THERMOMETRIC TITRATION OF POLYHYDRIC PHENOLS - APPLICATION TO THE DETERMINATION OF THE TANNIN CONTENT OF WINES

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 t o be the most suitable oxidants for the purpose. The use of excess pot assium permanganate or cerium(IV) for subsequent back-titration with M ohr's salt was ruled out on the grounds of kinetic considerations. Pot assium hexacyanoferrate(III) and potassium permanganate (first equival ence point) provided consistent results; however, the former gave rise to sharper equivalence points in titrations involving increased polyh ydric phenol concentrations. Titrations of tannins present in wine wit h potassium hexacyanoferrate(III) or cerium(IV) provided results that were also consistent with those obtained by using the classical Lowent hal volumetric method and the Folin-Ciocalteu method. Thermometric tit rations with potassium hexacyanoferrate(III) or cerium(IV) have the ad ded advantage that, unlike the Lowenthal method, they do not require o xidation restrictors or matrix correction. In addition, the presence o f sulfur dioxide or reducing sugars in the wine interferes much less s everely with the thermometric titration than it does with the Folin-Ci ocalteu method. If ascorbic acid were present it would interfere.