Division 4.9 Two determinations of the sugar content must be carried out:

Test determination

Transfer 10 ml of a mixture of equal parts of Fehling's solutions into a 200-ml to 250-ml Erlenmeyer flask placed on a wire mesh. (Equal quantities of Fehling's solutions A and B should be mixed together a few minutes before the determination.) Using the burette, add about 25 ml of the sugar solution. Boil for 15 seconds.

Add further quantities of the solution every 10 seconds, until the blue colour becomes pale.

Add one or two drops of the methylene blue indicator and continue to add sugar solution until the indicator changes colour completely.

The boiling liquid turns reddish brown.

Final determination

Place 10 ml of a mixture of equal parts of the Fehling's solutions in a 200- to 250-ml Erlenmeyer flask, then add directly the quantity of sugar solution which was used up during the test titration, less 0,5 ml. Bring the mixture to the boil and simmer for exactly two minutes. Add one or two drops of methylene blue, then add the remaining sugar solution, two or three drops at a time, at 10-second intervals for about one minute, until the blue colour of the indicator turns reddish brown.

Let A be the quantity of sugar solution used up, expressed in 0,1 ml.

As this is an empirical method, all the instructions given above must be followed rigorously.