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.
Two hundred c.c. of the wine are neutralised with solution of barium hydroxide, then very slightly acidified with three or four drops of a 3 per cent, solution of acetic acid, and concentrated to 15 c.c. under diminished pressure.
To the residue are added 2 c.c. of a solution of barium acetate (30 per cent.), and sufficient strong alcohol to give about 80 per cent. strength in the mixture. The precipitate thus obtained is filtered and washed with 80 per cent. alcohol: it contains barium tartrate, malate, and succinate; gums, pectins, albuminoids and tannins. It is taken up with water acidified with sulphuric acid (about 3 per cent.), of which 12 to 15 c.c. suffice to liberate the acids from their combination with the barium.
Strong alcohol is now added to the mixture, bringing the total volume up to 100 c.c. The gums, pectins, and albuminoids are precipitated, leaving the acids and tannins in the alcoholic solution.
1 "Vies," p. 136.
To eliminate the tartaric acid, 80 c.c. of the liquid are filtered off, some potassium chloride and potassium acetate added, and the volume is made up to 100 c.c. with water. This mixture is set aside for two or three days to allow of the deposition of the tartaric acid as cream of tartar. It is then filtered, and the precipitate washed with 65 per cent. alcohol. The filtrate retains only traces of tartrate, approximately 001 gram per 100 c.c., for which a corrective deduction should be made eventually.
In the filtrate, the malic and succinic acids'and the tannins are again precipitated by adding solution of barium hydroxide in slight excess, faintly acidifying again with acetic acid, and adding strong alcohol until the mixture is of about 80 per cent. alcoholic strength. The precipitate is filtered off, washed with alcohol, and dissolved in water slightly acidified with hydrochloric acid.
The estimation of the malic acid is made by oxidising the acid with potassium permanganate, but before this can be done it is necessary to remove the tannins. This is effected by adding a little solution of mercury and ammonium acetates made up in the following proportions: -
Mercuric acetate, 25 grants; ammonium acetate, 100 grams; water to 1 litre.
After filtering off the precipitate produced and expelling the small quantity of alcohol present, the filtrate is acidified with sulphuric acid and titrated with N/5 .permanganate solution. Only the malic acid is oxidised; the succinic acid is unaffected. To make the end.point more distinct, a known quantity of oxalic acid may be added to the solution before titration, and the result corrected accordingly. One c.c. of N/5.permanganate = 000223 gram of malic acid.
The titrated solution is mixed with clean sand, evaporated to dryness, and then extracted with ether to recover the succinic acid. The residue left on evaporating the ether is then titrated with standard alkali. If any small quantity of sulphuric acid has been co-extracted with the succinic acid, the amount should be determined gravimetrically and deducted from the titration result.