Chapter 7 



ACIDITY 



So many acids and bases, both organic and inorganic, occur in living 

 organisms that only a few of them can be considered here. Some of 

 the more common organic acids are citric in citrus fruits, tartaric in 

 grapes, malic in apples, and oxalic in rhubarb and spinach. In some 

 cases these acids are present in the plant tissues in the free condition. 



but often they occur as salts. Thus lemon juice contains about 5 per cent 

 of free citric acid, but much of the tartaric acid in grape juice has had 

 its acidity partially neutralized by potassium. Crystals of potassium 

 acid tartrate, "cream of tartar," often are deposited from grape juice 

 or wine on long standing. Similarly, the oxalic acid in some plants is 

 free, and in others exists as calcium oxalate. This is a matter of con- 

 siderable consequence because free oxalic acid, as well as its water-soluble 

 salts, is a strong poison. The halogeton weed, which grows in Nevada 

 and several other Western states, contains up to 18 per cent of soluble 

 oxalates on the dry weight basis. Livestock, particularly sheep, have 

 been killed by the thousands by eating this weed (see Fig. 7-1). Fortu- 

 nately, the oxalic acid in rhubarb and spinach is present largely in the 

 form of the very insoluble calcium oxalate which is nontoxic. Rhubarb 

 leaves, however, are said to contain harmful concentrations of soluble 

 oxalates. 



A whole series of organic acids is involved in the normal metabolism 

 of carbohydrates and fats in the animal body (Chap. 13). Among the 



