SOP: Volumetric Estimation of Tannins

GIFSQ — Global Institute for Food Safety & Quality

Principle

Tannins are estimated by their oxidisability with potassium permanganate. An aliquot of the sample is titrated with standard permanganate in the presence of indigo carmine as the redox indicator, giving the total oxidisable (tannin-like) matter. A second aliquot is first treated with gelatin in acid sodium chloride, which precipitates the true tannins; titration of the filtrate gives the non-tannin oxidisable matter. The difference between the two titrations is the permanganate consumed by the true tannins, which is converted to a tannin percentage using the permanganate equivalence (one ml of 0.1 N KMnO₄ = 0.0042 g of tannin as gallotannic acid).

Apparatus required

• Burette
• Porcelain dish
• 250-ml volumetric flask
• Conical flask

Reagents

a. 0.04 N potassium permanganate solution.

b. Indigo carmine solution: dissolve 1.5 g of indigo carmine (free from indigo blue) in 1 litre of water containing 50 ml of conc H₂SO₄.

c. Gelatin solution: soak 25 g of gelatin for 1 hr in saturated sodium chloride solution, heat until the gelatin dissolves, cool and dilute with saturated sodium chloride solution to 1 litre.

d. Acid sodium chloride solution: to 975 ml of saturated sodium chloride solution, add 25 ml of conc H₂SO₄.

e. Filter aid (kaolin, Kieselguhr or Supereel, as printed).

Procedure

To an aliquot of the filtered juice (10–20 ml containing 0.01 g of tannins) in a porcelain dish, add 20 ml of the indigo carmine solution and about 500–750 ml of water

↓

Add the potassium permanganate solution from a burette, 1 ml at a time with vigorous stirring, until the colour becomes light green

↓

Then add dropwise until the colour changes to bright yellow or to a faint pink at the rim — note the millilitres of potassium permanganate used (A)

↓

To 50 ml of the clear filtered juice in a 250-ml flask, add 25 ml of the gelatin solution and make up to volume with acid sodium chloride solution

↓

Transfer to a conical flask, add a little filter aid, shake for 15 min and filter

↓

To 50 ml of the filtrate (= 10 ml of juice), add 20 ml of the indigo carmine solution and about 500–750 ml of water, and titrate with potassium permanganate solution as before — note the millilitres used (B)

Calculation

A = total tannin-like materials; B = non-tannin materials; A − B = true tannins.

% tannin (as gallotannic acid) = (Titre × 100 × g of tannin per ml of KMnO₄ solution) ÷ ml of sample taken

Where,

• Titre = millilitres of potassium permanganate solution used for the true tannins, i.e. A − B
• g of tannin per ml of KMnO₄ solution = 0.0042 g per ml of 0.1 N KMnO₄, as printed
• ml of sample taken = millilitres of sample taken for the titration

Note: the book gives the equivalence as “one ml of 0.1 N KMnO₄ = 0.0042 g of tannin (gallotannic acid)” while the titrant prepared in the procedure is 0.04 N potassium permanganate; carried as printed.

References

Official Methods of Analysis, Association of Official Analytical Chemists, Washington, D.C., p. 240 (1970).

Note: the extracted source reads “D.C..urth edn.” — the edition number is illegible; carried without it.

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.