196 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Special Tests. 



(1) Phloroglucinol Reaction. On adding an equal volume of 

 concentrated hydrochloric acid and a small quantity of phloroglucinol 

 to a solution of a pentose and heating the mixture in a boiling water- 

 bath, it gradually becomes cherry red in colour and turbid and a pre- 

 cipitate is formed. The precipitate is dissolved by amyl alcohol, if 

 the cold solution be shaken up with this solvent ; the amyl alcohol 

 solution will show on examination with a spectroscope an absorption 

 band between D and E. 



The formation of a precipitate is not itself sufficient evidence for 

 the presence of a pentose, since a precipitate may be formed on heating 

 other substances with acid and phlorogucinol. 



(2) Ordnol Reaction (Tollens). If a mixture of equal parts of con- 

 centrated hydrochloric acid and pentose, or pentosan solution, be heated 

 with a little orcinol, the solution becomes red, then violet, finally blue or 

 blue-green with the separation of a precipitate. The appearance of 

 the green colour may be hastened by adding a drop of ferric chloride to 

 a portion of the solution. The remainder &i the solution on being shaken 

 with amyl alcohol imparts a bluish-red colour to the amyl alcohol, 

 which gradually becomes green. The solution on examination with 

 a spectroscope shows an absorption band between C and D but near D. 



Bial's Modification. On adding Bial's reagent (p. 614) drop by 

 drop to about 5 c.c. of a boiling solution of a pentose, a bright green 

 colour is produced. This is soluble in amyl alcohol, as above. 



F. GLYCURONIC ACID. 



Glycuronic acid resembles the pentoses in its reactions. 



A solution of glycuronic acid may be prepared by boiling a smally quantity 

 of Indian yellow with dilute hydrochloric acid, cooling, filtering off the 

 euxanthone and neutralising the solution. 



(1) It reduces Fehling's solution. 



(2) It gives the phloroglucinol and orcinol reactions. 



(3) It does not ferment. 



(4) The free acid is dextrorotatory, but when combined with euxanthone 

 or other compounds it is laevorotatory. 



