Misconception: Direct detection without homogenization of the sample Avoidance: Before detection, the water sample should be thoroughly shaken, and samples containing suspended solids should be taken immediately after being stirred evenly to avoid settling suspended particles that may cause low detection results; High concentration samples need to be diluted as required, and after dilution, they need to be mixed again. It is forbidden to directly take the upper clear liquid for testing.
Misconception: Chaotic order of reagent addition/dosage deviation Avoidance: Strictly add oxidants, catalysts, and other reagents in the order specified in the instrument or reagent instructions, and do not reverse them; Use specialized pipettes to accurately measure reagents, avoiding over adding, under adding, or dripping. Pay special attention to the standard addition of strong oxidizing reagents to prevent affecting the completeness of the reaction.
Misconception: Failure to set the digestion temperature/time as required Avoidance: Set the corresponding digestion temperature (usually 165 ℃) and time (usually 15-30 minutes) based on the type of water sample (such as domestic sewage, industrial wastewater) and reagent requirements. Do not arbitrarily shorten the digestion time or lower the temperature, otherwise the organic matter cannot be completely oxidized, resulting in distorted results; After digestion, it needs to be cooled to the specified temperature before colorimetric analysis.
Misconception: Unclean contamination of the colorimetric cell. Avoidance: Rinse the colorimetric cell 2-3 times with distilled water before colorimetric, and then rinse the test sample once to remove residual impurities or moisture; The outer wall of the colorimetric cell needs to be wiped dry with dust-free paper to avoid fingerprints and stains blocking the light path, which may cause abnormal absorbance detection.
Misconception: Skip calibration and directly test to avoid: After the first use, replacement of reagent batches, or long-term disuse, standard COD solutions (such as 50mg/L, 100mg/L, 500mg/L) must be used for calibration to establish a standard curve; It is recommended to insert calibration points into each batch of samples during daily testing to ensure the accuracy of instrument detection.
Misconception: High chlorine water samples have not undergone dechlorination treatment. Avoidance: For water samples with a chlorine content>1000mg/L, dechlorination reagents such as mercuric sulfate should be added first to mask chloride ions and avoid interference with detection results caused by the reaction between chloride ions and oxidants; The dechlorination reagent needs to be added in sufficient amount and wait for the specified reaction time before digestion.
Misconception: Starting the instrument directly without preheating Avoidance: After turning on the power, the COD analyzer needs to be preheated for 15-30 minutes. After the instrument's optical path is stable and the temperature is constant, sample testing can be carried out to avoid data fluctuations caused by unstable instruments.
Misconception: Direct colorimetric avoidance without cooling after digestion: The digested sample needs to be cooled to room temperature (or the colorimetric temperature specified by the instrument), and it is forbidden to put it in the colorimetric cell at high temperatures, otherwise it will damage the colorimetric cell, affect detection accuracy, and may also cause poor absorbance due to temperature differences.
Misconception: Continuing to use expired reagents. Avoidance: Regularly check the shelf life of COD analyzer reagents. If oxidants, catalysts, and other reagents become turbid, discolored, or precipitate, they should be replaced immediately; Reagents should be stored away from light and sealed as required to avoid failure and affect the test results.
Misconception: Failure to clean the instrument in a timely manner after testing. Avoidance: After the experiment is completed, rinse the reaction tube, colorimetric cell, and sampling channel with distilled water to remove residual reagents and samples; Before turning off the instrument power, ensure that all components are dry and clean. If not in use for a long time, clean the residual liquid inside the instrument to avoid corrosion of the components.