Take 100 c.c. of the wine, expel the carbon dioxide, and neutralise the free acids, if necessary, with standard alkali so far as to obtain an acidity of 05 gram per 100 c.c, calculated as tartaric acid. (Take the total acidity, already determined on another portion of the wine, as a guide to the amount of alkali to be added.)
Add 1 c.c. of a 40 per cent. solution of sodium acetate, and then, drop by drop, a 10 per cent. solution of ferric chloride until no further precipitate is produced. Each drop of the ferric chloride solution precipitates 0.05 gram of tannin.
For the more precise determination of the tannin in wine the method (Pi's) described under "Cider " (p. 483) may be used, with the following modification.
Ten c.c. of the wine (or 5 if rich in tannin) are placed in a porcelain basin, and 5 c.c. of the ammoniacal acetate of zinc added. The mixture should be alkaline, and turn brown; if not, a few drops of ammonia solution are added. Evaporate the liquid on the water bath until the volume is reduced to one-third or less, then add boiling water equal in amount to the original volume of liquid evaporated (15 or 10 c.c.), raise to boiling over a flame, and filter. Wash the precipitate thoroughly with hot water, and dissolve it in dilute sulphuric acid, including any traces remaining adherent to the basin.
The acid solution thus obtained is diluted to a litre, 10 c.c. of the standard indigo solution are added, and the mixture titrated with the permanganate solution. Let c be the number of c.c. of permanganate required.
If a = the number of c.c. of wine taken; b the number of c.c. of permanganate required to oxidise 10 c.c. of indigo solution, and x the tannin content of the wine, in grams per 100 c.c., x = (c-b) X 0.001 X 100 / a = c-b / 10a.
The method has been found to give 93 per cent. of the tannins, and the result is therefore corrected accordingly, so that finally x =100/93 x c-b/10a = 0.107 xc-b/a