Several Principles of Dissolved Oxygen Meter
Principle:
1. Iodometry: It is a benchmark method for measuring dissolved oxygen in water, using chemical detection methods with high measurement accuracy. It is one of the earliest methods used for detecting dissolved oxygen. The principle is to add manganese sulfate and alkaline potassium iodide to the water sample to generate manganese hydroxide precipitate. At this point, the properties of manganese hydroxide are extremely unstable, and it quickly dissolves and oxidizes with water to form manganese manganese oxide. Concentrated sulfuric acid is added to react the combined dissolved oxygen (in the form of MnMnO3) with potassium iodide added to the solution to precipitate iodine. Then, starch is used as an indicator to titrate the released iodine with sodium thiosulfate to calculate the content of dissolved oxygen. This method is suitable for various water samples with dissolved oxygen concentrations greater than 0.2mg/L and less than twice the saturation of oxygen (about 20mg/L). When the water may contain nitrite, iron ions, and free chlorine, it may interfere with the determination.
2. Current measurement method (Clark dissolved oxygen electrode) The current measurement method determines the content of dissolved oxygen (DO) in water based on the diffusion rate of molecular oxygen through the film. The thin film of dissolved oxygen electrode can only pass through the gas and diffuse the oxygen in the gas into the electrolyte, immediately undergoing a reduction reaction on the cathode (positive electrode): oxidation reaction occurs on the anode (negative electrode), such as the silver chloride silver electrode: the measurement speed of the current measurement method is faster than that of the iodine measurement method, the operation is simple, and the interference is less (not affected by the color, turbidity, and interfering substances in the chemical titration method of the water sample), and it can be automatically and continuously detected on site. However, due to its oxygen permeability membrane and electrode being prone to aging, when the water sample contains substances such as algae, sulfides, carbonates, and oils, the oxygen permeability membrane can be blocked or damaged. It is necessary to pay attention to protection and timely replacement because it relies on the electrode itself. The characteristic of measuring oxygen concentration by undergoing redox reactions under the action of oxygen requires the consumption of oxygen during the measurement process, So during the measurement process, the sample should be continuously stirred, with a general speed requirement of at least 0.3m/s, and the electrolyte needs to be replaced regularly, which limits its measurement accuracy and response time due to diffusion factors. The humidity sensor probe, stainless steel electric heating tube, PT100 sensor, flow electromagnetic valve, cast aluminum heater, and heating coil are all limited.
3. The determination of fluorescence quenching method is based on the principle of quenching effect of oxygen molecules on fluorescent substances. It is also necessary to choose a suitable dissolved oxygen instrument according to the needs of the sample solution.
