⑴ Potassium dichromate standard method 1. Principle: Add a certain amount of potassium dichromate and catalyst silver sulfate to the water sample, heat and reflux in a strong acidic medium for a certain period of time. Part of the potassium dichromate is reduced by oxidizable substances in the water sample. Titrate the remaining potassium dichromate with ammonium and calculate the COD value based on the amount of potassium dichromate consumed.

2、 Instrument 1: 1.250mL all glass reflux device 2. Heating device (electric furnace) 3.25mL or 50mL acid burette, conical flask, pipette, volumetric flask, etc.

3、 Reagent 1. Potassium dichromate standard solution (c1/6K2Cr2O7=0.2500mol/L) 2. Ferrous indicator solution 3. Ammonium standard solution [c (NH4) 2Fe (SO4) 2 · 6H2O ≈ 0.1mol/L] (calibrated before use) 4. Sulfuric acid silver sulfate solution

4、 Measurement steps * Ammonium calibration: Accurately draw 10.00mL of potassium dichromate standard solution into a 500mL conical flask, dilute with water to about 110mL, slowly add 30mL of concentrated sulfuric acid, shake well, cool down, add 3 drops of ferrous indicator solution (about 0.15mL), titrate with * ammonium solution, and the color of the solution changes from yellow to blue-green to reddish brown, which is the endpoint.

5、 Measurement: Take 20mL of water sample (if necessary, add less water to 20 or dilute before taking), add 10mL of potassium dichromate, plug in the reflux device, add 30mL of silver sulfate, heat and reflux for 2 hours, cool down, rinse the condenser wall with 90.00mL of water, and remove the conical flask After the solution cools down again, add 3 drops of ferrous indicator solution and titrate with ammonium standard solution. The color of the solution changes from yellow to blue-green to reddish brown, which is the endpoint. Record the amount of ammonium standard solution used At the same time as measuring the water sample, take 20.00mL of distilled water and perform a blank experiment using the same procedure. Record the amount of ammonium standard solution used for titration blank.