Quantifying Ascorbic Acid in Soft Drink
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 $\ce{I-}$:

$$\ce{I2 + I- &lt;=&gt; I3-}$$

The redox reaction then is:

$$\ce{C6H8O6 + I3- -&gt;C6H6O6 + 3I- + 2H+}$$

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:

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


  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). 

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


  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.