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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Emas on May 24, 2005, 06:17:23 AM

Title: How fast is Vitamin C Oxidized?
Post by: Emas on May 24, 2005, 06:17:23 AM

Hello,

In a few weeks time I am going to be doing a titration of Vitamin C, determining the amount of it in juice(or another similar substance) over time. This will experiment be repeated over a period of time(maybe 10 days?), hopefully showing that Vitamin C content drops over time because it is 'rapidly oxidized by air'(or so I have read).

The particular juice I am looking at doing has approx. 200mg of Vitamin C per litre.

What I am wondering, is am I actually going to see the effects of oxidation over say and 10 day time period? There will probably be two samples, one open to air at room temperature, and one open to air refridgerated.

thanx.

Title: Re:How fast is Vitamin C Oxidized?
Post by: xiankai on May 24, 2005, 08:21:30 AM

yes vitamin c is readily oxidised by oxygen.

what are u gonna titrate vitamin c with?

Title: Re:How fast is Vitamin C Oxidized?
Post by: Emas on May 24, 2005, 03:48:48 PM

I'll be doing a back titration (excess of Iodine).

Vit. C + I2 --> [Oxidized Vit C.] + 2H⁺ + 2I^-

IO3^- + 5I^- + 6H⁺ --> 3I₂ + 2I^-

then

2s₂0₃^2- + I2 --> 2I^- + S206^2-

Title: Re:How fast is Vitamin C Oxidized?
Post by: constant thinker on May 24, 2005, 08:49:30 PM

[edited]  Smelling may work also. Measuring amounts I don't know how to do that but I'd imagine the concentration of Vitamin C in the solution and wheather Vitamin C  will be suspended in the solution. Though Emas is right about Vitamin C oxidizing. I believe it is an anti-oxidant in the body.





Edited to remove that one line there constant.  You should NEVER do what you had suggested.  I know you put the warning there, but we can't take the risk of someone actually trying that and dropping dead.

Title: Re:How fast is Vitamin C Oxidized?
Post by: xiankai on May 25, 2005, 08:34:09 AM

pardon me for asking but i just wanna learn smth...

what is the 2nd equation for?

Title: Re:How fast is Vitamin C Oxidized?
Post by: constant thinker on May 25, 2005, 04:14:26 PM

Ok. I'm sorry. You are right and I'll keep it mind. I forget how dumb people can be.

Title: Re:How fast is Vitamin C Oxidized?
Post by: Emas on May 27, 2005, 01:21:10 AM

Vit. C + I2 --> [Oxidized Vit C.] + 2H+ + 2I-

IO3- + 5I- + 6H+ --> 3I2 + 2I-

then

2s2032- + I2 --> 2I- + S2062-

The second equation should really be first. That is, firstly, I am going to create an excess of Iodine (IO3- + 5I- + 6H+ --> 3I2 + 2I-) then I add the Vitamin C, which reacts with the Iodine (IO3- + 5I- + 6H+ --> 3I2 + 2I-) and finally i measure the amount of Iodine still present(left over), which will let me determine the amount of Vitamin C that has been oxidized.