The Folin–Ciocâlteu reagent (FCR) or Folin's phenol reagent or Folin–Denis reagent, is a mixture of phosphomolybdate and phosphotungstate used for the colorimetric in vitro assay of phenolic and polyphenolic antioxidants, also called the gallic acid equivalence method (GAE). It is named after Otto Folin, Vintilă Ciocâlteu, and Willey Glover Denis. The Folin-Denis reagent is prepared by mixing sodium tungstate and phosphomolybdic acid in phosphoric acid. The Folin–Ciocalteu reagent is just a modification of the Folin-Denis reagent. The modification consisted of the addition of lithium sulfate and bromine to the phosphotungstic-phosphomolybdic reagent.
The reagent does not measure only phenols, but will react with any reducing substance. It therefore measures the total reducing capacity of a sample, not just phenolic compounds. This reagent is part of the Lowry protein assay, and will also react with some nitrogen-containing compounds such as hydroxylamine and guanidine. The reagent has also been shown to be reactive towards thiols, many vitamins, the nucleotide base guanine, the trioses glyceraldehyde and dihydroxyacetone, and some inorganic ions. Copper complexation increases the reactivity of phenols towards this reagent.
This reagent is distinct from Folin's reagent, which is used to detect amines and sulfur-containing compounds.
Because it measures anti-oxidant capacity in vitro, the reagent has been used to assay foods and supplements in food science. The oxygen radical absorbance capacity (ORAC) used to be the industry standard for antioxidant strength of whole foods, juices and food additives. Earlier measurements and ratings by the United States Department of Agriculture were withdrawn in 2012 as biologically irrelevant to human health, referring to an absence of physiological evidence for polyphenols having antioxidant properties in vivo. Consequently, the ORAC method, derived only from in vitro experiments, is no longer considered relevant to human diets or biology.
- Singleton, Vernon L.; Orthofer, Rudolf; Lamuela-Raventós, Rosa M. (1999). " Analysis of total phenols and other oxidation substrates and antioxidants by means of folin-ciocalteu reagent".  Analysis of total phenols and other oxidation substrates and antioxidants by means of folin–ciocalteu reagent. Methods in Enzymology. Vol. 299. pp. 152–178. doi:10.1016/S0076-6879(99)99017-1. ISBN 9780121822002.
- Bärlocher, Felix; Graça, Manuel A. S. (2020), Bärlocher, Felix; Gessner, Mark O.; Graça, Manuel A.S. (eds.), "Total Phenolics", Methods to Study Litter Decomposition, Cham: Springer International Publishing, pp. 157–161, doi:10.1007/978-3-030-30515-4_18, ISBN 978-3-030-30514-7, retrieved 2022-09-29
- Ikawa M, Schaper TD, Dollard CA, Sasner JJ (2003). "Utilization of Folin–Ciocalteu phenol reagent for the detection of certain nitrogen compounds". J. Agric. Food Chem. 51 (7): 1811–5. doi:10.1021/jf021099r. PMID 12643635.
- Everette, Jace D.; Bryant, Quinton M.; Green, Ashlee M.; Abbey, Yvonne A.; Wangila, Grant W.; Walker, Richard B. (2010). "Thorough Study of Reactivity of Various Compound Classes toward the Folin−Ciocalteu Reagent". J. Agric. Food Chem. 58 (14): 8139–44. doi:10.1021/jf1005935. PMC 4075968. PMID 20583841.
- Oliver H. Lowry; Nira J. Rosebrough; A. Lewis Farr; Rose J. Randall (1951). "Protein Measurement with the Folin Phenol Reagent". J. Biol. Chem. 193 (1): 265–275. PMID 14907713.
- http://www.garfield.library.upenn.edu/essays/v13p045y1990.pdf[bare URL PDF]
- Cao G, Alessio H, Cutler R (1993). "Oxygen-radical absorbance capacity assay for antioxidants" (PDF). Free Radic Biol Med. 14 (3): 303–11. doi:10.1016/0891-5849(93)90027-R. PMID 8458588. Archived from the original on 2018-07-24. Retrieved 2019-09-12.
- Ou B, Hampsch-Woodill M, Prior R (2001). "Development and validation of an improved oxygen radical absorbance capacity assay using fluorescein as the fluorescent probe". J Agric Food Chem. 49 (10): 4619–26. doi:10.1021/jf010586o. PMID 11599998.
- "Withdrawn: Oxygen Radical Absorbance Capacity (ORAC) of Selected Foods, Release 2 (2010)". United States Department of Agriculture, Agricultural Research Service. 16 May 2012. Retrieved 13 June 2012.
- Prior R, Wu X, Schaich K (2005). "Standardized methods for the determination of antioxidant capacity and phenolics in foods and dietary supplements". J Agric Food Chem. 53 (10): 4290–302. doi:10.1021/jf0502698. PMID 15884874.