16 FRUITS AND FRUIT PRODUCTS. 



DETERMINATION OF TARTARIC, CITRIC, AND MALIC ACIDS. a 



Use the filtrate from the alcohol precipitate in the determination 

 of the organic acids. After evaporating the alcohol and taking up 

 the acids with water, add lead subacetate until the solution is alkaline, 

 then filter and wash the precipitate until only a slight amount of 

 lead remains in the washings. Wash the precipitate off the filter 

 paper into a beaker with hot water, precipitate the lead with hydro- 

 gen sulphid and filter the lead sulphid while hot, washing with hot 

 water. Evaporate the filtrate which contains the free organic acids to 

 about 50 cc and neutralize exactly with potassium hydroxid, using 

 phenolphthalein as indicator. Add an excess of strong solution of 

 neutral calcium acetate with constant stirring, and allow to stand from 

 six to twelve hours. Throw the precipitate of calcium tartrate on a 

 filter paper and wash until filtrate and washings make exactly 100 cc; 

 ignite the filter paper and precipitate, determine the lime by titration, 

 and calculate the tartaric acid therefrom. A correction of 0.0286 

 gram of tartaric acid which is held in solution in the 100 cc of wash- 

 ings as calcium tartrate must be added. Evaporate the filtrate to about 

 20 cc, and if a precipitate of calcium citrate is formed filter it hot, 

 wash with hot water, ignite, and titrate the lime. From this calculate 

 the citric acid. Again evaporate the filtrate to about 20 cc and add 3 

 volumes of 96 per cent alcohol by volume, which will throw down the 

 calcium salt of tartaric acid held in solution, the rest of the citrate, 

 and the malate and succinate. Filter this, ignite, titrate, and after 

 subtracting the tartaric acid present, calculate the rest as malic acid, 

 since the amount of citric and succinic acid present is very small. 



DETERMINATION OF TARTARIC ACID. b 



To 100 cc of the fruit juice add 2 cc of glacial acetic acid, 2 or 3 

 drops of a 20 per cent solution of potassium acetate, and 15 grams of 

 pure, finely powdered potassium chlorid. Dissolve this by shaking and 

 then add 20 cc of 96 per cent alcohol. Stir vigorously for one minute, 

 rubbing the walls of the beaker with the glass stirring rod to start the 

 crystallization of the potassium bitartrate, and allow to stand for fif- 

 teen hours at room temperature. Filter and transfer the precipitate 

 to a Gooch crucible with a thin asbestos felt and wash with a mixture 

 of 15 grams of potassium chlorid 20 cc of alcohol and 100 cc of water, 

 using the vacuum pump to aid filtration. The beaker is rinsed three 

 times with a few cubic centimeters of this solution, and the precipi- 

 tate is also washed with it, but in such a way that not more than 20 cc 

 in all of the wash solution are used. The precipitate and asbestos 

 filter are washed into the beaker with water and heated to boiling. 



a A modification of Schmidt and Hiepe's method. U. S. Dept. of Agric., Div. of 

 ('hem. Bui. 46, revised, p. 67. Ztsch. anal. Chem., 1882, 21, 534-541, 

 h Halenke and Moslinger, Zteehr. anal. Chem., 1895, 34, 283. 



