[OhyesOhno69]

I did an experiment where i had to determine the vitamin c content in fresh mango and dry mango. The method I used was an iodimetric titration.

I cut the mango into slices, 10 grams each, put it into a blender then I added water around 10 ml and mixed it. Then I transferred the juice into a 100 ml beaker and add water until the juice is 100 ml.

Before titrating, I put 60 ml H2SO4, 2 gr of solid KI and 50 ml of 0.001 KIO3 into a flask that will soon be used for titration.

I put the juice into the flask containing H2SO4, KI, and KIO3. I titrated using sodium thiosulfate 0.1 M. It required an average of 111.3 ml of sodium thiosulfate to titrate the fresh sample.

I dried the remaining mango (I did not cut all the mango) for around two weeks, then treated the dried mango using the same method as above.

Strangely enough, the average thiosulfate required to titrate the dried sample is 107.1 ml.

This is all the equation associated with the experiment.

Equation 1: potassium iodate and potassium iodide
IO3 + 5I + 6H+ => 3I2 + 3H2O
Equation 2: ascorbic acid and iodine
C6H8O6 + 2H2O + I2 => C6H6O6 + 2I + 2H3O+
Equation 3: iodine and thiosulfate
I2 + 2S2O32 => 2I + S4O62

After processing the data (finding the mol of thiosulfate then divide it by 2 to find the mole of iodine, which is also the mole of ascorbic acid, then multiplying the mole with the molar mass of ascorbic acid, 176.13 g/mol) I obtained that the vitamin c content in the fresh sample is 0.92 gr, and 0.7 gr for dried sample. This is weird, personally, since research stated that there's 16 mg for every 100 g of mango. (this is where i found that info: http://www.bawarchi.com/health/mango.html).

When standardized, the thiosulfate used for fresh sample is 0.094 M and 0.07 M for dried sample.

Has there been a mistake in the process? Or in the calculation? why doesn't the experimental value match the theoritical value?

[Borek]

Something is obviously wrong, recheck your numbers. What you basically state is that fresh mango contains almost 10% of vit C.  No way can the vitamin content be that high.

Where did you get the procedure from? Using over 100 mL of titrant is insane, you should aim at about 80% of burette capacity - be it for example 20 mL or 40 mL, depending on the glass used.

the mole of iodine, which is also the mole of ascorbic acid

It is not, it was back titration.

[OhyesOhno69]

The method is obtained from this website: http://daphne.palomar.edu/ngeetha/210%20Stuff/Iodimetric%20Titration%20of%20Vitamin%20C.htm

those numbers are what i had obtained. I was confused myself as well. Could it be an error in the method?

If the mole of ascorbic acid is not the same with iodine, then how do we get the mole of ascorbic acid?

thanks again for the help

[Borek]

Calculate amount of iodine produced through KI/KIO₃ reaction, calculate how much iodine was left after mixing with mango juice. Difference is what reacted with ascorbic acid.

[OhyesOhno69]

I re-do my experiment... and I used around 40 ml to titrate the fresh sample, while 38 ml to titrate the dry sample... after i calculated, the inital mol of iodine is smaller (0.00195) than the end mol of iodine (0.00206) (this is for the fresh sample). For the dry sample, I was able to find the mol reacted (0.00004 mol) and when I multiply it with the molar mass of ascorbic acid (176.13 gr), the vitamin c content (0.007 gr per 10 gr) is still larger than the theoretical value (1.6 mg per 100 gr). Was there anything wrong? is this a random or systematic error?