Preparation of COD powder pack reagents(Skip this section if using ready-made reagents)
◆ COD reagent (A) 100 samples: pour the COD reagent (A) powder packet into a 250ml beaker, add 80ml of distilled water to the beaker, and stir constantlySlowly add 5ml of pure concentrated sulfuric acid along the wall of the cup, dissolve, cool and put into a 100ml reagent bottle for later use;
◆ COD reagent (B) 100 samples: pour the COD reagent (B) powder packet into a 250ml beaker, add 80ml of distilled water to the beaker, and stir constantlySlowly add 5ml of pure concentrated sulfuric acid along the wall of the cup, dissolve, cool and put into a 100ml reagent bottle for later use;
◆ COD reagent (C) 100 samples: take the whole package of COD reagent (C), crush it first if there is a lump, put it in a 500ml fine-mouth brown reagent bottle, and add 250mlAnalyze pure concentrated sulfuric acid, stir until completely dissolved or place in a dark place to dissolve (slow dissolution) for later use.
Fabrication of COD precast reagents (Skip this section if using ready-made reagents)
◆ 0-200mg/L COD precast reagent production: take a number of clean and dry digestion colorimetric tubes and put them on the test tube rack, and add 2.5mL respectively to prepareCOD reagent (C) and 0.8mL of prepared COD reagent (A), cover, tighten, shake well, and store at room temperature and in a dark place for later use.
◆ 200-20000mg/L COD precast reagent: take several clean and dry digestion colorimetric tubes and put them on the test tube rack, and add 2.5mL respectively to prepareCOD reagent (C) and 0.8mL of prepared COD reagent (B), cover, tighten, shake well, and store at room temperature and in a dark place for later use.
COD operation process
Digestion device preheating:Turn on the power of the digester, select the COD digestion mode (165°C, 20min), press confirm (or start), and the digester will automatically heat up and preheat;
Concentration prediction:Before testing the water sample, the COD value of the water sample is first predicted, and the COD prereagent corresponding to the range is selected for operation, for example, predictionCOD concentration values between 0-200mg/L require the selection of pre-made reagents in the range of 0-200mg/L; If the predicted COD concentration value is 200-12000mg/L, it is chosenChoose pre-made reagents in the range of 200-2000mg/L.
Remark:The shielding ability of this reagent to chloride ions is: less than or equal to 1000mg/L. Beyond this concentration, dilute below 1000mg/L before proceeding.
1. When the predicted value is in the range of 0-200mg/L
Precast reagents in the range of 0-200mg/L are selected; Digestion time: 20min; Analytical procedure: CODLR.
2. When the predicted value is in the range of 200 - 2000mg/L
Choose precast reagents in the range of 200-20000mg/L; Digestion time: 20min; Analytical Procedure:CODMR pre。
3. When the predicted value is in the range of 2000-20000mg/L
Choose precast reagents in the range of 200-20000mg/L; Digestion time: 20min; Analytical Procedure:CODHR Pre。
4. When the predicted value exceeds 15000 mg/L
The water sample needs to be diluted to the corresponding concentration value in sections 1-3 above, and then manipulated according to the corresponding range, and the determination result is multiplied by the dilution factor.
5. Test steps on the computer
(1) Select the corresponding COD analysis program and press Confirm to enter the blank calibration interface
(2) Wipe the blank sample clean, put it into the colorimetric cell and press the zero point
(3) Wipe the water sample clean, put it into the colorimetric pool and measure
6. Precautions
◆ If there is COD, the treated water sample will show green, and the darker the color, the higher the concentration value of the water sample.
◆ After adding the water sample, the test tube will be hot, and when the cap is shaken, it is necessary to avoid scalding (you can wrap the cap with a wiping cloth), tighten the cap, and shake all the sediment at the bottom.
◆ Prefabricated reagent tubes of different ranges cannot be mixed, and the same range of reagents must be used for blanks and samples.
◆ Pay attention to cross-contamination, use different measuring utensils for different liquids (pay attention to changing the tip when using a pipette).
◆ Use a suitable pipette to take the liquid, do not use a 5mL pipette to remove 2mL and less liquid, less than or equal to 2mL of liquid, it is recommendedPipette 2 times with a 1mL pipette.
◆ When using a pipette, it is necessary to calibrate the pipette frequently (it is recommended to do so once a week), and pay attention to replacing the tip during use.
◆ The protective cover must be covered during the heating and digestion process of the digester. During the digestion process, beware of scalding, pay attention to the high temperature area of the digester.
◆ After the digestion tube is taken out from the digester, take one piece at a time, it is forbidden to take out multiple tubes at the same time, pay attention to the high temperature, and prohibit shaking.
◆ The cooling process can be natural cooling or water cooling, but the temperature must be below 120°C for water cooling.
◆ The digested COD sample tube must be clear and translucent (colored, but must not be turbid, no suspended solids, precipitated at the bottom, and shaken)
7. Removal of chloride ion interference
Chloride is the main interference in the determination of COD, and COD prereagents are able to shield chloride ions up to 1000mg/L in water samples when chloride ions are highAt this concentration range, chloride ions can be diluted below 1000mg/L by appropriate dilution before testing.
If the dilution results in a low COD concentration that affects the accuracy of the test (the diluted COD concentration is recommended to be no less than 50mg/L), before adding the sample,Chloride ions were shielded with mercury sulfate (HgSO4) in the water sample, and 0.5g of mercury sulfate per 10mL of water sample was added to a beaker and stirred with a glass rodAfter 3-5 minutes and a few minutes of standing, the supernatant is taken for testing.
The shielding ability of COD reagents against chloride ions can be referred to the interference and levels in the table below.
Whether the chloride ion exceeds 1000mg/L
Add 2.00mL of specimen to the tube, then 0.5mL of silver nitrate solution (Note 1), mix well, and finally add 2 drops of potassium chromate solution (Note 2),Shake well, if the solution turns red, the chloride ion solution is less than 1000mg/L; If it is still yellow, the chloride concentration is higher than 1000mg/L. You can also press GB/T 11896Methods: The concentration of chloride ions in water samples was determined.
◆ Note 1: Silver nitrate solution: c(AgNO3)=0.1mol/L. Dissolve 17.1g of silver nitrate in 1000mL of distilled water.
◆ Note 2: Potassium chromate solution: ρ(K2CrO4)=50g/L. Dissolve 5.0g of potassium chromate in a small amount of water, add silver nitrate solution dropwise until a red precipitate is formed,Shake well, let stand for 12h, filter and dilute the filtrate to 100mL with water.
