HYDROXY-, KKTO- AND DIBASIC ACIDS 115 



Malic Acid. 



Malic acid is contained in apples, pears and other fruits. It is 

 usually prepared from rhubarb stalks or unripe mountain ash berries. 

 The juice is boiled with milk of lime ; the neutral calcium salt is 

 precipitated. The salt is recrystallised from dilute nitric acid and the 

 acid salt is so obtained. It is decomposed with the calculated quantity 

 of sulphuric acid, the liquid is filtered from calcium sulphate and 

 evaporated. Malic acid crystallises out. 



Malic acid crystallises in groups of colourless 4 or 6-sided prisms. 

 It is deliquescent and readily soluble in water, alcohol and ether. 



On heating to about 180, it melts and loses water yielding 

 fumaric and maleic acids. 



Malic acid is optically active and contains one asymmetric carbon 

 atom. The natural form is /-malic acid. 



The salts of malic acid resemble those of citric, oxalic and 

 tartaric acids. Calcium malate is not precipitated in the cold. On 

 boiling in neutral and concentrated solution calcium malate is pre- 

 cipitated ; alcohol precipitates calcium malate from dilute aqueous 

 solution. A mixture of oxalic, tartaric, citric and malic acids may 

 thus be separated. The oxalate and tartrate are precipitated from 

 dilute solution in the cold ; on boiling the filtrate calcium citrate is 

 precipitated, and on adding 2 volumes of alcohol to the filtrate 

 calcium malate is thrown down. 



Tartaric Acid. 



Tartaric acid occurs in certain plant juices ; its only important 

 source is grape juice. During fermentation a deposit forms on the 

 bottom lees and a crystalline crust on the sides tartar or argol of 

 the cask. The argol consists mainly of potassium hydrogen tartrate and 

 calcium tartrate. Their precipitation is due to their insolubility in 

 the alcohol as it is produced. If the crude argol be boiled with water 

 and filtered and the solution crystallised, cream of tartar separates out, 

 the term cream of tartar having arisen from the fact that the salt 

 collects in crusts on the surface during the evaporation. 



Preparation. 



Tartaric acid is prepared from tartar by dissolving it in water and 

 neutralising with lime. Insoluble calcium tartrate is thrown down and 

 from the solution, which still contains tartaric acid, insoluble calcium 

 tartrate is precipitated by adding calcium sulphate or calcium chloride. 

 The insoluble calcium salt is decomposed with sulphuric acid and tar- 

 taric acid isolated from the solution by crystallisation. 



