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Based on whether phenols can be distilled off with water vapor, they are classified into volatile phenols and non-volatile phenols. Generally, phenols with a boiling point below 230℃ are considered volatile phenols (monohydric phenols), while those with a boiling point above 20℃ are considered non-volatile phenols.
Phenols are highly toxic substances; ingestion of a certain amount can cause acute poisoning symptoms. Long-term consumption of water contaminated with phenol can cause dizziness, itching, anemia, and neurological disorders. When the phenol content in water exceeds 5 mg/L, it can cause fish poisoning and death. The main sources of phenol pollution are wastewater from oil refining, coking, gasification plants, wood preservation, and certain chemical industries (such as phenolic resins).
The main analytical methods for phenol include volumetric methods, spectrophotometry, and chromatography. Currently, the 4-aminoantipyrine spectrophotometric method is widely used in various countries; high-concentration phenol-containing wastewater can be treated with the bromination volumetric method. Regardless of whether the bromination volumetric method or spectrophotometric method is used, when oxidants, reducing agents, oils, and certain metal ions are present in the water sample, they should be eliminated and pre-distilled. For example, free chlorine can be reduced by adding ferrous sulfate; sulfides can be precipitated by adding copper sulfate, or released as hydrogen sulfide under acidic conditions; oils can be removed by extraction with organic solvents. Distillation serves two purposes: first, to separate volatile phenols; and second, to eliminate interference from color, turbidity, and metal ions.
4-Aminoantipyrine Spectrophotometric Method
Phenolic compounds react with 4-aminoantipyrine (4-AAP) in a medium with pH 10.0 ± 0.2 in the presence of potassium ferricyanide to form an orange-red p51 phenol antipyrine dye, which has a maximum absorption at a wavelength of 510 nm. Quantification is performed colorimetrically. The reaction equation is as follows:
The colorimetric reaction is affected by the type, position, and number of substituents on the phenolic ring. For example, phenols with para-substitution by alkyl, aromatic, ester, nitro, benzoyl, nitroso, or aldehyde groups, but unsubstituted at the ortho-position, do not produce a colorimetric reaction with 4-aminoantipyrine. This is because these groups prevent the oxidation of phenols to quinone structures. However, phenols with para-substitution by halogens, sulfonic acids, hydroxyl, or methoxy groups do produce a colorimetric reaction with 4-aminoantipyrine. The reactions of ortho-nitrophenols and meta-nitrophenols with 4-aminoantipyrine are different; the former is colorless, while the latter produces a slightly colored reaction. Therefore, the phenols determined by this method are not total phenols, but only those that develop a colorimetric reaction with 4-aminoantipyrine, using phenol as the standard, and the result is calculated based on the phenol content.
The lowest detectable concentration (DMR) using a 20 m²/d cuvette is 0.12 m²/L. If extraction with chloroform after color development is performed, and the DMR is measured at a wavelength of 460 nm, the DMR can reach 0.002 m²/L; the upper limit of detection is 0.12 m²/L. Furthermore, in the direct spectrophotometric method, the colored complex is not stable enough and should be measured immediately; the colored complex is stable for 3 hours using the chloroform extraction method.
