The bluish-greenish color in your samples during the Phenol-Sulfuric Acid method could be due to interfering substances or side reactions. Proteins, peptides, or phenolic compounds in the sample may react with phenol and sulfuric acid, causing this color change. Oxidation reactions, sample matrix interference, or contamination of reagents could also be factors. Ensure your sample is free of interfering substances and that reagents are fresh and correctly concentrated. Temperature and timing control are crucial; deviations can lead to unexpected colors. Consider running control experiments with known standards to identify the cause of the issue.

thanks for your consideration Dr Balamurugan Krishnamoorthy