82] Estimation of Sugar 53 



and run in 5 c.c. of the sugar solution, then bring it back over 

 the burner. A red precipitate will form. After boiling about 

 thirty seconds, slide away from the flame and allow the 

 precipitate to settle. If the liquid be still blue, more sugar 

 must be added ; if colourless, too much has been added 

 already. The colour of the liquid is best seen by allowing the 

 precipitate to settle completely, and then tilting the basin up 

 sideways so that the white side of the basin may be seen 

 through the liquid. In this case, however, the liquid will be 

 still blue. Add another c.c. of sugar solution. Boil for half- 

 a-minute, and allow to settle again. Repeat this operation 

 until the blue colour has nearly gone, then add the solution, a 

 drop or two at a time, boiling after each addition and testing a 

 drop of the liquid for copper by placing it on a porcelain tile 

 together with a drop of potassium ferrocyanide solution in 

 dilute HC1. A brown colouration shows that there is still 

 excess of copper. In this case add more sugar, boil, and 

 test again. 



To tell when the reaction is complete without the aid 

 of K 4 Fe(CN) 6 requires considerable practice, therefore the 

 student is advised to use it always. 



When the reaction is complete, read off the amount of sugar 

 solution which has been added. Now, the probability is that 

 this result will be higher than the truth. The reason for this 

 may be seen by allowing the basin to stand for about five 

 minutes after the determination has been finished, when the 

 solution will have become distinctly blue again from the re- 

 oxidation of the Cu 2 0. 



It is necessary, therefore, to do the estimation as rapidly as 

 possible. After having got an idea of the quantity which is 

 necessary by the preliminary operation just described, clean 

 out the dish, take a fresh 5 c.c. of solution I. and another of 

 solution II. as before. Mix with the rod. Heat to boiling, 



