[Sean13]

I get two different outcomes, even when using the same exact quantities of everything each time.
The first outcome has a dark blue/black endpoint and the same blue black color appears (and quickly goes away) before the endpoint.
The second outcome has the same dark blue/black endpoint, but when left overnight something precipitates and goes to the bottom, and the solution becomes clear/colorless (I assume this is the starch-triiodide complex.) The precipitate is blue/black, and when it is stirred it goes back into solution. Also, before the endpoint, instead of the blue/black color appearing and going away, a yellow/orange appears and goes away.
What could be the reason for this difference? Also, can both outcomes be considered a success? I use a previously prepared solution of potassium iodate and HCL, but weigh and add starch, iodide, and ascorbic acid each time.

these are the equations of what happens:
generation of I2
KIO3(aq) + 6 H+(aq) + 5 I- (aq) à 3 I2(aq) + 3 H2O(l) + K+(aq)

oxidation of vitamin-C
(2) C6H8O6(aq) + I2(aq) à C6H6O6(aq) + 2 I- (aq) + 2 H+
(aq)

The ascorbic acid solutions contains potassium iodide, HCL, starch, and between 100 and 150mL of water .
The titrant is potassium iodate.

[Borek]

Please elaborate of the exact procedure you are following, and exact procedures that lead you to the observed "outcomes". It is not clear what you did, nor why one sample was kept overnight and the second was not (or was it?).

[Sean13]

First, I prepare the ascorbic acid solution to be titrated. I add the ascorbic acid sample, add 1 molar HCL and potassium iodide in excess of what is needed ( 3.33mL in excess and 0.4g/0.002 moles in excess, respectively). Then I titrate it to the endpoint with a 0.01 molar Potassium iodate, always swirling the flask to keep everything in solution.

The first outcome that I sometimes get has everything in solution at the endpoint (nothing goes to the bottom). I left it overnight and nothing changed. The second outcome, at the endpoint, after a few minutes, begins to deposit something. When I left it overnight, it became clear and colorless and there was a dark black colored precipitate on the bottom. There is no relation between the two outcomes; it appears to happen randomly. Sometimes it will happen, other times it will not.

edit: the potassium iodate and HCL are two different solutions, not one, that I use for all of the titrations. 

[Borek]

Perhaps a difference in the amount of starch added, there is nothing inorganic that could precipitate here.

Do you add more titrant after reaching the endpoint? Or, in other words - is the total amount of titrant added always identical and stoichiometric, or can be it in a random excess?

What is the sample? Pure ascorbic acid, or something with an organic matrix?

[Sean13]

I add the same amount of starch every time. I only add the titrant until the endpoint-- once it reaches that, I stop adding it. Logically, the titrant could be in excess up to one drop, as we do not know how much of that drop was required to reach the endpoint. The sample is a Vitamin C tablet meant to be taken as a vitamin supplement. I grind it up in a mortar and pestle and take a portion of it.

I assume it is the starch that goes to the bottom, because I do not think that anything else could form that dark black color, and there is no color left in solution, which means that the starch is not present in solution anymore. So the question is: why is the starch sometimes soluble and why is it sometimes not?

Thanks