108 PUBLIC HEALTH CHEMISTRY 



at a temperature of 20 C. Clarify dark-coloured solutions 

 with basic lead acetate. If cloudy only, add 3 c.c. cream 

 of alumina, mix well, add one drop of PbAc, shake, and 

 filter. If yellow, repeat, adding more PbAc. If brown 

 or black, add 2 c.c. of 10 per cent Na 2 SO 3 , then PbAc 

 solution till no further precipitate. 



2. Estimation by Fehling's Solution. The sugar must 

 first be inverted by boiling with a dilute acid and 

 then estimated either volumetrically or gravimetrically. 

 (a) Volumetrically. Take I grm. of sample, dissolve in 

 50 c.c. aq. dest., add 5 c.c. concentrated HC1, and heat to 

 70 C. for 10 to 20 minutes to invert 



C j 2 H 2 2 O ! j + H 2 O = C 6 H x 2 O 6 -f- C 6 H ! 2 O 6 

 Saccharose Dextrose Laevulose 



Cool, and neutralize with NaOH, make up to 100 c.c. 

 (i.e., i per cent), and put into a burette. Take 10 c.c. of 

 Fehling's solution in a flask or porcelain dish, add 40 c.c. 

 of water, and bring to the boil. Run in the dilute sugar 

 solution carefully until all the copper is reduced, keeping 

 the liquid at the boiling-point all the time. The end point 

 is the chief difficulty in this method. Filtration is helpful, 

 but an indicator is commonly used. Allow to settle, 

 remove a drop of the supernatant fluid, and test on a 

 white tile with potassium ferrocyanide until a brown 

 precipitate is no longer given on acidifying with acetic acid. 

 Or, using Harrison's indicator, take several drops of a 

 freshly-made mixture (of 0-05 grm. of starch boiled with 

 water, 10 grm. of KI added, and the bulk made up to 

 100 c.c.) on a white tile, and from time to time a drop of 

 the hot liquid (precipitate and all) is removed and placed 

 on a spot and a drop of acetic acid is superimposed. A 

 blue colour develops until all the copper is reduced. 10 c.c. 

 of Fehling's solution = 0-0475 grm. inverted cane sugar 

 and 0-0500 grm. dextrose, laevulose, or invert sugar, and 

 0-0678 grm. lactose and 0-0807 S rm - maltose, (b) Gravi- 

 metrically. Proceed as above until the I per cent inverted 

 saccharine solution is obtained. Thereafter take 20 c.c. 

 to 50 c.c. of Fehling and boil. Add a measured quantity 

 of the i per cent solution (but always less than will com- 

 pletely precipitate the Fehling solution) and continue the 



