[goanna_s]

I have performed a back titration on a vitamin C tablet to determine the amount of ascorbic acid present.

The method is basically to react excess iodine with the ascorbic acid and titrate the remaining iodine with thiosulfate.

What I want to know is why a back titration was performed, rather than titration directly with iodine. Is it because the endpoint is hard to determine with the direct method and therefore less accurate?

[mike]

How would you know the end point?

[goanna_s]

With the direct method? I guess when you start to see the brown colour of iodine appear.

[Borek]

Check kinetics of the ascrobic acid/iodine reaction - if the reaction is slow it is much better to add excess of reagent, wait till equilibrium (15 minutes is often enough) then reverse titrate remaining excess. Think how slowly reacting solution will behave near the endpoint - you add a drop of titrant and you see color change... only to dissapear after several seconds.

If I recall correctly iodometry endpoint is never as sharp as in some other titrations.

End point can be found in both cases - it is either colorless solution getting blue, or blue solution getting colorless. Near endpoint amount of free iodine is so low that solution will be not brown, but - at best - pale yellow.

[GCT]

It may be beacuse the end point of pertaining to the latter two titrants is preferred, or an excess of the first titrant is required due to stoichiometry.