OXALIC ACID. 153 



1. Oxalic Acid. 



Occurrence. Very widely distributed in nature ; in 

 the form of the acid potassium salt in the different 

 varieties of Oxalis ; in the form of the calcium salt in 

 a number of plants ; in urine (some of the urinary cal- 

 culi consist entirely of this salt) ; in the form of the 

 ammonium salt in guano. 



Formation. By the action of finely divided sodium 

 on dry carbonic anhydride at 350-360 ; by heating 

 sodium formate; by the decomposition of cyanogen 

 with water ; by the heating of cellulose (paper, linen) 

 with potassium hydroxide ; the most important method 

 of formation is, however, by the oxidation of a great 

 many organic substances with nitric acid, hyperman- 

 ganic acid, etc. 



Preparation. The expressed juice of oxalis plants is 

 precipitated by means of a solution of sugar of lead, 

 the precipitate decomposed with sulphuric acid or sul- 

 phuretted hydrogen, and the filtrate evaporated to 

 crystallization. Or 1 part sugar or starch is heated 

 with 8 parts of nitric acid (specific gravity, 1.38) until 

 action has ceased, and the solution then evaporated to 

 dryness. On the large scale it is also produced by 

 heating sawdust with caustic potassa or soda. 



Properties. Colorless prisms, soluble in 15 parts 

 water, more easily soluble in alcohol. It contains 2 

 molecules water of crystallization, which are given off 

 at 100. "When carefully heated up to 150, the efilo- 

 resced crystals can be completely sublimed ; when 

 rapidly heated, it is partially resolved into carbonic 

 anhydride, carbonic oxide, formic acid, and water. 

 Oxidizing agents transform it into carbonic anhydride 

 and water. Sulphuric acid resolves it into water, car- 

 bonic anhydride, and carbonic oxide. Nascent hydrogen 

 (zinc and hydrochloric acid) converts it into glycolic 

 acid (p. 145) and acetic acid. 



Strong, bibasic acid. Its salts, with the exception of 



