Chapter 2 The nutrients in food
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I am comparing the concentration of ascorbic acid in different apple products (such as eating apples, cooking apples and apple juice). I have results from my titrations of DCPIP with the apple solution. I am really confused about how you work out how much vitamin C is present with the raw data I have. Do you have a method of how to calculate the vitamin C concentration in mg? Thanks.
Igloo replies ...
I assume that, before you carried out your titrations with your various samples, you standardised your DCPIP solution with a solution containing a known concentration of vitamin C, for example one containing 0.25 g dm-3 (250 mg dm-3).
It is likely that that you measured the vitamin C solutions by pipette into the titration flask, and that the DCPIP solution was run in from a burette. Let’s say that 25.0 cm3 of this standard vitamin C solution required 20.0 cm3 of the DCPIP solution.
When the titration is repeated using 25.0 cm3 of the fruit juice, let us assume that the titration reading is now only 12.0 cm3. This means of course that the fruit juice is less concentrated in vitamin C than the standard one containing 0.25 g dm-3, and the precise concentration of vitamin C in the fruit juice will therefore be (12.0/ 20.0 x 0.25) = 0.15 g dm-3 (150 mg dm-3).
The calculation can also be carried out via what is known as the “dye factor”. However, I think that the method of proportion which I used above is the most straightforward way to tackle the calculations.
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updated: 07 April 2006
