Calibration method of dissolved oxygen meter for water quality monitoring
The content of dissolved oxygen in water is one of the important indicators to measure the self-purification energy of water and judge the quality of water. Dissolved oxygen content is closely related to the composition and distribution of biological communities in water bodies. For fish in water, dissolved oxygen needs to be greater than 4 mg/L to ensure their normal life activities. Therefore, during the water quality monitoring process, a high degree of attention is paid to the indicator of dissolved oxygen.
The main methods for measuring dissolved oxygen in water are: iodometric method, electrochemical probe method, conductivity method and fluorescence method. Iodometry is the earliest method used to measure dissolved oxygen in water and is also the benchmark method for measuring dissolved oxygen in water. When the water contains some reducing substances such as sodium nitrite, sulfide, thiourea, humic acid or tannin, it will cause greater interference to the iodometric titration data. In this case, the electrochemical probe method is recommended. . Electrochemical probe method instruments are widely used in practical work due to their fast measurement speed, stable results, accurate data, and low interference. Its working principle is that when oxygen molecules in the water pass through the diaphragm, they are reduced by the working electrode. This process produces a The oxygen concentration is directly related to the diffusion current, and the dissolved oxygen concentration in the water is calculated by measuring this diffusion current. The current method standard for electrochemical probe method to measure dissolved oxygen is HJ 509-2009. This article mainly discusses and analyzes the operation and maintenance details of the electrochemical probe method for measuring dissolved oxygen in water, so as to fully understand the performance characteristics of the instrument and thereby improve the measurement accuracy of the dissolved oxygen meter.
Dissolved oxygen meter range calibration
Laboratory method 1 (saturated air water method): At room temperature of 20°C, fill 1L of ultrapure water in a 2L beaker, aerate the water for 2 hours, stop aeration and let it stand for 30 minutes, and then perform range calibration of the instrument. Select the magnet speed to be 500rpm or stir the electrode repeatedly in the water phase to start the calibration. Select saturated air and water calibration on the instrument interface. After completion, the full scale will be displayed as 100%.
Laboratory method 2 (water-saturated air method): At room temperature of 20°C, add water to the sponge in the probe protective sleeve until it is completely wet, absorb the water on the probe electrode membrane with filter paper, and then put the electrode back to the protective sleeve. After balancing in the sleeve for 2 hours, start the calibration. Select water-saturated air calibration on the instrument interface. After completion, the full scale is displayed as 102.3%. Generally speaking, the calibration results using the water-saturated air method and the saturated air-water method are consistent. After calibration, the saturated air-water or water-saturated air is back-tested, and the measured value is around 9.0mg/L.
On-site calibration: The instrument needs to be calibrated before each use. It is difficult to do it in an environment of 20°C when used outdoors. Therefore, you can choose the water-saturated air method to calibrate in the probe sleeve on-site. Use this method to calibrate the instrument for measurement. The indication error is within the acceptable range and can be used normally.
Zero point calibration
Use freshly prepared anaerobic water, dissolve 0.25g sodium sulfite and 0.25g cobalt chloride hexahydrate in 250mL ultrapure water, put the probe into the anaerobic water and move it gently, start the zero point calibration, and wait until the instrument value stabilizes. Click Finish after zero. Instruments with zero point compensation do not need to perform zero point calibration.
