192 PHOTOSYNTHESIS 



heating an alcohol solution of the components is sparingly soluble and 

 melts at 204-205°." 



l-Xylose. The hydrazones of 1-xylose are in general more soluble 

 than those of 1-arabinose. A characteristic compound of 1-xylose is the 

 double salt cadmium bromide xylonate, (C5H906)2Cd . CdBra . 2H2O, 

 which is sparingly soluble. Arabinose does not form an analogous com- 

 pound. The xylose in about a 20 per cent solution is treated with an 

 equal weight of bromine and twice the weight of cadmium carbonate, 

 heated gently and allowed to stand for twelve hours. Then the mixture 

 is evaporated almost to dryness, a small amount of water is added, 

 filtered and washed. The filtrate is concentrated to a small volume and 

 an equal volume of alcohol is added. After some time the crystals are 

 formed which can be analyzed for cadmium and bromine and have an 



20 



(a) — = -f 7.4°. The reaction succeeds even when in the sugar mix- 

 ture only about one-fifth is xylose. ^^ 



d-Ribose is still an exceedingly rare pentose and has been found as a 

 constituent of the pentoside vernin and also of the nucleinic acid investi- 

 gated by Levene and Jacobs. ^^ 



e. Quantitative Methods of Pentose Determination. 



The formation of furfural from pentoses on treatment with hydro- 

 chloric acid has been used very extensively for the quantitative determina- 

 tion of this group of carbohydrates. The method consists essentially in 

 heating the sample in a distillation apparatus with 12 per cent hydro- 

 chloric acid, adding more acid as this distills over and determining the 

 furfural in the distillate. For a number of reasons the method is not en- 

 tirely satisfactory and many modifications of the original Stone and 

 Tollens ^° method have been devised. These modifications consist largely 

 in the method of determining furfural in the distillate, in the method of 

 heating the sugar mixture with the acid and in the method of distillation. 



One of the first difficulties in the quantitative determination of pen- 

 toses by means of the furfural method is occasioned by the fact that 

 not only pentoses but also hexoses yield furfural-like substances. The 

 amount of the latter formed from sucrose has been variously reported 

 as yielding 0.2 to 1.5 per cent. Hexoses form oxymethylfurfural and 

 methylpentoses methylfurfural. These substances behave so much like 

 furfural, that in the analytical methods employed they can ordinarily 



'^Neuberg Ber. chem. Ges., 33, 2243 (1900) ; Zeit. Physiol. Chem., 35, 38 (1902). 

 Miither and Tollens, Ber. chem. Ges., 37, 312 (1904). Tollens and Maurenbrecher, 



' ' ^'Bertrand, Bull'. Soc. chim. (3). 5, 546, 554 (1891). Widtsoe and Tollens, 

 Ber chem Ges.. 33, 136 (1900). Nef, .^nn. Chem.. 403, 253 (1914). 



^Levene and Jacobs, Ber. chem. Ges.. 42, 2102, 2469, 2474, 3247 (1909). Levene 

 and Clark, Jour. Bio. Chem., 46, 22 (1921). Braun, Ber. chem. Ges.. 46, 3949 



(1913) 



~ Stone and Tollens, Ber. chem. Ges., 21, 2150 (1888). 



