112 ORGANIC AGRICULTURAL CHEMISTRY 



polarized light to the right, i.e. it is dextro-rotatory and on this 

 account is known also as dextrose. This physical property, 

 being constant and definite in amount, is used as a means of 

 determining the quantity of glucose in a solution. That is, 

 the amount in a solution can be calculated by measuring the 

 angle through which it turns the plane of polarized light to the 

 right. This will be referred to again when we consider ordinary 

 or cane sugar. It is also a strong reducing agent. An am- 

 moniacal solution of silver nitrate is reduced to metallic silver 

 and an alkaline solution of copper sulphate is reduced to red 

 cuprous oxide, CU2O. This last reaction is also used as a 

 method of quantitatively determining the amount of glucose in 

 solution by weighing the cuprous oxide formed. The copper so- 

 lution used in this determination is made up of definite amounts 

 of copper sulphate, CUSO4, potassium sodium tartrate or 

 Rochelle salt, KNaC4H406, and sodium hydroxide, NaOH. 

 It is known as Fehling^s solution,^ smd may be used volumetrically 

 by titration, as in urine analysis, or gravimetrically by weighing 

 the cuprous oxide formed, as is done in the case of ordinary 

 solutions containing glucose. Fehling's solution may be used 

 in determining the amount of one of the other monosaccharoses, 

 viz. fructose, and also the two disaccharoses, malt sugar and 

 milk sugar. Glucose is a soHd, crystallizing in masses more or 

 less wax-like and containing one molecule of water of crystalliza- 

 tion. As ordinarily obtained under the name of glucose sirup 

 it is a thick, viscous liquid. It is readily soluble in water and 

 almost insoluble in absolute alcohol. It tastes only slightly 

 sweet. It has numerous industrial applications, in dyeing, in 

 the making of confectionery and jellies and in pharmaceutical 

 preparations. It is not ordinarily obtained from its natural 

 sources, but is made by chemical transformation from other 

 members of the carbohydrate group. All three of the common 



^ Allihn's modification of Fehling's solution is made as follows : 



Solution A : CuS04 • 5 H2O, 69.2 g. per litre. 



Solution B : (Rochelle salt, 346 g. + KOH, 250 g.) per litre. 



A and B are mixed in equal volumes and the mixed solution used freshly prepared. 



