10 The Estimation of Carbohydi^ates 



of basic lead acetate solution is not allowed to exceed about 5 c.c. 

 in 300 to 500 c.c. of the solution, there is no loss whatever of laevulose 

 or other reducing sugars. Care must always be taken that the excess 

 of basic lead acetate is not left for any length of time with the sugar 

 solution; the latter after precipitation should be immediately filtered 

 on a Buchner funnel, and after thoroughly washing, the excess of lead 

 should be precipitated as soon as possible with sodium carbonate or 

 sodium sulphate. The solution can then be diluted to a known volume, 

 when it remains perfectly stable without any change occurring in the 

 sugars present^. 



Experiment has shown that a mixture of saccharose, reducing 

 sugars and tannin can be precipitated in this way without any loss of 

 sugar occurring if care be taken to avoid using any considerable excess 

 of basic lead acetate during tlie 'preci'pitaiion. Parkin^ has given an 

 example of this kind in which the following results were obtained in 

 a test analysis: 



Tannin added and 

 precipitated with 5 c.c. of Control in 

 basic lead acetate distilled water 



Saccharose ... ... 0-487 grm. 0-489 grm. 



Dextrose 0-204 „ 0-201 „ 



Laevulose 0-244 „ 0-244 „ 



Total sugar 0-935 „ 0-934 „ 



In this case tannin was added to the solution in amount just sufficient 

 to be carried down by the 5 c.c. of basic lead acetate ; the slight excess 

 of lead present was not removed and it is seen not to have influenced 

 the result. 



In agreement with the view the writer puts forward that no loss of 

 laevulose ever occurs by actual precipitation, but that when such 

 appears to take place it is caused by a transformation of the sugar 

 by the alkaline lead solution, are the facts recently recorded by 

 Le Docte^. The latter showed that whereas in the hot aqueous digestion 

 process of extracting sugars from sugar beet in presence of basic lead 

 acetate, the polarisation due to laevulose (which was purposely added) 

 disappeared entirely, it was not changed at all or only very slightly when 



1 If the solution is to be kept for any time a little toluene should be added and the 

 mixture well shaken. If sodium carbonate is used any large excess should be avoided, 

 tests being made during the precipitation until exactly the right quantity has been added. 

 When these precautions are observed the solution can be kept for months without any 

 change, even inversion of saccharose, taking place. 



2 Biochem. J. 1911, 6, 12. ^ Sucrerie Beige, 1912, 275. 



