190 PHOTOSYNTHESIS ■ 



mixture of substances, the identification and quantitative determination 

 of any particular substance becomes very difficult. The admixture of vari- 

 ous substances makes the ordinary reactions or tests uncertain or com- 

 pletely inhibits them. Plants always contain a mixture of carbohydrates, 

 their separation and quantitative determination cannot be attained with 

 great precision. This is also due to the fact that the proportion of differ- 

 ent carbohydrates is often such that one is present in very much lower 

 concentration than another. Thus, the amount of glucose and fructose 

 is usually very much greater than that of either arabinose or xylose ; this 

 adds to the difficulty of the determination of the latter. In many cases a 

 qualitative identification of the dififerent carljohydrates present is of con- 

 siderable value. 



For the identification of the pentoses and methyl pentoses and the dif- 

 ferentiation of these a number of color reactions have been devised. 

 Most of these reactions depend upon the fact that when treated with 

 strong acids pentoses yield furfural, methylpentoses yield methylfurfural 

 and hexoses oxymethylfurfural. These reactions are especially valuable 

 for the determination of the presence or absence of a group of carbo- 

 hydrates. At the same time, because the reactions are at times afifected 

 by the presence of other carbohydrates, a negative result does not always 

 establish the absence of a particular carbohydrate. We cannot take up the 

 detailed manipulation and interpretation of the various tests but must refer 

 to the original literature and si>ecial publications on the subject.-^ 



Phlaroglucin reaction. To the solution to be tested is added sufficient 

 hydrochloric acid to make about an 18 per cent solution, a small quantity 

 (about 0.1 gram) phloroglucin is added and the mixture is heated in 

 the boiling water bath for 10-15 minutes. In the presence of pentoses 

 a red-violet coloration is produced. For identification the absorption spec- 

 trum of the colored solutions produced is invaluable. The solution is 

 shaken out with amylalcohol (free from furfural!) and the extract ex- 

 amined spectroscopically. One absorption band, between the D (sodium) 

 and E (thallium) lines, is characteristic for pentoses. The reaction is also 

 produced by glucuronic and galacturonic acids. In the presence of im- 

 purities and hexoses a precipitate of varying density may be formed. 

 The cooled solution may, under these circumstances, be filtered and the 

 precipitate washed. The filter paper is washed with alcohol and the clear 

 solution examined spectroscopically. The reaction is very sensitive, the 

 absorption band is not produced by hexoses or methyl pentoses, but these 

 in large excess, diminish its sensitivity. The absence of the characteristic 

 band does not definitely exclude the. presence of pentoses.-^ 



Orcin reaction. There are various forms of this reaction ; the modifica- 



**Van der Haar, A. W., "Anleitung zum Nachweis. zur Trennung und Bestim- 

 mung der Monosaccharide und Aldehydsauren," Berlin, 1920. Abderhalden, E., 

 "Handbuch der Biochemischen Arbeitsmethoden," II, p. 95, Berlin, 1910. Tadokoro, 

 Jour. Col. Agri. Hokkaido Univ., 10, pt. 6. 141 (1923). 



"Wheeler and Tollens, Ann. Chem., 254, 329 (1889). Tollens, Ber. chem. Ges., 

 29, 1202 (1896). Pinoff, ibid., 38, 766 (1905). 



