Vintage titrations: tannin in wine

White, or red, science is always the perfect vintage. 

Equipment

Materials per group

• Samples of red wine (for white wine see below), 50 cm³
• Potassium manganate(VII), 0.004 mol dm^–3, 50 cm³
• Activated charcoal, 1 g 
• Deionised water 
• Indigo carmine indicator solution, 0.5%, 10 cm³. (The indigo carmine indicator is made up by dissolving 0.5 g of the dyestuff in 60 cm³ warm deionised water. The solution is cooled, 4 cm³ of conc H₂SO₄ is added, and the volume made up to 100 cm³ with deionised water. The solution is filtered through a No. 42 Whatman paper). 

Equipment per group

• Burette, 50 cm3 
• Pipettes, 5 cm3 and 2 cm3 
• Funnel
• Filter paper
• Measuring cylinders, 10 cm³, 25 cm³ and 250 cm³ 
• Conical flasks, 250 cm³
• Beaker, 50 cm³
• White tile
• Safety glasses

Health, safety and technical notes

• Read our standard health and safety guidance here. 
• Wear eye protection.
• Solutions themselves are of low hazard, but eye protection is advisable when heating any liquid.
• This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete.

Commentary

The colour of red wine is due to the presence of anthocyanidins, a class of flavonoids.

Tannin is a collective name for other largely colourless but bitter flavonoids which are also present in the wine. In making red wine the crushed grapes are put into vats. Some of the stems, the skins, and the pulp remain with the juice forming a residue which is known as the must.

The alcohol in the fermenting juice extracts colour from the skins and the longer the juice is in contact with the must the darker the wine. In the process tannin is also extracted into the wine.

White wine, which should not pick up any colour from the grape skins, is made by pressing the grapes as quickly as possible and the juice alone is then set to ferment. The level of tannin in white wines is only about one-tenth of that found in red wine.

The procedure described below is based on that described by Professor G.W.A. Fowles of University of Reading.2 During trialling most students needed a lot of guidance to work through this procedure. 

Procedure

Actual titration

A funnel is placed in the neck of a 250 cm³ conical flask. 5 cm³ of wine is pipetted and 10 cm³ of deionised water is added. The flask is heated gently until the volume of the wine and water is reduced to 5–7 cm³. The alcohol will now have boiled off. 25 cm³ of cold deionised water is now added and 2 cm³ of indigo carmine indicator is added by using a pipette. (As the indicator uses up some potassium manganate(VII) it is important to measure it out carefully so that it can be allowed for in the ‘blank’.) 0.004 mol dm³ KMnO₄ is placed in the burette and the mixture is titrated. A golden yellow colour appears at the end point. Let this titre be A cm³.

Blank titration

20–25 cm³ of the wine is placed in a beaker with 1 g of activated charcoal and the mixture is stirred thoroughly. The mixture is then filtered and 5 cm³ of the decolorised wine is transferred to a 250 cm³ conical flask using a pipette. The procedure described in ‘Actual titration’ is repeated. This blank titration will allow for the indicator and for any oxidisable substances in the wine apart from the anthocyanidins and the tannins. Let the volume of the blank titre be B cm³.

Calculation

The amount of potassium manganate(VII) used in oxidising the tannins (and anthocyanidins) is A – B = C cm³.

Tannins are of variable composition. The titration is referred to a standard tannin solution, for which 1 cm³ of 0.004 mol dm^–3 KMnO₄ = 0.0832 mg tannin.

Therefore, % tannin in wine = 0.01664 C

The level of tannin for burgundies and clarets will be in the range 0.15 – 0.4%.

Extension 

In his book Chemistry in the Marketplace Ben Selinger describes a series of experiments on wine analysis with Australian wines which are designed for first year undergraduates.

Notes

This resource is part of a collection of problem-solving activities, designed to engage learners in small group work. Find out how to use these resources, and obtain a list of suggested ‘junk items’ here.