VI] THE VEGETABLE ACIDS 87 



B. Take 2-3 cm. of a strong solution of tartaric acid, acidify with glacial acetic 

 acid and add a little potassium acetate solution. A white crystalline precipitate of 

 potassium hydrogen tartrate will be formed. 



Expt. 87. Identification of tartaric acid in grapes. Take 150-200 gms. of unripe 

 gi-apes (early July) and boil them well with the minimum amount of water in an 

 evaporating dish. As they soften they should be well stirred and crushed. Then 

 filter and squeeze the mass through strong linen. Neutralize the filtrate with caustic 

 soda, heat to boiling and filter on a pump. Cool the filtrate, and add 2-3 c.c. of 

 saturated calcium chloride solution. Allow the mixture to stand for 24 hours. 

 A crystalline precipitate will separate out. Under the microscope this will be seen 

 to consist of needles and octahedra. The needles are a double salt of ci?-tartaric and 

 ^-malic acid (Ordonneau, 2) ; the octahedra consist either of tartaric acid or racemic 

 acid or a mixture of both. Filter off" this precipitate and heat in 50 ^Jq acetic acid. 

 The double salt and the tartaric acid will dissolve, but octahedra of racemic acid (if 

 present) will remain undissolved. Filter and make the following tests with the filtrate : 



(a) Add to a small quantity in a test-tube, resoreinol and sulphuric acid as in 

 Expt. 86 A{d); a positive result is given. 



(6) Evaporate down the remainder on a water-bath and add potassium acetate 

 and acetic acid as in Expt. 86 ^ ; potassium hydrogen tartrate crystallizes out. 



If octahedra are left undissolved after treating with 50% acetic acid, racemic 

 acid is present. Heat this residue with dilute hydrochloric acid. It will go into 

 solution. Neutralize a portion with ammonia, and the acid will crystallize out at 

 once. Test another portion with resoreinol as in (a) ; a positive result will be given. 



Of the tribasic acids, citric acid, C3H4 • OH * (C00H)3, is the most 

 important. 



Citric acid occurs in large quantities in fruits of the genus Citrus, i.e. 

 in the Orange, Lime, Lemon, etc. Also in many other fruits, such as the 

 Gooseberry, Currant, Tomato, etc. 



Expt. 88. Tests for citric acid. A. Take a 1 % solution of citric acid, neutralize it 

 with caustic soda (or use a soluble citrate) and make the following tests : 



(a) Add 5 o/^ calcium chloride solution. No precipitate is given. Heat to boiling 

 and a white precipitate of calcium citrate is formed. Calcium citrate is soluble in 

 cold water but insoluble in hot water. 



(6) Add 5 «/o lead acetate solution. A white precipitate of lead citrate is formed. 

 Add an equal quantity of acetic acid and warm : the precipitate is soluble. 



B. Take 5 c.c. of a 2 % solution of citric acid and add 3 c.c. of Denig^s' reagent 

 (prepared by dissolving with the aid of heat 1 gm. of mercuric oxide in a mixture of 

 4 c.c. of strong sulphuric acid and 20 c.c. of distilled water). Boil, and add a 2 % 

 solution of potassium permanganate drop by drop. The permanganate is at first 

 decolorized, but, on further cautious addition, the colour persists. Finally the liquid 

 becomes turbid and a white precipitate forms. This is due to a mercury compound 

 of acetone-dicarboxylic acid, resulting from the oxidation of citric acid by the per- 

 manganate. 



