1. Introduction

Chemical oxygen demand (COD) is a chemical method used to measure the amount of reducing substances in a water sample that require oxidation. It represents the oxygen equivalent of substances (generally organic matter) that can be oxidized by strong oxidants in wastewater, wastewater treatment plant effluent, and polluted water. Therefore, COD is often used as an indicator of the amount of organic matter in water; a higher COD indicates more severe organic pollution. In the study of river pollution and the properties of industrial wastewater, as well as the operation and management of wastewater treatment plants, it is an important and relatively quick-to-measure parameter of organic pollution, and one of the most important control indicators in my country's total water pollutant discharge control plan. Measurement methods include the dichromate method, the potassium permanganate method, and spectrophotometry. This paper uses the dichromate method, according to the standard "HJ 828-2017 Water Quality—Determination of Chemical Oxygen Demand—Dichromate Method," to determine the COD values ​​of wastewater from the secondary sedimentation tank and equalization tank of a factory.

2. Instruments and Reagents

2.1 Instruments

JH-T6 fully automatic potentiometric titrator, microwave digester, platinum composite electrode, silver composite electrode

2.2 Reagents

Ferrous ammonium sulfate solution (0.05mol/L), mercuric sulfate solution (200g/L), potassium dichromate solution (0.25mol/L), silver sulfate-sulfuric acid solution (10g/L), silver nitrate solution (0.1mol/L), potassium hydrogen phthalate standard solution (2.0824mol/L)

3. Experimental Methods

3.1 Experimental Procedures

(1) Standardization of Ferrous Ammonium Sulfate

Pipette 5mL of potassium dichromate solution (0.25mol/L), add 50mL of water, slowly add 15mL of concentrated sulfuric acid, cool, and titrate with ferrous ammonium sulfate solution to the endpoint. (2) Chloride Determination of Samples

Take 5 mL of water samples from both the secondary sedimentation tank and the equalization tank, add 50 mL of pure water, and titrate with 0.1 mol/L silver nitrate to the endpoint.

(3) Determination of Samples and Standards

Secondary sedimentation tank sample: Transfer 5 mL of water sample to the digestion vessel using a pipette, add 5 mL of primary water, 4 mL of mercuric sulfate solution,

accurately add 5 mL of potassium dichromate solution, and slowly add 15 mL of silver sulfate-sulfuric acid solution to begin digestion. After the digestion vessel cools, transfer the digested solution to a titration cup, rinse the walls of the digestion vessel with 40 mL of primary water, and transfer the rinsing solution to the titration cup as well. After cooling, titrate with standardized ferrous ammonium sulfate to the endpoint. Use 10 mL of primary water instead of a blank sample.

Sample Preparation: Transfer 0.5 mL of water sample into the digestion vessel using a pipette, add 9.5 mL of primary water, 1 mL of mercuric sulfate solution, accurately add 5 mL of potassium dichromate solution, and slowly add 15 mL of silver sulfate-sulfuric acid solution to begin digestion. After the digestion vessel cools, transfer the digested solution to a titration cup, rinse the walls of the digestion vessel with 40 mL of primary water, and transfer the rinsing solution into the titration cup as well. After cooling, titrate to the endpoint with standardized ferrous ammonium sulfate. Use 10 mL of primary water instead of a blank sample.

Standard: Accurately pipette 10 mL of potassium hydrogen phthalate standard solution into the digestion vessel. Other operating procedures are the same as for the sample test. The theoretical COD value of the standard solution is 500 mg/L.

3.2 Parameter Settings

3.2.1 Digestion Instrument Parameters
 

3.2.2 Titrator Parameters

Calculation formula:

In the formula:

X --- Sample COD value, in milligrams per liter (mg/L);

V1 --- Volume of ferrous ammonium sulfate standard solution consumed by the sample, in milliliters (mL);

V0 --- Volume of ferrous ammonium sulfate standard solution consumed by the blank, in milliliters (mL);

c --- Concentration of ferrous ammonium sulfate standard solution, in moles per liter (mol/L);

V --- Sample volume, in milliliters (mL);

8000 --- Conversion value of ¼ molar mass of oxygen in mg/L.

4.2 Spectrum

Secondary Settling Tank Equalization Tank

4.3 Conclusion

The results show that the relative deviation of COD in the secondary settling tank sample determined by potentiometric titration is 0.4%, and the relative deviation of COD in the equalization tank sample is 3%, which meets the national standard requirements for precision. This also proves the feasibility of microwave digestion for sample pretreatment.

Precautions

1. The national standard applies to the determination of COD values ​​less than 700 mg/L and chloride content less than 1000 mg/L for water samples. The COD of the secondary settling tank sample used in this paper is around 300 mg/L, and the COD of the equalization tank sample is around 10000 mg/L. The chloride content of the water samples in both tanks is around 5000 mg/L, so dilution is required before measurement in both tanks. 1. Due to the low COD value and high chloride concentration in the secondary sedimentation tank, and considering the instrument's detection limit for COD, the sample in the secondary sedimentation tank was diluted by half and the amount of mercuric sulfate added was increased. The conditioning tank had high COD and chloride content; therefore, the sample in the conditioning tank was diluted 20 times for determination.

2. Potassium dichromate solution should be added accurately using a pipette during the experiment. Potassium dichromate standard reagent should be dried to constant weight in a 105℃ oven before use.

3. When transferring the digested sample from the digestion vessel to the titration cup, the digestion vessel needs to be repeatedly rinsed with primary water to ensure no residual sample solution remains.

4. It is recommended to recalibrate ferrous ammonium sulfate with potassium dichromate solution before daily use.

5. Both mercuric sulfate and potassium dichromate solution added during the experiment are environmentally harmful reagents; waste liquid after the experiment should be collected and disposed of uniformly.

6. The order of adding reagents in the pretreatment process is as follows: sample to be tested, primary water, mercuric sulfate, potassium dichromate, silver sulfate.

References

[1]. HJ 828-2017 Water Quality - Determination of Chemical Oxygen Demand - Potassium Dichromate Method [S].