DIBASIC AND TRIBASIC ORGANIC ACIDS. 289 



Calcium oxalate, CaC 2 4 , is, in small quantities, a normal con- 

 stituent of urine. Ferrous oxalate, ferri oxalas, FeC 2 4 .H 2 0, is 

 made by adding potassium or ammonium oxalate to ferrous sul- 

 phate, when double decomposition takes place, and the ferrous 

 oxalate is precipitated as a pale yellow, crystalline, nearly in- 

 soluble powder. 



Dibasic acids with alcoholic hydroxyl. 



/OH 



Malic acid = C 4 H 6 O 5 or C 2 H 3 CO 2 H 



Tartaric acid = C 4 H 6 O 6 or C 2 H 2 



\\CO,H 

 \C0 2 H 



In the various acids heretofore considered, the hydrogen is 

 derived either from the unsaturated hydrocarbon residue, or 

 from the hydroxyl in the carboxyl. As shown by the graphic 

 formulas of the above two acids, they contain also hydrogen in 

 the hydroxyl form not in combination with .CO. This hydrogen 

 whilst not replaceable by metals, may be replaced by alcohol 

 radicals; in other words, it behaves like the hydroxyl hydrogen 

 in alcohols. In order to indicate this difference in the function 

 of the hydrogen, malic acid is said to be dibasic, but triatomic ; 

 tartaric acid is dibasic and tetratomic. A few other acids be- 

 have in a similar manner, as, for instance, lactic acid. 



Malic acid, H 2 C 4 H 4 5 , occurs in the juices of many fruits, as 

 apples, currants, etc. 



Tartaric acid, Acidum tartaricum, H 2 C 4 H 4 6 = 150. Frequently 

 found in vegetables, and especially in fruits ; grapes contain it 

 as potassium acid tartrate, which is obtained in an impure state 

 as a by-product in the manufacture of wine. During the fer- 

 mentation of grape-juice, the sugar is converted into alcohol; 

 potassium acid tartrate is less soluble in alcoholic fluids than in 

 water, and is, therefore, gradually deposited, forming the crude 

 tartar, or argol of commerce, a substance containing chiefly potas- 

 sium acid tartrate, but also calcium tartrate, some coloring 



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