Q1: How to quickly understand the approximate concentration range of COD samples?
Answer: Take 2.5mL of COD water sample, add D/E consumables and shake it continuously. You can judge the approximate concentration of COD in the water sample by color. The closer the color is to blue-green, the greater the concentration is, and the closer the color is to the blank sample, the lower the concentration is.
Q2: What should be paid attention to when the COD concentration is too high and needs to be diluted?
Answer: When the COD water sample needs to be diluted, the principle is to take as much mother liquor as possible. When the dilution multiple is small (about 5 times dilution), the mother liquor is generally taken. More than 50mL, when the dilution multiple is large (such as dozens or hundreds of times), the mother liquor volume must not be less than 10mL (to avoid large human errors caused by dilution). When testing, the original water sample should be diluted in a volumetric flask and then the dilution liquid should be taken for testing. The concentration value of the original water sample is the measured value of the dilution liquid multiplied by the dilution multiple. It should be noted that when diluting, the water sample must be shaken before sampling, and the supernatant cannot be used.
Q3: How to determine whether the chloride ion content in the water sample exceeds 1000mg/L?
Answer: Add 2mL of water sample to the reaction tube, add 0.5mL of silver nitrate solution (17g/L), mix thoroughly, add two drops of potassium chromate indicator solution (50g/L), and shake well. If the solution turns red, it indicates that the chloride ion content is less than 1000mg/L. If it is still yellow, it indicates that the chloride ion content is greater than 1000mg/L.
Q4: When the chloride ion concentration in the COD water sample is very high, how to perform dilution detection?
Answer: You can choose the dilution strategy according to the following situations: 1. After adding D/E consumables, it becomes turbid, or directly changes color or becomes milky. At this time, the original water sample generally needs to be diluted more than 10 times to eliminate interference; 2. After digestion, it becomes turbid when 2.5ml of dilution water is added. At this time, the original water sample generally needs to be diluted more than 5 times to eliminate interference; 3. The digestion colorimetric solution becomes turbid when it is finally poured into the cuvette. At this time, the original water sample generally needs to be diluted about 3 times to eliminate interference. The COD concentration of the original water sample is the measured value multiplied by the dilution multiple. When using the instrument of Lianhua Technology LHOS operating system, the dilution multiple can be entered to directly display the COD value of the water sample.
Q5: The COD value in the water sample is not high, but the chloride ion concentration is very high. How to detect this water sample?
Answer: In this case, the method of diluting the water sample cannot be considered, because the COD value is already very low, and further dilution of the test result may produce a large error. When testing this kind of water sample, you can choose to add mercuric sulfate to mask the chloride ions. Theoretically, 0.04g of mercuric sulfate can mask 1000mg/L of chloride ions, but the larger the amount added, the better. Too much will interfere with the determination. Or take 10mL of water sample and add it to a test tube, add solid silver nitrate until no precipitation is produced, and take the supernatant for the experiment. A simpler and more effective way is to choose low-range high-chlorine resistant consumables.
Q6: The theoretical concentration of the sample is very high, why can’t the instrument measure the value or the measured value is particularly low?
Answer: COD masking agent may exist in the water sample. The main component of the masking agent is sodium chlorate. Under acidic and high temperature conditions, sodium chlorate can oxidize the reducing substances in the water, resulting in low potassium dichromate consumption, making the final result lower than the actual value. Sodium chlorate has no COD removal effect. It is only during the test that the reducing substances are oxidized in a strong acid environment, resulting in the inability of the dichromate method to be measured normally. Sodium chlorate does not have a COD removal effect under normal conditions.
Q7: How to detect water samples containing COD masking agent sodium chlorate?
Answer: Take 2.5mL of water sample and add 4.8mL of E consumables. Be careful not to do it too fast (to prevent splashing). At this time, the solution turns yellow, bubbles are generated, and yellow gas CLO2 is released. Do not shake the reaction tube at this time, as splashing is likely to occur. After leaving it for 20 minutes, shake it again when fewer bubbles are generated. After shaking it, put the reaction tube on the digester and digest it at 100 degrees for 2 hours (it can be slightly extended). When the color of the solution turns colorless and no bubbles are generated, add 0.7mL of D consumables, shake it and digest it at 165 degrees. The rest is operated according to the normal detection process. Note that all operations must be performed in a fume hood. Do not seal the digestion, because the water sample will release gas and generate high pressure during the sealed digestion process, which may cause the tube to burst.
Q8: COD water samples may contain volatile substances or alcohol-like substances. How to detect it?
Answer: First, make a preliminary judgment on the COD concentration. If the concentration is low, you can try sealed digestion. If there is a lot of volatile gas in the water sample and you are worried about the risk of bursting the pipe, you can add consumables first to see the color. If the concentration is high, you can dilute it before conducting the experiment.
Q9: What is the reason for the blank to appear green during COD testing?
Answer: 1. Check whether the consumables are expired. Expired consumables will cause the consumables oxidation rate to decrease, and the blank may appear green. 2. Check whether sulfuric acid is used when preparing consumables. If hydrochloric acid is used to prepare consumables, the chloride ions in the hydrochloric acid will be oxidized, causing the blank to turn green. 3. Detergents or alcohol are used when cleaning utensils. Both of these are organic matter. If the utensils are not cleaned after use, the blank will also turn green.
Q10: What is the reason for negative values when COD is detected using a photometric instrument? How to solve it?
Answer: 1. If the water sample and blank are mixed up, the order needs to be reversed; 2. If the digestion tube and other utensils are not clean, they can be soaked in dilute acid and then cleaned with a large amount of distilled water; 3. The distilled water may be contaminated and needs to be replaced; 4. When using low-range consumables, improper operation will pour the precipitate into the colorimetric dish. It is necessary to wait until the precipitate sinks to the bottom before colorimetric comparison; 5. The COD concentration of the water sample is extremely low and exceeds the lower limit of the range. A slight error in operation will result in a negative number; 6. Before the experiment, the beakers, pipettes, and colorimetric dishes used in the experiment need to be cleaned; when absorbing different water samples, the pipette must be cleaned before absorbing other water samples; 7. The instrument fails and needs to be returned to the factory for repair.
Q11: COD water samples are digested using open tubes. What is the situation when some tubes are hanging on the wall?
Answer: The reaction tube may not be cleaned well. Soak it in dilute sulfuric acid to clean it.
Q12: If the digestion tube breaks during COD digestion and the liquid spills into the digestion hole, how should it be handled?
Answer: Turn off the power of the instrument and unplug it, and clean the acid in the digestion hole with a wet rag. The digestion hole is sealed and the acid will not seep into the instrument. If it is really difficult to wipe it clean, you can use a dropper to drip a small amount of water into the digestion hole, but not more than half, clean it with a brush and then suck it out with a straw, wipe it dry and then power it on for testing without short circuit and continue to use it.
Q13: Why does the COD water sample turn black when silver nitrate is added?
Answer: There may be halogen elements in the water sample, which will generate silver halide precipitation, which will be oxidized and blackened in the air. Including silver chloride precipitation is white in the early stage, and it will turn black over time.
Q14: How to deal with the waste liquid after the COD experiment?
Answer: 1. Outsourcing: There are special waste liquid treatment companies in each place, which can be entrusted to them for treatment. The waste liquid contains hexavalent chromium, sulfate, and silver ions, and the water sample is acidic. 2. Self-treatment: Add NaHSO3 crystals in small amounts in batches until the solution changes from yellow to green, stirring while adding (if using an oxidation-reduction photoelectric meter for determination, it is very convenient); if there are other metals besides Cr, after confirming the conversion of Cr (VI), treat it as waste liquid containing heavy metals; add 5% NaOH solution and adjust the pH to 7.5-8.5 (note that the precipitate will dissolve again if the pH is too high); leave it overnight, filter out the precipitate and store it properly (if the filtrate is yellow, it must be reduced again); perform a full chromium test on the filtrate, and confirm that the filtrate does not contain chromium before it can be discharged.
The above are the common problems and solutions related to the COD experiment when you use Lianhua Technology’s water quality testing instrument. If you encounter other difficult problems in the COD experiment, please leave a message and ask questions. Our after-sales service team will answer you in detail!
Post time: Feb-17-2025
