THE OPTICAL ASSAY OF SUGARS. 



60 cc. of a 5 per cent, solution of caustic soda; the material after addition 

 to the sugar solution is immediately neutralized. It is stated that an excess 

 has no action on the opticity of the sugars. 



The Effect of the Precipitate produced by Lead 

 Compounds in Clarification. In the majority of the ahove detailed 

 schemes for clarification aa insoluble precipitate is formed which occupies an 

 appreciable volume, so that if, after clarification, the solution be made up to 

 100 cc. the actual volume is 100 cc. less the volume occupied by the precipi- 

 tate ; prima facie, an error is thus introduced, though that this is the case 

 is denied by certain chemists. H. Pellet 5 in particular claims that the 

 precipitate formed by the addition of basic acetate of lead entrains sugar, and 

 that this entrainment compensates for the volume occupied by the lead 

 precipitate. In his experiments he shows that a weight of sugar material 

 dissolved in water and made up to 100 cc. in the presence of its precipitate 

 gives a reading of, say, 50, the same weight of sugar material made up to 

 200 cc. in the presence of its precipitate will give a reading exactly half the 

 first, in this case, 25 ; if the lead precipitate exercised an influence propor- 

 tional to its volume the first solution would be more than twice as concentrated 

 as the second, and hence the first reading should be more than twice as large as 

 the second ; this phenomenon he attributes to the entrainment of sugar by the 

 lead precipitate, and claims that it is unnecessary to apply a correction for its 

 volume. 



The writer in investigating the same subject found also that a fixed weight 

 of sugar material made up to different volumes in the presence of the precipi- 

 tate tends to give identical polarizations independent of the dilution, and 

 explains the apparent non-influence of the lead precipitate by an increase in 

 the specific rotation of cane products with dilution. 



Home's very detailed experiments 6 also point to the conclusion that the 

 lead precipitate introduces a positive error and that sugar is not entrained. 



Correction for the volume of the lead precipitate is made by the following 

 methods : 



1. ScheiUer's Method. 1 The material under analysis is first made up to a 

 volume of 100 cc. in the presence of its precipitate, and the reading taken; a 

 second reading is taken under identical conditions, except that the volume 

 is now made up to 200 cc. 



Let x be the volume of the precipitate; let a be the reading in 100 

 apparent cc., and 5 the reading in 200 apparent cc. 



Then (100 x) a (200 x) I. 

 Solving this equation x is found. 



Assuming that there is a change in the rotation of cane products with 

 dilution this method is inaccurate, and the writer modifies it thus : 



449 



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