Determination of Volatile Phenols in Water - 4-Aminoantipyrine Spectrophotometric Method
**Keywords:** Determination of volatile phenols in water; 4-aminoantipyrine spectrophotometry; US analytical instruments; 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 mainly monohydric phenols and are more likely to evaporate into the air compared to non-volatile ones. **(2) Sources:** These compounds often come from industrial wastewaters such as those from petroleum refining, gas purification, coking, papermaking, ammonia synthesis, wood preservation, and chemical manufacturing. **(3) Hazards:** Volatile phenols are toxic and can cause acute poisoning if ingested in large amounts. Long-term exposure to contaminated water may lead to symptoms like dizziness, skin rashes, itching, anemia, and neurological issues. At low concentrations (0.1–0.2 mg/L), they can cause a fishy odor in water, while at higher levels (>1 mg/L), they can be lethal. High phenol content in wastewater is not suitable for irrigation, as it can harm crops or reduce yields. Chlorination of water containing trace phenols may produce a specific chlorophenol smell, which is both unpleasant and potentially harmful. --- ### 2. Selection of Analytical Methods The 4-aminoantipyrine spectrophotometric method is widely used globally. For samples with high phenol concentration (>0.5 mg/L), the direct method is applied. When the concentration is low (<0.5 mg/L), the chloroform extraction method is preferred to improve detection sensitivity. --- ### 3. Sample Collection Water samples should be collected using glass containers to avoid contamination. After collection, the presence of oxidants should be checked immediately. If oxidants are detected, excess ferrous sulfate can be added. Then, phosphoric acid is used to adjust the pH to 4.0 (using methyl orange as an indicator). A small amount of copper sulfate (1 g/L) is also added to inhibit microbial oxidation. The sample should be stored at 5–10°C and analyzed within 24 hours. --- ### 4. Pre-Distillation and Pretreatment Distillation helps remove color and turbidity from the sample. However, if the sample contains oxidants, sulfides, oils, or reducing agents, proper pretreatment is necessary before distillation. **(1) Oxidants:** If the sample contains oxidizing agents, adding excess ferrous sulfate can neutralize them. A potassium iodide-starch test paper can be used to detect oxidants. **(2) Sulfides:** For low sulfide levels, adjust the pH to 4.0 with phosphoric acid and add copper sulfate to precipitate copper sulfide. For high levels, aerate the sample in a fume hood to release hydrogen sulfide. **(3) Oils:** Transfer the sample to a separatory funnel and separate the oil. Add sodium hydroxide to raise the pH to 12.0–12.5 and extract with carbon tetrachloride. Repeat the extraction twice and discard the organic layer. The aqueous phase is then heated to remove residual solvent and adjusted to pH 4.0. **(4) Reducing Agents:** For substances like formaldehyde or sulfite, use diethyl ether or dichloromethane to extract the phenols. Back-extract with sodium hydroxide to transfer the phenol into the aqueous phase, then remove the solvent and dilute the solution to the original volume. Always perform this in a fume hood and avoid open flames due to the flammable nature of ether. --- ### 5. Principle of 4-Aminoantipyrine Direct Photometric Method In a pH 10.00±0.2 environment, phenolic compounds react with 4-aminoantipyrine in the presence of iron to form an orange-red indophenol dye. This dye has maximum absorption at 510 nm. The reaction is sensitive to substituents: para-hydroxy groups may hinder the reaction, while nitro groups in certain positions may prevent it. The minimum detectable concentration is 0.1 mg/L using a 20 mm path length cuvette. --- ### 6. Reagents **(1) Phenol Standard Stock Solution:** Weigh 1.00 g of pure phenol and dissolve in water, then dilute to 1000 mL. Store at 4°C for at least one month. **(2) Calibration Solution:** Dilute the stock solution to 10 μg/mL on the day of use. **(3) Buffer Solution (pH 10):** Dissolve 20 g of ammonium chloride in 100 mL of ammonia water. Store in a refrigerator. **(4) 2% 4-Aminoantipyrine Solution:** Dissolve 2 g of 4-aminoantipyrine in water and dilute to 100 mL. Store in a refrigerator for one week. **(5) 8% Iron Solution:** Dissolve 8 g of iron in water and dilute to 100 mL. Store in a refrigerator for one week. **(6) Copper Sulfate (1 g/L)** **(7) Phosphoric Acid** **(8) 5% Ferrous Sulfate Solution:** Dissolve 5 g of solid ferrous sulfate in 100 mL of water. --- ### 7. Analytical Procedure **(1) Calibration Curve Preparation:** In 8 50 mL colorimetric tubes, add 0.00, 0.50, 1.00, 3.00, 5.00, 7.00, 10.00, and 12.50 mL of 10 μg/mL phenol standard solution. Dilute each to 50 mL with water. Add 0.5 mL buffer solution and mix. Then add 1.0 mL of 4-aminoantipyrine solution and 1.0 mL of iron solution. Let the mixture stand for 10 minutes. Measure the absorbance at 510 nm using a 20 mm cuvette. Use distilled water as the blank. **(2) Sample Measurement:** Take an appropriate volume of the distillate, dilute to 50 mL, and follow the same procedure as the calibration curve. Subtract the blank value from the measured absorbance. **(3) Data Processing:** After correcting for the blank, plot the absorbance versus phenol concentration (μg). Use the regression equation to calculate the volatile phenol content in the water sample. Finally, convert the result to mg/L and plot the calibration curve with measurement 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 solution using dichloromethane. The dye shows maximum absorption at 460 nm. It is suitable for determining volatile phenols in drinking water, surface water, groundwater, and industrial wastewater. The minimum detectable concentration is 0.002 mg/L, and the upper limit is 0.12 mg/L. --- **Keywords:** Determination of volatile phenols in water; 4-aminoantipyrine spectrophotometry; US analytical instruments; UV-1100; UV-1200
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