100 PLANT COMPOUNDS 



88. Some of the Common Organic Acids.— (a) Citric Acid, 



CH 2 .COOH 



I 



HO.C.COOH 



I 

 CH2.COOH 



is found in the free state in lemons, limes, currants, goose- 

 berries, cranberries, etc. It is obtained from lemons and 

 limes for commercial purposes, and is used in medicine and 

 for calico printing. It forms large rhombic crystals with 

 one molecule of water of crystallization, and is soluble in 

 water and in alcohol. On boiling with lime a tricalcium 

 salt is precipitated. 



(b) Malic Acid, 



HO.CH.COOH 



I 

 CH-.COOH 



occurs free in unripe apples, grapes, gooseberries, and in many 

 other fruits; as the acid potassium salt in some cherries and 

 rhubarb; and as the acid calcium salt in mountain ash 

 berries. It is prepared from the latter, and used to some 

 extent in medicine. It forms deliquescent crystals, soluble 

 in alcohol. The normal calcium salt is insoluble in alcohol. 



(c) Oxalic Acid, 



cooh 



I 



COOH 



is found in very many plants, frequently as the insoluble 

 calcium salt in the form of crystals (Fig. 3, d). It occurs 

 as the soluble acid calcium salt as well as the soluble acid 

 potassium salt in sorrel and rhubarb. It forms monoclinic, 

 efflorescent prisms with two molecules of water of crystal- 

 lization. It is soluble in water and alcohol. 



(d) Tartaric Acid, 



HO.CH.COOH 

 HO.CH.COOH 



is found as the acid potassium salt in grapes; and as the 

 acid potassium and acid calcium salts in pineapples. When 



