Determination of Volatile Phenols in Water - 4-Aminoantipyrine Spectrophotometric Method
**Keywords:** determination of volatile phenols in water; 4-aminoantipyrine spectrophotometry; US analytical instrument; UV-1100; UV-1200 **1. Sources and Hazards of Volatile Phenols** (1) **Definition**: Volatile phenols are those that can be distilled with steam, typically having boiling points below 230°C. They are usually monohydric phenols. Non-volatile phenols cannot be distilled under these conditions. (2) **Sources**: These compounds are commonly found in industrial wastewater from processes such as oil refining, gas production, coal coking, papermaking, ammonia synthesis, wood preservation, and chemical manufacturing. (3) **Hazards**: Volatile phenols are highly toxic. Ingesting large amounts can cause acute poisoning, while long-term exposure to low concentrations (0.1–0.2 mg/L) may lead to symptoms like dizziness, skin rashes, itching, anemia, and nervous system issues. High concentrations (>1 mg/L) can be fatal. Phenol-contaminated water can also cause a fishy odor and is harmful to crops when used for irrigation. Chlorination of water containing trace phenols can produce chlorophenol-like odors. **2. Selection of Methods for Determination of Volatile Phenols** The 4-aminoantipyrine spectrophotometric method is widely used globally. For higher concentrations (>0.5 mg/L), the direct method is applied. For lower concentrations (<0.5 mg/L), the chloroform extraction method is preferred. **3. Sampling and Preservation** Water samples should be collected using glass containers. Upon collection, check for oxidants. If present, add excess ferrous sulfate, then adjust pH to 4.0 using phosphoric acid (indicated by methyl orange). Add copper sulfate (1 g/L) to inhibit microbial oxidation. Store the sample at 5–10°C and analyze within 24 hours. **4. Pre-Distillation and Pretreatment** Distillation helps remove color and turbidity. However, if the sample contains oxidants, sulfides, or oils, pretreatment is necessary: - **Oxidants**: Acidify the sample and test with potassium iodide-starch paper. A blue color indicates oxidants; excess ferrous sulfate can neutralize them. - **Sulfides**: Adjust pH to 4.0 and add copper sulfate to precipitate sulfides. For high levels, aerate the sample to release hydrogen sulfide. - **Oils**: Use a separatory funnel to separate the oil, then extract with carbon tetrachloride. For high petroleum content, n-hexane is more effective. - **Reducing agents**: Acidify the sample and use ether or dichloromethane for extraction. Back-extract with sodium hydroxide to isolate phenols. Note: Ether is flammable and must be handled in a fume hood. **5. Principle of 4-Aminoantipyrine Direct Photometric Method** Phenolic compounds react with 4-aminoantipyrine in a pH 10.00±0.2 solution in the presence of iron to form an orange-red indophenol dye. The maximum absorption occurs at 510 nm. The reaction is affected by substituents on the phenol ring—para-hydroxy groups inhibit the reaction, while ortho-nitro groups prevent it. The detection limit is 0.1 mg/L using a 20 mm cuvette. **6. Reagents** (1) **Phenol standard stock solution**: Dissolve 1.00 g of phenol in water and dilute to 1 L. Store at 4°C for at least one month. (2) **Calibration**: Use potassium bromate-potassium bromide for standardization. (3) **Intermediate solution**: Dilute the stock solution to 10 μg/mL. Prepare fresh daily. (4) **Buffer solution (pH 10)**: Mix 20 g ammonium chloride in 100 mL ammonia water. (5) **2% 4-aminoantipyrine solution**: Dissolve 2 g in 100 mL water. Store in the fridge for a week. (6) **8% iron solution**: Dissolve 8 g iron in 100 mL water. (7) **Copper sulfate (1 g/L)** (8) **Phosphoric acid** (9) **5% ferrous sulfate**: Dissolve 5 g in 100 mL water. **7. Analytical Procedure** (1) Prepare a calibration curve using 8 tubes with 0.00, 0.50, 1.00, 3.00, 5.00, 7.00, 10.00, and 12.50 mL of 10 μg/mL phenol standard. Add buffer, 4-aminoantipyrine, and iron solution. Let stand for 10 minutes, then measure absorbance at 510 nm. (2) For the sample, dilute the distillate to 50 mL and follow the same procedure. Subtract the blank value. (3) Use the regression equation to calculate phenol concentration in mg/L. Plot the calibration curve and mark the sample data points. **8. 4-Aminoantipyrine Extraction Spectrophotometry** This method involves extracting the orange-red dye formed by the reaction of phenols with 4-aminoantipyrine in a pH 10.00±0.2 medium using dichloromethane. Maximum absorption occurs at 460 nm. It is suitable for analyzing drinking water, surface water, groundwater, and industrial wastewater. The detection limit is 0.002 mg/L, with an upper limit of 0.12 mg/L. **Keywords:** determination of volatile phenols in water; 4-aminoantipyrine spectrophotometry; US analytical instrument; UV-1100; UV-1200
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