Frequent users often appear during the operation of COD measurement:
(1) The error is too large
(2) poor repeatability
(3) The measurement result shows a negative value.
For these problems, it is necessary to check whether the instrument detection is normal. Refer to the inspection steps in the manual for checking the core components of the analyzer. Secondly, if you want to see if the curve is still in reality E10, you can enter the curve to be marked. If you have the conditions, you can also enter the calibration curve yourself.
A further solution:
Most of these phenomena are not caused by the instrument, mainly due to the operation. In order to facilitate the majority of users to solve the above problems, the operation skills and methods are summarized as follows:
(1) The direction of the colorimetric tube. When the colorimetric tube is inserted into the colorimetric aperture, it must be ensured that the colorimetric white oval label faces the operator.
(2) Carefully wipe the outside of the colorimetric tube before measuring. After dissolving for 1 min - 2 min, whether to rinse the colorimetric tube with water, carefully wipe the outside of the tube with dry filter paper or paper towel.
(3) Error caused by the colorimetric tube. A more wearable colorimetric tube will produce an absolute error of ±15 mg/L, and a newer tube with an absolute error of ±5 mg/L. In the process of measuring low COD values ​​(less than 20 mg/L), the colorimetric tube may cause a large error. The solution is: wash all the colorimetric tubes with distilled water, add 5mL of distilled water, carefully wipe the outside, take one as a blank, record the COD value as 0, then measure the COD values ​​of other colorimetric tubes and compare them. . The difference between the measured values ​​is less than ±5 mg/L, and the same set of test tubes are used for each measurement, which can reduce the error.
(4) Precipitation during the cooling of the water sample also seriously affects the measurement results. A: When you are not in a hurry to measure, remove the colorimetric tube and leave it for 1-2 minutes after the digestion. First wipe the outside of the tube with a wet filter paper and then dry the filter paper. Then place it at rest (do not shake) 1H-2H and measure again, you can get more accurate results. B: When rushing to measure, the colorimetric tube can be taken out for 1 min after the digestion, and then chilled with cold water for about 2 minutes. Immediately after cleaning with filter paper, the measurement is completed within 5 minutes, and good results can be obtained.
(5) Operation of reagent addition. Add reagent No. 1 to the water sample and shake well. Add reagents Nos. 2 and 3. No. 3 must be added vertically without flowing into the water sample along the wall. Shake well and dissolve.
(6) If the chloride ion content is low, the number 1 can be omitted and a low-range curve can be made separately, so that the repeatability is better.
(7) Wipe the pipe clean before dissolving. If there is dirt on the glass, it is likely to be solidified on the glass during digestion, resulting in inaccurate readings. Make sure the lid of the tube is tight. If the cover is not tightly closed, it may cause air leakage, resulting in a large or unstable data deviation. You can see if the liquid level of each tube is at a similar height when the digestion is completed. If there is a significant decrease in the liquid level of the pipe, it means that the gas leakage has occurred.
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