Frequent users often encounter issues during COD (Chemical Oxygen Demand) measurements. These problems typically include:
(1) Large measurement errors
(2) Poor repeatability of results
(3) Negative values displayed in the readings.
To address these issues, first check if the instrument is functioning properly. Refer to the manual for inspection procedures and verify the performance of key components of the analyzer. Additionally, ensure that the calibration curve used is accurate and up-to-date. If possible, manually input your own calibration curve to improve accuracy.
A more practical approach:
Most of these problems are not caused by the instrument itself but rather by improper operation. To help users solve these issues effectively, here are some essential operational tips:
(1) Pay attention to the direction of the colorimetric tube. When inserting it into the instrument, make sure the white oval label on the tube is facing the operator. This ensures consistent light transmission and accurate readings.
(2) Before measuring, carefully wipe the outside of the colorimetric tube with a clean, dry filter paper or tissue. After digestion (typically 1–2 minutes), rinse the tube with distilled water and wipe the outer surface thoroughly to avoid contamination.
(3) Colorimetric tubes can introduce significant errors, especially when measuring low COD values (<20 mg/L). A worn tube may have an absolute error of ±15 mg/L, while a new one has an error of ±5 mg/L. To minimize this, wash all tubes with distilled water, add 5 mL of distilled water, wipe them dry, and use one as a blank (recorded as 0 mg/L). Compare the remaining tubes—ensure the difference between readings is less than ±5 mg/L. Always use the same set of tubes for consistency.
(4) Precipitation after cooling can affect results significantly. If time allows, let the tube sit undisturbed for 1–2 hours after digestion. Wipe the outer surface with a wet filter paper first, then dry it before measuring. For faster results, remove the tube after digestion, cool it in cold water for 2 minutes, wipe it immediately, and measure within 5 minutes. This method also yields reliable results.
(5) Proper reagent handling is crucial. Add reagent No. 1 to the sample and shake well. Then add reagents No. 2 and No. 3. Be careful when adding reagent No. 3—it should be added vertically to prevent it from flowing down the side of the tube. Shake the mixture thoroughly until fully dissolved.
(6) If chloride ion concentration is low, you can skip reagent No. 1 and create a low-range calibration curve. This can improve repeatability and reduce variability in results.
(7) Clean the pipette and test tubes before each measurement. Any residue on the glass can solidify during digestion, leading to inaccurate readings. Ensure the tube lid is tightly sealed. An improperly sealed lid can cause air leakage, resulting in unstable or incorrect data. Check that the liquid levels in all tubes are similar after digestion. A significant drop in level indicates gas leakage, which must be addressed before proceeding.
By following these steps, users can significantly improve the accuracy and reliability of their COD measurements. Consistent practice and attention to detail will lead to better results and greater confidence in the process.
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