Science Buddies: "Ask an Expert"

I'm testing different types of orange juice with starch-iodine solution to see how much vitamin c each type has.
I dropped orange juice in the starch-iodine solution until it turned colorless.
I was researching on many different sites and some showed that the more drops it took you to get the solution colorless means there's more vitamin C in it. However, some said that the more drops it took, the lesser the vitamin C.
I'm confused because websites are saying the complete opposite of each other.

Is it the more drops to get the solution colorless the less vitamin C, or the more it took, the more vitamin c it has?
Please answer ASAP since my project is due in less than 2 days.
Thank you.

chennyyoon - I think I might understand your confusion. Depending on what site is describing this experiment, you could see the indicator change color for two different reasons. In one case you have titrated an amount of iodine mixture into an orange juice/starch mixture to change an already "clear" indication to "blue" indicating that you have reacted with all the ascorbic acid in the solution and now have neutralized the ascorbic acid. The more drops of iodine mixture required, the higher the vitamin C (ascorbic acid) was in the mixture. This is the Science Buddies methodology of performing the titration.

Another method is to introduce orange juice (ascorbic acid) mixture into an already "blue" indicated reagent containing iodine and starch until it turns "clear" another variant of titrating the reagent. This is sort of the reverse. In this case, the more drops of orange juice required to change the indicator color (iodine neutralizing the ascorbic acid) indicates a "low" quantity of ascorbic acid in that sample. Just the reverse of the Science Buddies procedure.

I guess the moral of the story is that there may be two ways to titrate a solution, but only follow the instructions of one of them, all the way through.

These are very sensitive tests and after you complete your experiment, you can reverse the color of your reagent (blue-to-clear, or clear-to-blue) by adding a drop or two of the titrated (not titrating) solution. It will show you how sensitive and precise the titration method is in measuring sampled solutions.

Here is a description of the Science Buddies background. Understand what it is describing and you will have an easier time understanding where you were confused.

So how can you use the iodine-ascorbic acid reaction to determine the amount of ascorbic acid (vitamin C)? If you start with a known concentration of iodine, and carefully measure the amount of the iodine solution that you add, you can calculate how much ascorbic acid was present. How do you know when the iodine-ascorbic acid reaction is complete? You add an indicator to the solution. In this case, the indicator is soluble starch. When iodine reacts with starch, it turns the solution a blue-black color. If ascorbic acid is present in the solution, iodine will react with it, and not with the starch, so the solution will not change color. However, once all of the ascorbic acid has been oxidized, added iodine will be free to react with the starch, producing a distinct color change.

Rick Marz