510 CONSIDERATION OF CARBON COMPOUNDS. 



tion. This fact indicates that the iron is held in a complex ion, Fe(C 2 O 4 ) 2 // , 

 since the color of simple ferrous salts in solution is usually pale green. 

 The salt has strong reducing properties and is used as a developer in 

 photography. 



Potassium ferric oxalate, K 3 Fe(C 2 O 4 ) 3 , gives a green solution, and the iron is 

 ,robably held in a complex ion, Fe(C 2 O 4 ) 3 /// . It is rapidly reduced by sun- 

 light, thus, 



2K 3 Fe(CA) 3 = 2K 2 Fe(C 2 4 ) 2 + K 2 C 2 O 4 + 2CO 2 , 

 and, therefore, is useful in making platinotypes in photography. 



Hydroxy-acids. 



In the acids heretofore considered, the hydrogen is derived either 

 from the hydrocarbon radical or from carboxyl. There are, however, 

 compounds containing as a third radical hydroxyl i. e., that radical 

 characteristic of alcohols. Consequently we may look upon these 

 compounds as acids into which alcoholic hydroxyl has been intro- 

 duced, or as alcohols into which carboxyl has been introduced. The 

 acid properties of these compounds are so predominating that the 

 compounds are spoken of as acids, and according to the number of 

 carboxyl groups present we have monobasic, dibasic, etc., acids. The 

 hydrogen of the carboxyl is, of course, replaceable by metals, while 

 the hydrogen of the alcoholic hydroxyl can be replaced by hydrocar- 

 bon radicals. In order to indicate this difference in the function of 

 the hydrogen the number of the respective groups present is given in 

 the name. Thus, tartaric acid, which contains 2 hydroxyl and 2 car- 

 boxyl groups, is designated as a dibasic hydroxy-acid, or as dihy- 

 droxy-dicarboxylic acid, while citric acid, which contains 1 hydroxyl 

 and 3 carboxyl groups, is a monohydroxy-tribasic acid or hydroxy- 

 tricarboxylic acid. 



Of the several methods known for obtaining hydroxy-acids only one shall be 

 mentioned. It corresponds to one of the methods used for the introduction of 

 hydroxyl into hydrocarbons ; in one case the halogen of a hydrocarbon, in the 

 other case the halogen of an acid is replaced by hydroxyl : 



CH 3 Br + H 2 : CH 3 OH + HBr, 



Brom-acetic acid. Hydroxy-acetic acid. 



It is evident from what has been said that we have running parallel to every 

 series of acids another series of hydroxy-acids. For instance thus : 



