What are the water quality dissolved oxygen detection methods (the principle of water quality dissolved oxygen detection)

At present, the methods and standards for the detection of dissolved oxygen in my country include 'Iodometry Method for Determination of Dissolved Oxygen in Water Quality' (GB7489-1987), 'Electrochemical Probe Method for Determination of Dissolved Oxygen in Water Quality' (HJ506-2009) and American ASTM standard (D888-05). ), the first two are China's national and industry-standard methods, and the latter is the US EPA-approved standard method. Let's introduce the detection principle in detail.

1. The principle of the method for the determination of dissolved oxygen in water by the iodometric method:

The determination of dissolved oxygen in water is generally performed by the iodometric method. Add manganese sulfate and alkaline potassium iodide solution to the water to form manganese hydroxide precipitate. Due to the extremely unstable nature of manganese hydroxide, it rapidly reacts with dissolved oxygen in the water to form manganese sulfate. After 15 minutes, concentrated sulfuric acid was added to make the brown precipitate react with the potassium iodide added in the solution, and iodine was precipitated. The more dissolved oxygen, the more iodine precipitated, and the darker the color of the solution. Take a certain amount of the water sample after the reaction with a pipette, use starch as an indicator, titrate it with a standard solution, and calculate the dissolved oxygen content in the water sample.

2. The principle of the method for the determination of dissolved oxygen in water by electrode polarography:

A constant voltage is applied between the two poles, electrons flow from the cathode to the anode, and a diffusion current is generated; 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; the instrument automatically converts the galvanometer reading to the dissolved oxygen concentration, and the dissolved oxygen value is displayed on the screen.

3. The principle of the method of fluorescence LDO determination of dissolved oxygen in water:

The modulated blue light hits the fluorescent material to excite it and emit red light. Since oxygen molecules can take away energy (quenching effect), the time and intensity of the exciting red light are inversely proportional to the concentration of oxygen molecules. Using the red light source synchronized with the blue light as the reference, measure the phase difference between the excitation red light and the reference light, and compare it with the internal calibration value to calculate the concentration of oxygen molecules. After some processing, the dissolved oxygen is output.

The iodometric determination steps are complicated, not suitable for on-site determination, and the detection time is relatively long. In the water body where algae flourish, due to the increase of oxygen release due to photosynthesis, the oxygen in the water may reach a supersaturated state. At this time, it is difficult to measure the dissolved oxygen in the water by the iodometric method, and the measurement result is not accurate enough.

Determination of dissolved oxygen in water by electrode polarography is simple and fast, the price of the instrument is relatively low, and it is a national standard method. But the formation of an oxygen gradient slows down the reaction as the oxygen is depleted and fouling on the membrane and electrodes. If the semipermeable membrane is damaged, the electrolyte is easily contaminated, which will cause the battery potential to drift, and the drift will be incorrectly displayed as the concentration of dissolved oxygen in the water sample. Therefore, the electrolyte and semipermeable membrane need to be replaced regularly.

The determination of dissolved oxygen in water by fluorescence LDO is simple and quick. Compared with the first two methods, the fluorescence LDO method does not require calibration for the determination of dissolved oxygen in water, the response time is fast, the measurement results are stable, and there is no requirement for flow, no interference, less cleaning frequency, and less maintenance. But the price of the instrument is relatively high, and it does not belong to the national standard method.