ORGANIC cHEMISTRY - III

Experiment 1

Principle

Glucose reduces Fehling’s solution to reddish brown cuprous oxide as

C₆H₁₂O₆ + 2Cu (OH)₂ ------> C₆H₁₂O₇ + Cu₂O + H₂O

The Fehling’s solution can be standardized using a standard solution of glucose (0.5 %) with methylene blue as the indicator (1% aqueous solution).The end point is just the disappearance of the blue colour. Glucose in the given solution can be estimated using standard Fehling’s solution.

Materials required

Apparatus: burette, pipette, conical flask, volumetric flask etc

Chemicals: Fehling solution A and B, methylene blue, glucose solution etc.

Procedure

Procedure:

i.To prepare standard glucose solution

About 1.2 g of analar dextrose is weighed accurately (‘a’) gram, dissolved in distilled water and made up to 250 ml in a standard measuring flask.

ii.To standardize the Fehling’s Solution

Exactly 10 ml of Fehling’s solution of A and B is pipetted out into a clean 250 ml conical flask. The standard glucose solution is run down from the burette. After the addition of 10 ml of the glucose solution from burette. The solution is heated to boiling. About 3 to 5drops of a 1% aqueous methylene blue indicator solution is added. The gentle ebullition is maintained for 2 minutes and the sugar solution is added in small quantities (say 1 ml at a time).The titration is continued while the solution being allowed to boil for about 1 more minute till the blue colour just fades away. The titration is repeated to get concordant values (‘b’ ml)

iii.To estimate glucose

The given glucose solution is made up to 100 ml in a standard measuring flask. Exactly 10 ml of Fehling’s Solution of A and B is pipetted out into a clean 250 ml conical flask. About 10 ml of the made up glucose solution is run down from a burette into the beaker. It is heated to boiling. To the hot solution, 3 to 5 drops of methylene blue indicator solution is added and the titration is continued while the blue colour just fades away. The titration is repeated to get concordant values (‘c’ml)

Calculations

Weight of glucose present in 250 ml of the standard solution is ‘a’ gram.

Volume of standard glucose consumed for 20 ml of Fehling solution is ‘b’ ml

Volume of given glucose consumed for 20 ml of Fehling solution is ‘c’ ml

The amount of glucose present in the given solution is 100*a*b/c*250

Result

Amount of glucose present in the whole of the given solution=

Reference

egyankosh.ac.in/bitstream/123456789/12041/1/Experiment-4.pdf

blamp.sites.truman.edu/files/2016/01/Fehling-final.pdf

Questions

1. Difference between reducing and non-reducing sugars?

2. What are Fehling A and fehling B solution?

3. Draw the structure of glucose in different forms (linear and cyclic)?

4. Why heating is required?

5. Structure of indicator?

6. Any other method for glucose estimation?