Describe the procedure you would follow to determine the concentration of Vitamin C in a sample if the volume of DCPIP required to reach the endpoint was more than the 25-mL flask could hold. Note the titration must still be carried out in the flask. Be sure to include any calculations you would need to perform.
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To determine the concentration of Vitamin C in a sample using titration with DCPIP (2,6-dichlorophenolindophenol), even if the volume of DCPIP required exceeds the capacity of the 25-mL flask, you can follow these steps:
1. Preparation:
- Prepare a stock solution of DCPIP by dissolving a known mass of DCPIP in a suitable solvent to make a known concentration, preferably in the range of 0.01 to 0.1 M.
- Prepare a standard solution of Vitamin C by dissolving a known mass of pure Vitamin C in distilled water and making it up to a known volume (e.g., 100 mL) in a volumetric flask. Calculate its concentration using the formula: Concentration (C) of Vitamin C = mass of Vitamin C / volume of solution (in L).
- Alternatively, you can use a commercial Vitamin C tablet by crushing and dissolving a known mass in distilled water in a volumetric flask. Note that if using commercial tablets, the composition of the tablet should be checked to make sure it only contains Vitamin C.
- Determine the equivalence point (endpoint) of your vitamin C-DCPIP titration by titrating standardized Vitamin C solution with DCPIP, recording the volume of DCPIP solution required until the endpoint is reached. Assume this is the endpoint volume.
2. Calculation:
- Calculate the number of moles of DCPIP used at the endpoint by multiplying the known concentration of DCPIP by its volume (in L) used at the endpoint.
- Calculate the moles of Vitamin C present in the sample. Since Vitamin C reduces DCPIP in a 1:1 ratio, the moles of Vitamin C will be equal to the moles of DCPIP used at the endpoint.
- Determine the diluted sample volume: Subtract the endpoint volume from the 25 mL (the flask capacity) to get the remaining volume of sample required.
- Calculate the concentration of Vitamin C in the sample using the formula: Concentration of Vitamin C = moles of Vitamin C / diluted sample volume (in L).
3. Procedure:
- Pipette an appropriate volume of the sample into the 25 mL flask, ensuring it is less than the flask's capacity.
- Add the necessary amount of DCPIP solution, ensuring you have an excess compared to the stoichiometry of the reaction.
- Perform the titration carefully, adding DCPIP solution gradually while swirling the flask gently to thoroughly mix.
- Observe the color change of the solution. The endpoint is reached when a defined color change occurs (e.g., from blue to colorless).
- Record the volume of DCPIP solution used at the endpoint.
- Apply the calculations mentioned above to determine the concentration of Vitamin C in the sample.