M. Celeste et al., THERMOMETRIC TITRATION OF POLYHYDRIC PHENOLS - APPLICATION TO THE DETERMINATION OF THE TANNIN CONTENT OF WINES, Analyst, 118(7), 1993, pp. 895-898
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.