82 THE VEGETABLE ACIDS [ch. 



Expt. 79. Tests for formic acid. Make a solution of formic acid (1 c.c. acid : 

 100 c.c. water) and perform the following tests : 



{a) Acidify 10 c.c. with a few drops of strong hydrochloric acid and add a little 

 magnesium powder. The formic acid will be reduced to formaldehyde. Filter and 

 test for the latter by Schryver's test (see p. 39). 



(6) Neutralize a few c.c. of the solution with dilute caustic soda and add a few 

 drops of 5 "/o mercuric chloride solution and heat. The mercuric salt is reduced to 

 mercurous chloride which is precipitated, being insoluble. 



Expt. 80. Detection of formic acid in the Nettle (Urtica dioica). Take a strong 

 filter-paper (about 10 cms. in diameter) of the best quality and soak it in a concen- 

 trated solution of barium hydroxide. Allow the paper to dry in air, whereby the 

 barium hydroxide is converted into carbonate. Take at least 200 Nettle leaves, and, 

 with gloved hands, carefully blot both sides of the leaves between the folded paper. 

 Break up the paper in about 40 c.c. of distilled water, warm and filter on the pump. 

 Wash with 10 c.c. of hot water. To the filtrate containing barium formate add 

 0'5 gra. of glacial phosphoric acid and distil with a water condenser. Add about 

 20 drops of strong hydrochloric acid to the distillate and then magnesium powder. 

 When hydrogen is no longer evolved, filter,and test for formaldehyde by Schryver's 

 reaction. A positive result will be obtained. 



Acetic acid has been found to occur in plants, both in the free state 

 and as salts and esters. Possibly, however, in some cases it may have 

 arisen from the decomposition of other substances during distillation. 



Propionic acid has rarely been detected in plants. Butyric, isobutyric 

 and caproic acids have been detected in a few plants. 



Isovaleric acid has been isolated from various plants, notably species 

 of Valerian ( Valeriana). 



Esters of the above acids form important plant constituents since 

 they are responsible for many fruit odours. Amyl acetate, for instance, 

 occurs in the fruit of the Banana (Musa sapientum): amyl formate, 

 acetate and caproate are probably present in the fruit of the Apple 

 (Pyrus Malus), etc. Such compounds are frequently classed with the 

 "essential oils" (see p. 108). 



The next group to be considered are the monohydroxy-acids of the 

 methane series. Of these glycollic acid may be mentioned. 



GlycoUic acid, or hydroxy-acetic acid, CHg'OH'COOH, has been 

 isolated from unripe fruit of the Grape and from the leaves of the Virginian 

 Creeper (Ampelopsis hederacea). Also from the Sugar-cane (Saccharum 

 ojfficinarum), the Lucerne (Medicago sativa) and the Tomato (Lycoper- 

 sicum esculentum). Its relationship to the amino-acid, glycine (see p. 134), 

 should be borne in mind. 



