THE PRODUCTS OF PHOTOSYNTHESIS 191 



tion of Bial has found much favor. The reagent consists of a solution 

 of 1 gram orcin in 500 cc. 25 per cent hydrochloric acid and 1.5 cc. of 

 a 10 per cent aqueous ferric chloride solution. The test is carried out 

 by adding sufficient reagent to the solution to be tested so that it con- 

 tains about 18 per cent hydrochloric acid and is boiled for a few minutes. 

 The solution turns blue-green or blue. It is shaken out with amylalcohol 

 (free from furfural) and examined spectroscopically. There are two 

 absorption bands between the Fraunhofer lines C and D. The weaker of 

 these, the one nearer the D line, is the only one which is produced only 

 by pentoses. The reaction is very sensitive but must be considered as 

 a test also for methylpentoses, glucuronic and galacturonic acids. It was 

 at first claimed the latter two acids do not give the test. This is only 

 the case when the acids are present in exceedingly small quantities. There 

 is a difference in the rate of reaction which also accounts for the differ- 

 ence in color formation. Xylose reacts faster than arabinose and this 

 faster than glucuronic acid.-* 



Naphfhoresorcin test for Glucuronic Acid. A means of differentiating 

 between pentoses and glucuronic acid has been devised by Tollens and 

 somewhat modified by Neuberg and Saneyoshi. Equal volumes of the 

 solution to be tested and concentrated hydrochloric acid are boiled one 

 minute with 1 cc. of a 1 per cent alcoholic solution of naphthoresorcin. 

 In the presence of pentoses and hexoses the solution rapidly colors. After 

 cooling the solution is shaken out with an equal volume of benzol and 

 the latter solution examined spectroscopically. Glucuronic acid yields a 

 red- violet solution with an absorption band at the D (sodium) line. 

 Tollens ^^^ obtained positive results with this test with some plant mate- 

 rial. Presumably the naphthoresorcin test applies also to galacturonic acid. 



A number of other color reactions have been devised for the detec- 

 tion of pentoses and methylpentoses depending upon the formation of 

 furfural and methylfurfural." Important among these is the reaction 

 of furfural with aniline acetate which gives a red color; methylfurfural 

 gives a yellow color. 



The identification of the pentoses can be accomplished under favor- 

 able conditions by the isolation of the sugar itself. But as crystalliza- 

 tion is often prevented by the presence of other substances, recourse must 

 be taken to special reactions. 



l-Arabinose. This sugar forms several characteristic substituted phenyl- 

 hydrazine compounds. The p-bromphenylhydrazone is quite insoluble 

 in water, recrystallized from alcohol or acetone it softens when heated 

 rapidly at 150° and melts at 165°.-° The diphenylhydrazone formed by 



»* Allen and Tollens, Ann. Chem., 260, 305 (1890). Bial, Deut. med. Wochschr., 

 28, 253 (1902). Lefevre and Tollens, Bcr. Chan. Gcs., 40, 4520 (1907). 



**" Tollens, Ber. chem. Ges., 41, 1788 (1908). Neuberg and Saneyoshi, Biochcm. 

 Zeit., 36, 56 (1911). Mandel and Neuberg, ibid., 13, 148 (1908). Van der Haar, 

 ibid., 88, 203 (1918). 



"Schiff, Ber. chem. Gcs.. 20. 540 (1887). 



^Fischer, Ber. chem. Ges., 27, 2490 (1894). 



