COD: Chemical oxygen demand, also known as chemical oxygen demand, refers to the dichromic acid consumed by soluble substances and suspended matter in water samples when subjected to oxidation treatment with potassium dichromate under certain conditions. The mass concentration of oxygen corresponding to the salt, expressed in mg / L, is an important indicator of the degree of water pollution. The smaller the value, the lighter the water pollution.

HJ828-2017 《Determination of chemical oxygen demand of water quality by dichromate method》

This standard specifies the dichromate method for the determination of chemical oxygen demand in water.

This standard applies to the determination of chemical oxygen demand in surface water, domestic sewage and industrial wastewater. This standard does not apply to chlorine

Determination of chemical oxygen demand in water with a compound concentration greater than 1000 mg / L (after dilution).

When the sampling volume is 10.0ml, the detection limit of this method is 4mg / L, and the lower limit of determination is 16mg / L. Undiluted

The upper limit for determination of water samples is 700mg / L. When the limit is exceeded, the determination should be made after dilution.

A known amount of potassium dichromate solution was added to the water sample, and the silver salt was used as a catalyst in a strong acid medium. After boiling and refluxing,

Titrate was used as an indicator to titrate the unreduced potassium dichromate in water samples with ferrous ammonium sulfate.

Calculate the mass concentration of oxygen consumed.

Take 10.0 ml of water sample in an Erlenmeyer flask, add mercury sulfate solution, 5.00 ml of potassium dichromate standard solution and several explosion-proof boiling glass beads in order, and shake well. Mercury sulfate solution is added at a mass ratio of m [HgSO4]: m [Cl-] ≥20: 1, and the maximum amount is 2 ml.

Connect the conical flask to the lower end of the condensing tube of the reflux device, and slowly add 15ml of silver sulfate-sulfuric acid solution from the upper end of the condensing tube to prevent the escape of low-boiling organics. Rotate the conical flask to mix it evenly. Maintain a slight boiling reflux for 2 h since the solution began to boil. In the case of a water-cooled device, condensate should be passed through before adding the silver sulfate-sulfuric acid solution.

After cooling under reflux, add 45 ml of water from the upper end of the condenser tube to rinse the condenser tube to make the solution volume about 70 ml. Remove the cone

Shaped bottle. After the solution was cooled to room temperature, 3 drops of a test solution of ferrous iron were added and titrated with a standard solution of ferrous ammonium sulfate. The color of the solution changed from yellow to blue-green to red-brown. Note the consumption volume V1 of the standard solution of ferrous ammonium sulfate.

Note: When the sample concentration is low, the sampling volume can be increased appropriately.

advantage:

Follow the national standard scheme, the operation is intuitive;

cheap price

Disadvantages:

High-power heating wire heating, large power consumption;

Direct water cooling, wasting precious fresh water resources;

The heating power is not easy to control, and the temperature rises too quickly and easily overboils;

Can't dissolve regularly, need to be watched manually in real time