Laboratory Procedure for Estimation of Glucose:
Standardisation of Fehling’s Solution:
Prepare a solution (known standard solution) of glucose AR by weighing accurately 1.25gm and dissolving it in 250 mL standard flask in water. Make up the volume to the mark.
Pipette out 20 mL each of Fehling’s A & B in a dry conical flask and shake thoroughly. Pipette out 20 mL of this freshly mixed Fehling’s solution in a clean conical flask and dilute it with 20 mL water. Heat the solution up to 70° over wire gauze. Take the standard solution of glucose prepared in a burette and run this solution slowly into the boiling Fehling’s solution until the blue colour has completely disappeared. Take care to maintain this temperature for every addition of glucose solution. Repeat the above titration by running the glucose solution steadily into the boiling Fehling’s solution until the end point is approached and then cautiously add glucose solution drop-by-drop till the end point is reached.
Alternatively to detect the end point more accurately, 5-6 drops of methylene- blue indicator may be added to the Fehling’s solution and then glucose solution added drop by drop. However, if methylene-blue is used as indicator the Fehling’s solution should not boil for more than 2-3 minutes at a stretch. The end-point here also is marked by the disappearance of the blue colour.
- Choose the titrant.
- Choose the titrate.
- Select the normality of the titrate.
- Select the volume of the titrate.
- Start titration.
- When the blue colour just fades Select the indicator.
- Continue the titration.
- End point is noted at the colour change of the solution.
- From the final reading the normality of titrant can be calculated by the equation:
- After finding the normality, the amount of substance in the whole of the given solution can be calculated by the equation:
- Note: 10 times dilute the stoke solution and that is used as titrant.
- Atomic weight of Glucose=180.1559 g/mol.
Observations and Calculations:
||Volume of Tirate (mL)
||Burette Reading (mL)
||Volume of Tirant used (mL)
Normality of Titrate used, N1=.............N.
Volume of Titrate used, V1 =..............mL.
Volume of Titrant Used, V2 =..............mL.
Therefore, the Normality of Titrant N2= =................N.
The amount of substance in the whole of the given solution =
- The amount of substance in the whole of the given solution =..............g.
Points to Remember while Performing the Experiment in a Real Laboratory:
- Always wear lab coat and gloves when you are in the lab. When you enter the lab, switch on the exhaust fan and make sure that all the chemicals and reagents required for the experiment are available. If it is not available, prepare the reagents using the components for reagent preparation.
- Properly adjust the flame of the Bunsen burner. The proper flame is a small blue cone; it is not a large plume, nor is it orange.
- Make sure to clean all your working apparatus with chromic acid and distilled water and ensure that all the apparatus are free from water droplets while performing the experiment.
- Make sure to calibrate the electronic weigh balance before taking the measurements.
- Clean all glass wares with soap and distilled water. Once the experiment completed recap the reagent bottles. Switch off the light, exhaust fan and Gas cylinder before leaving the lab.
- Discard the used gloves in a waste bin.