COD (chemical oxygen demand) water sample pretreatment is an important step to ensure the accuracy of COD measurement. The main purpose is to remove interfering substances in the water sample and adjust the state of the water sample to make it suitable for subsequent COD measurement. In the determination of COD (chemical oxygen demand) water samples, the presence of chloride ions often has a significant impact on the determination results, so eliminating the interference of chloride ions is an important step to ensure the accuracy of the determination. The following are some commonly used methods to eliminate the influence of chloride ions in COD water samples:
1. Applicable conditions of dilution method: When the chloride ion content in the water sample is high, but the COD content is relatively low, the concentration of chloride ions can be reduced by diluting the water sample, thereby reducing its interference with the determination results.
Note: The dilution multiple should be appropriate. Too high or too low dilution multiples may affect the accuracy of the determination results. At the same time, the COD value of the water sample should be re-determined after dilution, and the corresponding calculation should be performed to obtain the COD value of the original water sample.
2. Mercury salt method (mercury sulfate complex method) Principle: Mercury sulfate (HgSO₄) reacts with chloride ions (Cl⁻) to form a complex [HgCl₄]²⁻, thereby eliminating the interference of chloride ions on COD determination.
Operation steps: Add an appropriate amount of mercury sulfate solution to the water sample to make the mass ratio of HgSO₄ to Cl⁻ reach 10:1 (or adjust the ratio according to the specific situation). Then measure according to the normal COD determination steps.
Note: Mercury sulfate is a toxic substance. Pay attention to safety when using it, and properly dispose of waste liquid to prevent environmental pollution.
3. Silver salt method principle: By adding silver nitrate (AgNO₃) solution to the water sample, silver ions (Ag⁺) react with chloride ions (Cl⁻) to form silver chloride (AgCl) precipitation, thereby removing chloride ions in the water sample.
Operation steps: Add silver nitrate solution drop by drop to the water sample until no white precipitate is produced. Then filter to remove the precipitate and measure the COD of the filtrate.
Note: If there are other substances that can react with silver ions (such as sulfate ions, etc.) in the water sample, it may affect the accuracy of the measurement results. In addition, silver nitrate is expensive, so it should be used sparingly.
4. Principle of standard curve correction method: By pre-configuring a series of water samples with different chloride ion concentrations and measuring their COD values, a standard curve of COD and chloride ion concentration is drawn. Then, in the actual measurement, the corresponding COD value is found on the standard curve according to the chloride ion concentration of the water sample for correction.
Operation steps: Configure a series of water samples with different chloride ion concentrations and measure their COD values; draw a COD-Cl⁻ standard curve; in the actual measurement, find the corresponding COD value on the standard curve according to the chloride ion concentration of the water sample and make corrections.
Note: The drawing of the standard curve should be accurate and reliable; during the correction process, attention should be paid to the dilution multiple of the water sample and the consistency of the measurement method.
5. Other methods In addition to the above methods, there are also methods to eliminate the interference of chloride ions, such as chlorine correction method, closed digestion method, low concentration oxidant method, KI-KMnO₄ oxidation method, bismuth absorbent dechlorination method, etc.
These methods have their own advantages and disadvantages and scope of application, and can be selected according to actual conditions. In order to eliminate the influence of chloride ions in the determination of COD water samples, it is necessary to select a suitable method according to the specific conditions of the water sample. At the same time, safety and accuracy should be paid attention to during the operation to ensure the reliability of the determination results.
Post time: Mar-06-2025
