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I have access to a GC/MS, FID, and a spectrophotometer. My first thought was to derivatize with trimethylsilylating agent, then run it by GC. Unfortunately the water is problematic. Heating it is likely to destroy the ascorbic. I don't have access to a manifold freeze-drier. I do have access to a rotovap, but I don't think that works well for removing water. I do have access to a freezer, but no way to vacuum the sample down to 0.006 atmospheres. Titration is out of the question because I don't have a clue what else is in the sample.

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  • $\begingroup$ Is there something wrong with just running the sample through the GC/FID, and then with added internal standard of ascorbic acid? HPLC would probably be better, though. See e.g. dx.doi.org/10.1021/jf071710s $\endgroup$ – ssavec Dec 2 '13 at 10:34
  • $\begingroup$ @ssavec - You cannot inject aqueous samples containing ionic species into a GC unless you want to clog your system with nonvolatile detritus. Ascorbic acid is very soluble in water, so doing an extraction first is out. $\endgroup$ – Ben Norris Dec 2 '13 at 12:35
  • $\begingroup$ @BenNorris thanks for info, I just thought that almost everything is reasonably volatile under GC conditions. Obviously wrong. I intuitively expected the HPLC to be much better for this problem and now I also know why. $\endgroup$ – ssavec Dec 2 '13 at 16:06
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Titration is in the question because ascorbic acid is likely the principal, if not only, reducing agent.

Using iodine

For example, [this site] describes a titration using the iodine-starch redox indicator (the presence of iodine and starch together creates a blue/black color).

The active oxidant is $\ce{I3-}$, formed from $\ce{I2}$ and $\ce{I-}$:

$$\ce{I2 + I- <=> I3-}$$

The redox reaction then is:

$$\ce{C6H8O6 + I3- ->C6H6O6 + 3I- + 2H+}$$

Add a small amount of a saturated solution of soluble starch to your soft drink sample and then titrate against a known solution that is equimolar in $\ce{NaI}$ or \$\ce{KI}$ and $\ce{I2}$. When the dark color persists in your soft drink sample, you have your end point.

This site provides a full procedure (PDF).

Using $\ce{KMnO4}$

Potassium permanganate is its own indicator, since $\ce{KMnO4}$ solutions are purple and the reduced version is colorless (technically an insoluble brown solid).

If your soft drink is dark-colored, you may want to dilute it so that you can see the color change.

The redox equation is:

$$\ce{2MnO4- +6H+ 5C6H8O6-> 2Mn^{2+} + 8H2O}$$

Spectrophotometrically

Ascorbic acid absorbs UV light (but then so does other things in your soft drink, like the preservative sodium benzoate). However, you could use ascorbic acid's reducing power to turn on an absorbance in the visible. Here is one example. This paper may be behind a paywall, but the abstract contains the essential idea. In this paper, the authors describe a system using copper(II) and phenanthroline(phen). Ascorbic acid reduces copper(II) to (copper(I), which creates a coordination complex with phenanthroline that has $\lambda_{\text{max}}=460 \ \text{nm}$. The authors claim a detection limit of 40 $\mu\text{g/L}$, which is pretty good (40 parts per million).

$$\ce{2Cu^{2+} + C6H8O6 -> 2Cu+ + C6H6O6 +2H+}$$ $$\ce{Cu+ + 3phen ->Cu(phen)3+}$$

Another colorimetric assay

The Folin-Ciocalteu assay is another nice colorimetric assay. Reducing agents react with the phosphomolybdate and phosphotungstate compounds in the reagent mixture and produce a blue color. This method needs to be calibrated against some ascorbic acid standards because the redox chemistry is complex and not fully understood. The absorbance max of the reduced FC reagent is in the 670ish range, if I remember correctly.

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