Behaviour of tannic acid from various commercial sources towards redox, metal complexing and protein precipitation assays of tannins

Abstract Tannic acid is the most commonly used standard for quantitation of tannins. Tannic acid from five commercial sources (Merck, Sigma, Aldrich, Fluka and Apin chemical companies) was tested towards a redox (Folin‐Ciocalteu method) and a metal complexing (ferric chloride method) assay, and three protein precipitation assays. Tannic acid from various commercial sources behaved differently towards these tannin assays showing that (i) tannic acid preparation differed from source to source, and (ii) tannic acid from different sources as a standard results in different tannin values for a given sample. It is suggested that the source of tannic acid used as a standard should be stated, and the assay values obtained by different workers should be compared with caution.