What is Dissolved Oxygen?

Dissolved Oxygen (DO) refers to the oxygen dissolved in the molecular state of water, that is, O2 in water, represented by DO. Dissolved oxygen is an indispensable condition for the survival of aquatic organisms. One source of dissolved oxygen is when the dissolved oxygen in the water is not saturated, the oxygen in the atmosphere penetrates into the water body; the other source is the oxygen released by the plants in the water through photosynthesis. Dissolved oxygen changes with changes in temperature, air pressure, and salinity. Generally speaking, the higher the temperature, the greater the dissolved salt, and the lower the dissolved oxygen in water; the higher the air pressure, the higher the dissolved oxygen in the water.

Dissolved oxygen is closely related to the partial pressure of oxygen in the air, atmospheric pressure, water temperature, and water quality. At 20°C and 100kPa, the dissolved oxygen in pure water is about 9mg/L. Some organic compounds biodegrade under the action of aerobic bacteria, consuming dissolved oxygen in the water. If the organic matter is calculated by carbon, according to C+O2=CO2, it can be known that every 12g of carbon consumes 32g of oxygen. When the dissolved oxygen value in the water drops to 5 mg/L, some fish have difficulty breathing.

Dissolved oxygen is not only consumed by reducing substances such as sulfide, nitrite, and ferrous ions in water, but also by the respiration of microorganisms in water and the oxidative decomposition of organic substances in water by aerobic microorganisms. Therefore, dissolved oxygen is the capital of the water body and the performance of the water body's self-purification ability.

The dissolved oxygen value is a basis for studying the self-purification ability of water. The dissolved oxygen in the water is consumed, and it takes a short time to return to the original state, indicating that the water body has a strong self-purification ability, or that the water body is not seriously polluted. Otherwise, it means that the water body is seriously polluted, the self-purification ability is weak, or even loses the self-purification ability.

What is the significance of measuring dissolved oxygen in water?

With the rapid development of industry and agriculture in the world today, a large amount of industrial wastewater and farmland drainage is discharged to rivers, lakes, and seas. At the same time, about 80% of urban domestic sewage is discharged directly without treatment, and most of the domestic sewage in small towns and vast rural areas is in a state of disorderly discharge has caused the water quality in many places to deteriorate day by day, water pollution and water resource shortage are becoming more and more serious, so it is urgent to monitor and effectively treat sewage in time. Among them, the dissolved oxygen content in water is an important indicator for water quality monitoring.

The dissolved oxygen in natural water is close to the saturation value (9ppm), and the dissolved oxygen content decreases when the algae multiply vigorously. Water bodies polluted by organic matter and reducing substances can reduce dissolved oxygen. For aquaculture, water body dissolved oxygen has a vital impact on the survival of aquatic organisms such as fish. When dissolved oxygen is lower than 4mg/L, it will cause the fish to suffocate and die. For humans, the dissolved oxygen content in healthy drinking water should not be less than 6mg/L. When the consumption rate of dissolved oxygen (DO) is greater than the rate of oxygen dissolved into the water body, the content of dissolved oxygen can approach 0. At this time, anaerobic bacteria can reproduce and deteriorate the water body, so the size of dissolved oxygen can reflect the water body is affected It is an important indicator of the degree of water pollution and a comprehensive indicator of water quality. Therefore, the measurement of dissolved oxygen content in water is of great significance for environmental monitoring and the development of aquaculture.

Methods of Measuring Dissolved Oxygen

1. Fluorescence Dissolved Oxygen Sensor

To measure dissolved oxygen, the first thing to talk about is the fluorescence (optical) dissolved oxygen sensor, which uses the fluorescence measurement principle: the modulated blue light shines on the fluorescent substance to excite it and emit red light, because oxygen molecules can take away energy (The quenching effect), so the time and intensity of the red light excitation are inversely proportional to the concentration of oxygen molecules.

2. Iodometric method

The iodometric method is a chemical detection method with accurate measurement. It is an earlier method used to detect dissolved oxygen. Add manganese sulfate and alkaline potassium iodide solution to the water to generate a manganese hydroxide precipitate. Due to the extremely unstable nature of manganese hydroxide, it quickly reacts with dissolved oxygen in the water to form manganese sulfate. After 15 minutes, concentrated sulfuric acid is added to make the precipitate react with the potassium iodide added to the solution, and iodine was precipitated. The more dissolved oxygen, the more iodine is precipitated. Take a certain amount of the reaction water sample, use the starch indicated, and titrate with the standard solution to calculate the dissolved oxygen content in the water sample.

Attention: The iodometric method is suitable for source water, surface water, and other clean water.

3. Amperometry

The measurement speed of the amperometric method is faster than that of the iodometric method, the operation is simple, the interference is less, and it can automatically and continuously detect on the spot. However, because its oxygen-permeable membrane and electrodes are relatively easy to age, when the water sample contains algae, sulfide, carbonate, oil, and other substances, the oxygen-permeable membrane will be blocked or damaged, and it needs to be protected and replaced in time. Because it relies on the redox reaction of the electrode itself under the action of oxygen to measure the oxygen concentration, oxygen consumption is required during the measurement process, so the sample must be continuously stirred during the measurement process, and the electrolyte needs to be replaced regularly.

4. Electrode polarography

Electrode polarography means that a constant voltage is applied between the two electrodes, and electrons flow from the cathode to the anode to generate a diffuse current; at a certain temperature, the diffusion current is proportional to the dissolved oxygen concentration; the quantitative relationship between the current and the dissolved oxygen concentration is established, and then the instrument can The ammeter reading is automatically converted to dissolved oxygen concentration.

Attention: Electrode polarography is a simple and quick method for measuring dissolved oxygen in water, and the price of the instrument is relatively low, which is a national standard method.