Re: Orange juice/Iodine titration results seem backwards

I am going to assume your procedure was fairly rigorous, like the one here.

The first thing that comes to mind is asking whether you are using an iodINE solution or an iodATE solution -- this is important because what may be happening is that your test reagent is changing, not what you're testing. Or perhaps both, but I would expect an iodine solution exposed to air to change faster than orange juice.

But your last example (that you expect frozen orange juice to have more vitamin C, and frozen orange juice takes more iodine solution to titrate) is consistent -- assuming everything else is the same, more iodine soution means more vitamin C. A way to check your process is by running a "control" titration every time; for example, before you titrate an orange juice sample, grind up and dissolve a vitamin C pill that contains a known amount of vitamin C (e.g 250mg, which is .25g), and see if that titration stays the same while the juice titration changes.