bacteria to efficiently break down organic pollutants.
Winkler Titration
The Winkler titration, developed in 1888, is the foundational chemical method for determining dissolved oxygen, often serving as a reliable reference standard for calibrating electronic sensors. This method relies on a precise sequence of chemical reactions to convert dissolved oxygen into a titratable compound. The process begins by “fixing” the oxygen in the water sample, achieved by adding manganese(II) sulfate and an alkali-iodide-azide reagent.
This initial step creates an alkaline environment where the dissolved oxygen oxidizes the manganese(II) ions, resulting in a brown precipitate of manganese(IV) oxide or hydroxide. The amount of this precipitate is directly proportional to the original oxygen concentration. Next, a strong acid, such as sulfuric acid, is added to the sample, which causes the manganese precipitate to dissolve.
The acidification step causes the oxidized manganese to react with the iodide ions, releasing free iodine. The quantity of liberated iodine is chemically equivalent to the amount of dissolved oxygen originally fixed in the sample. Finally, the liberated iodine is measured through titration, where a sodium thiosulfate solution is added until a color change indicates the reaction’s endpoint.
A starch solution is used as an indicator near the end of the titration, turning the sample a deep blue-purple color in the presence of iodine. The titration continues until the blue color disappears completely, indicating that all the iodine has reacted with the thiosulfate. The total volume of sodium thiosulfate solution required is then used to calculate the dissolved oxygen concentration.
Electrochemical Measurement
Modern field and laboratory measurements often utilize electrochemical probes, which provide rapid and continuous readings of dissolved oxygen concentration. These sensors, commonly of the Clark-type or polarographic design, operate by measuring the electrical current generated during oxygen reduction. The probe consists of a cathode and an anode submerged in an electrolyte solution, enclosed by a gas-permeable membrane.
Dissolved oxygen molecules diffuse across this membrane and into the internal electrolyte solution. Once inside, the oxygen is chemically reduced at the cathode surface, a process that consumes electrons. In a polarographic sensor, an external voltage is applied to drive this reduction, creating a measurable electrical circuit.
The resulting flow of electrons, or current, from the anode to the cathode is directly proportional to the partial pressure of oxygen. A higher concentration of dissolved oxygen outside the membrane results in more oxygen diffusing in, leading to a greater rate of electron consumption and a stronger electrical current. Galvanic sensors operate on a similar principle but generate their own voltage through the difference in electrode metals, eliminating the need for an external power source.
Because the measurement process consumes a small amount of oxygen, electrochemical probes require the water sample to be moving or the probe to be constantly stirred. This ensures a continuous supply of fresh, oxygenated water to the membrane surface, preventing the sensor from inaccurately reading localized oxygen depletion. The simplicity of use and real-time data output have made these probes a standard tool for instantaneous DO monitoring.
Optical Sensors
Optical sensors, also known as optodes, offer a less maintenance-intensive alternative to electrochemical probes, employing luminescence quenching to measure dissolved oxygen. The active component is a sensing element coated with a luminescent dye, known as a luminophore. When the luminophore is excited by a pulse of blue light from an internal light-emitting diode (LED), it absorbs the energy and momentarily enters a higher energy state.
As the excited luminophore returns to its ground state, it naturally emits light, typically at a longer wavelength. If an oxygen molecule collides with the excited luminophore, the oxygen absorbs the excess energy, preventing light emission—a process called luminescence quenching. The degree of quenching is directly proportional to the oxygen concentration.
Instead of measuring a current, the sensor measures the intensity or the decay time of the emitted light. A faster decay time or a weaker light signal indicates a higher concentration of dissolved oxygen, as more oxygen molecules are present to quench the luminescence. This method does not consume oxygen during measurement, eliminating the need for continuous stirring or water flow.
Optical sensors require less frequent calibration and maintenance compared to electrochemical probes, as they do not rely on an internal electrolyte solution or a fragile membrane that needs regular replacement. This robust design makes them well-suited for long-term, unattended deployment in environmental monitoring applications.
Ensuring Accurate Results
Obtaining reliable dissolved oxygen measurements requires careful attention to procedural details and environmental factors. Regular calibration is necessary for any electronic probe, typically involving a two-point check using a zero-oxygen solution and air-saturated water. This process aligns the sensor’s electronic output with known oxygen concentrations, compensating for signal drift over time.
Temperature compensation is necessary because oxygen solubility decreases significantly as water temperature increases. Most modern probes include a built-in thermistor to automatically correct the DO reading based on the measured water temperature. Without this correction, a reading taken in warm water may appear artificially low, even if the water is fully saturated with oxygen.
Proper sample handling is essential, especially when collecting samples for the Winkler method or laboratory analysis. Care must be taken to minimize agitation and prevent the introduction of air bubbles, which can artificially inflate the DO measurement. If samples must be transported, they should be sealed and kept cool to suppress biological activity that would otherwise consume the dissolved oxygen.
For electrochemical probes, the condition of the semipermeable membrane and the electrolyte solution must be routinely inspected and maintained according to manufacturer’s specifications. Contaminated or damaged membranes can impede oxygen diffusion, leading to inaccurate readings. Allowing the sensor to stabilize and reach thermal equilibrium with the water sample before recording the final value ensures precise data.