Vitamin C investigation (volumetric analysis)

Concentration (%m/m) of vit-C in the commercial tablets.


Consider the bottle of vitamin-C tablets shown on the right.
Using a 0.100 M NaOH standardised solution, what is the theoretical value of the average titre expected for the titration of one tablet? The balanced chemical equation for the reaction between ascorbic acid (vit-C) and NaOH is shown below.

C6H8O6(aq) + NaOH(aq) --> NaC6H7O6(aq) + H2O(l)

Solution

 

Equipment
- 3 X 200 mL conical flasks
- mortar and pestle
- phenolphthalein
- Brand of Vit-C tablets
- Stock solution of 0.100M NaOH
- electronic balance
- Burette (50mL)
- funnel
- retort stand
- white tile
- distilled water

 

 

 
   
Step 1 Weigh a 200 mL conical flask. Record its mass in table1.
   
Step 2 Crush a Vitamin C tablet into a fine powder using a mortar and pestle.
   
Step 3 Place the crushed vitamin tablet in the conical flask and reweigh. Record the result in table 1.
   
Step 4 Place approximately 30-40 mL of distilled water and completely dissolve the powder.
   
Step 5 Fill the burette with the 0.100M standardised solution of NaOH. Record the burette reading in table 2.
   
Step 6 Place two or three drops of indicator in the the conical flask and titrate against the standard solution until the end point.
   
A permanent colour change of the solution in the conical flask indicates the end point.
   

Step 7 Take the final reading from the burette and record the result in table 2.

Step 8 Repeat steps 1-7 twice more.

1. Calculate the mass of vit-C (molar mass 176 g/mol) for each trial . Record, the result in table 3.

2. Calculate the concentration (%m/m) of vit-C (molar mass 176 g/mol) for each trial . Record, the result in table 4 .

3. Are the results repeatable and reproducible? Explain.

4. Are the results reliable? Explain

5. View the label shown on the right from the container of the brand of Vitamin-C tablet. What can you say about the validity of the results? Solution

 

Questions you may wish to investigate.

1. Do all fresh citrus fruit have the same %(m/m) of ascorbic acid (vitamin-C)?

2. How does boiling vegetable impact on the vitamin-C content of the food?

3. Does heat destroy vitamin-C?

4. How does the content of a cheap generic brand of orange juice compare with a similar well known Brand?

5. When cooking vegetables which method is better in preserving vitamins, steaming or boiling?