Determination of Chemical Oxygen Demand of Water by Potassium Dichromate Method
Introduction:
The Chemical Oxygen Demand (COD) of water is an important indicator that reflects the amount of organic microbes and pollutants in the water, and is widely used as an important assessment index of water environment quality. In current, chemical oxygen demand is usually determined by the biochemical oxygen demand method and the potassium dichromate method. In this experiment, the potassium dichromate method is used for COD determination in water samples.
Experimental Methods:
The chemical oxygen demand of the water samples was determined by a potassium dichromate method using the method GB11914-89. The samples were collected from both natural and synthetic sources. The samples were filtered through a 0.45-µm membrane filter before analysis.
1. Equipment requirements:
The analytical instruments used in this experiment include an electric mixer, pH meter, Kjeldahl apparatus, constant temperature water bath, thermostatic drying oven, thermostatic incubator, spectrophotometer, test tubes, graduated pipettes, and other common laboratory equipment.
2. Reagents and Materials:
The materials and reagents used in this experiment were potassium dichromate (K2Cr2O7), sulfuric acid (H2SO4), hydrogen peroxide (H2O2), silver nitrate (AgNO3), ferrous sulfate (FeSO4) and disodium hydrogen phosphate (Na2HPO4).
3. Sample Preparation:
Approximately 10 mL of water sample was taken in a test tube and the pH of the sample was adjusted to 2 with 1 M H2SO4 solution. To the sample 3 mL of 0.5 M H2SO4 and 0.2 g K2Cr2O7 were added and the mixture was kept for 15-20 minutes in an electric mixer for stirring. Then, 0.2 mL of H2O2 was added to each sample and the mixture was kept for 10 minutes for stirring. After that, a known volume of the sample was taken in a test tube and a known volume of AgNO3 was added in the same tube and the mixture was kept for 10-15 minutes with mixing. A known volume of FeSO4 was then added in this tube and the mixture was kept for 5 minutes with stirring. The sample was filtered through a 0.45-µm membrane filter and the filtrate collected was used for the determination of the COD.
4. COD Determination:
Following the preparation steps of the sample, the COD in the sample was measured by spectrophotometric method. A spectrophotometer was used to measure the absorbance of the sample at a wavelength of 490 nm. A standard curve was prepared by plotting the absorbance values of different concentrations of the potassium dichromate solution and the COD of the sample was then estimated from the standard curve.
5. Results and Discussion:
The results obtained from the COD determination are tabulated in Table 1 for both the natural and synthetic sources. The COD values for the natural sources of water samples were found to range from 8.6 mg/L to 10.2 mg/L, while for the synthetic sources, the values ranged from 6.4 mg/L to 8.2 mg/L. The COD for the natural sources of water samples was found to be higher than the COD values of the synthetic sources, indicating the presence of a higher amount of organic matter in the natural sources.
Table 1: COD values (mg/L) of the water samples
Source Natural Synthetic
Sample 1 8.6 6.4
Sample 2 9.2 7.2
Sample 3 10.2 8.2
Conclusion:
In this experiment, the potassium dichromate method was used to determine the COD values of water samples. For both natural and synthetic sources of water, the COD values were found to be within the acceptable range for safe water. The COD values for the natural sources were found to be higher than those for the synthetic sources, indicating the presence of a higher amount of organic matter in the natural sources.
