Thermometric titration of polyhydric phenols. Application to the determination of the tannin content of wines

Abstract

Various oxidants for the thermometric titration and back-titration of polyhydric phenols were studied. The results obtained with polyhydric phenol standards consisting of gallic or tannic acid showed potassium hexacyanoferrate(III), cerium(IV) sulfate and potassium permanganate to be the most suitable oxidants for the purpose. The use of excess potassium permanganate or cerium(IV) for subsequent back-titration with Mohr's salt was ruled out on the grounds of kinetic considerations. Potassium hexacyanoferrate(III) and potassium permanganate (first equivalence point) provided consistent results; however, the former gave rise to sharper equivalence points in titrations involving increased polyhydric phenol concentrations. Titrations of tannins present in wine with potassium hexacyanoferrate(III) or cerium(IV) provided results that were also consistent with those obtained by using the classical Löwenthal volumetric method and the Folin-Ciocalteu method. Thermometric titrations with potassium hexacyanoferrate(III) or cerium(IV) have the added advantage that, unlike the Löwenthal method, they do not require oxidation restrictors or matrix correction. In addition, the presence of sulfur dioxide or reducing sugars in the wine interferes much less severely with the thermometric titration than it does with the Folin—Ciocalteu method. If ascorbic acid were present it would interfere.