516 CONSIDERATION OF CARBON COMPOUNDS. 



tartrate. From the solution somewhat evaporated, tartar emetic 

 separates in colorless, transparent rhombic crystals : 



2KHC 4 H 4 O 6 + Sb 2 O 3 = = 2KSbO.C 4 H 4 O 6 + H,O. 



Potassinm acid Antimonous Tartar emetic, 



tartrate. oxide. 



The fact that not antimony itself, but the group SbO, replaces the 

 hydrogen, has led to the assumption of the hypothetical radical SbO, 

 termed antimonyl. 



Tartar emetic is soluble in water, insoluble in alcohol ; it has a 

 sweet, afterward disagreeable metallic taste. 



Action of certain organic acids upon certain metallic oxides. The solu- 

 tion of a ferric salt (or certain other metallic salts) is precipitated by alkali 

 hydroxides, a salt of the alkali and ferric hydroxide being formed. When a 

 sufficient quantity of either tartaric, citric, oxalic, or various other organic 

 acids has been added previously to the iron solution (or to certain other metallic 

 solutions) no such precipitate is produced by the alkali hydroxides, because 

 organic salts or double salts are formed which are soluble, and from which the 

 metallic hydroxides are not precipitated by alkali hydroxides. Upon evapora- 

 tion no crystals (of the organic salt) form, and in order to obtain the com- 

 pounds in a dry state, the liquid, after being evaporated to the consistence of 

 a syrup, is spread on glass plates which are exposed to a temperature not 

 exceeding 60 C. (140 F.), when brown, green, yellowish-green, amorphous, 

 shining, transparent scales are formed, which are the scale compounds of the 

 U. S. P. 



Instead of obtaining these compounds, 'as stated above, by adding the 

 organic acids (or their salts) to the inorganic salts, they are more generally 

 obtained by dissolving the freshly precipitated metallic hydroxide in the 

 organic acid. 



The true chemical constitution of many of these scale compounds has not 

 as yet been determined with certainty. 



Of official scale compounds containing tartaric acid may be mentioned the 

 iron and ammonium tartrate, and the iron and potassium tartrate. The first com- 

 pound is obtained by dissolving freshly precipitated ferric hydroxide in a solu- 

 tion of ammonium acid tartrate ; the second, by dissolving ferric hydroxide in 

 potassium acid tartrate. The clear solutions, after having been sufficiently 

 evaporated, are dried, as mentioned above, on glass plates. 



Citric acid, Acidum citricum, H 3 C 6 H 5 O 7 .H 2 O = 2O8.5. Citric 

 acid is a tribasic acid containing three atoms of hydrogen replaceable 

 by metals ; its constitution may be expressed by the graphic formulas : 



/OH CH 2 .C0 2 H 



/>C0 2 H | 



C 3 H 4 or COH.CO 2 H 



\\C0 2 H | 



\C0 2 H CH 2 .C0 2 H 



Citric acid is found in the juices of many fruits (strawberry, rasp- 

 berry, currant, cherry, etc.), and in other parts of plants. It is 

 obtained from the juice of lemons by saturating it with calcium car- 



