Science Buddies: "Ask an Expert"

Hi, I'm doing the wonderful science project on your website, the one testing vitamin C in oranges. But I have stumbled upon a problem, and that is I'm not sure exactly when the titration is considered complete. The solution starts to turn a murky brown, not blue, and it starts off light brown then darkens. I'm not sure when to stop recording the number of drops of iodine.
Also, I found out from a friend that you can also do it the other way around, drip in drops of the orange juice into a vitamin C indicator solution of starch and iodine. Would this be more accurate? I can't seem to find the background information on why this happens, what chemical reaction causes the indicator solution to slowly turn clear as the orange juice is dripped in drop by drop, so I'm not sure. Thanks.

Hi Wendy,

I think you are doing this really great project on the Science Buddies website:

https://www.sciencebuddies.org/science- ... p044.shtml

The murky appearance you are describing is normal for this titration, but I’m not sure why you are not seeing a clearer endpoint on the titration. The end-point should be a permanent trace of a blue-black color, so the end-point is when the sample turns from brown to dark.

The titration is normally done by adding the iodine solution to a fixed volume of orange juice as this allows you to quantitate the vitamin C in your samples. This makes it a little easier to graph your results. I would not recommend adding the orange juice to the indicator solution, as I don’t think this will solve the problem you are having.

Here are some questions that might help me give you some suggestions for improving your results.

1. Did you purchase the kit mentioned in the project description? The concentration of iodine required for the titration is difficult to obtain from other sources, so this may be the source of the problem.

2. If you did not purchase the kit, please describe how you made all of the solutions for the titration.

3. How did you dissolve the starch solution? Is your starch solution clear?

4. Are you working in an area that is well lighted?

5. Are you having a problem with both the standard vitamin C pills and with the orange juice samples? The standard should be clearer so it should be easier to read the end point. If you haven't tried a standard yet, do try one as this should give you some experience in reading the endpoint.

Let me know the answers to the questions, and I’m sure that I will have some additional suggestions.

Donna Hardy

Thanks for replying
I made the starch solution by placing 1gram of potato starch in 200mL of distilled water to create a 0.5% starch solution. I let it nearly boil on the stove until the starch had assimilated into the water. When it cooled and I took it out, it was a milky colour throughout. For the iodine solution, on the bottle it said 'W/V 1% available iodine'. For the juices, I simply juiced the oranges and measured out 20mL for each sample.
My starch solution was dissolved through putting it in water and stirring until it was near boiling, taking it off the stove and allowing it cool. It was not clear like water, but a milky translucent colour throughout.
I work in an area that is very well lighted.
I tried the standard already. It worked perfectly, the endpoint was perfectly clear.
However, when I tried with the orange juice, I added 10 drops of the starch solution, started the titration and continually reaches a point where it simply turns a murkier and murkier brown the more iodine I add to it. However, if I add some more random squirts of starch solution while it is a dark murky brown (I know you are not meant to, but I was getting fustrated and anxious about the due date of this project) the solution turns blue-black. Tiny little blue particles are also present in the solution when it is murky brown.
Is it because the iodine solution is deteriorating from exposure to air? Is vitamin C in the orange juice deteriorating?
I'm very confused.

Hi Wendy,

Thanks for the detailed information about your experiment. I think there are two possible reasons for the problem you are having:

1. The starch may not be in solution. Dissolved starch solution should be clear and translucent, not milky white. If the starch is not heated enough, then the starch won’t dissolve. Try boiling your sample again, while stirring continuously, let it come all the way to a boil. You will see a visible change in the appearance of the starch solution when is dissolves and this will require heating until there are visible bubbles.

2.If you did not purchase the kit recommended in the project then the iodine solution may not be concentrated enough. Here is a website that explains the chemistry of the iodine solution used for vitamin C analysis.

http://www.instruction.greenriver.edu/k ... aminc.html

Where did you get your iodine? Is it a Lugol’s iodine solution? Is it a reddish brown color? If you bought the kit on the Science Buddies website, however, then this is not the problem. However, if you did not get it from the kit, then the iodine may be too dilute to give you a clear endpoint.

So, try boiling the starch, and add a drop of iodine to the dissolved solution. The color should turn blue black. If this happens, then you will be able to see a clear endpoint when you titrate your samples.

Let me know if these suggestions help resolve the problem.


Donna Hardy