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potassium dichromate standard method First
the principle: In the water samples by adding a certain amount of potassium dichromate and catalyst silver sulfate, heated in a strong acidic medium refluxed for some time, part of potassium dichromate was oxidizable substances in water samples The remaining potassium dichromate is titrated with ferrous ammonium sulfate and the COD is calculated from the amount of potassium dichromate consumed.
Second, the instrument 1 1.250mL all-glass reflux device 2. Heating device (electric furnace) 3.25mL or 50mL acid burette, conical flask, pipette, volumetric flask.
Third, the reagent 1. Potassium dichromate standard solution (c1 / 6K2Cr2O7 = 0.2500mol / L) 2. Try ferrous spirit indicator solution 3. Ferrous ammonium sulfate standard solution [c (NH4) 2Fe (SO4) 2 · 6H2O ≈ 0.1mol / L] (calibration before use) 4. Sulfuric acid - silver sulfate solution
Fourth, the determination of ammonium ferrous sulfate calibration: Accurately draw 10.00mL potassium dichromate standard solution in 500mL conical flask, diluted with water to about 110mL, 30mL concentrated sulfuric acid slowly, shake, add 3 drops after the test Ferrous spirit indicator solution (about 0.15mL), titration with ammonium ferrous sulfate solution, the color of the solution from yellow through the blue-green to reddish brown is the end.
V. Determination: Take 20mL water samples (if necessary, take less water to take 20 or dilute and then take), add 10mL of potassium dichromate, plug back the device, then add 30mL silver sulfate sulfate, heated to reflux 2h After cooling, Rinse the condenser tube with 90.00 mL of water and remove the conical flask. After the solution is cooled again, add 3 drops of ferrous spirit indicator solution and titrate with ferrous ammonium sulfate standard solution. The color of the solution changes from yellow to blue-brown to red-brown That is the end, record the amount of ammonium ferrous sulfate standard solution.Watch the water at the same time, take 20.00mL double distilled water, according to the same procedure for a blank experiment. Record the amount of ammonium ferrous sulfate standard solution titration blank.
Sixth, calculation CODCr (O2, mg / L) = [8 × 1000 (V0-V1) · C] / V Seven, matters needing attention 1, the use of 0.4g mercury sulfate complex chloride ion up to 40mg, With 20.00mL water samples, that is, the highest concentration of 2000mg / L chloride ion concentration of water samples. If the chloride ion concentration is lower, less mercury sulfate may be added to maintain the mercury sulfate: chloride ion = 10: 1 (W / W). If a small amount of mercury chloride precipitation, does not affect the determination. 2, the method for the determination of COD in the range of 50-500mg / L. For chemical oxygen demand of less than 50mg / L of water samples, should use 0.0250mol / L potassium dichromate standard solution. Drop back with 0.01mol / L ammonium ferrous sulfate standard solution. For COD greater than 500mg / L of water samples should be measured before dilution. 3, water samples heated reflux, the remaining amount of potassium dichromate solution should be added to the amount of 1 / 5-4 / 5 is appropriate. 4, with potassium hydrogen phthalate standard solution to check the quality of reagents and operating techniques, the theoretical CODCr per gram of potassium hydrogen phthalate is 1.176g, so dissolved 0.4251g potassium hydrogen phthalate (HOOCC6H4COOK) in Distilled water, transferred to 1000mL volumetric flask, diluted with distilled water to the mark, making it 500mg / L CODcr standard solution. When used with the new. 5, CODCr measurement results should retain three significant figures. 6, each experiment, ammonium ferrous sulfate standard titration solution should be calibrated, especially at room temperature when the concentration changes. (You can also add 10.0ml potassium dichromate standard solution to the blank after titration and titrate with ferrous ammonium sulfate to the end point.)
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