Do you know what reagents, calibration methods and principles are needed to measure COD?

When measuring COD, some reagents need to be used, including potassium dichromate standard solution, ferrous spirit indicator and ferrous ammonium sulfate standard solution, etc., which are mixed and used in a certain proportion. So do you know the calibration method and principle of measuring cod?

1. Potassium dichromate standard solution: pre-dry 12.258g standard product or high-quality pure potassium dichromate at 120℃ for 2h, dissolve in water, dilute it in a 1000mL volumetric flask, and move it to the marked line, and shake well.

2. Test Ferroline Indicator: Dissolve 1.485 g of phenanthroline and 0.695 g of ferrous sulfate in water, dilute it to 100 ml, and store in a brown bottle.

3. Standard solution of ferrous ammonium sulfate: Dissolve 39.5g of ferrous ammonium sulfate in water, stir and slowly add 20ml of concentrated sulfuric acid, cool to 1000ml volumetric flask, add the diluent to the pipeline and shake well. Before use, calibrate with potassium dichromate standard solution. What is the calibration method for measuring COD?

Accurately absorb 10 ml of standard potassium dichromate solution into a 500 ml conical flask and dilute to about 110 ml. Then 30 ml of concentrated sulfuric acid was added slowly and mixed well. After cooling, add 3 drops of 0.15 ml test solution, titrate with ferrous ammonium sulfate, the color of the solution is from yellow to blue-green to red-brown as the end point. What is the principle of measuring COD by the potassium dichromate method?

In the highly acidic solution, accurately add excess potassium dichromate standard solution, heat to reflux, and oxidize by water reducing agent. Excess potassium dichromate is indicated by the ferrous essential oil, and ferrous ammonium sulfate is used as standard The solution was dripped, and the chemical oxygen demand (cod) was calculated by the consumption of potassium dichromate standard solution.

Cr₂O₇+14H+6e 2Cr+7H₂O (oxidation of water samples)

Cr₂O₇+14H+6Fe 2Cr+6Fe+7H₂O (titration)

Fe+ test ferrite (indicator) → reddish-brown (end point)