Determination of phenol in the presence of sulfite (sulfur dioxide) by the 4-aminoantipyrine spectro-photometric method

The interference of sulfite (sulfur dioxide) with the 4-aminoantipyrine (4-AAP) spectrophotometric method for phenol is discussed for procedures without distillation, and with distillation in the absence and presence of copper(II) sulfate. Sulfite represses the color development in all these procedures. Without chloroform extraction, the maximum tolerable amounts of sulfite in the procedures without distillation, distillation without copper(II) sulfate, and distillation with copper(II) sulfate are 15, 10, and 20 mg/100 ml, respectively. For the extraction method, the limits are 4.0, 4.0, and 10 mg/ 100 ml, respectively. Copprer(II) sulfate catalyzes the air-oxidation of sulfite. Phenol can be determined in the presence of large amounts of sulfite by treating with sulfide to form polythionates and thiosulfate; excess of sulfide is removed with copper(II) sulfate, sulfuric acid is added, the phenol is distilled, and the 4-AAP method is applied. Improvements are made in the overall accuracy of the 4-AAP method.