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Help with Vitamin C project calculations

Postby JEDDY » Thu Dec 20, 2007 3:32 am

I have finished my project - but I can't figure out a calculation... I added drops of orange juice (from different varieties) to 25ml of iodine/starch solution and counted the number of drops added until the solution turned from blue to clear.

I understand that the Vitamin C standard concentration is 1mg/1ml... also, there are 20 drops of water in 1 ml ... and it took 15 drops to turn my Vitamin C standard solution clear.

So - # of drops in a ml = 20
# of drops of Vit C standard to complete titration = 15

15/20 = .75 so the concentration of Vitamin C mg/ml is .750

NOW ... how to I use this to find the concentration of the juices?

ex: Navel -#1 - took 14 drops of juice to complete the titration...

What do I do now to find the concentration of vitamin c in the juice? Do I use the vit C standard (15) in this instance too?

Help! I'm missing a step here, or something...

Former Expert
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Joined: Sat Sep 03, 2005 10:20 am

Concentration questions

Postby paulsdecarli » Thu Dec 20, 2007 3:02 pm

I guess I don't understand the starting point or basic assumptions of your calculation. Does it take one mg of vitamin C to neutralize one cc of test solution?

If that is the case, then there would have to be 25 mg of C in the x drops of orange juice. Of course, I'm assuming that your OJ dropper has been calibrated so that you really do know how many drops per cc. you have.

Assume 20 drops per ml and then 15 drops needed to neutralize would contain 25 mg of C. 25 mg/.75 ml = 33.33 mg/ml = 25 grams/liter WOW!

That's a lot of vitamin C!

What I have just done is to demonstrate what we scientists call a sanity check. Take a close look at the numbers and see whether they make sense.

I have caught a lot of my own blunders (like multiplying when I should have divided) this way.
Will other constituents of orange juice hace aN EFFECT

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Occupation: science journalist

Re: Help with Vitamin C project calculations

Postby adance » Mon Dec 24, 2007 11:07 am

Hi Jeddy,

I found a paper online that I think explains what you're trying to do:

The way it's written, it may be too advanced for you, but I wanted you to know where my info is coming from and it's something you could show a teacher if you need more help.

Have you covered moles in chemistry class yet? That's a concept you need to understand for this project. A mole is 6.02 x 10^23 molecules. Depending on the size of the molecule, a mole of different compounds will have a different mass in grams. If I remember my chemistry correctly, a molecule's mass in g/mol=atomic mass in Daltons, which you get by adding up the atomic weight of each individual atom.

For example, water=2 hydrogen + 1 oxygen. H=1 dalton, O=16 daltons, so water=1+1+16=18 daltons, or 18 g/mol. That is, if you have 18 grams of water, you'd have a mole--6.02 x 10^23 individual water molecules. (You can get these numbers from any periodic table.)

From this paper, it looks like every molecule of vitamin C neutralizes one molecule of iodine in the starch solution. So, what you need to do is figure out how many molecules, or moles, of iodine are in your solution. That will be the same as the number of vitamin C molecules it took to neutralize it. The molecular weight of vitamin C is 176.123 g/mol (from the paper), so you can use that number to convert from moles to grams. Then, you can use your drop # to figure out how many grams in a ml of juice.

Hope this helps!

Amber Dance
Science Buddy

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