APPARATUS AND MATERIALS:
· 1 dm3 of a solution containing 5% orthophosphoric acid (H3PO4)
· 100 cm3 of aqueous 2, 6- dichlorophenolindophenol (dcpip) (0.4 gdm-3)
· 75 cm3 of ascorbic acid (0.20 gdm-3) in 5% orthophosphoric acid solution
· Deionised water, boiled to remove dissolved oxygen which could otherwise interfere with the results if the Vitamin C determination
· Green cabbage
· Filter funnel
· Filter paper
· 50 cm3 burette
· 250 cm3 conical flask
· 500 cm3 measuring cylinder
· 250 cm3 beaker
· Bunsen burner
· Pestle and mortar
· Safety glasses
Preparation of 2,6- dichlorophenolindophenol solution
Dcpip is a dye which when placed in water turns blue, in acid it turns red, and when placed in ascorbic acid, it turns into a colourless compound. In order to prepare this solution, firstly, you need to dissolve 0.4 g of dcpip in deionised water. Then filter this and make the volume of the solution up to 1 dm. Since the dye easily spoils, it is best to keep it in a chilled and dark spot.
Standardization of the 2,6- dichlorophenolindophenol solution
Seeing that it is very difficult to prepare the solution accurately, the most prudent thing would be to standardize it. Therefore, you would first have to transfer 25.0 cm of standard ascorbic acid solution to a conical flask and titrate it immediately with the dcpip. As the dye mixes in, its blue colour changes to give a colourless solution. When the pink colouration due to the dcpip remains the same for around 10 seconds, it is considered to be the end point.
Testing the Vitamin C content in the water:
Firstly, weigh out 10 g of finely chopped cabbage, which you would then place in 20 cm3 of boiling water for 5 minutes. While waiting, pour 50 cm3 of orthophosphoric acid in two different containers each. After the 5 minutes, filter out the cabbage but do not spill out the water. Place the water from the boiled cabbage into a beaker and measure the volume. Lets call this value ‘Vc’. Pour the 50 cm3 of orthophosphoric acid into the water. Next, transfer 10 cm3 of this solution to a conical flask. This should be done using a pipette. And then, titrate it with the dye (dcpip) Record this value.
Repeat the above experiment for different boiling times, i.e. 4, 3, 2 and 1 minute
Testing Vitamin C content in the boiled cabbage:
After filtering out the cabbage, you need to liquidize it. Pour the orthophosphoric acid from the other beaker into the liquidized cabbage and stir the mixture. Measure this mass and lets call this value ‘Mc’. Remove about 1/12th of this mixture, you can estimate this value; you do not have to be very precise. This fraction of the mixture should then be weighed. Lets call this value ‘mc’. Next, filter this mixture. The filtrate should be made up to about 5 cm3. Lastly, titrate the filtrate with the dcpip. And hence, record the value.
Repeat the above procedure for cabbage boiled for 4, 3, 2 and 1 minute each.