540 CHAPTER XXVI 



: It is evident that the greatest percentage errors occur when the cane sugar 

 is in large excess, as in the analysis of raw sugars. The remarks already 

 made concerning the necessity of standardization under the exact working 

 conditions apply equally here. 



Detection of Small Quantities of Reducing Sugars. — Since cane sugar 

 itself slightly reduces Fehling's solution this material is not adapted to detect 

 small quantities of reducing, sugars in the presence of cane sugar. This may 

 be done by means of Soldaini's solution, which, as used by the U.S. Bureau 

 of Standards, contains 297 grams of potassium bicarbonate and i gram 

 copper sulphate in 1,000 c.c. Ten grams of cane sugar give on two minutes' 

 boiling with 50 c.c. of this solution only i • i mgrms. of cuprous oxide. A 

 ver}^ delicate test for the purity of a sample of cane sugar is also afforded by 

 this means. 



Optical Assay of Fructose. — The rotation of fructose falls very rapidly 

 with rise of temperature. Hence by observation of the optical activity 

 at different temperatures the amount of fructose can be estimated. For 

 each 1° Centigrade rise in temperature and for i gram fructose in 100 c.c. 

 the rotation falls 0-0357° Ventzke. Assuming that the other sugars present 

 are not affected, the amount of fructose follows directly. 



Individual Estimation of Reducing Sugars in Mixtures. — Aldose sugars 

 may be estimated in the presence of ketose sugars, and vice versa, based on 

 their different behaviour towards the halogens, the former being readily 

 oxidized, while the latter are but little affected. Romijn's^^ method is as 

 below: — Ten grams of iodine and forty grams of borax are made up to 

 1,000 c.c. Twenty-five c.c. of this solution are mixed with the same quantity 

 of a solution of tKe mixed sugars containing not more than 0-15 gram. 

 The mixture is then kept for from 16 to 22 hours in a stoppered flask in a 

 thermostat at 25° C. After oxidation is complete the excess of iodine 

 remaining is determined by means of sodium thiosulphate, as in the iodo- 

 metric determination of copper described already in this chapter. For 

 two atoms of iodine, one molecule of an aldose sugar is accepted. 



A more direct and convenient method of applying this reaction is that 

 of Herzfeld and Lenart,^^ conducted as follows : — To 50 c.c. of a solution 

 containing not more than i per cent, of ketose sugar, bromine is added in 

 quantity i c.c. for each gram of aldose sugar present. After standing 

 24 hours at room temperature the excess of bromine is evaporated off, and 

 the ketose sugar determined in the residue. 



In either of these methods determination of the total sugars present 

 gives data to calculate the undetermined sugar. 



Separation of Glucose and Fructose from Sucrose. — Ammoniacal lead 

 acetate, prepared by adding ammonia to lead acetate until the opalescence 

 which forms just disappears, precipitates glucose and fructose from solution; 

 the sucrose remains in solution as a soluble lead compound. The precipi- 

 tated lead-sugar compounds are suspended in water through which is passed 

 a current of carbon dioxide ; the lead glucose compound is decomposed, 

 and is removed by filtration ; the lead fructose compound may then be 



