86 THE VEGETABLE ACIDS [ch. 



Of the dihydroxy-dibasic acids, tartaric acid is the best known. It 

 should be noted that tartaric acid is dihydroxy-succinic acid. Thus the 

 three acids are related as follows: 



Succinic acid COOPI • CHg • CHg ' COOH 

 Malic acid COOH -CHOH 'CHa 'COOH 

 Tartaric acid COOH • CHOH • CHOH • COOH 



Tartaric acid is widely distributed in plants, often in the form of 

 the calcium or potassium salts. It occurs in many fruits, as for instance, 

 those of the Grape (Vitis vinifera), Tomato (Lycopersicum esculentum). 

 Mountain Ash (Pyrus Aucuparia) and Pineapple {Ananas sativus); it 

 has also been detected in the leaves and vegetative parts of many plants. 

 Tartaric acid is easily soluble in water from which it crystallizes in colour- 

 less prisms. Calcium tartrate is only slightly soluble in cold water, though 

 more so in hot. On adding calcium chloride to a soluble tartrate, calcium 

 tartrate is precipitated, more or less rapidly according to the strength of 

 the solution, and sometimes as a crystalline precipitate. The crystals may 

 occur as characteristic rhombic prisms with octahedral faces or as needles. 

 The precipitate is soluble in acetic acid. The acid potassium salt of 

 tartaric acid is soluble with difficulty in water and hejice is used in 

 identification of the acid. 



Racemic acid, which is a combination of dextro- and laevo-tartaric 

 acids, is also found in certain varieties of the Grape. Calcium racemate 

 is insoluble in acetic acid. It is soluble in hydrochloric acid from which 

 it separates out rapidly in a crystalline state on neutralizing with ammonia 

 (tartrate only separates slowly). 



Expt. 86. Tests for tartaric acid. A. Take a 1 % solution of tartaric acid, 

 neutralize with caustic soda (or use a soluble tartrate) and make the following tests r 



(a) Add a few drops of 5 % calcium chloride solution. A white precipitate of 

 calcium tartrate is formed. Add an equal volume of glacial acetic acid and warm ; 

 the precipitate dissolves. 



(6) Add a few drops of 5 % lead acetate solution. A white precipitate of lead 

 tartrate is formed. Add acetic acid and warm ; the precipitate dissolves. 



(c) To 2-3 c.c. in a test-tube add a few drops of ferrous sulphate solution. Place 

 the test-tube in a beaker of cold water, and add a few drops of hydrogen peroxide 

 followed by an excess of caustic soda solution. A deep violet or blue colour is obtained. 

 The colour is due to the formation of dihydroxymaleic acid and the reaction of this 

 with the ferric salt present. 



{d) To one drop of tartrate add 2 drops of a 2 % solution of resorcinol and then 

 3 c.c. of strong sulphuric acid. Heat gently. A rose colour is formed which deepens 

 to a violet-red. 



