THE DETERMINATION OF CANE SUGAR 507 



into which is weighed a sugar of known polarization. This weight of sugar 

 is then made up to an apparent definite volume in the presence of the pre- 

 cipitate and a polarimetric reading taken. The apparent increase in the 

 polarization of the sugar affords data to calculate the volume of the pre- 

 cipitate. 



4. Wiechmanns Method** The precipitate is collected, washed free 

 of sugar, dried and weighed. Its specific gravity is then obtained with the 

 pycnometer, benzene being the liquid used ; from its weight and density the 

 volume of the precipitate is calculated. 



5. Home's Method** Home eliminates the error due to the volume of 

 the lead precipitate by making the solution of sugar product up to definite 

 volume, and clarifying by the addition of dry basic acetate of lead in powdered 

 form, and assuming that the volume of the acetyl radical which goes into 

 solution is compensated by the volume of the material precipitated. This 

 method has met with considerable approval, and may be considered as a 

 standard process. 



All the above methods are applicable to the analysis of juices measured 

 by volume and not by weight. 



Action of Basic Lead Acetate on Sucrose. The effect of lead acetate on 

 cane sugar is small, and is given by Bates and Blake 50 as under, pure sugar 

 being tested in normal concentration : 



Basic lead Difference in Basic lead Difference in 



acetate added. Polarization. acetate added. Polarization, 



cc. Ventzke. cc. Ventzke. 



0-5 . . o -09 . . 4-0 . . o -06 



1 -o .. 0-13 .. 5-0 .. 0-93 

 1-5 . . o -io .. 6-0 .. o -oo 



2 -O .. 0-13 .. 7-0 .. 0-05 



2-5 . . o -06 . . 8 -o . . o -09 



3-0 .. o -08 .. io -oo .. 0-19 



Action of Basic Lead on Reducing Sugars. The left-handed rotation of 

 fructose is diminished by the presence of lead acetate in alkaline solution, 

 so much so that in great excess the opticity may become positive. This 

 observation was first made by Gill. 51 It has also been shown by Davis 52 

 that the position of equilibrium is not obtained instantaneously, and that a 

 slow, progressive change in the polarization of products containing fructose 

 takes place in the presence of basic lead acetate. Neither glucose nor fructose 

 is precipitated by basic lead salts in pure solution, but, in the presence of 

 bodies which form insoluble combinations with lead, both glucose and fructose 

 are carried down, probably in the form of lead glucosate and fructosate. 

 This observation was first made by Lagrange, 53 and has been further studied 

 and proved by Geerligs, 54 Pellet, 55 Bryan 56 and Deerr, 57 since the absence of 

 precipitate in the system, fructose-water-basic lead acetate, has led to much 

 confusion and misunderstanding. 



It is also to be remembered that this lead compound is not broken up 

 by the addition of sodium sulphate or other precipitant of lead. Actually 

 the precipitation of an excess of lead in this way leads to a further precipita- 

 tion of reducing sugars and accentuates the error. The error is also intro- 

 duced b}' the use of neutral lead acetate followed by sodium carbonate to 

 remove an excess of lead. 



