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1. Can COD blanks be reused?
Multiple experiments using the same curve can reuse the same blank. This blank must be kept in the dark. Measure the absorbance of the blank over time to check that the results are stable.
2. Can the sample be diluted to measure COD?
OK. Dilute the sample, then add the appropriate amount of diluted sample to the bottle. Multiply the final assay result by your dilution factor.
3. Reasons for the green color in COD experiments
Whether sulfuric acid is used in the preparation of the reagent, if the reagent is prepared with hydrochloric acid, the chloride ion in the hydrochloric acid will be oxidized, causing the blank to become a chloride ion;
If the concentration of the sample to be tested is high, if the detection exceeds the range of the reagent, re-test after appropriate dilution;
Use detergent or alcohol when cleaning the utensils, both of which belong to organic substances. If the utensils are not cleaned after use, the blank will turn green.
4. Reasons and solutions for negative values in COD experiments
If the water sample and the blank are mixed up, the order needs to be changed;
Distilled water may be contaminated, and the distilled water needs to be replaced;
Digestion tubes and other utensils are not clean, they can be soaked in dilute acid and then washed with a large amount of distilled water;
Improper operation when using low-range reagents, pour the precipitate into the cuvette, and wait for the precipitate to sink to the bottom before colorimetry;
The COD concentration of the water sample is extremely low, exceeding the lower limit of the range, and a negative number will appear if there is a slight error in the operation;
Before the experiment, the beakers, pipettes, cuvettes, etc. used in the experiment should be cleaned; when drawing different water samples, such as using a pipette, be sure to clean the pipette before drawing other water samples; if using a pipette The gun needs to be replaced.
5. How to verify the accuracy of COD test results?
Use a COD standard solution that falls within the concentration range you are using. Test this standard solution as if it were your sample. If your test results are within the margin of error, you can be confident that your equipment and reagents are working properly and that your test is operating accurately.
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