This section is from the book "Alcohol, Its Production, Properties, Chemistry, And Industrial Applications", by Charles Simmonds. Also available from Amazon: Alcohol: Its Production, Properties, Chemistry, And Industrial Applications.
This is equal to the difference between the total tartaric acid and the cream of tartar, expressed as tartaric acid.
Alternatively, the German official process may be used. In this, the total tartaric acid is determined according to the method given below, and from this is deducted the alkalinity of the ash of the wine, calculated as tartaric acid. The ash from 50 c.c. (or 25 c.c. if rich in sugar) is dissolved in 20 c.c. of iV/4.HCl and 20 c.c. of water, heated just to boiling over a small flame to expel carbon dioxide, and then titrated with N/4.NaOH, using delicate litmus paper as indicator. Each c.c. of N/4-HCl used up = 0.0375 gram of tartaric acid, since one molecule of potassium carbonate is produced from two molecules of cream of tartar.
A disadvantage of the foregoing process for total tartaric acid is the length of time required. If quicker results are wanted, the following method may be used: -
To 100 c.c. of the wine add 2 c.c. of glacial acetic acid, 05 c.c. of a 20 per cent. solution of potassium acetate, and 15 grams of powdered potassium chloride. Stir well to hasten the solution of the last ingredient. Add 15 c.c. of alcohol (95 per cent.), and rub the sides of the vessel well with a glass rod for about a minute to promote the crystallisation of the cream of tartar; then let the whole stand for at least fifteen hours.
Decant the liquid through a thin asbestos filter in a Gooch crucible, transferring no more of the precipitate than is necessary. Wash the precipitate and filter three times with a small quantity of a mixture of potassium chloride (15 grams), alcohol (20 c.c.), and water (100 c.c.), using not more than 20 c.c. of the mixture altogether. For this purpose, a test-tube fitted with a wash-bottle arrangement of blow-off tubes is convenient. Transfer the filter and the precipitate to the beaker in which the precipitation was effected, wash out the crucible with hot water, add about 50 c.c. of hot water to dissolve the cream of tartar, heat to boiling, and titrate the solution with decinormal alkali, using delicate litmus tincture or paper as indicator. To the actual number of c.c. required add 1.5 as correction for the solubility of the precipitate, and multiply the result by 0.015 to give the total tartaric acid in grams per 100 c.c. of the wine.