The amount of dissolved oxygen in water is related to many things, such as atmospheric pressure, water temperature, salt content and so on. Under normal circumstances, unpolluted water, dissolved oxygen is more saturated state.
The factors that affect dissolved oxygen in water mainly include two aspects. One is the oxygen consumption formed when dissolved oxygen in water drops. For example, when aerobic organic matter degrades, dissolved oxygen in water is consumed. The second is the reoxygenation of increased dissolved oxygen, such as the dissolution of oxygen in the air and the photosynthesis of plants in the water. Under the combined action of these two factors, the content of dissolved oxygen in water will change to different degrees.
The general methods for the determination of dissolved oxygen in water are:
1. Determination of dissolved oxygen by iodimetry
Iodimetry is one of the main methods for the determination of dissolved oxygen in water.
Manganese hydroxide is formed by adding manganese sulfate and basic potassium iodide to the water sample. Manganese hydroxide is chemically unstable and reacts quickly with dissolved oxygen in water to form manganese sulfate.
After standing for 15 to 20 minutes, concentrated sulfuric acid is added to promote brown precipitation and potassium iodide in the solution to fully react, so as to gradually precipitate iodine. The more dissolved oxygen in the water, the more iodine is extracted, and the darker the solution. By taking out a certain amount of water sample after reaction through a high-precision pipette, and then using starch as an indicator, titrating through standard solution, we can know the specific content of dissolved oxygen in water.
Advantages and disadvantages: iodimetry is more procedures, more suitable for professional laboratory. If there are more algae and other plants in the water, certain oxygen will be released under photosynthesis, and the dissolved oxygen in the water will reach saturation. In this state, the error of dissolved oxygen measurement is large by iodine method.
2. Determination of dissolved oxygen by electrode polarography
Compared with iodimetry, electrode polarography is more advanced.
The main mechanism is to add a constant voltage between the poles to promote electrons from the cathode to the anode, thus forming a certain amount of diffusion current.
The content of dissolved oxygen in the water can be known by measuring the diffusion current, because at a certain temperature, the diffusion current in the water sample is proportional to the concentration of dissolved oxygen in the water. Through quantitative analysis, the instrument can read the specific value of dissolved oxygen in the water sample.
Advantages and disadvantages: electrode polarography determination of the steps is relatively less, the use of equipment is relatively low price, is the current application of the standard dissolved oxygen in water determination method.
3. Determination of dissolved oxygen by fluorescence method
The main mechanism of fluorescence determination is to use the quenching effect of fluorescent substances to reduce the fluorescence intensity and shorten the maintenance life of fluorescence, so as to know the content of dissolved oxygen in water.
The more dissolved oxygen in the water, the shorter the lifetime of the fluorescence. The modulated blue light is irradiated to the fluorescent material, and the corresponding red light can be emitted. Dissolved oxygen in water can take away fluorescence energy, so the duration and intensity of red light are inversely proportional to the concentration of dissolved oxygen. By measuring the phase difference between red light and reference light, the content of dissolved oxygen in water can be known.
Advantages and disadvantages: Fluorescence method is the most simple and convenient method for the determination of dissolved oxygen in water at present. Compared with iodine method and electrode polarography, fluorescence method does not need to specially calibrate dissolved oxygen in water, and the measurement speed is relatively fast, the whole measurement process is relatively stable, the measurement results are relatively accurate, and there are no strict requirements on the flow rate, and the interference from external factors is relatively small. The cleaning frequency can be reduced and the maintenance cost is low
