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Determination of COD
Author：hngoob Renew time：2020-02-22
Chemical oxygen demand (COD) refers to the amount of oxidant consumed when water is treated with strong oxidant under certain conditions. COD reflects the degree of water pollution by reducing substances. Reductive substances in water include organic matter, nitrite, ferrous salt, sulfide, etc., so COD determination can reflect the content of organic matter in water.
First, the principle of potassium dichromate method (CODCr) In strong acidic solution, add excess potassium dichromate standard solution accurately, heat and reflux, oxidize reducing substances (mainly organic matters) in water sample, use ferriferol as indicator for excess potassium dichromate, drip back with ammonium ferrous sulfate standard solution, and calculate chemical oxygen demand of water sample according to consumed potassium dichromate standard solution amount.
II. Instruments
1,500 ml all-glass reflux device.
2. Heating device (electric furnace).
3, 25ml or 50ml acid burette, conical flask, pipette, volumetric flask, etc.
Iii. reagents
1. Potassium dichromate standard solution (C1/6K2Cr2O7); According to take in advance at 120℃ drying 2h benchmark or high quality pure potassium dichromate 12.258g dissolved in water, into a 1000ml volumetric flask, diluted to the standard line, shake well.
2. Ferrous indicator liquid test: weigh 1.485g of phenanthroline (C12H8N2•H2O) and 0.695g of ferrous sulfate (FeSO4•7H2O) in water, dilute to 100ml, and store in a brown bottle.
3. Standard solution of ammonium ferrous sulfate (C (NH4) 2Fe (SO4) 26H2O): 39.5g ammonium ferrous sulfate is weighed and dissolved in water, 20ml concentrated sulfuric acid is slowly added while stirring, cooled and transferred into a 1000ml volumetric flask, diluted with water to the marking line, and shaken well. Before use, use potassium dichromate standard solution for calibration. Calibration method: accurately absorb 10.00ml potassium dichromate standard solution in a 500ml conical flask, add water to dilute to about 110ml, slowly add 30ml concentrated sulfuric acid, and mix well. After cooling, add 3 drops of test ferrous spirit indicator solution (about 0.15ml) and titrate with ammonium ferrous sulfate solution. The color of the solution changes from yellow to cyan to reddish brown. C=0.2500×10.00/V In the formula: C-concentration of ammonium ferrous sulfate standard solution (mol/L); V-dosage of ammonium ferrous sulfate standard solution (ml).
4. sulfuric acid-silver sulfate solution: add 5g of silver sulfate to 500ml of concentrated sulfuric acid. Place for 1-2 days and shake occasionally to dissolve it.
5. Mercury sulfate: crystal or powder. Four, determination steps
1. Take 20.00ml of evenly mixed water sample (or a proper amount of water sample diluted to 20.00ml) and place it in a 250ml grinding-mouth reflux conical flask. Accurately add 10.00ml of potassium dichromate standard solution and several small glass beads or zeolites. Connect the grinding-mouth reflux condenser tube. Slowly add 30ml of sulfuric acid-silver sulfate solution from the upper opening of the condenser tube. Gently shake the conical flask to mix the solution evenly and heat the reflux for 2 hours (counting from the beginning of boiling). For waste water samples with high chemical oxygen demand, first take 1/10 of the volume of waste water samples and reagents required for the above operation in a 15×150mm hard glass test tube, shake well, and observe whether it turns green after heating. If the solution is green, the sampling amount of waste water should be appropriately reduced until the solution does not turn green, thus determining the volume to be taken for waste water sample analysis. When diluting, the amount of waste water sample taken shall not be less than 5ml. If the chemical oxygen demand is very high, the waste water sample shall be diluted several times. When the chloride ion content in wastewater exceeds 30mg/L, 0.4g of mercury sulfate should be added to the reflux conical flask first, and then 20.00ml of wastewater (or a proper amount of wastewater diluted to 20.00ml) should be added and shaken up.
2. After cooling, rinse the condenser tube wall with 90ml of water and remove the conical flask. The total volume of the solution shall not be less than 140ml, otherwise the titration end point is not obvious due to too much acidity.
3. After the solution is cooled again, add 3 drops of ferrous sulfate indicator solution and titrate with ammonium ferrous sulfate standard solution. The color of the solution changes from yellow to cyan to reddish brown. Record the dosage of ammonium ferrous sulfate standard solution.
4, determination of water samples at the same time, take 20.00ml of distilled water, according to the same operation steps for blank test. Record the dosage of ammonium ferrous sulfate standard solution when the determination is blank.
V. calculation: In the formula:
C-concentration of ammonium ferrous sulfate standard solution (mol/L);
VO--- dosage of ammonium ferrous sulfate standard solution when titration blank (mL);
V1--- the dosage of ammonium ferrous sulfate standard solution when titrating water sample (mL);
V---- volume of water sample (ml); 8-molar mass of oxygen (1/2O) (g/mol). Matters needing attention
1. The maximum amount of chloride ions complexed with 0.4g mercury sulfate can reach 40mg. If 20.00ml of water sample is taken, the water sample with a maximum chloride ion concentration of 2000mg/L can be complexed. If the concentration of chloride ion is low, mercury sulfate can also be added less to keep mercury sulfate: chloride ion = 10: 1 (w/w). If a small amount of mercuric chloride precipitates, the determination will not be affected.
2, water sample volume can be in the range of 10.00-50.00ml, but reagent dosage and concentration need to be adjusted according to the following table, also can get satisfactory results.