§ 210. ESTIMATION BY POLARIZATION. 223 



long) should be — 15° ( = 15 x l"0),i and for dextrose alone T'Oß". 

 Supposing now the angle of rotation observed to have been 

 — 5 "202°, that is -9 "798° less than levulose alone, the amount 

 of the latter present can be calculated from the statement 



2883-3: 1883-3:: 9-798 :,->; 



■where 2883-3 is the difference between the rotation-constants of 

 grape- and fruit-sugar {1883-3- (- 1000-)j, and 1883-3 the con- 

 stant of grape-sugar ; multiplying out 



a; = 0-65317x 9-798 

 = 6-4 



which is the percentage of levulose, leaving 15-0-6-4 = 8-6 per 

 cent, of dextrose. 



§ 210. Cane- and Invert-sugar. — A similar course may be pursued 

 in other cases involving the simultaneous estimation of two sugars. 

 For cane- and invert-sugar, as they occur for instance in beet-juice, 

 it is usual to determine the rotatory power both l)efore and after 

 inversion with a dilute acid. If cane-sugar is present, more invert- 

 sugar will be found than would correspond to the rotatory power 

 first observed, 



According to Haughton,- the rotatory power of mixtures of cane- 

 and invert-sugar cannot be correctly determined unless the excess 

 of the basic acetate of lead, used to clarify the solution, is first 

 removed. He thinks that the lead forms a dextro-rotatory 

 compound with levulose. Such solutions containing lead are 

 also said to yield inaccurate results Avith Fehling's solution. 



If the use of basic acetate of lead alone is insufficient to render 

 such liquids as beet- juice, etc., clear and colourless enough for 

 examination in the polariscope, it may sometimes be advantage- 

 ously combined with solution of alum. It is frequently undesirable 

 to decolourize with charcoal, as that is capable of retaining some 

 of the sugar. 



The presence of asparagine can be the source of considerable 

 error : firstly, because that substance is itself optically active ; and 

 secondly, because its rotatory power varies as the solution is acid 

 or alkaline. Moreover, it is resolved, by boiling with hydrochloric 



^ Calculated from equation a = j-5^" . Neubauer takes the rotatory power of 

 levulose to be - 100° instead of - 106°. 



^ Journ. Chem. Soc. ix. 85; Zeitschr. f. anal. Chem. x. 490, 1871. 



