Dissolved oxygen (DO) in water can be found using several methods, primarily chemical analysis and electrochemical or optical sensors. Modern techniques increasingly rely on electrochemical probes for their convenience and accuracy.
Methods for Measuring Dissolved Oxygen
Here's a breakdown of the common methods:
-
Chemical Analysis (Titrimetric Method): The Winkler titration is a classic method. It involves chemically fixing the dissolved oxygen in the water sample and then titrating the fixed oxygen with a reducing agent. The amount of reducing agent needed is directly proportional to the amount of dissolved oxygen.
-
Electrochemical Methods:
- Galvanic Probes: These probes generate their own current based on the amount of oxygen present. The oxygen diffuses through a membrane and reacts at the electrodes, producing a current proportional to the DO concentration.
- Polarographic Probes: These probes require an external voltage to operate. Oxygen diffuses through a membrane and is reduced at the cathode, generating a current proportional to the DO concentration.
-
Optical Dissolved Oxygen Sensors: These sensors use a fluorescent dye that is sensitive to oxygen. The dye is excited by a light source, and the presence of oxygen quenches the fluorescence. The amount of quenching is related to the DO concentration. These are also referred to as luminescent dissolved oxygen (LDO) sensors.
-
Colorimetric Methods: These methods involve a chemical reaction that produces a colored solution, where the intensity of the color is proportional to the DO concentration. This is generally less precise than other methods.
Modern Techniques: Electrochemical Probes
Electrochemical probes, particularly galvanic and polarographic probes, are the most widely used techniques due to their ease of use, portability, and relatively low cost. They provide real-time measurements and are suitable for both laboratory and field applications. Optical DO sensors offer high accuracy and low maintenance but are typically more expensive.
Choosing the Right Method
The choice of method depends on several factors, including:
- Accuracy requirements: Optical and electrochemical sensors are generally more accurate than colorimetric methods.
- Cost: Titration and colorimetric methods can be less expensive initially, but electrochemical and optical sensors offer long-term benefits and reduced labor.
- Portability: Electrochemical and optical sensors are easily portable for field measurements.
- Sample conditions: Certain methods may be more suitable for specific water types or conditions.
Example Use Cases
| Method | Application | Advantages | Disadvantages |
|---|---|---|---|
| Winkler Titration | Laboratory analysis, calibration | Accurate if performed correctly, good for low DO concentrations | Time-consuming, requires skilled technician, can be affected by interferences |
| Galvanic Probe | Field monitoring, wastewater treatment | Simple to use, self-powered, relatively inexpensive | Membrane fouling, requires regular calibration |
| Optical DO Sensor | Aquaculture, environmental monitoring | High accuracy, low maintenance, fast response | More expensive than other methods |
| Polarographic Probe | General lab use, environmental studies | Accurate, relatively fast, good for varied DO concentrations | Requires external power, membrane fouling, calibration needed |
