51. What are the various indicators that reflect toxic and harmful organic matter in water?
Except for a small number of toxic and harmful organic compounds in common sewage (such as volatile phenols, etc.), most of them are difficult to biodegrade and are highly harmful to the human body, such as petroleum, anionic surfactants (LAS), organic Chlorine and organophosphorus pesticides, polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), high-molecular synthetic polymers (such as plastics, synthetic rubber, artificial fibers, etc.), fuels and other organic substances.
The national comprehensive discharge standard GB 8978-1996 has strict regulations on the concentration of sewage containing the above toxic and harmful organic substances discharged by various industries. Specific water quality indicators include benzo(a)pyrene, petroleum, volatile phenols, and organophosphorus pesticides (calculated in P ), tetrachloromethane, tetrachloroethylene, benzene, toluene, m-cresol and 36 other items. Different industries have different wastewater discharge indicators that need to be controlled. Whether the water quality indicators meet the national discharge standards should be monitored based on the specific composition of the wastewater discharged by each industry.
52.How many types of phenolic compounds are there in water?
Phenol is a hydroxyl derivative of benzene, with its hydroxyl group directly attached to the benzene ring. According to the number of hydroxyl groups contained on the benzene ring, it can be divided into unitary phenols (such as phenol) and polyphenols. According to whether it can volatilize with water vapor, it is divided into volatile phenol and non-volatile phenol. Therefore, phenols not only refer to phenol, but also include the general name of phenolates substituted by hydroxyl, halogen, nitro, carboxyl, etc. in the ortho, meta and para positions.
Phenolic compounds refer to benzene and its fused-ring hydroxyl derivatives. There are many types. It is generally considered that those with a boiling point below 230oC are volatile phenols, while those with a boiling point above 230oC are non-volatile phenols. Volatile phenols in water quality standards refer to phenolic compounds that can volatilize together with water vapor during distillation.
53.What are the commonly used methods for measuring volatile phenol?
Since volatile phenols are a type of compound rather than a single compound, even if phenol is used as the standard, the results will be different if different analysis methods are used. In order to make the results comparable, the unified method specified by the country must be used. The commonly used measurement methods for volatile phenol are the 4-aminoantipyrine spectrophotometry specified in GB 7490–87 and the bromination capacity specified in GB 7491–87. Law.
4–Aminoantipyrine spectrophotometric method has fewer interference factors and higher sensitivity, and is suitable for measuring cleaner water samples with volatile phenol content <5mg/L. The basic principle is that in the presence of potassium ferricyanide and water with a pH value of 10, phenolic compounds react with 4-aminoantipyrine to form an orange-red dye with a maximum absorption value at a wavelength of 510nm. If the generated orange-red dye is extracted with chloroform, it will have a maximum absorption value at a wavelength of 460nm, which can reduce the minimum detection concentration of 4-aminoantipyrine spectrophotometry from 0.1mg/L to 0.002mg/L. .
The bromination volumetric method is simple and easy to operate, and is suitable for determining the amount of volatile phenols in industrial wastewater >10 mg/L or effluent from industrial wastewater treatment plants. The basic principle is that in a solution with excess bromine, phenol and bromine generate tribromophenol, and further generate bromotribromophenol. The remaining bromine then reacts with potassium iodide to release free iodine, while bromotribromophenol reacts with potassium iodide to form tribromophenol and free iodine. The free iodine is then titrated with sodium thiosulfate solution, and the volatile phenol content in terms of phenol can be calculated based on its consumption.
54. What are the precautions for measuring volatile phenol?
Since dissolved oxygen and other oxidants and microorganisms can oxidize or decompose phenolic compounds, making the phenolic compounds in the water very unstable, the method of adding acid (H3PO4) and lowering the temperature is usually used to inhibit the action of microorganisms, and a sufficient amount of sulfuric acid is added. The ferrous method eliminates the effects of oxidants. Even if the above measures are taken, water samples should be analyzed and tested within 24 hours, and water samples must be stored in glass bottles rather than plastic containers.
Regardless of the bromination volumetric method or the 4-aminoantipyrine spectrophotometric method, when the water sample contains oxidizing or reducing substances, metal ions, aromatic amines, oils and tars, etc., it will have an impact on the accuracy of the measurement. interference, necessary measures must be taken to eliminate its effects. For example, oxidants can be removed by adding ferrous sulfate or sodium arsenite, sulfides can be removed by adding copper sulfate under acidic conditions, oil and tar can be removed by extraction and separation with organic solvents under strongly alkaline conditions. Reducing substances such as sulfate and formaldehyde are removed by extracting them with organic solvents under acidic conditions and leaving the reducing substances in water. When analyzing sewage with a relatively fixed component, after accumulating a certain period of experience, the types of interfering substances can be clarified, and then the types of interfering substances can be eliminated by increasing or decreasing, and the analysis steps can be simplified as much as possible.
Distillation operation is a key step in the determination of volatile phenol. In order to completely evaporate the volatile phenol, the pH value of the sample to be distilled should be adjusted to about 4 (the discoloration range of methyl orange). In addition, since the volatilization process of volatile phenol is relatively slow, the volume of the collected distillate should be equivalent to the volume of the original sample to be distilled, otherwise the measurement results will be affected. If the distillate is found to be white and turbid, it should be evaporated again under acidic conditions. If the distillate is still white and turbid for the second time, it may be that there is oil and tar in the water sample, and corresponding treatment must be carried out.
The total amount measured using the bromination volumetric method is a relative value, and the operating conditions specified by national standards must be strictly followed, including the amount of liquid added, reaction temperature and time, etc. In addition, tribromophenol precipitates easily encapsulate I2, so it should be shaken vigorously when approaching the titration point.
55. What are the precautions for using 4-aminoantipyrine spectrophotometry to determine volatile phenols?
When using 4-aminoantipyrine (4-AAP) spectrophotometry, all operations should be performed in a fume hood, and the mechanical suction of the fume hood should be used to eliminate the adverse effects of toxic benzene on the operator. .
The increase in the reagent blank value is mainly due to factors such as contamination in distilled water, glassware and other test devices, as well as the volatilization of the extraction solvent due to rising room temperature, and is mainly due to the 4-AAP reagent, which is prone to moisture absorption, caking and oxidation. , so necessary measures should be taken to ensure the purity of 4-AAP. The color development of the reaction is easily affected by the pH value, and the pH value of the reaction solution must be strictly controlled between 9.8 and 10.2.
The dilute standard solution of phenol is unstable. The standard solution containing 1 mg phenol per ml should be placed in the refrigerator and cannot be used for more than 30 days. The standard solution containing 10 μg phenol per ml should be used on the day of preparation. The standard solution containing 1 μg phenol per ml should be used after preparation. Use within 2 hours.
Be sure to add reagents in order according to standard operating procedures, and shake well after adding each reagent. If the buffer is not shaken evenly after adding it, the ammonia concentration in the experimental solution will be uneven, which will affect the reaction. Impure ammonia can increase the blank value by more than 10 times. If the ammonia is not used up for a long time after opening the bottle, it should be distilled before use.
The generated aminoantipyrine red dye is only stable for about 30 minutes in aqueous solution, and can be stable for 4 hours after extraction into chloroform. If the time is too long, the color will change from red to yellow. If the blank color is too dark due to the impurity of 4-aminoantipyrine, the 490nm wavelength measurement can be used to improve the measurement accuracy. 4–When the aminoantibi is impure, it can be dissolved in methanol, and then filtered and recrystallized with activated carbon to refine it.
Post time: Nov-23-2023
