Quantification of total phenols in bio-oil using the Folin–Ciocalteu method

Bio-oil from fast pyrolysis of biomass contains phenolics derived from the lignin portion of the biomass. Traditional testing for total phenolics in bio-oil is based on either a rough estimate of the weight percent water-insolubles in bio-oil or on tedious liquid–liquid extraction methods. We have evaluated the Folin–Ciocalteu (FC) colorimetry method used for quantifying total phenols in wine to determine total phenols in bio-oil. This method, based on the oxidation of phenolic compounds by the FC reagent, is fast and easy to perform. This study evaluated its accuracy relative to interferents by the use of positive and negative controls. Positive controls included phenol, 4-methylphenol, 3-ethylphenol, guaiacol, 2,6-dimethoxyphenol and eugenol. The negative controls included sugars, furfural, and acids. Potential interferents with the quantification of total phenols by the FC method was calculated for all positive and negative controls by using data obtained when adding the contributor (positive controls) and the interferent (negative controls) into bio-oil using typical concentrations found in bio-oil. The positive and several of the negative controls produced strongly correlated linear relationships between the indicated phenolic content of the bio-oil and the amount of contributor or interferent added. However, the slopes of these relationships for the negative controls were much smaller than those for the positive controls, indicating that the error in the prediction of phenolic content was small even for large concentrations of interferent compounds. For typical concentrations of non-phenolic compounds in bio-oil, the error in predicted phenolic content as a result of their presence was ≤5.8%. Total phenolic content in bio-oil detected by the FC method was comparable to the quantity of total phenolics obtained by liquid–liquid extraction. All results fell within the margin of error and the uncertainty of the measurement by the FC method indicating there was no significant difference in the results between the two methods. The FC method uncertainty of measurement was ±1.1% at the 95% confidence level.