CARBOHYDRA TES 



53 



oxide is rapidly filtered through a weighed Gooch 

 crucible, washed with boiling well-boiled water, and 

 dried. The residue may be weighed direct as CugO, 

 ignited to CuO, or dissolved off with nitric acid, and 

 estimated volumetrically or electrolytically. 



The factors used to determine the weight of dextrose 

 or cane sugar (see below) from the weight of copper are 



(2) Cane Sugar {Sucrose). — The aqueous extract 

 obtained above (i) is purified by treating with excess of 

 basic lead acetate^ solution, any precipitate filtered off, 

 and the excess of lead in solution removed by passing 

 in HgS. The HgS in solution is removed by bubbling 

 air through the liquid. The cane sugar is now 

 estimated polarimetrically by inversion (see 19, p. 33), 

 or the reducing sugars formed determined by means of 

 Fehling's solution as above. 



146. {z) Starch. 



The insoluble residue from (i) above is hydrolysed 

 to dextrose by boiling for two to three hours with dilute 

 hydrochloric acid (20 c.c. HCl, 200 c.c. water) under a 

 reflux. The digest is cooled, neutralised with sodium 

 carbonate, and the dextrose estimated as above. 



147. ij) Pentosans. 



The pentosans are insoluble carbohydrates, which are the 

 mother substances of the pentose sugars, such as xylose. Their 



^ See footnote, p. 168. 



