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Common sources of error and quality control measures for COD chemical oxygen demand rapid determination instrument
Date: 2025-12-12Read: 52
COD chemical oxygen demand rapid analyzerAlthough the detection efficiency has been greatly improved, the results are affected by various factors, and if not controlled, it can easily lead to data distortion.
1、 The main sources of error include:
1. Incomplete digestion: Insufficient heating time, low temperature, or presence of difficult to oxidize organic compounds (such as pyridine) in the sample, resulting in low COD;
2. Chloride ion interference: Cl ⁻ in high chloride wastewater (such as seawater and pickling wastewater) is oxidized by potassium dichromate, causing a virtual increase in COD;
3. Colorimetric error: Staining, scratching, or mismatched blank on the colorimetric dish can affect the absorbance reading;
4. Reagent failure: After prolonged storage of the digestion solution, Cr ⁶⁺ is reduced and the slope of the standard curve decreases.
2、 The quality control measures are as follows:
1. Standard sample validation: Insert certified standard substances (such as GSB 07-3161) into each batch of testing to ensure a recovery rate of 90% -110%;
2. Chloride ion masking: For samples with Cl ⁻>1000 mg/L, add mercuric sulfate masking agent (HgSO ₄: Cl ⁻=10:1), or switch to low chloride specialized reagents;
3. Standardize the operating process: strictly control the digestion time according to the instructions (usually 15-20 minutes at 165 ℃), cool to room temperature before color comparison;
4. Regular calibration: Draw a new standard curve using a standard potassium hydrogen phthalate solution every week, R²≥0.999；
5. Vessel maintenance: Soak the colorimetric dish in 10% nitric acid and rinse with ultrapure water to avoid cross contamination.
In addition, it is recommended to establish an internal quality control chart to monitor long-term testing stability. Through systematic quality control,COD chemical oxygen demand rapid analyzerIt can meet the accuracy requirements of standard methods such as HJ/T 399-2007 while ensuring speed.
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